Western Historic Radio Museum

Rebuilding the Hammarlund SP-600

~ The Molded Capacitor Problem ~

~ Receiver Disassembly and Capacitor Replacement ~

~ Rebuilding Suggestions and Mechanical Restoration ~

 ~ Thorough Cleaning and RF/IF Alignment ~

~ Performance Expectations ~

by: Henry Rogers - WA7YBS - WHRM

photo right: The 1953 Hammarlund SP-600 JX-21

The Hammarlund SP-600 Series of Receivers

General Information about the SP-600

Introduced in 1950 and selling for nearly $1000 at that time, the SP-600 was intended for the military and commercial user market. It was a very popular receiver and many thousands were built, especially for military applications. Though most versions were built throughout the 1950s, the SP-600 continued to be produced in smaller numbers up into the early 1970s. The standard SP-600 tunes from .54 to 54MC in six bands. A "J" suffix indicates JAN parts were used in the construction and an "X" suffix indicates a switchable crystal LO. Hammarlund also offered a "JL" version with 100-400KC substituted for the .54-1.35MC band and a "VLF" version that covered 10-540KC. Hammarlund made over 40 variations of the SP-600 that were each assigned a numerical suffix which identified the particular circuit, mechanical changes or sometimes the end-user. The last in the "time-line" was the model variation SP-600 JX-21A from 1969-1972 which utilized a product detector circuit, two additional tubes and some other changes to make it "compatible" with SSB operations.

Most versions use a 20 tube double conversion superheterodyne circuit with a rotating turret bandswitch. The receivers also feature enormous proportions, robust construction and oversize controls - along with a super-smooth tuning system that only adds to the enjoyment of operating these fine receivers. Double conversion is switched in above 7.4MC and uses a 3.5MC crystal-controlled conversion oscillator. Though the SP-600 has two dials, it has no bandspread - the right side dial is a logging scale allowing precise retuning of desired stations. On the left is the main tuning dial and the mechanically articulated dial pointer that indicates which tuning scale is in use (along with the small window between the dials that shows which tuning range is selected.) The tuning arrangement was an up-dated version of the "Continuous Bandspread" system introduced in RCA's AR-88 series receivers in the 1940s. The frequency readout accuracy is vague which is why a precise logging scale system is incorporated into the SP-600 design. The S-meter is not illuminated and a switch is provided to indicate either carrier level or audio output. Most (but not all) SP-600 model numbers usually will have a suffix with "J" or "JX" followed by a numeral. As mentioned above, suffix "J" indicated that, as much as possible, military level components and construction were used. Suffix "X" indicated that a switchable six-position, fixed-frequency crystal-controlled oscillator was installed that allowed the user to install HC-6/U type crystals for specific desired LO frequencies. The user could switch to any of the crystal LO frequencies for increased stability for that particular frequency - however, the receiver still has to be "tuned" to the desired frequency for the RF and Mixer stages to be in tune. The number following the suffix generally indicates specific features for that version, e.g., contract or end user, circuit changes, etc., with the number ranges being more or less chronological for introduction until the last of production. Though the numbers assigned are chronological for their introduction, many of the numbered versions were produced over a relatively long period of time that saw many engineering changes incorporated into the receivers. As an example, early 1953 JX-21 receivers are different from the 1956 USAF documented JX-21 and both of these versions are very different from the JX-21A of the late sixties. Many SP-600 receivers were set-up for diversity operation and the standard diversity model was the JX-17 version. This version was produced in large numbers and can be easily spotted by observing that it has two extra controls and uses three red colored knobs. The SP-600 Audio output is about 2 watts from a single 6V6 using a balanced split-winding audio output transformer for 600 ohms Z. The audio quality from a rebuilt SP-600 is impressive.

For some reason, Hammarlund advertising and literature rarely mentioned the SP-600's "Super Pro" lineage. The receivers were never called "Super Pro" and usually only referred to as "SP-600."  The influence of the earlier Super Pro receivers on the SP-600 design and construction is obvious. In 1931, Hammarlund sky-rocketed to the top of communications receiver technology with the introduction of their Comet Pro receiver, the first successful shortwave superheterodyne receiver. Then, in 1936, they jumped to the top again with the introduction of the Super Pro. The Super Pro remained basically unchanged from 1939 up to the late forties and, by that time, it was apparent to most users that the SP-400 Super Pro was just a repackaged SP-200. Hammarlund, once again, had to come up with a tremendously innovative receiver and, in 1950, they once again "hit it out of the park" with the SP-600. The new receiver became a favorite of both military and commercial users. Nowadays, many rebuilt SP-600 receivers have found new homes in vintage AM ham stations around the world. Admired for its great audio reproduction, ultra-smooth tuning, great sensitivity and sheer dominance of the ham station landscape, SP-600 receivers are sought-after, rebuilt, restored and then finally, once again, put to work.

photo left: Top view of the 1953 SP-600 JX-21 chassis.

