Completion of the Electronic Rework

Sometimes the .05uf dual capacitors were used for the AC line filter but dual .01uf capacitors were also used on some later receivers. Also, there are two black (sometimes silver) tubular capacitors mounted against the front of the chassis on stand-offs. These are .25uf for the AVC delay in CW mode and .05uf for the Noise Filter circuit. These can be replaced with appropriate "Orange Drop" capacitors.

The B+ filter capacitor rebuild involves cutting the can of this triple 20uf electrolytic to remove the old capacitors inside. Mark the can before cutting to assure it goes back together in alignment. Cut slightly above the base crimp. Once the cap is in two pieces use a heat gun to get the black tar hot enough that the old insides just slide out of the can. The same treatment will remove any black tar in the bottom piece that has the bakelite terminal base. Wear heavy gloves when using the heat gun. Once the can is cleaned out you can install three new 22uf 450wvdc capacitors with the negative leads tied together. You will have to stack the caps as seen in the photo to the right for everything to fit into the can. Use sleeving to insulate the leads and electrician's tape to hold the three caps in alignment. You'll have to drill four small holes (.060") in the bakelite base of the capacitor near the terminal posts to allow the new capacitor's leads to be connected to the proper terminals. You'll notice that the terminals that are on the inside would make great connections but unfortunately they are made out of aluminum and can't be soldered to. Don't drill through the aluminum as it is so soft it will clog the drill flutes and break the drill bit - drill through the bakelite. Wrap the new capacitor's leads around the proper terminals and solder. Now you'll have to join the top of the can back to the base. Use some paper card stock to act as a backing for the epoxy that is used to glue the can back together. Coat both sides of the card stock with lots of epoxy and fit the top back in place making sure that the marks on the can are in alignment. Wipe the excess epoxy off the outside of the can. Wrap the can with masking tape to hold the two pieces in alignment until the epoxy sets up. Paint the glue joint with silver paint to hide the seam. This isn't a perfect, undetectable rebuild of the B+ filter cap but it makes the installation of new electrolytics much neater.

The tub mount 10uf  electrolytics do not use PCB oil but are filled with the old black tar so rebuilding is easy. An old style, really large, high wattage soldering iron with the huge 3/4" inch wide size tip is required. These old behemoths transfer enough heat to remove the bottom plate by melting the solder and prying up the bottom plate, one side at a time. Once the plate is off, dig out the tar and the old capacitor and replace with a new electrolytic as shown in the lower photo right. It's not necessary to replace the bottom plate. This process is next to impossible without using a very large soldering iron. If you don't have one, just replace the old tubs with new electrolyics installed on new tie strips for the bias supply and at the existing tie point for the driver tube and at the 6V6 tube socket for its cathode by-pass.

The BFO from the JX-21 receiver showing the detached end-cap and that the blade spring load is fully extended. Who knew I'd run into this same problem on this same BFO fifteen years later?

Shown in the photo to the left is the BFO from the JX-21 receiver. I had noticed that the BFO wouldn't hold alignment and many times when turning the BFO knob, the BFO frequency wouldn't change. As can be seen in the photo, the metal end cap that has the threaded rod passing through it had become detached from the coil form. This allowed the blade spring which provides an end load on the slug adjustment to become fully extended. This resulted in erratic movement of the slug and BFO frequency. The repair was to reattach the end cap using epoxy making sure that the end cap was fully seated onto the coil form and held in place while the epoxy cured.

On "non-X" optioned receivers, working on the BFO is easy only requiring removal of the side panel for good access and visibility. Then removing the BFO can to observe, repair and test.

I always try to return the receiver circuitry to the original design and layout. I always try to use components that are the same values and ratings as the original. It is acceptable to replace a 10% resistor with a 5% - but not vise versa. Replacing the molded capacitors is the exception and, when replacing the molded caps, be sure to use all the same type of new capacitors - it looks like a professional rework job then. I have found through restoring many communications receivers that the original design works best and many functional anomalies can be avoided by adhering to the original component type, original placement and original wiring layout. Once the receiver is returned to original, now all of the original documentation - no matter where it comes from - can be used and will be accurate for all future users of the receiver. Additionally, the manual's operational instructions will be accurate and the receiver will perform as described in the manual.

Although I don't really think it's necessary, a minor reshaping of the audio can be accomplished. If just a few minor changes are installed it usually won't limit the SP-600's ability to demodulate CW and SSB signals. Paralleling the two 5100pf coupling capacitors in the audio section with .01uf capacitors then paralleling the audio output tube's cathode bypass with a 10uf electrolytic and adding a 10uf electrolytic in parallel with the 1st AF Amplifier's 1K cathode resistor will give the audio output a response that sounds like a typical, older communications receiver. This simple approach will satisfy most users that feel that the original 125hz at -3db limited the bass too much and these few minor changes don't affect the ability of the SP-600 to function normally in the CW and SSB modes.

I think that the 600Z ohm output transformer also affects the bass response because a matching transformer is necessary when using a typical 4Z ohm or 8Z ohm loudspeaker. A high quality matching transformer will help. Also, a good quality loudspeaker and enclosure would help. Also, the loudspeaker should be placed several feet away from the operating position as the distance will reduce the level of perceived audio highs and enhance the audio bass response.

If your version of the SP-600 has the Diode Load terminals on the rear chassis, a shielded cable can be connected to the "diode jumper" and the shield connected to chassis. This can be routed to a high fidelity amplifier "sound system" and that will definitely help the audio quality. You'll lose the Noise Limiter (no great loss) and a 600 ohm resistor should be connected across the 600 ohm audio output terminals but the audio results should be impressive.

Cosmetic Restoration of the SP-600
 

Next, the individual captive shafts need to be dismounted. The knobs usually don't need to be removed since just the captive bushing nut on the backside of the panel can be removed. Keep track of which captive shafts go where since they are slightly different. The BFO shaft has the rotation stop pin, for example. In others, the shaft may be slightly longer or shorter. Since the knobs can be left mounted, that will identify the captive shafts. Dismount the CL meter. Dismount the Tuning Lock mechanism. Dismount the two dial escutcheons. Dismount the plexiglass from the backside of the panel that covers the band indicator. The front panel is now stripped of all parts and can either be cleaned and polished or it can be prepped for painting.

If you want to preserve the original paint then cleaning can be done with dish soap and water or any other sort of mild cleaner. Avoid using Windex because it has ammonia as its active cleaner which can remove some kinds of paint used for panel nomenclature. Glass Plus which is ammonia free can be used. Grease or oil film can be removed using WD-40 as a solvent followed by Glass Plus to removed the WD-40 residue. Most panels will have some chipping that can be touched up using matched paint. The panel can then be lightly polished or waxed if desired since the original paint is super tough.

Removing the original paint is difficult but repainting isn't too complicated since only the front of the panel is painted. The edges are also painted but the entire backside of the panel is left bare aluminum.

The original paint is very, very tough and will require a couple (at least) applications of heavy-duty methylene-chloride epoxy stripper (this type of stripper usually isn't available anymore, in 2024, so use any type of epoxy stripper that's available and hope for the best.) After stripping, make sure the panel is super clean and degreased before painting. Avoid sanding or brushing with a steel brush because originally the panel was chemically etched so a paint primer wasn't necessary. Since you're going to have to fill the engraving after the panel is painted you don't want to have anything that's going to add more paint than is necessary. Spray a couple of coats and see how the engraving looks. If it's still pretty sharply defined and the panel coverage looks good then stop. Too much paint will fill the engraving and prevent proper nomenclature filling. Two coats should be the maximum. Most off the shelf "rattle can" paints do have "fillers" in the paint to act as primers to fill scratches. These "fillers" aren't used in automotive spray paint. Also, the automotive paint will "shrink" somewhat as it is drying. At first, while the paint is not fully dry, the nomenclature will look like there is too much paint applied but wait at least one day and see how the paint looks after it has fully dried. It should shrink and the engraved nomenclature should then look sharply defined. Wait at least two days before doing the nomenclature fill. This gives the new paint a change to cure and harden.

