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. 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 automotive paints do have "fillers" in the paint to act as primers to fill scratches. These "fillers" are found in "off the shelf" spray can paint. 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 actually see the IF passband and to really see what the various adjustments performed in the IF section actually accomplishes. 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 more 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,...sometimes 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 reason (that would be believable) for the green panel, other than the one SP-600 I saw in K6DGH's collection. 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'm going 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.

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 returning the panel to that 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.

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" 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.

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 9.95mc instead of 10.0mc.,which is still within spec.

All of my testing while I had the SP-600 on the bench was using a W6LVP shielded magnetic loop antenna that was given to me. The performance is good enough considering how easy it is to have a decent low-noise antenna indoors for the test bench. 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 receiver XSQ 16.985mc, a marine beacon out of mainland China at about 1600hrs with a RST 579 (CW ID with trilling data pulses.)

The SP-600 is visually an impressive receiver due to its size 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 very different receiver (and not like a Heathkit,...well, maybe just a little.)

SP-600-JX-21 sn: 21649  2024 photo

SP-600 General Performance Expectations

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.

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 <>.

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. Collins utilized a precision PTO 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. 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.   >>>

 
SP-600-J25 - 1953
This SP-600-J25 was purchased at a local ham swap meet for $50 in 2010. The receiver's nonfunctionability accounted for its very reasonable price (that, and the fact that it was almost 15 years ago.) I've fully rebuilt the receiver internally but cosmetically it's still pretty much in original condition, including the excellent condition original tuning and logging dials. I did have to replace the Crystal Filter Phasing knob because of a broken section on the bakelite part of the knob, otherwise the cosmetics are original. The SP-600-J25 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 J25 was the updated replacement for the J5. The "CAUTION" tag in the upper left corner of the panel is not original but,... hey, it fits. The missing data plate designated the receiver as R-483/FRR. This original Hammarlund cabinet is medium gray wrinkle finish and is a good match for the over-all gray and black color scheme of this receiver. The cabinet was not with the receiver at the swap meet.

An individual's impression of the SP-600's audio quality is another subjective judgment. Some users find the audio to be standard "communications grade" with no 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 mild 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. Strong AM BC or AM SW stations might sound okay in 13kc but I like 3kc better,...kinda mellow. 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.

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.  >>>

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.)

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.

While the SP-600 may not provide the dial accuracy of an R-390A, you won't strain your wrist tuning in stations while band cruising (although you might find it difficult to "zero in" on tuning some stations due to the wide tuning span and fairly fast tuning rate.) You'll hear the same stations that the R-390A would receive and they may even sound a little bit better,...but probably not.

1. Limited Dial Accuracy - No surprise since each tuning range covers megacycles. It's actually the dial resolution that's vague but usually a well-aligned SP-600 will be fairly accurate on "marker frequencies."

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 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 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 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 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 fairly stable.

6. 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.

7. 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.

8. 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. Be sure to take into consideration that the SP-600 was originally intended for surveillance and data reception by the military, not as a listening pleasure band-cruiser.

9. 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.

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 only found on the JX-17 version.

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 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 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-J25 receiver and it's an automotive type acrylic enamel paint. The color seems dark but that's how the original J25 looks. The gloss came from a very slight polishing I did. I also returned the cabinet color to the standard 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 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" (the Hammarlund SP-600  predecessors,) read our article "The Incredible 'Super Pro' Receivers" which details each of the early Hammarlund models - the Comet Pro, and 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. Navigation link in Home Index.

Hallicrafters' Super Pro Receiver - If you are 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 (with a surprising result.) Navigation link in Home/Index.

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," and if you've found the the VLF's manual is no help for alignment instructions, the 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 link in Home/Index 

 Return to Home Index