Some Preliminary Rebuilding Information

The Caveat - Though most SP-600 receivers look pretty much the same on the exterior, inside there are several differences between the many variations with different value components, additional parts or lack of parts being commonly encountered. The receivers shown in this article are two different versions of the early variety. The SP-600 JX-21 has the switchable crystal controlled oscillator and all of the modifications to incorporate the "JX" option. This is an early version (1953) JX-21, so it is somewhat different than the later 1956 USAF versions and completely different from the 1972 JX-21A version. The other receiver is the SP-600-25C. This receiver has the 25-60 cycle power transformer and lacks the switchable crystal controlled oscillator. There are several minor differences between these two types of SP-600 receivers and certainly many differences with the other 38 or so variations of the SP-600. So, if your SP-600 doesn't exactly conform to what is stated here, don't be too surprised - I won't be either.

 Manuals and Schematics - There are about 40 different versions of the SP-600 but the receivers can be roughly divided between early versions and later versions. This division is not based on the number suffix but rather when the particular receiver was manufactured. There aren't manuals for every single numbered version but it's fairly easy to find a manual and schematic that is proper for your particular version.

The Military came out with several different manuals for many of the variations, so it's possible to find both Signal Corps TMs (TM-11-851) and several types of USAF manuals. However, with some of the military manuals you will find there are earlier manufactured versions of the receiver than what is covered in the manual. This is particularly true of the JX-21 USAF manual which is dated 1956 and is for the later versions of the JX-21. The JX-21 was first built in early 1953, which means the 1956 USAF manual will have several differences from the early JX-21 receivers. This is true for almost all SP-600 receivers versus the available manuals. It is more important to use the manual that was issued closest to the date of manufacture of the receiver rather than going by the particular numerical version of the receiver. In other words - not all specifically numbered versions of the SP-600 were identically built.

With the early versions of the schematic as in Issue 1 and 2 of the Hammarlund manuals no component values are on the schematic. You'll have to use the parts list as a cross-reference to determine the correct value of the component shown on the schematic. Later schematics, from Issue 3 up, do have the component values on the schematic.

BAMA (Boat Anchor Manual Archive) has about four of the six issues of the Hammarlund manual online plus other SP-600 information. Generally, Hammarlund Issue 1 and 2 are for the earliest versions while Issue 3 is sort of a transition version. Issues  4, 5 and 6 are for the later versions. This is generally the case and it's best to down load all of the versions available to see which is closest to your particular SP-600.

There are some dealers doing business on eBay that sell repro manuals if you are interested in having hard copies of the Air Force issues and of the Signal Corps TM (around 200 pages, so it's expensive.) If your SP-600 was modified or has missing components you will absolutely need to use the manual and schematic that is correct for your receiver. Though it is possible to work through the SP-600 with general information from almost any manual, if the receiver has been compromised with modifications, you'll need accurate documentation to return the receiver to its original configuration.

photo above:  How the typical SP-600-JX receiver looks before restoration. Lots of dirt and corrosion on this one.

Photo above: A look under the chassis shows a lot of molded capacitors (even some replacement caps that are molded types.) Other serious problems are surely lurking inside this typical unrestored  SP-600 JX.

1. RF Platform - 20 molded capacitors - all are .01uf - 400wvdc

2. Turret Bandswitch - 6 molded capacitors - all are .01uf - 400wvdc - only the 1RF, 2RF and Mixer modules for the .54-1.35mc band and the 1.35-3.45mc band have molded capacitors that need to be replaced.

3. T-1 455kc/3.559mc Mixer Transformer - 1 molded capacitor. This unit is mounted to the side of the RF box next to the filter chokes. Inside the unit is one .01uf - 400wvdc molded capacitor.

4. Crystal Conversion Oscillator - 3 molded capacitors. This unit is mounted under the chassis. All capacitors inside are .022uf - 400wvdc.

5. IF Transformers - 2 molded capacitors inside the Conversion IF transformer, value is .022uf - 400wvdc. 1 molded capacitor inside two of the 455kc IF transformers, value is .022uf - 400wvdc. The Crystal Filter/IF transformer doesn't have molded capacitors inside.

6. Switchable Crystal Controlled Local Oscillator Assembly ("X" Option) - 2 molded capacitors inside this unit. Both are .01uf  - 400wvdc.

Additionally, most receivers will have two tub units that are double .05uf at 600wvdc capacitors. There is also a .05uf  tubular and a .25uf tubular (Sprague built units usually) that are mounted on standoffs on the front apron of the chassis. The usual procedure is to replace these with individual components when doing the rebuild. All of the other under-the-chassis molded capacitors are easy to access and replace. All total, the amount of molded capacitors that need to be replaced will exceed 50.

Components - When Hammarlund eliminated the molded capacitors from production they went to ceramic disk capacitors. That choice as new replacements would be fine, as would SBE Orange Drop or metalized film "Yellow-Jacket" types of replacement capacitors. The choice is not critical since any of these types of capacitors are at least a 1000 times better than the original molded capacitors.