Spray a small amount of EOOC into a cup and using an acid brush or Q-Tip, apply a coating to the chassis. EOOC does react with aluminum but you aren't going to leave it on the surface for more than just a few minutes. Wipe off the EOOC and then swamp the area with water using either a brush or Q-Tip. Gently wipe and repeat the water rinse a few times to neutralize the EOOC. Application of the EOOC should be performed in a well-ventilated area. The surface aluminum will be left very clean AND will have the flat finish of the original aluminum. This treatment leaves the aluminum very clean but doesn't "polish" or scratch to remove the oxides and dirt. >>>

Components like the power transformer and the chokes can be cleaned with any mild cleaner then touched-up as necessary.

It appears that the original method used to finish the dials was to chemically apply a black oxide coating to the dial. This might have been black anodizing but it could also just be an oxide treatment. Next, the white paint was applied into the areas that are to be the background color. This had to be a relatively slow drying paint that was allowed to settle leaving the high points - the dial numerals - slightly above the paint. Then a squeegee type of device skimmed the high points to reveal the black oxide underneath.

I haven't tried this method of dial restoration since spare original dials are so expensive and I don't want to take a chance on ruining a decent set of dials. However, I do now have a spare set of SP-600 dials that are in poor condition and sometime in the future maybe I'll attempt this type of restoration. For the time being though, the only accepted solution for poor condition dials is a photo paper overlay that is glued to the back of the dials and then reverse mounting the dials. These never look original and are only a "stop-gap" solution until good condition original dials are found or someone develops a good restoration method for the SP-600 dials.

And on that note,...over the years since I wrote about restoring SP-600 dials, I've come across many, many good condition original dials. They're pretty easy to find. Keep an eye on eBay as they turn up there often. Also, cheap SP-600 parts sets at ham swaps often have decent dials under the dirt. With just a little searching of the Internet, it's easy to find good condition original SP-600 dial sets for sale.

Meter Before

Meter After

The later Marion Electric RF/AF Meter

Cabinets are usually wrinkle finished and can be found in several different shades of gray but the original paint was a slightly brownish-gray color. The cabinets can be cleaned with Glass Plus and a soft brass brush, like a suede brush. Soak each surface in turn with the Glass Plus and then work the brass brush gently in a circular motion to work the dirt and grime out of the wrinkle convolutions. Wipe the area with a paper towel and repeat until the paper towel wipes are fairly clean. Don't scrub too long as the paper towels will never be perfectly clean and after a few times scrubbing you are really seeing some "dead" paint as color on the towels. Usually two applications is enough. Let the cabinet dry thoroughly and then do any touch-up that is necessary and let that dry. Finally, apply 10W Machine Oil to a clean cotton cloth and wipe the cabinet down. Then wipe off the excess oil with a dry cotton cloth. The cabinet will look beautiful.

A complete repaint is difficult. You can sand the original paint for prep and try wrinkle finish painting but it's very difficult to achieve even coverage and even wrinkling over such a large area and with multiple sides involved. It's possible to do a light over-spray using a satin or matte finish spray paint. Be careful as this usually looks much worse than expected. It's okay if the color sprayed is close to original but requires a bit of care if it's a "color change." Of course, the final method (and probably the best for such a large cabinet) would be to take the cabinet to a professional powder coating business. Nowadays, there are several types of wrinkle finishes that powder coaters can apply. Their black wrinkle finish is difficult to distinguish from original. Grays are somewhat problematic and depends on what the powder-coater has for paint when it comes to matching the original gray. I've only seen excellent black wrinkle from powder-coaters but this is always changing as more and more paints become available.

Mechanical Restoration Work

It's common to find the tuning mechanism dirty and many times this is what is causing the tuning slippage. Remove the front panel along with the main tuning dial, the logging dial and the band indicator dial. Spray the brass gears with WD-40 and work the dirt out with an acid brush. WD-40 is cheap and a good de-greaser. Dry any WD-40 residue with paper towels. When clean and dry, lubricate only the bronze bearings - sparingly, they are oil-impregnated but a drop or two of 10W machine oil will help since most of them are pushing 70 years old. The brass gears really don't need lubrication and too much oil or grease on the gear teeth are just "dust traps" that end up causing wear and other problems. Be sure to clean the rim of the logging dial with denatured alcohol. Sometimes you'll find the the rim of the logging dial has a thin film of grease on it. This will cause slippage when tuning. The rim of the logging dial must be super-clean. Additionally, the rim-surface of the logging dial must be "perfect." In other words, the can't be any dents, warps, gouges on the rim. In addition to a visual inspection, be sure to run your finger tip along the rim to see if you feel any imperfections. Any rim issues need to be corrected by lightly using a very fine file to dress down the imperfection so the surface is even and smooth. When dressing down the imperfections be sure to maintain the proper shape of the dial rim and don't remove any material other than the defects. Clean the rim afterwards with denatured alcohol.

The contact areas where the drive wheel engages the reduction wheel and the groove where it engages the logging dial rim must be super clean. Clean all of these brass wheels thoroughly with denatured alcohol. Again, check these mating surfaces for imperfections and correct any that are found. Usually the drive wheels are in good condition but it depends on the condition of the receiver and how it was stored.

Reinstall all three dials. Inspect the logging dial and how it fits into the drive wheel groove. Check the tuning action. It should be super-smooth. Once in a while, after cleaning, there might still be some tuning slippage. At this point, you might suspect the "S" tensioning spring that loads the drive wheel against the rim of the logging dial. The "S" spring sometimes looses its "springiness" but you can remove the "S" spring and bend it outwards to increase the loading then re-install it. Sometimes the "S" spring has just "had it" and it needs to be replaced with a new one (or a good one from a parts set.) Most of the time the "S" spring is okay and other problems are causing the slippage.

The FIX that WORKS - If you've tried everything,...thoroughly cleaning, checking the logging dial rim, readjusting the "S" spring and the dial still slips,...then the problem is likely excessive polish on the surface of the logging dial rim due to wear. If the slipping is still present, then apply a solution of rosin and denatured alcohol (about the viscosity of water) into the groove of the drive wheel using a small paint brush. You don't need much, just brush a little of the rosin-mix into the groove. Rotate the dial using the tuning knob to transfer the rosin solution on to the dial rim and then let the solution dry (alcohol evaporates.)  The "grip" of the rosin will assure that the dial will not slip anymore. If in the future the rosin seems to loose its grip, just reapply the rosin-mix to the drive wheel groove and tune the receiver to transfer the rosin to the dial rim. Easy.

If you have trouble finding rosin powder, try using a rosin block that's for a violin bow. Just use a small file and file a small amount of the bow rosin into a cup. Add a little denatured alcohol and stir until the rosin is dissolved in the alcohol. The viscosity of water is about right. If you don't know anyone that plays a string instrument, then try buying a rosin block at a musical instrument store.

When everything is "right" with the tuning mechanism its operation is better than "velvet smooth" and there will be no slipping of the dial drive.

NOTE: UG-103/U versus SO-239:  I received an e-mail from Hammarlund SP-600 historian Les Locklear that provided information that Hammarlund approved the substitution of a SO-239 for the UG-103/U in a revision sheet dated September 13, 1951. SO-239 receptacles appear on receivers as early as the SP-600 JX-8 versions. The conclusion is that it's possible to find a SP-600 with a SO-239 antenna receptacle that is correct and is an approved end-user installation.

The original antenna connectors for the SP-600. The UG-103/U twin-ax receptacle and the PL-102 (UG-102) twin-ax mating plug.