Rebuilding the SP-600 Assemblies

The RF Platform

This is the most difficult assembly to remove, rebuild and then re-install. The bandswitch must be placed in a position exactly in-between any of the "lock-in" band select positions. This allows the short contact pins from the RF units in the turret to not be engaged into the contact fingers of the RF Platform. Remove the top cover over the tuning condenser and unsolder ALL of the connections to the tuning condenser. There are 16 connections that need to be de-soldered here. Then remove the top cover from T-1 and unsolder the wires that are connected to the terminals there. There are from five to seven wires that need to be unsoldered depending on which version is being worked on. Next remove the four mounting screws on the top of the RF Platform. Now, carefully lift the RF Platform up while guiding the T-1 wire cable as they pass through the opening to T-1. With the RF Platform out of the receiver, you'll be able to see 20 molded caps that need to be replaced. Be sure to watch the alignment holes versus the placement of the new capacitors. You want to have clearance from the alignment holes down through the holes in the ceramic mounts and down to the adjustments on the RF units of the turret. Make sure your placement of the new capacitors doesn't block alignment tool access. Try to mount the new capacitors in exactly the same position and same orientation as the originals. Check the contact fingers for corrosion and clean with De-Oxit applied with a Q-tip - nothing abrasive. Don't move the bandswitch control on the receiver until you have reinstalled the RF Platform. Re-installation requires that the wire cable be guided through the access hole to T-1 as you lower the RF Platform into place. Reconnect the wires to the T-1 terminal board and reconnect all 16 connections to the tuning condenser. Replace the mounting screws. Check for proper operation of the bandswitch-turret and smooth engagement of the contact pins of the turret into the RF Platform contacts.

photo above: The RF Platform showing the original Cornell-Dubilier molded capacitors. (SP-600-25C)

photo above:  Close-up of the broken contact finger on the RF Platform, 1st RF stage, Antenna input. These contact fingers are very fragile at the points where they enter into the ceramic mounts. The right-angle bends tend to be weak points.  Hammarlund soldered the bottom lugs on the contact fingers to lock them in place and to prevent "rocking" as the turret pins are engaged.  (SP-600 JX-21)

Removing the contact finger requires careful workmanship. You must use solder wick and remove absolutely all of the solder from the contact finger lugs. Next, insert an 18ga. wire into the "T" slot in the ceramic and push gently down on the tang that holds the contact finger in the ceramic. Once the tang is down, that side of the contact finger can be pushed slightly up. Insert the 18ga. wire in the other "T" slot and push down on that tang, then that side can be pushed slightly up. Once the two tangs are released the contact finger can be removed from the top of the ceramic mount. These contact fingers are very fragile and will break easily, so go slowly and carefully, then successful extraction is fairly easy. To install the good contact finger just push it into place in the slots and the tangs will lock. Then solder the bottom lugs to lock in place.

One can also note from the close-up photo to the left that there is some corrosion on the fingers and also a lot of dirt or "tuner grease." This should all be cleaned with denatured alcohol and a Q-tip. Stubborn areas may require a brass brush to remove the corrosion. When everything is clean then go over the contact fingers with De-Oxit D-5, which contains a corrosion cleaner along with a lubricant that works fine for these kind of contacts.

Careful, meticulous rework to the RF Platform is essential to regain the superior performance that the SP-600 is capable of.

The Bandswitching Turret

This is a really great piece of engineering. As the bandswitch control on the front panel is turned, the turret rotates and places into an engagement position the contact pins of four front-end modules that make up the 1RF, 2RF, Mixer and LO sections of the front-end for each tuning range of the receiver. Each front-end module is built onto a ceramic mount that has the tunable coil, the trimmer capacitor and a couple of resistors and fixed capacitors that make up each circuit. The connections are to short pins on the back of the ceramic mount that engage the RF Platform fingers when the turret is rotated into position. Each front-end module can easily be removed from the turret since they are only held in their mounted position by two arched clips.
 

Six of the RF modules use molded capacitors that will need to be replaced. All of the others have mica caps installed. The .54mc-1.35mc section will require replacement of a capacitor in the 1RF, 2RF and Mixer modules and the same units require a capacitor replacement in the 1.35mc-3.45mc band. Each module has one molded capacitor of .01uf at 400wvdc. While you are working on the turret, it's a good idea to check the resistors along with the other components installed in all of the front-end modules. The Mixer sections especially seem to sometimes have burned resistors but they do turn up in the other modules also. Once and a while you'll find someone has inadvertently injected high level RF into the receiver antenna input and burned the 1RF coil. Check the pins of each RF units for corrosion and cleanliness. Don't use anything abrasive unless corrosion is present, then you might have to use a brass brush and denatured alcohol to clean. Follow with De-Oxit applied with a Q-tip.

Be sure to check the tightness on all of the tensioners that are on the inductance adjustments. These are circular metal disks with slots along the edge that are threaded onto the inductance threaded shaft. These tensioners provide a spring load on the inductance adjustment so it will stay set. Sometimes these tensioners loosen and when that happens you'll find that the inductance adjustments don't seem to hold adjustment when doing your alignment. Check them - they should be a little more than snug but not over-tight. Too tight and you'll have difficulty adjusting the inductance with the alignment tool. The tensioner should be just tight enough to hold its position and not move when the inductance shaft is adjusted.

NOTE: Early modules will have different value resistors compared to later ones.

Photo left:  RF front-end module with the original C-D molded capacitor installed (SP-600-25C)
 

photo above: 2RF module (3.45-7.4mc) with burned 510 ohm resistor (SP-600 JX-21)

The IF Transformers

There are molded capacitors located in each of the IF transformers with the exception of T-3, the crystal filter/IF transformer assembly.  IF transformers T4 and T5 have one .022uf 400wvdc molded capacitor inside that will need to be replaced. T-2 has two .022uf 400wvdc molded capacitors inside that will need to be replaced. The IF transformer shields are held in place by four 6-32 screws that mount the shield to the chassis by way of right angle brackets on the shields. Removing the screws and the shield allows you access inside of the IF transformer. There is a blade spring that provides a load on the IF adjustment screws. Be sure to keep track of the spring and replace it when you re-install the shield. The molded capacitors are very easy to replace in all of the IF transformers. Be sure to check the value of the carbon resistors that are located inside the IF transformer shields since any leaky capacitors may have compromised the resistor value.

NOTE: Be aware that the IF transformers change slightly in the later versions of the SP-600 and will have slightly different connections to the terminals under the chassis.

photo right: The IF section of the SP-600 with the shields removed allowing access to the molded capacitors. The nearest IF transformer has already had the new SBE "Orange Drop" replacement capacitor installed. Note here that the 2200 ohm resistor is 5% - I had to replace the burned up original 10% resistor because of the leaky original capacitor (SP-600-25C.)

T-1 455kc and 3.955mc Mixer Transformer

This is the small assembly that is mounted on the right side of the RF tuning box near the power supply filter chokes. It is held to the RF box by six screws that are difficult to access in the  non-"X" receivers because of their proximity to the power supply filter chokes and impossible to access in the receivers with the "X" option. In "X" optioned receivers, the Crystal Controlled Oscillator unit has to be removed first. Unsolder the four wires that connect to the Filter Capacitor/Filter Choke mounting assembly, remove the four screws and dismount the assembly. You don't have to unsolder the wires that connect to the Crystal Controlled Oscillator unless you are going to remove it for rebuilt, then unsolder them. Unscrew the fours screws that mount the Crystal Controlled Oscillator and move it out of the way (or remove it for rebuild if you've unsoldered the wires.) Now you should have easy access to T-1. On non-"X" receivers all you have to do is remove the mounting screws from the nearest filter choke and move it back and to the side to allow the cover of T-1 to be removed. Once the cover of T-1 can be removed, then you can access the single .01uf 400wvdc molded capacitor located inside. After replacement of the capacitor, the cover can be re-installed. If you are not going to rebuild the Crystal Controlled Oscillator then remount it (but if you have then resolder those wires) and then remount the filter choke-capacitor assembly and resolder those wires.

The 3.5mc Conversion Oscillator

This assembly is located under the chassis. The shield is mounted to the chassis using four 6-32 screws. Removing the screws and the shield allows access to the 3.5mc oscillator circuit. There is a vertically mounted fiber board that has some components mounted to it. There are three .022uf 400wvdc molded capacitors used in this circuit that will need to be replaced. When re-installing the shield note that there are three notches that allow the circuit wiring to pass under the shield when mounted without pinching. Be sure the wires pass through the notches. Also note that there is a notched fiber spacer that sets on top of the vertical fiber board. This must be in place when re-installing the shield since it prevents vibration movement of the vertical board that might mechanically affect the 3.500mc crystal.

The Crystal Controlled Switchable Local Oscillator ("X" option)

This unit allows the operator of an "X" optioned SP-600 receiver to substitute a crystal LO for the regular LO for increased stability. Six crystal sockets allowed the user to install HC-6 crystals for fixed frequency operation. A six position switch allowed selecting any of the crystals installed. A vernier frequency control was also provided. The crystal switch and the frequency vernier were brought out as two extra controls added to the upper right section of the front panel.

When selecting a Crystal Controlled LO frequency, only the LO is changed - you will still manually have to tune the receiver to the selected operating frequency in order to have the RF and Mixer stages also tuned correctly. You'll need to install the correct frequency HC-6 crystal which is not the same frequency that you want to tune in. The manuals will provide the formula to calculate the crystal frequency required for the desired receive frequency. Basically, 455kc or 3.955mc is added to the desired receive frequency to obtain the correct crystal frequency depending on the band and whether the receiver is operating single or double conversion. Also, on the higher frequency bands the result is divided by 3 or 4 for operation on the crystal frequency harmonic.

Removing the Crystal Controlled Switchable LO unit requires removing the knobs and any shaft couplers. Remove the filter choke/filter capacitor sub-chassis to access the wiring harness connections. Five wires have to be unsoldered there. There is also a connection to the switch and the RF Platform to unsolder. Remove the screws from under the chassis that mount the hex stand-offs that are attached to the bottom of the unit.  You should now be able to extract the entire unit from the receiver. Several screws need to be removed (and the sheet metal cover sides slightly bent outwards to remove it) in order to gain access to the Crystal Oscillator. Once inside you'll find two molded capacitors that need to be replaced. Check the resistors for drift and clean the switch to complete the rebuild. Re-assemble and re-install in the reverse order.