IF/RF Alignment of the SP-600

Crystal Filter - This circuit seems to have a lot of critics that claim it doesn't function very well, if at all. It WILL operate like a conventional crystal filter but there are a couple of set ups that should be checked before alignment. The PHASING control is a special dual stator-single rotor that should be verified as to its correct position. This will require taking the shield cover off of the Crystal Filter assembly. With a flashlight and a dental mirror the position of the rotor can be seen. The rotor should be at half mesh. It doesn't matter which side of rotation as long as the rotor is at half-mesh. Now verify that there's a red painted dot on top of the PHASING shaft visible when the shield is installed. If there isn't a red dot then paint one there for future reference (with the red dot at "12 o'clock" then the knob should be on the <>.) Now the shield can be reinstalled. Without changing the position of the PHASING shaft install the knob so it is on the <> or 5 on the scale. Now, after a full IF peak alignment, the Crystal Filter should operate as follows. At <> is maximum selectivity and closely around the <> will be the heterodyne attenuation. As the PHASING is adjusted to a position high or lower than the <>, then the IF passband broadens. It very important to have the PHASING knob installed with the correct relationship to the PHASING condenser in order to have the Crystal Filter actually work as it should.

What is really nice about aligning the SP-600 is that all of the adjustments are done from the top of the chassis - no need to have the receiver on its side and have to look at the tuning dial sideways.

Performing a Conventional IF Sweep Alignment on the SP-600

 Sweep aligning the IF section of any receiver allows the technician to have a view of the IF passband and to really see what the various adjustments performed in the IF section will actually accomplish. Peak aligning will get the receiver very close but there's always a slight "tweak" that's necessary for a good, symmetrical IF passband and just watching the audio output meter or the detector output line (or diode load) doesn't provide the accuracy needed. The reason is only the "peak" or "tip" of the passband curve shows on the VTVM. With the sweep alignment and the 'scope, the entire shape of the passband can be seen and adjusted for best symmetry.

Also required will be a dual trace oscilloscope that has X vs Y capability. Most 'scopes that were considered "high tech" in the 1980s or so will have this function. While it's not an absolute necessity to have the XvsY function, it's difficult to achieve the proper waveforms without additional equipment being required. I have three 'scopes here that can provide XvsY, a Tektronix 475, a B&K 1472C and a Leader LBO 505. Again, these were test-bench gear in the 1980s and nowadays are priced very reasonably,...many times they'll be free. Of course, theses aren't the only 'scopes that will work, it just happens to be what I have. For this particular sweep alignment, I used the Leader LBO 505.
 

Oscilloscope Probes - Using 'scope probes are the best cables for the 'scope connections,...if you have a set of them. The 'scope probes have compensation built-in that cancels the capacitance that's inherent in a shielded cable. That capacitance has a tendency to "roll off" higher frequencies causing distortion to RF waveforms. All oscilloscope probes will have a trimmer adjustment in the probe base-body and most lab-type 'scopes will have an on board calibration square wave generator that can be used to adjust the probe trimmer for a perfect square wave. The disadvantage of 'scope probes is that most are usually 10:1 probes input to output so a 2v p-p waveform input would only show as a 0.2v p-p waveform on the 'scope. Most of the time, the vertical amplifier gain (both X and Y) can be adjusted to give the display the correct proportions. If just shielded cables are used it's possible that there might be very slight distortion present because of the capacitance involved. If only shielded cables are available, then use the shortest lengths possible for minimum capacitance-effect. Using unshielded cables might result in stray pick-up or coupling problems since the input Z of the 'scope is very high.

The Sweep - You'll want to sweep from about 550kc down to about 350kc. On the HP 3312A, the dial's frequency setting is the highest frequency and the sweep goes down in f, so set the HP 3312A dial to 550kc and adjust the delta f (deviation) for about 200kc. Although these settings aren't all that critical, it does take a little "playing around" with the adjustments to get the proper looking passband image on the 'scope. Also, sometimes the Y input on the 'scope has to be inverted so that the passband image has the "nose" or peak at the top. Some 'scopes don't have an "invert" function so the image of the IF passband will be "nose down." This really isn't a problem since the shape of the passband is what is important. Additionally, the passband curves shown in the manual will have the "nose" down.

Photos - These are of the Leader LBO 505 'scope images. 

XTAL PHASING - The crystal filter phasing knob position is very important for a "peak alignment" but all sweep alignments will be in the "NON-CRYSTAL 3KC" position so the PHASING knob's position isn't important.

First do an accurate Peak IF Alignment - Perform a complete peak alignment of the IF section before doing the sweep alignment. Besides T4 and T5, the peak alignment will adjust L31 and L32 of T1 for accurate conversions. It isn't really necessary to sweep these two adjustments. Same with peaking T2. For sweep alignment be sure the receiver is in single conversion or set to 3.0mc Band 3. The sweep adjustments are going involve the two trimmers on T5 and the two trimmers on T4. The resulting symmetry at the detector output with the sweep input going to the 1st Mixer (photo 3) is what is important because this is the signal that then goes to the audio output reproduction.
 

2024 Rebuild of SP-600 JX21 sn:21649

This project started because I wanted to change the front panel color on this receiver back to a sage green I had painted it in 2010. I hadn't been using the receiver much and it had been sitting in the ham shack closet for the past five years. A quick check found that the BFO didn't function. Further problems were discovered and that resulted in a thorough "going over" of what I did to the receiver as an electronic rebuild almost 20 years ago.

I had the green panel SP-600 as the ham station receiver in the Western Historic Radio Museum, so it was essentially "on display" since the vintage ham shack was part of the museum. I think I only had the green panel job on display for a couple of days when a ham visitor to the museum commented, "Is that a Heathkit receiver? It looks like a Heathkit color." - he was referring to the SP-600. I was flabbergasted. Speechless,...I couldn't really think of a good comeback for the green panel, so I just told the guy it was an "old Hammarlund color" and left it at that. I hadn't thought that the panel color resembled any of the Heathkits but for the rest of the day, every time I looked at the SP-600, I thought "Heathkit!" I took the SP-600 out of the hamshack the next day and thought, "I have to change this panel back to Hammarlund gray." - an impetuous decision but that's what I did. Photo of the gray panel repaint is below in the "Collector's Gallery" as the JX-21. As can be seen in that photo, it was "a lot of gray." I regretted the gray repaint of the receiver and cabinet immediately but did nothing about it, until 2024.

The SP-600 SN:21649 was used occasionally and was part of my 630M station in 2018 that paired the HF SP-600 with my SP-600-VLF receiver. However, for the last five years, SN:21649 has been in the hamshack closet,...unused. Every time I looked at the photo of the SP-600 when it had the green panel, I thought about how much I liked the color combo. Pretty soon I was thinking about returning the panel to that "green" color. I still had the same "matched color" paint (I thought) and the spray can was still good (yes, I tested it.) All I needed to do was disassemble the SP-600 and strip down the front panel. But, a little receiver pretesting turned up some problems that needed to be taken care of before painting.

The first green panel from 2010

Power supply filter chokes and filter capacitor wiring discrepancy - Oct 18, 2024 - Since I had to remove this assembly to work on the BFO, I thought I would "clean up" the wiring that was looking a little frayed. As I was checking the actual wiring versus the wiring diagrams, I saw that all of the wires going into the circuitry should connect to the chokes BUT all of these wires went to the filter capacitor instead. When I rebuilt this receiver about 20 years ago, I think that I just resoldered the wires in exactly the same spot as I had unsoldered them, not double-checking (just assuming all was correct since the receiver worked.) The connections going to the filter capacitor are electrically correct but I would rather have the receiver reflect what is shown on the Hammarlund wiring diagrams (wiring to the chokes) since that avoids later confusion (like I had for a while when I was inspecting this hook-up,...20 years later.)

Broken Cap on SCO - This capacitor runs from the Oscillator Switch over to the Selectable Crystal Oscillator. In moving the SCO box this cap broke one lead right at the body,...impossible to fix. The cap is only in use when the SCO is on, which is never,...but I'll replace it anyway. 12pf cap.

BFO Problem - the coil form detached from the rear cup

This isn't the JX-21 panel, it's one I found in a SP-600 spare parts box

Stripping - Since this was a repaint job from probably 20 years ago or so, the paint wasn't anything special. As soon as the stripper made contact with the paint it began to disintegrate and was entirely removed in about 10 minutes. A nylon scrub brush was used to remove the stripper and paint. This was followed by a water rinse using a garden hose. The stripping was done outside. With the paint removed it was obvious that holes for mounting two data plate tags had been filled (located over the CL meter and over the Main Tuning dial escutcheon) and a deep gouge around the Selectivity switch mounting hole had been filled. With the paint removed, the panel showed very minor imperfections that almost any paint would cover. Stripping took place on Oct 20, 2024.

Nomenclature Fill - This is just tedious work. It takes patience because each bit of engraving has to be done at least twice. Once for the main filling and then a second time for best appearance. I must have used at least 60 or 70 pieces of 1.5" x 2.5" of paper towels that are wetted with Glass Plus and only used once and then discarded. The process is described in the section "Cosmetic Restoration" section of this write-up. The most difficult section to do are the two ovals that encircle the Xtal Chart and the MC Range opening. I had to use blue masking tape to define the border, then paint, let dry and remove the masking. The broad recessed area for the paint is too wide and too shallow for any other method to work.

ID/SN Data Plate from the JX-21

2024 Green Panel with Escutcheons Mounted
This particular paint mix is a slightly different hue than what I used in 2010. It has a bit more blue-gray in the mix,...interesting. It's still basically "sage green."

Functional Test and Operation - I connected the SP-600 JX-21 to the Collinear Array and it performed fine. I had to rebuild the Twin-ax and coax antenna cable. I switched over to a shielded magnetic loop antenna and performance was still fine. I had to let the receiver remain in operation for about half and hour before the drifting settled down. I can tell that the IF isn't aligned very well. It's been over 15 years since the last alignment, so maybe it's due. A sweep alignment should help but the peak alignment needs to be performed first.

Changing the 600 Series Cabinet to Black - This cabinet was black when it was given to me by N7RCA. I had thought for a while that it was an original color option but sometime later I took off the latch and saw the standard gray color. About 14 years ago, I changed that black color to Hammarlund Gray. Now, I'm going back to black. I'll do an Artist's Acrylic "wash" which is how I had applied the Hammarlund Gray over the first black paint job. The process is covered in the section "Changing the Cabinet Paint Color" in a section above. 

The sweep IF alignment has resulted in the passband staying "centered" when going from 3kc to 8kc and then to 13kc. Also, tuning across a signal like WWV will have the carrier level remain constant as the signal is tuned through. This type of IF alignment was a very noticeable improvement in the quality of reception and reproduction. 

The RF Tracking alignment really improved Band 4 performance since both the HFO and RF1 were off quite a bit. Now 20M performance is as expected. WWV on 2.5mc, 5.0mc, 15.0mc and 20.0mc are right on the index mark. WWV 10.0mc is slightly low but the alignment points are perfect, so probably a slight anomaly in the coil of that module. It shows 9.990mc instead of 10.0mc.,which is still within spec (it's about the width of the tip of the index pointer.)

My work bench test antenna is a shielded magnetic loop that's inside the work area. Once I had moved the SP-600 to the ham shack and had it running with the Collinear Array the limitations of a small indoor loop (even with a LNA) were obvious. With the Collinear Array as the antenna, I was able to receive XSQ 16.985mc, a marine beacon out of mainland China at about 1600hrs with a RST 579 (CW ID with trilling data pulses.) Also, heard Trenton Military on 15.035mc USB with aviation weather out of Trenton, Ontario, Canada. Heard both XSG and XSQ operating on 12.6mc, Chinese marine CW beacons.

The SP-600 is visually an impressive receiver due to its physical size, its enormous control knobs and the dual "portholes" that are used as the dial escutcheons. Now, add to that a sage green panel with a charcoal gray-black cabinet and the SP-600 looks like a different receiver. Not military in appearance but now looking like a European coastal marine station receiver - very cool!

SP-600-JX-21 sn: 21649    2024 photo
I know, you want to know what's in the background. On the left behind the JX-21 is the Hammarlund RBG-2 and matching loudspeaker. Center background is the Pixel Loop and the UK Union Jack. On the right farthest back is the black wrinkle HRO-7T, then the ART-13A and nearest right is the Collins R-389.

SP-600-25C sn:2793 - Back on the Workbench After 12 years in a Garage

When I closed the Western Historic Radio Museum in Virginia City in 2012 and moved to Dayton Valley, I put this SP-600-25C into the front garage of the workshop for storage. It was wrapped up in a heavy cloth for protection. I had rebuilt this 600-25C in 2009, replacing all of the molded capacitors and several burned resistors to get the receiver operational. Then, down in Dayton Valley, I did a quick operational check. I remember that there were two problems. First, the BFO was dead. Second, when I had reassembled the receiver, I didn't mechanically synchronize the tuning dial to the tuning condenser. I had performed a complete alignment and then discovered the problem. Since that was a "front panel off" to reposition the dial correctly and then a "redo the RF tracking alignment" type of fix, I didn't do the repair then. I just wrapped it up and put it on a small work table in the garage/shop. Now, with 12 years of storage in the mostly unheated part of the shop, it'll be interesting to see what else doesn't work.
 

The SP-600-25C receivers were built for the Signal Corps and have a very large 25-60 cycle power transformer and don't have the switchable crystal controlled oscillator option. The "CAUTION" tag in the upper left corner of the panel is not original but,... hey, it fits. The missing data plate might have designated the receiver as R-483/FRR. But, what is stamped on the serial number plate on the tuning condenser cover is "TYPE SP-600-25C" and that indicates that JAN parts weren't used in the assembly making one think this might have been a commercial version receiver. However, right behind the serial number tag is an orange Signal Corps acceptance stamp.

Extraction - Nov 8, 2024 - After twelve years, a lot of stuff gets piled up in the way of where I had the SP-600-25C stored. It was sitting on a small but stout work table, wrapped up in a gray heavy cloth. But, there were at least two layers of other stuff in front of the receiver that had to be moved first. Once I cleared a path, I removed the cloth and was glad to see that nothing physically had happened to the receiver,...it was just as clean as it was when I wrapped it up twelve years ago. The 25~ power transformer adds a little bit of weight to the receiver so I'd guess it weighs about 70 pounds instead of the normal 65 pounds. I used a hand truck with pneumatic tires to move the receiver from the shop to the house. Getting the 70 pound chassis up stairs is just about at my limit of lifting and carrying things up or down stairs anymore but it was accomplished without injury.

SP-600-25C sn: 2793 - 1953
This 2009 photograph shows the SP-600-25C installed into a 600 Series Hammarlund table cabinet. The cabinet was given to me by N7RCA and it had been painted black. I think the combination of gray panel, black escutcheons and knobs with the black cabinet looks great. This cabinet was repainted gray in 2010 but has been repainted black again in 2024. Now it houses the green panel SP-600-JX-21.

SP-600-25C Top of Chassis
Note the Signal Corps acceptance stamp just below the ID/SN tag. Note that the antenna input is a field installed SO-239 connector. Also, note how the 25~ power transformer actually "over-hangs" the rear chassis.     2024 photo

Inspection - Nov 9, 2024 - SN:2793 looked very nice. There were a few very light spider webs here and there but nothing really got through the heavy cloth that I had wrapped the receiver in while it was in the shop-garage. I put the receiver on the workbench and connected a 600Z speaker, the loop antenna and plugged it into a Variac in order to "soft-start" the AC voltage input. Soft-starting is a ramping-up of the AC input that takes about 30 seconds from 0vac to 115vac input (the action of the 5R4G rectifier tube filaments taking about 15 seconds to come up to temperature also acts as a "soft-start" on the B+ if the AC power is just "switched on.") I had background noise and a RF indication on the meter. I tuned in WWV on 10mc and then a few SW stations on the 24 meter band. On AM, the operation was okay considering the receiver hadn't had power applied in 12 years. I switched on the BFO and it had a very weak output. I thought I remembered that it didn't work at all. I replaced the 6C4 BFO tube - no change. I replaced the 6BA6 BFO buffer tube - no change. Besides the weak output, the heterodyne oscillation is very unstable and almost erratic sounding. Obviously, more troubleshooting will be required. On AM, tuned to 20mc WWV, the signal is very stable, so the problem is in the BFO (or so it seemed.)