Once the Crystal Oscillator unit back in the receiver and is operational, it's doubtful this particular option will ever be used unless you want to listen on one frequency - exactly. When the SP-600 is running on its own LO it doesn't drift after a 10-15 minute warm-up, so only the most demanding frequency-stable military-type requirements would benefit from using the Crystal Controlled LO.

Completion of the Electronic Rework

Sometimes one of the .05uf dual capacitors is 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.

Rebuilding the Electrolytic Capacitors - The SP-600 uses a triple 20uf electrolytic B+ filter capacitor with a built-in resistor connected between the negative terminal and the capacitor's metal case. The way in which this electrolytic capacitor is connected, with negative connected to chassis, eliminates the need for this particular internal component (it isn't shown on the schematics either.) These were extremely high quality electrolytic capacitors and many are still fully functional today. If you want to use the originals, then check them carefully for leakage and then reform them if the receiver has not been powered up for several years. Also, most SP-600 receivers have several 10uf electrolytic capacitors that are mounted in "bath-tub" containers. Again, these were very high quality units and many are fully functional BUT check them carefully. If you feel uncomfortable using 50 year old electrolytic capacitors (and maybe you should,) just replace them. I usually install the new electrolytics inside the original housings since it results in a much better under-chassis appearance.

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 one-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.

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.

photo left: The BFO from the JX-21 receiver showing the detached end-cap and that the blade spring load is fully extended.

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 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.

A good example of an acceptable enhancement would be a minor adjustment to the audio frequency roll-off of the SP-600. With the 125Hz 3db roll-off, the SP-600 audio is "high quality communications" audio and it is noticeably lacking in bass response. It is very easy to parallel the two existing .0015uf coupling capacitors with .01uf or .02uf capacitors and parallel the cathode bypass electrolytic capacitor on the 6V6 audio output tube with a 50uf to 100uf electrolytic capacitor. One published audio enhancement also adds negative feedback which does require that the audio output transformer's earphone winding connections be reversed which is a minor change that is easily reversed if desired. The negative feedback decreases AF gain by 8db which then requires that the AF Gain control be advanced significantly more than normal. The overall effect of most of the audio enhancements is to add the bottom octave to the audio response and add to the enjoyment of the use of the receiver.

The AVC circuit in early SP-600 receivers is also the subject of some criticism and does cause audio distortion on some AM signals when the RF gain control is set to maximum. Resistor values can be changed to enhance the AVC to RF gain ratio but it is also much easier to just reduce the level of RF gain which then allows the AVC to function correctly.

In studying any proposed circuit enhancement, be sure that no components are removed, no physical damage is necessary to install the enhancement and everything is easily and totally removable.

Mechanical Restoration Work

Install ALL of the Screws - The SP-600 uses a lot of screws to mount various shields, panels and other assemblies. All of these screws are important for mechanical stability and for shield electrical connections. It is very common to find an SP-600 missing many of the screws for these parts. Most "hamsters" think that only two screws are required to mount panels (one screw, if it's really tight!) ALL of the screws are necessary for the receiver to have its mechanical strength and stability, so be sure to install all of the screws and replace any that are missing.

NOTE on 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 after 1951, it is possible to find a SP-600 with a SO-239 antenna receptacle that is an original installation.

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

IF/RF Alignment

The IF Section - The IF is straight forward in alignment except for the second conversion frequency adjustment of 3.955mc. This requires the 455kc IF to have been very accurately aligned since the conversion oscillator operates with a 3.500mc crystal. Center the IF using the Crystal Filter crystal frequency and you'll end up very, very close. Be careful on the 3.955mc adjustment of T-1. It is possible to adjust this to a harmonic and everything seems okay - until you try to operate the receiver above 7.4mc and the instability lets you know something is wrong. Also, the Crystal Filter adjustment of T-3 can be adjusted to a harmonic with the result that the Crystal Filter won't resonate correctly. This is a common problem on some SP-600 receivers. Check the height of the adjustment screws on all of the IF adjustments and be suspicious of any adjustments that "peak" when they are at the limits of either end of adjustment.

Crystal Filter - This circuit seems to have a lot of critics that claim it doesn't function very well, if at all. Most of the problems with the Crystal Filter operation can be traced to either poor alignment of the IF section or improper placement of the Phasing knob in relation to the phasing condenser's position. The IF alignment frequency must be determined with the Crystal Filter set to the sharpest position. It will probably not be exactly 455 kc but will be very close and this is the frequency that all of the single conversion IF transformers are adjusted to. Align the IF with the selectivity in the 3kc position. To set the Phasing knob's position, inject an unmodulated 455kc signal into the 1st Mixer stage. Don't over-drive the IF with a strong signal. Switch to CW and tune the BFO for about 1kc audio output, set the selectivity to 0.5kc and then rotate the Phasing control slowly. When rotating the Phasing knob, you'll notice that the selectivity varies from wide to very narrow and near the narrowest position you'll eliminate the 1kc tone. Set the Phasing knob so that the widest bandwidth is at 10 and the narrowest is at 0. Elimination of heterodynes will occur around 2 or so on the Phasing scale. The "diamond" mark on the Phasing scale indicates mid-range on the selectivity.