Those are the two main problems but I'm sure a few more will show up after I run the receiver for awhile (this ended up being quite an understatement.)

At this time, I also used a small paint brush and applied DeOxit to the tuning condenser rotor contacts and then ran the tuning from low to high and back (to spread the DeOxit.) It also appears that I didn't clean these dials,...at least not very well. The fact that the dials were not fully cleaned and that the hub screws were ultra-tight makes me think that I never really had the dials off and this mechanical misalignment has been with this SP-600-25C for quite a long time. So, this time I cleaned the dials with Glass Plus (I was really surprised at how much crud came off of the dials.) Afterwards, I ran the main dial down to the Index mark just to recheck all parts of the mechanical alignment. I also applied DeOxit to the AF Gain pot and the RF Gain pot since both had been noisy in operation during the test. I shot a minimal amount of DeOxit down the barrels of the toggle switches because the AVC switch seemed to not be making good contact. The condenser box cover was reinstalled.

Installation of a Later Marion Electric RF/AF Meter - I had an extra one of these high-quality meters that was just sitting in a drawer. I thought that I might as well install it into this receiver. I still have three decent examples of the older style meters with the paper scales, just in case I want to "go back to original" (but, why would I?) The Marion Electric meter installs the same way that the older meters did. The scaling is slightly different on the RF scale but the AF scaling is identical to the older meters.

The BFO Problem,...or is it the BFO? - The BFO is working somewhat but the output level is low and the frequency stability very poor. I checked the voltages at V12 BFO and V13 BFO Buffer. I apparently had a couple of poor solder joints as a couple of voltages were not present or were not the expected voltage. This might have been a contact problem since all of the original solder joints are MFP coated. Soldering the joints involved got the voltage to read correctly but still the instability was present. Tubes were replaced with the same result. To confirm that the instability problem was in the BFO, I tuned to WWV on 10mc and then coupled an external RF signal generator to the input to put a heterodyne on the 10mc signal. There wasn't any instability in that heterodyne hook-up and that seemed to confirm that the problem was in the BFO itself. I had a "known-good" BFO assembly that only needed the coil form glued to be usable. I repaired this BFO assembly and then did a "R&R" on the BFO. Although this improved the BFO output substantially, the instability problem is still present under some conditions.

The BFO isn't the Problem but What Is? - Nov 11, 2024 - Further clues to this problem came when I attempted a RF tracking alignment. I peak aligned the IF first. It was very close and didn't require much adjustment. I started the RF tracking at the AM-BC (Band 1,) then Band 2 and then Band 3. No problems were encountered except every adjustment was significantly off because I had changed the physical position of the tuning dial so that it was now mechanically synchronized with the tuning condenser. Next, I started Band 4 by injecting 14.5mc to align the HFO. I couldn't get the receiver to remain on frequency at all. Just very erratic. Starting at Band 4, the receiver is in dual conversion. I switched back to 40M on Band 3, which is single conversion. The SSB signals with the BFO on were absolutely stable. I switched to 5mc WWV with the BFO on and everything was stable. So, in single conversion the receiver operates as it should. When in dual conversion, it's completely unstable. There are a few things to check. First would be to replace the 6C4 3.500mc Crystal Oscillator tube and the 6BE6 2nd Mixer tube and then check the 3.500mc crystal oscillator to see if it's stable. It could also be the dual conversion switch acting erratically maybe not making good contact,...clean and check. T2 is another possibility. I assume the reason that 10mc WWV, which is dual conversion, with a RF signal generator also at 10mc to provide a heterodyne, sounds stable is because essentially the two signals are being processed as one AM signal. The second conversion instability isn't apparent in the AM mode because the frequency variation is very slight and the combination of WWV 10mc and the RF generator at 10mc is actually "one signal." Since dual conversion starts with a 3.955mc signal that mixes with a 3.500mc crystal oscillator, the 455kc IF is what becomes unstable in dual conversion and since the BFO heterodynes with the IF and detector at 455kc, the instability shows up much more in the CW mode. Further investigation of this problem is required. New tubes,...no change. Cleaned dual conversion switch,...no change. Also had to recondition and solder two of the wires to dual conversion switch but still no change. Visual inspection of the 3.5mc Crystal Oscillator didn't really reveal any issues. Resoldered a few connections,...no change.

Replace T2? - A close inspection of T2 revealed that there was extensive corrosion underneath T2 that had started on top of the chassis. Whatever had caused the corrosion had migrated under T2. I dismounted T2 to have a look. It was bad enough that I had to use a brass brush to remove the corrosion on the chassis and the underneath of T2 actually had a lot of corrosion residue and discoloration. I had another T2 assembly in the spare parts, so I installed this T2 on the cleaned chassis. I realigned the second conversion (3.955mc) at T1 and T2,...to my surprise, no change. The instability was still present.

A 3500kc Crystal from 1968 Installed? - Since single conversion works perfectly and that uses the entire RF platform, the entire 455kc IF and detector out, the BFO and on through the AF stages, it seems the only circuits involved in the instability are the 3.500mc Crystal Oscillator that is switched "on" with the dual conversion switch and T2 (and it has been replaced.) The part of T1 that provides the 3.955mc conversion is also in the dual conversion circuit. Next is to substitute the RF signal generator for the 3.5mc Crystal Oscillator and see what changes (if any) occur. The substitution of the RF signal generator didn't change the instability. I went into the 3.5mc Crystal Oscillator again and this time I pulled the crystal. I was surprised to find the date code of 5-68 on the crystal (May 1968, the receiver had molded capacitors and was built in 1953.) It's certainly a replacement crystal showing that someone had worked in this same area earlier (of course, I didn't install the 1968 crystal but I had replaced the three capacitors inside the Crystal Oscillator housing.) The soldering of the crystal itself wasn't very good and I found a few other solder joints that looked a little sloppy. I cleaned the terminals were the crystal was mounted and then soldered it in place. I resoldered all of the joints inside the Crystal Oscillator and on the 6C4 and 6BE6 tube sockets. Finally, the SP-600 was stable when in dual conversion. I think the main problem had been the 3.5mc crystal's solder joints but since I did all of the Crystal Oscillator reworking at the same time, I'm can't be sure. But, with dual conversion now stable, I could proceed with the RF tracking alignment of Bands 4, 5 and 6.

RF Tracking on Bands 4, 5 and 6 - No problems encountered with the dual conversion bands alignment. The biggest problem when doing the RF Tracking is engaging the alignment tool into the slot of the trimmer C or L. You can't see down to the adjustment slot through the .375" diameter alignment holes and only a flashlight provides enough illumination. Then getting the tool into the slot requires "sighting" down the alignment tool for proper engagement. I assume that sometime during the SP-600 production there were proper alignment tools available. These would have a recessed metal blade down and inside a barrel that allowed placing the opening of the barrel over the L adjustment for centering and then turning the tool until the recessed blade "drops into" the slot. That way engagement was precise, easy and it couldn't slip out of the slot. The C adjustment tool would be somewhat the same but a different diameter to allow a good fit over the C-trimmer slot. I haven't seen any alignment tools that are exactly like that (the ones seen are too short or too large in diameter.) Also, the material of the tools I have (which is some type of plastic) reacts with the LC resonance adjustment tuning from 14mc on up. When adjusting the HFO, I have to determine just how far to detune the adjustment to have it "drop into tune" when the alignment tool is removed. Where the normal RF tracking alignment should only take 10 or 15 minutes, it takes about a hour and a half because of difficulty in having to see the adjustment slots and getting to tools to fit properly and to compensate for their reaction with the HFO.  

Under the chassis of the SP-600-25C - this is after the rebuild back in 2009 that replaced the molded tubular capacitors with SBE Orange Drops.     2009 photo

Close-up of the 25~ Power Transformer showing how the pylon is cut and a rear bracket installed on the chassis to help support such a heavy, over-sized transformer   2024 photo

Performance - The SP-600-25C performs just about like the JX-21. There's a slightly different RF Gain pot taper so the 25C has a better adjustment span when in MAN-CW. Everything else is just like the JX-21, including the slightly low dial reading for 10mc WWV. The 25C is a nice performing SP-600 and it's kind of nice to not have the upper right corner of the panel congested with "never used" controls. This receiver now functions very well and is accurate in the tuning dial's index points but since these are usually in 10kc increments it's impossible for "to the kilocycle accuracy." The audio is like all properly operating SP-600s and would certainly benefit from using a large speaker that's located several feet away from the listener.

SP-600-25C  SN:2793    2024 photo
The lack of the JX option leaves the upper right quadrant of the front panel looking a little vacant. But, since the JX controls are never used, they aren't missed when operating the 25C. This original condition front panel is spectacular. Just the usual "rack rash" that's common on original paint panels. The "CAUTION" tag isn't original to the receiver but it was used to fill the space that would have been occupied by the missing R-483/FRR data plate. Note that the RF/AF meter is now a late-version Marion Electric type.

SP-600 General Performance Expectations

Sensitivity - The SP-600 can be a very sensitive receiver. It's specs were 2uv in AM and 0.75uv in CW for a signal to noise ratio of 10db. It's sensitivity above 20mc all the way up to 50mc is impressive, especially for a receiver that was designed in the post-WWII, late-forties era. To achieve the rated sensitivity however, the receiver will require a full rebuild, NOS or equivalent tubes and MOST IMPORTANTLY,...an accurate IF/RF alignment using laboratory-quality equipment. Additionally, a resonant or matched antenna for the tuned frequency is necessary to achieve the highest level of sensitivity. When these criteria are met, the SP-600 will hear just about anything on the air, with propagation conditions, QRM or QRN being the only limitations. 

Selectivity - The selectivity is very good at the 3kc position for SSB and CW signals. AM signals can have the bandwidth expanded to 8kc and if the station is very strong, try 13kc for the ultimate in wide bandwidth audio response. I find that the 3kc bandwidth is actually good for strong AM signals since the upper audio frequency attenuation tends to enhance the bass response a little resulting in a mellow sounding audio. A sweep IF alignment will really help in getting the proper relationship between the 3kc, 8kc and 13kc bandwidths. With a sweep IF alignment, strong AM stations sound very nice in the 13kc bandwidth.

The Crystal Filter has it critics but I find that it works very well on CW but even SSB or AM can benefit from its use when adjacent frequency interference is a problem. Accurate alignment of the crystal filter is crucial for its proper operation and the correct alignment depends on accurately setting the PHASING knob to agree with the dual-stator/single rotor PHASING condenser. When set up correctly with a full IF alignment, the Crystal Filter works just like one would expect. The <> is the maximum selectivity and heterodyne action takes place around and near the <>.

Dial Resolution Vagueness and Rapid Tuning - Many users feel that the tuning dial's vague readout accuracy is the main disadvantage to using the SP-600 receiver and generally you'll find these critics comparing the SP-600 to the R-390A. These two receiver's utilize design approaches that couldn't be more diverse - not to mention that the R-390A cost was at least two and a half times that of the SP-600. The SP-600 covers large slices of the spectrum in each tuning range. You just couldn't get one kilocycle accuracy when each tuning range covered several megacycles using a variable-C type of tuning. Collins utilized a precision PTO (permeability-tuned oscillator) with a tuning range of just 1000kc in ten turns. That was mixed with a multiple-crystal-controlled oscillator and variably tuned IFs that resulted in double or triple conversion and 1kc or better tuning accuracy. It was expensive to build the R-390A and it could be difficult to maintain. The SP-600 was easy to maintain, although parts of it are difficult to disassemble, it's easy to work on (sort of) and, when in use, it allowed rapid coverage of wide areas of the spectrum - something necessary for certain kinds of surveillance. This rapid tuning though does make tuning in some stations difficult, especially at the higher frequencies. The logging scale was provided to allow a means of accurate resetability and its use will really help in finding net frequencies for monitoring. Once you've found the net, make a note of the logging scale reading and you'll always be able to retune right to the correct frequency.

Drifting - What can you expect from Hammarlund? The typical SP-600 doesn't drift excessively after a 30 minute warm-up (it might never stop drifting but it's a minimal drift after 30 minutes.) As mentioned elsewhere, the Crystal Controlled Switchable Oscillator - the "X" option - is rarely used. It requires specific crystals for the desired tuned frequency and then you still have to tune the receiver to the desired frequency so that the RF stages and the Mixer are in tune.

Audio Reproduction - Another often heard complaint is the SP-600 audio. An individual's impression of the SP-600's audio quality is obviously a subjective judgment. Some users find the audio to be standard "communications grade" with limited bass response while others listeners find the audio to sound "high fidelity." The stock bass response is rolled off so that the -3db point is at about 125hz resulting in a moderate bass at best. Use 3kc bandwidth for upper audio attenuation to enhance the bass somewhat. Be sure to match the 600 ohm output for the best audio reproduction and use a large speaker in a good enclosure. The audio has a lot of highs when the bandwidth is in the 13kc position and this tends to really de-emphasize the bass response but again, it depends on the loudspeaker being used. A lot of the listening experience is dependent on the loudspeaker location. If the loudspeaker is directly in front of you and only about two feet away you'll be hearing the high frequency audio overpowering the bass response. Try locating the loudspeaker about ten feet away. It will require the AF Gain to be somewhat higher but the loudspeaker's distance tends to roll-off the audio highs and, since low frequency audio has more power, the bass response is improved. The advanced AF Gain also tends to improve bass response.

SSB Reception Issues - The calibrated BFO can be very accurate and very handy for SSB. You do have the capability of selecting upper or lower sideband by which side of "0" you set the BFO. BFO-drift is just about non-existent after about a 30 minute warm-up. To set the BFO to the correct sideband, first set the BFO to "0" and then tune in the SSB station to "zero beat" or where the audio sounds "muffled and bassy" - then, by adjusting just the BFO to one side or the other of "0," the audio will clear up and sound normal. Whichever side that is will be the correct setting for whatever sideband that station is using and more than likely for all the SSB ham stations on that particular band. The SP-600 will provide excellent SSB audio. And you don't need a Product Detector or a CV-591-type of external SSB IF unit either. Good SSB demodulation requires that the detector has the proper ratio of BFO injection to the incoming signal level. Best audio will require that the AVC is in the OFF position and that the RF gain control used to set the correct ratio. Turn the AF gain up to 6 or 7. Turn the RF gain to a level where the incoming received background noise is moderate - usually about 7 or 8. Selectivity should be on 3kc. With an SSB be signal tuned in, reduce the RF gain if the audio sounds distorted. At the proper RF gain setting, SSB signals sound fine on the SP-600. The disadvantage is that every signal has a different strength and the RF gain usually has to be adjusted each time. With "one on one" QSOs, it's usually not a problem but Net Operations, with many different stations "checking in," can be somewhat frustrating. You can operate with the AVC on to limit the maximum sensitivity (with the RF gain set to about 7) but strong SSB stations might still distort. Most versions of the SP-600 provide an adjustable BFO Injection adjustment located on the rear apron of the receiver. Be aware that when the BFO Injection is adjusted to maximum, the BFO Buffer tube's cathode is grounded and the tube will tend to run somewhat hotter than usual. If the BFO output is typical for a good condition SP-600, ample BFO injection for SSB will be with the Injection set to about 50%. A very low-level of BFO injection was provided for CW signals and prevented "masking" of very weak CW signals.