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.

Cosmetic Restoration of the SP-600

Repainting the Front Panel - This should only be done if the panel is really in poor shape and doesn't look very good after cleaning and polishing. First, if you want to maintain originality, have the front panel paint matched at an automotive paint shop. This way you end up with high quality paint (with hardener catalyst added) that's the correct color. Many automotive paint stores will fill spray cans with the matched paint for a nominal fee. This makes painting the panel much easier.

Many SP-600 enthusiasts don't like the original Hammarlund gray panel color and choose paint the panel another color. This is certainly an option since many examples of  SP-600s have turned up with green, kahki and several different shades of gray front panels that all seem to be original paint jobs.

The original paint is very, very tough and will require a couple (at least) applications of heavy-duty epoxy stripper. Make sure the panel is super clean and degreased. Original SP-600 panels look like they didn't use a primer and since you are going to have to fill the engraving you don't want to have anything that is 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, spray another coat. Three coats should be the maximum. Let this dry for at least a day or two.

For the nomenclature fill, I mix Artists Acrylics of White, Raw Sienna and a little Black to end up with a Manila color. "Pure white" will look way, way too bright. Do only one or two sets of engraving at a time. Apply the paint with a brush into the engraving and you don't have to be real careful - just paint it in. Let the paint set for about two minutes. Use a small piece of paper towel folded a couple of times that has been dampened (not dripping wet) with Glass Plus. Don't use Windex - only Glass Plus works for removing the excess fill paint without damaging the panel paint. Lightly rub the dampened paper towel in a circular motion with the paper towel absolutely flat on the panel. As the excess paint is beginning to come off, flip the paper towel over to the clean side to continue to remove the excess paint. I always have several pieces of towel ready because you'll need at least two pieces (four sides) to remove the paint. Let the paint in the engraved area set for a few minutes more and then wipe the area down again with another dampened paper towel to fully clean the area  I usually have to apply some more fill paint and repeat the process to get the lettering to really look professional and proper. Let the fill paint dry for a day. Finally, use carnauba wax and apply a nice matte polish to the panel. It will look great.

photos right:  The different steps of painting the SP-600 panel. The top photo shows the panel stripped and ready to paint. The next photo shows the panel painted without the nomenclature filled. Note that the panel is painted green. This happens to be a Hammarlund green color that was computer matched from a military BC-1004/SP-200 panel by an automotive paint shop. The paint is very high quality and is remarkably better that the general purpose paint purchased at the hardware store. I waited several days to allow the paint to fully cure before doing the nomenclature full.

 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.

Nobody wants a "polished to a mirror" finish on a receiver chassis. Most restorers want the aluminum chassis to appear as it did when the receiver was fairly new. The EOOC treatment ends up with the aluminum looking "flat" original. The IF transformer shields and all of the the other removable aluminum pieces including the side gussets can be taken outside and, one at a time, sprayed with EOOC, left for three minutes and then rinsed with water from a hose. This ends up with the aluminum pieces looking very clean and with a flat finish like they had when they were new. Be sure to rinse all the pieces thoroughly with water. Also, don't rub the aluminum pieces with anything - just let them air dry. Rubbing will actually end up slightly polishing the aluminum piece and will ruin the flat finish you are trying to achieve. When cleaning the IF, BFO & T-1 shields, be careful of the lettering on top of the cans. The lettering is very delicate and almost any cleaner seems to destroy it. Avoiding the lettering works best. This EOOC trick only works well on aluminum. It doesn't have too much of an effect on steel - it cleans (as it's supposed to clean your oven) but that's all.

Components like the power transformer and the chokes can be cleaned with any mild cleaner then touched-up as necessary. If they look a little dull, they can be given a coat of Armor-All.

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 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 someone develops a good restoration method for the SP-600 dials.

Escutcheons and Plexiglass Screws - The escutcheons seem to take a lot of hits and are usually scratched up. Repainting is sometimes necessary but usually they will touch up fine and look more original that way. I use satin black spray paint that is sprayed into a small cup and then applied with a very small brush. The satin black seems to blend better than gloss, even though the escutcheons are somewhat glossy. If you've repainted the front panel, you'll probably want to repaint the escutcheons, too. The screws that hold the plexiglass dial covers always seem to be rusty. Just wire brush and paint with flat black paint. Don't torque these screws tight or you'll crack the plexiglass. Just barely snug is all that is needed. 

photos left: Before and after showing the early RF/AF Level Meter scale restoration. Staining of the paper scale can be seen in the before photo (SP-600 JX-21).

SP-600 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. 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. Selectivity is very sharp in the crystal filter position and you can also vary the phasing from center to reduce heterodynes or peak CW signals.

Many users (or maybe potential users) feel that the tuning dial's vague readout accuracy is the main drawback to the 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 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. The only way Collins could get 1.0kc accuracy was to limit the coverage of each tuning range to one megacycle. If you want a 30 band receiver, you can have the 1.0kc accuracy. The SP-600 allows rapid coverage of wide areas of the spectrum - something necessary for certain kinds of surveillance.