Fixed Antenna Input Impedance - By using the ham station antenna you will be providing the SP-600 with a matched antenna that will allow it to perform at its best. Many casual listeners using unmatched end-fed wires may be disappointed at the SP-600s performance, especially when considering the high noise levels experienced with end-fed wires. The SP-600 was designed to operate with a matched antenna and this set up will give the best performance results. There isn't any sort of Antenna Trimmer control provided because Hammarlund expected the end-user to provide a matched antenna (dipole usually) with approximately 100 ohms impedance (50Z or 75Z will also work fine.)

Remote Standby Relay - Most SP-600 versions will have a RELAY socket that looks like an AC receptacle. This parallels the SEND-REC switch and allows the receiver to be put into stand-by remotely. This can be done with the auxiliary contacts on a Dow-Key type relay and it makes using the SP-600 as the station receiver very practical. Be sure to leave the SEND-REC switch in the SEND position when using the remote stand-by function. An exception to this RELAY socket is the popular SP-600 JX-17, which, since it was for diversity reception, does not include a remote standby option.
 

1. Limited Dial Resolution - No surprise since each tuning range covers megacycles. Although the resolution is vague, dial accuracy is usually very good with "marker frequencies" generally being right on the pointer-index.

2. No Crystal Calibrator - Since the relationship of the dial index pointer to the tuning dial can't be changed, a crystal calibrator would be of very little benefit. Hammarlund assumed that the receiver would be used by the military or commercial users and a heterodyne frequency meter would be used if accurate frequency setup was necessary.

3. No Antenna Trimmer - The antenna input is designed for a balanced line of 95Z ohms nominal. Hammarlund expected that impedance-matched balanced antennas (dipoles, or similar) would be used. See the next complaint that's related,...

4. Unusual Twin-Ax Antenna Input Connector - These were installed for using balanced 100Z shielded dual conductor cable (called Twin-ax.) The PL-102 connectors aren't difficult to find but it was accepted by Hammarlund that the UG-103/U could be replaced with a SO-239 UHF coax connector in the field (by the end-user.) One of the two wires that went to the two pins of the UG-103/U would be grounded with the remaining wire connected to the center pin of the SO-239 for an unbalanced input suitable for other types of antennas,...verticals, for example. Most users nowadays just make the proper connection inside the PL-102 with a RG-58 coaxial cable extension with a PL-259 on the opposite end. NOTE: The Twin-ax type of coaxial cable has very high capacitance per foot losses that increase substantially as the frequency increases. This high-loss limited the usefulness of this type of cable for antenna feedline purposes.

5. Frequency Drift - Typical of ALL Hammarlund oscillator designs, the HFO and the BFO tend to drift quite a bit. I don't know if they really ever settle down but after 30 minutes of warm up, the frequency drifting is about as good as it's going to get. Certainly the military use involving RTTY mandated employing the JX option for crystal-control of the HFO but Hammarlund assumed that the lower frequency of the BFO operation would be stable enough. Some of the post-WWII Signal Corps BC-794 modifications to "JX" control did also involve a crystal-controlled BFO since these receivers were specifically for RTTY. For typical ham operations, especially in a vintage equipment station, the SP-600 frequency drifting is not noticeable on AM signals and, after 30 minutes warm up, SSB and CW signals are also more or less stable enough.

6. Rapid Tuning Rate - The SP-600 tuning ranges cover wide sections of the spectrum. Good for surveillance or quick checks on known stations. Amateur CW and SSB signals, especially when tuning above 10mc, can be difficult to tune in due to this rapid tuning rate. Often, one gets close with the tuning and then does a fine adjustment using the BFO. Of course, this isn't as noticeable on AM signals. The very large tuning knob does help a lot.

7. Crystal Filter Easily Misaligned - The Crystal Filter requires careful set up for it to work correctly. The position of the PHASING knob is critical to the Crystal Filter's proper and expected operation. Because of the ease at which the Crystal Filter can become mis-aligned, it has gained a reputation of "not working very well." When set up correctly, the Crystal Filter will operate like a typical crystal filter.

8. Selectivity Switch Problems - For some reason the Selectivity switch always seems to have contact problems with switching noise being the symptom. Frequent cleaning with DeOxit will help.

9. Audio Bandwidth and Output Z - The audio is "communications grade" with the -3db roll-offs at 125hz and 4500hz and a "flat response" from 200hz to 2500hz. When selecting 13kc IF bandwidth, the increased higher audio frequencies tend to make the audio response sound thin without much bass. Better overall audio response for typical AM stations will be experienced in the 8kc bandwidth, or even in the 3kc bandwidth for a very mellow sound. It does depend on the quality of loudspeaker system used and where the loudspeaker is located. The audio output impedance of 600Z ohms requires a matching transformer and the quality and design of the matching transformer also can affect the low-end audio.

10. Size, Weight and Cabinets - The SP-600 is a physically large receiver that takes up a lot of bench space. Install the receiver into a cabinet and it's even bigger. Then there's the carrying handles on the sides of the cabinet that always seem to limit what's put next to the receiver. Weight of the chassis is 65 pounds but put that into the Hammarlund cabinet and the weight goes up to 92 pounds. Genuine Hammarlund Series 600 cabinets are moderately difficult to find.

11. A Final Note - So, are these serious enough operational issues that the SP-600 wouldn't be considered as THE station receiver in a vintage ham station set up? I think the SP-600 can perform quite well but there are a few things that can affect the enjoyment of using this receiver. Size and weight might be one issue. It's difficult to integrate such a large receiver onto a desk top with other equipment. Another perceived problem is limited dial resolution making "to the kilocycle" tuning is impossible. Then there's the problematic audio reproduction that might affect the enjoyment of AM signals. I think another important issue would be the difficulty in rebuilding an early SP-600 and, even after that formidable task is finished, then there's the "all important" proper and correct alignment. A careful and accurate alignment WILL make ALL the difference in whether the SP-600 attains an acceptable performance level or not. It's difficult to find one really important issue that makes the SP-600 undesirable as THE station receiver. Who knows? Maybe it's the SP-600's unique "dual portholes" appearance. When considering the SP-600 as a vintage ham station receiver, try to remember that the receiver was designed over 75 years ago. The receiver was at the upper end of top-performers in 1950, when it was introduced. The SP-600 was a competitive receiver at that time but it can't compete with 75 years of receiver design and electronics evolution. When listening to an AM signal using the SP-600, be sure to take into consideration that the receiver was originally intended for surveillance and data reception by the military,...not as a listening-pleasure band-cruiser.

Collectors Photo Gallery of SP-600 Receivers

SP-600 JX from the RCA David Sarnoff Research Center

This incredible SP-600 is owned by Jeff James W2NBC. It was purchased by RCA's David Sarnoff Research Center in 1957. Since the panel is RCA Umber in color and the cabinet is also the grayish-brown that RCA used in their equipment, it seems likely that this receiver was part of a special order from Hammarlund and painted in the RCA colors. The RCA "meatball" logo is fifties vintage but the tag in the upper left corner uses the later "block" RCA letters (came about in 1964.)  There was also a calibration tag above the RCA meatball that was last dated in the sixties. Jeff has restored this special SP-600 but the panel is original paint. The cabinet was matched to the original color found under the grab handles.

SP-600 JX-14

SP-600 JX-14 with a light greenish-gray panel and black nomenclature. RACAL popularized this panel color that they referred to as Admiralty Grey.

Photo from eBay.

SP-600 JX

Though this SP-600 is in fairly rough condition one can't help noticing that the front panel has been painted light blue and the dial escutcheons painted dark blue. Obviously, when doing an end-user repaint,...anything goes. With the white cabinet, this was probably a visually striking SP-600 after it had just been repainted.