 

photo above: This 1953 SP-600-25C was purchased at a local ham swap meet for $50. The receiver's nonfunctionability accounted for its very reasonable price. 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-600-25C 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. This receiver was probably designated R-483 by the SC. 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.

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.

The SP-600's audio is high grade communications, that is, not really super high fidelity but so good compared to other communications receivers you'll think it's high fidelity. 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. In the 13kc position you will note that the IF gain is slightly reduced, this is normal and part of the design for best audio reproduction with expanded bandwidth. Bass is rolled off so that the 3db point is at about 125hz. This gives a bass response that is somewhat apparent but not "booming." The stronger the station is the better the audio will sound. Audio enhancements that add the bottom octave to the receiver's audio response will make AM ham stations that are running some power sound fabulous and the retired AM BC transmitters operated on 160M or 75M will be a pleasure to listen to. SWBC and regular AMBC stations are greatly improved with a minor audio enhancement.

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. Drift is just about non-existent after about a 15 minute warm-up. To set the BFO to the correct sideband, first 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 8 or so. 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, lower the RF gain if the audio sounds distorted. At the proper RF gain setting, SSB signals sound great 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. Most versions of the SP-600 provide an adjustable BFO Injection adjustment located on the rear apron of the receiver. This can be adjusted to a higher level if you plan on mostly SSB reception rather than CW, although the highest level is still a lower injection level than what is required for SSB demodulation with high level signals. Low-level BFO injection was normally used for CW signals and prevented "masking" of very weak CW signals. Some modifications will increase the BFO coupling capacitor value to increase the total injection level at the detector but this is really unnecessary since it's much easier just to reduce the RF gain level. 

Using the SP-600 as a ham AM station receiver will provide you with the best in sensitivity along with the best in audio reproduction. On early versions of the SP-600 you will have to watch the level of RF gain while in AVC and MOD mode. Too much RF gain will override the AVC causing some distortion to moderate to high level AM signals. Just back the RF gain down until the Carrier Level meter starts to drop and the distortion will clear up. It is minor distortion and may not be noticeable to some ears.

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 at the higher frequencies. However, the SP-600 was designed to operate with a matched antenna and this set up will give the best performance results.

 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 and you'll hear the same stations that the R-390A would receive and they'll really sound a whole lot better.

Rebuilding the 1953 SP-600 JX-21...again!

Jan/3/10 -  I've had this particular JX-21 receiver for a few years. I did do a cosmetic restoration and a partial re-cap when I first got it. After the partial re-cap, I found that the receiver, though basically operational, had its quirks. It didn't seem to hold IF alignment and would sometimes almost break into oscillation in the 3KC bandwidth position. Cosmetically, the receiver still had a badly corroded logging dial and a pretty bad main tuning dial. Also, the band indicator dial was discolored as was the little plastic tag for logging the crystal oscillator frequencies. I wasn't really satisfied with the color I'd painted the front panel, either. As can be seen in the header photo this SP-600 had a lot of potential. It just needed to be totally rebuilt - correctly this time! I'd found an excellent condition main tuning dial and logging dial. Also, an excellent condition band indicator dial and the crystal oscillator frequency tag. With these cosmetic additions, along with a repaint of the front panel to a better color (Hammarlund green!) and a thorough rebuild that tracks down ALL of the problems that this vintage SP-600 has, we should end up with an excellent performing and cosmetically beautiful receiver. I started the rebuild of this 1953 SP-600 JX-21 receiver at the beginning of the new year. 

Jan/6/10 - In trying to separate the modifications from the original circuit, I found the Issue 6 manual I was using was just not close enough. After downloading several versions of the Hammarlund SP-600 manuals and the USAF manual for the JX-21 versions, I've come to the conclusion that this receiver is the earliest version of the JX-21. Many of the components are date-coded 1953. It is quite different from the 1956 USAF manual's JX-21. The chassis wiring and component values are virtually identical to those in the Hammarlund manual Issue 2. Now to remove the many modifications and sloppy repairs.

Jan/10/10 - More and more I am appalled by the incredibly poor quality rework found in this JX-21. The RF Platform was loaded with unsoldered joints, cold solder joints, broken parts and sloppy work. There were several modifications that looked like the receiver was used to drive some kind of device from the IF output. In fact, there was a 5K load resistor across the audio output transformer secondary. I'm pretty sure this receiver was not used by the military but was probably used somewhere else like government or commercial use where poorly trained technicians performed the maintenance on the receiver. Every area of the receiver had some rework performed sometime in the past and all of the work was sloppy and poorly done. The amazing thing is that this JX-21 seemed to be basically working. It had quirks but, still, it did seem to be a functional receiver. I guess it shows that the SP-600 design is very forgiving and the receivers will work even when they are loaded with internal problems.