Photo from eBay

SP-600 JX-17

SP-600 JX-17, standard configuration. The JX-17 was probably the most produced version of the SP-600. It was designed specifically for diversity operation and has a slightly different AVC system because of that. The red knobs are specifically for diversity operation options and are only found on the JX-17 version. Since the JX-17 was for diversity set-ups, a remote relay socket isn't provided on the rear chassis. The cabinet shown in this photograph has been repainted to a cream-color.

Photo from eBay.

SP-600 JX-17 Console

Here is a SP-600 JX-17 that is installed into a wooden console made out of plywood that has been stained and finished. The large speaker area is integral to the cabinet and the receiver is mounted from the front into its area. This SP-600 console is in Berlin, Germany and was used by the US Army "Berlin Brigade," possibly at Andrews Army Base where it was placed in the soldier's dayroom as an "entertainment radio."

The photo is from Nicolas von Moellendorff, who is the current owner. Nick obtained this SP-600 from the last manager of the US Army Laundry in Berlin. It has a 120vac to 240vac transformer external to the receiver for operation on European line voltage (and interfacing to Euro-plugs and receptacles.)

Nick had a small museum in Berlin for US Army items and equipment. He currently has a militaria website. The URL is www.berlin-military.com

SP-600 VLF31

The SP-600-VLF31 receiver tunes from 10kc to 540kc and is built along the lines of the HF SP-600 receiver. The "VLF" is NOT a SP-600 with LF coils installed in the turret. It is a completely different circuit designed specifically for low frequency operation. First, it's a single-conversion superhet that is using 705kc as the IF. It employs two TRF amplifiers in the front end and uses a total of 21 tubes. There's a 1160kc conversion oscillator circuit that mixes with the 705kc IF output to provide a 455kc IF output to run RTTY or similar devices. The Selectable Xtal oscillator has four positions and uses FT-243 crystals. There are only five selectivity positions and all of them go thru the dual stage bandpass and phasing crystal filter. Four IF amplifiers are used. While there are significant differences in the circuit, the construction and most of the mechanics are the same as the HF SP-600. Turret band switching, same dial drive system, similar layout, etc. The side panels are similar to the R-390A receiver and don't have the typical SP-600 "pylons." The chassis and side panels are gold iridite (aka alodine) finish.

What about performance? If you're familiar with how vintage, high-performance VLF receivers operate and you are located in a RF-quite area and you're using a good design loop antenna (either remote tuned or shielded magnetic,) the SP-600-VLF-31 is an unbeatable LF receiver. I've logged well-over 250 NDB (Airport Beacons) from all over North America with this receiver operating with a six-foot remotely tuned loop antenna. I used this receiver for a 2XQSO with NO3M in Pennsylvania and WØSD in South Dakota on 630M CW. The SP-600-VLF shown was built in 1958.

A lot more detail about this SP-600-VLF31 is in the write-up in "Hammarlund SP-600VLF Receiver" - use the Home-Index to navigate

SP-600-JX-21

I've owned this receiver for about 16 years and it has undergone several restorations during that time. I bought it from Jack Brower N7ID (now SK) who brought it down to Virginia City from his QTH in south-eastern Idaho in 2008. He enjoyed driving, I guess, because he insisted on delivering the receiver "in person" (maybe he wanted to see the WHRM too.) This receiver was from a humid location or it was stored poorly. I've had to do a lot of rework that involved replacing corroded parts, including replacing the entire SELECTIVITY switch, replacing the tuning and logging dials, replacing the CL meter, replacing a lot of the wiring,...on and on. The first restoration got the receiver working and all of the molded caps replaced. The second restoration was all cosmetic. In 2010, I painted the front panel green but that was quickly changed back to gray. I had the gray front panel paint matched from the SP-600-25C receiver's original paint panel and it's an automotive type acrylic enamel paint. The color seems dark but that's how the original 25C looks. The gloss came from a very slight polishing I did. I also returned the cabinet color to the standard gray (as soon as I installed the new panel color JX-21 into the new gray cabinet I knew "that's a lot of gray!") Later, I replaced the CL meter with a NOS Marion Electric meter (not installed in this photo.) In 2024, I found a good condition spare SP-600 front panel. I painted it sage green and have installed it on this JX-21. Luckily, the spare panel allowed me to save this excellent gray panel,...just in case,...if I want to change back to a gray panel some day, it becomes a relatively easy swap.

WA7YBS 75M & 630M Station

The station consists of two Hammarlund SP-600 receivers. The top receiver is the SP-600VLF-31 tuning from 10kc up to 540kc. The bottom receiver is the Hammarlund SP-600 JX-21 tuning from 540kc up to 54mc. The small box to the left of the JX-21 is the remote tuning for the six-foot loop antenna used on 630M for reception. The HF receiver output is to a floor speaker. The LF receiver output is to 600Z ohm 'phones. 

The transmitter is an ART-13A with O-17/ART-13A LFO installed allowing operation from 200kc up to 600kc. On top of the ART-13A is the CU-32/ART-13A Antenna Loading Coil. The CU-32 allows the ART-13A to match a variety of antenna types on LF. Both the HF output and the LF output from the ART-13A are routed through the CU-32. The output of the CU-32 is connected to the HF antenna when "FIXED ANT" is selected and is routed to the 630M antenna when "TRAILING ANT" is selected. The silver box to the left of the ART-13A is an auxiliary condenser that aids in loading the transmitter on 75M. The J-38 hand key is for LF-CW and the mike is for HF phone. LF transmitting antenna is an end-fed wire 163 feet long. HF antenna is a 135' CF Inv-vee with 96' of ladder line to a Viking KW tuner.

Unfortunately, I had to disassemble this station. A new "modern, efficient, modulated" furnace that has a "smart" thermostat doesn't like any type of RF field near it. Especially RF in the MW spectrum. I have to move all 630M operations out to the shop and that hasn't happened yet.

Conclusion

1. If you have a SP-600 that has a panel painted some color other than Hammarlund gray, send a photo. It's certainly an end-user repaint but vintage repaints are interesting. So far, I've only seen green, blue and khaki. There's a maroon front panel on the Internet. Also, a black panel that might be anodized. Any other colors out there?

2. Does anyone own a SP-600 J-4 or the R-320 version of the SP-600? The receiver has an IF Gain control where the JX option normally is. Used for triple-diversity setup. I've never seen one.

3. Does anyone own a SP-600-JLX? It covers 100kc to 400kc and the HF bands. I've never seen one.

For Sending You SP-600 Photos or Other Information - Here's the e-mail link: WHRM - SP600 INFO

and, there's more Hammalund info in these articles,... 

Pre-War Super Pro Receivers - If you want to know more about how the SP-600 came into being by reading about its "older brothers," read our article "The Incredible 'Super Pro' Receivers" which details each of the early Hammarlund models - the Comet Pro, all of the pre-war Super Pros, the SP-10, the SP-100, the SP-200 and the post-WWII SP-400 receivers, along with the WWII Military versions, the Power Supplies plus lots of other Hammarlund information.

Hallicrafters' Super Pro Receiver - If you're interested in Hallicrafters' Super Pro version , the R-274, go to "Hallicrafters' Super Pro R-274 Receiver" where, after reading the article, you can vote on whether your favorite receiver is the Hammarlund SP-600 or the Hallicrafters' R-274. The vote tally is presented at the end of the article (and it's a surprising result.)

Hammarlund's SP-600-VLF Receiver - If you're interested in the VLF version of the SP-600, go to "Hammarlund SP-600-VLF Receiver." If you've found the the VLF's manual is absolutely no help for alignment instructions, this article includes a newly-written procedure for sweep aligning the VLF's IF that's easy to follow and doesn't require any special equipment. Navigation links are in Home/Index for all three articles

SP-600-JX-21   ca: 1953   2011 photo

This is the earliest version of the JX-21. The JX-21 was produced mainly for the USAF and there were several orders for these receivers throughout the 1950s. In the late-1960s, the last SP-600 version was released, the JX-21A featuring a product detector, different knob styles and different front panel engraving.

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