Jan/17/10 - The JX-21 is now fully reassembled as of 1/16/10. Initial power up resulted in nothing - no output. B+ checked fine. A check of the 6V6 grid and cathode voltage showed the problem was the 6V6. The 6V6 had a 300 ohm short from the grid to ground - a grid to cathode short? This was odd since the receiver was functional before disassembly. Well, a NOS 6V6 got the SP-600 up and running. Looked like everything was tracking very close just as is with no alignment (I had aligned it two years ago.) However, tuning in local AM BC stations showed there was significant audio distortion when the RF gain was advanced above 5 or 6. I performed a complete IF alignment but that didn't correct the distortion. Problem was in the AVC and was caused by an unsoldered joint - the 10uf electrolytic on the bias to the driver tube - oops! (remember, don't be in a hurry when doing the rebuild.) I found some other suspicious looking solder joints which I corrected. Now the SP-600 sounds great with no distortion on local AM BC, even with the RF gain at 10. Next is an RF tracking alignment which really looks like it is unnecessary but it's always a good idea to check all bands anyway.

Jan/20/10 - Though the RF alignment seemed like it was going to be unnecessary, it really did need to be done. I found several adjustments on all bands were quite a bit off, especially in the 2RF stage. Also, while aligning the 7.4-14.7mc band I found that the LO inductance tensioner had loosened up and I couldn't get the lower end of the band to stay in adjustment. Well, the only way to correct this is to remove the bottom cover on the turret and rotate the band switch until the LO module is accessible and then tighten the tensioner. Sounds more difficult than it is. Took about 10 minutes to correct, then the alignment of the LO on 7.4mc held adjustment as it should. The rest of the alignment was completed without any problems.

Jan/21/10 - While incorporating the JX-21 into the ham station I discovered that the Remote Stand-by didn't work. I looked at the wiring to the RELAY socket and that looked stock. Up front where the SEND-REC switch is located revealed the problem - another dreaded modification. This one was easy to remove. The original SEND-REC switch wires were soldered to an insulated stand-off so they were permanently in receive. The SEND-REC switch had added wires that were traced back to the power transformer. The mod had changed the SEND-REC switch to a B+ ON-OFF switch by switching the chassis to CT connection on the power transformer. Correction was to remove the non-original wires and reconnect everything back to the original configuration.

Jan/28/10 - I've been using the JX-21 in the ham station for about a week now. It sounds absolutely great! All of the problems that caused erratic and quirky reception are now gone. Just an amazing receiver. And the green panel, too - WOW!

photo above:  The finished 1953 version of the SP-600 JX-21 with green panel. The green paint was computer-matched from a WWII Hammarlund BC-1004 Super Pro receiver so there is a Hammarlund connection with the color chosen for the panel. The original Hammarlund SP-600 cabinet is painted dark charcoal gray.

May 2010 - Some updates and Additions - Though the restoration cleared up the operational problems with the SP-600 JX-21, the audio did sound somewhat "thin" without too much bass response. Only the ham stations running retired AM BC transmitters sounded great and all of the other AM transmitters sounded a bit flat. I researched a couple of audio enhancements and chose N3FRQ's that adds the bottom octave to the audio response and does not damage or even remove any original parts. This is a particularly good enhancement because it is easy to install and easy to remove and does no damage with its installation. Negative feedback is used and this does reduce the overall audio gain by 8db. This means that instead of the AF Gain being at around "2" you have to run it at about "5" for the same volume. Also due to the 8db reduction, SSB and CW signals will have to have the RF gain substantially increased for the same audio level. However, the benefit of this enhancement is that the SP-600 audio now sounds incredible on AM BC, SW BC and all AM ham stations. Here's a link to the audio enhancement: SP600 AUDIO

I was also the recipient of a very nice, original SP-600 Carrier Level Meter. The meter happened to be the later Marion Electric type and the quality of these meters is greatly improved over the earlier meters with the paper scales. It's worth looking for these later meters.

Photo Gallery of Other SP-600 Receivers

SP-600 JX-14

SP-600 JX-14 with a light gray panel and black nomenclature. 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.

Conclusion

More Information Wanted:   We always try to present the most accurate information available. We depend on Hammarlund SP-600 collectors, restorers and fans to provide information or corrections for this article. This results in accurate information that helps all enthusiasts in the preservation and operation these marvelous receivers. We are looking for the following information.

1. Want to add your SP-600 rebuilding experiences, ideas or photos? I always give full credit for your contributions if I incorporate your info into this webpage article. Performance thoughts?

2. If you have an original SP-600 that has a panel painted some color other than Hammarlund gray, send a photo. I would like to have better confirmation that other panel colors can be found and that they were original. I've seen original (or at least vintage) green and khaki but don't have photos.

3. If you've painted your SP-600 front panel something other than gray, send a photo, I think the non-gray panels look great. As to originality, tens of thousands of SP-600s are available with their original gray panels in great condition. To find out what the original Hammarlund gray looked like is pretty easy. It isn't really much of an issue to paint the panel some other color since there is evidence that Hammarlund did produce some SP-600 receivers with non-gray panels. Besides, you can always strip and repaint to original Hammarlund gray if your conscience bothers you.

For Photos or Other Information Here's the e-mail link: SP-600 Rebuilding Info

If you want to know more about how the SP-600 came into being by reading about its "older brothers" (the Hammarlund predecessors,) read our article "The Incredible Pre-war 'Super Pro'" 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 and the SP-200  receivers, along with the WWII Military versions, the Power Supplies and lots of other Hammarlund information. Navigation link below.