~ General Information about the SP-600 ~
~ The Molded Capacitor Problem ~
~ Receiver Disassembly and Capacitor Replacement ~
~ Electronic Rebuilding and Mechanical Restoration ~
~ Fixing the Chronic Dial Slippage Problem ~
~ RF/IF Alignment ~
~ Performance Expectations ~
|The Hammarlund SP-600 is certainly a favorite receiver of many vintage radio operators. The SP-600's massive proportions, super-smooth tuning and great performance usually result in the receiver assuming a prominent position in the ham station landscape. Unfortunately, all early production SP-600 used leakage-prone molded capacitors in the circuitry. For top-performance and reliability, these early receivers must be rebuilt. Unfortunately, over half of the fifty capacitors that need to be replaced are hidden in various assemblies within the receiver. These assemblies will have to be removed for molded capacitor replacement. This web-article provides detailed information (with plenty of photographs) on the hidden capacitor locations, the rework necessary for the various assemblies and other important rebuilding information. Henry Rogers Dec. 11, 2009|
Rebuilding the Hammarlund SP-600 Series of Receivers
General Information about the SP-600
Hammarlund released an advertisement in 1948 that introduced their newest receiver, the SPC-600X. The receiver was to sell for $395 but, as Hammarlund had done before, this advertisement was really just to let hams and interested commercial users know what was going to be available in the future. No SPC-600X receivers were ever created other than maybe a prototype or two. It was over a year later, in 1950, that Hammarlund introduced the SP-600 Series and the selling price had dramatically risen to nearly $1000. This new SP-600 was primarily intended for the military and commercial user market. It was a very popular receiver and many thousands were built, especially for military applications. It's likely that much of the SP-600 design criteria actually came from the U.S. Army Signal Corps, especially the JX crystal oscillator concept. The use of the selectable crystal oscillator is seen on some "customized" SP-200 WWII Super Pro receivers and was even designated as an "Improvement Kit" with the part number MC-531 from the Signal Corps. Sometimes these receivers were "custom" rebuilds done by various engineering companies for the Signal Corps. These "custom" SP-200 Super Pro receivers usually date from 1947 up into the 1950s. It's likely that the turret band changing was also a specification from the Signal Corps and this is shown by the Hallicrafters' R-274 receiver, built for the Signal Corps and sharing many similarities to the SP-600, including the turret band switching concept. So, although Hammarlund advertising appears to ignore the Signal Corps input, there is just too much evidence available that shows much of Hammarlund's design concepts for the SP-600 actually came from the Signal Corps. The first contract issued was in 1950 for the R-483/FRR for the Signal Corps and was actually a SP-600 JX-5 and was not actually built until November 1951. The first built SP-600s were R-274A/FRR receivers (SP-600 JX-1) built on a 1951 contract in September 1951.
Most SP-600 versions were built throughout the 1950s but 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 "JLX" version with 100-400KC substituted for the .54-1.35MC band and a "VLF" version that covered 10KC-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 Carrier/Audio Level 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. A rebuilt SP-600 will provide an impressive "communications grade" type of audio reproduction.
The Molded Capacitor Problem
|Why You Should
Rebuild Your SP-600 Receiver - The Hammarlund SP-600 series of receivers were built for
commercial-military users who required a receiver designed for continuous use with
maximum performance capabilities. However, that was over 60 years ago!
Nowadays, all SP-600 receivers built in the early fifties to the
mid-fifties, that have not
been rebuilt at sometime in the past, will certainly need all of the
leakage-prone molded tubular capacitors replaced. The SP-600 receiver
circuit seems to be able to keep on functioning even with many leaky
capacitors. This has caused many operators to continue to use their
SP-600 thinking everything is okay, BUT the receiver isn't
working to specifications and sooner or later (likely sooner) there will
be a failure that could cause serious damage to the receiver. On that
note, the leaky capacitors not only cause
performance issues but are also likely to cause failure of other
components in the receiver circuit. It is quite common
to find several burned resistors in any early SP-600 that has not been
rebuilt but was operated long hours with leaky capacitors. The burned resistors are due to the excessive leakage current
that the molded capacitors develop after aging a decade or two. Two
types of molded capacitors are found in early SP-600 receivers. Sprague
manufactured the "Black Beauty" - a black plastic encapsulated,
color-coded capacitor and Cornell-Dubilier supplied a gray plastic body,
color-coded molded capacitor. Both types were used interchangeably by
Hammarlund and both types exhibit high current leakage today (the C-D
gray body, color code banded capacitor is perhaps worst capacitor ever
built for developing leakage current - well, maybe Micamolds are worse.) The molded capacitors were only
used in early production SP-600 receivers - from 1950 up to around 1955.
After that, more reliable ceramic disk capacitors were used. However,
the largest production runs of SP-600 receivers date from 1950 up to
about 1958 with somewhat smaller production runs until the early
sixties. Thereafter, production levels were fairly low. Consequently,
you are most likely to find an early production SP-600 unless you
specifically look for the later versions (and they are usually the more expensive receivers.)
This article covers all of the normal problems encountered when
rebuilding the early SP-600 receivers.
photo left: Top view of the 1953 SP-600 JX-21 chassis.
Some Preliminary Rebuilding Information
|Difficulty, Experience and Workmanship
- Rebuilding an SP-600 receiver is daunting. But it is a project that is
well within the capabilities of most communications receiver restorers.
You will have to perform extensive disassembly
of the receiver and some of its assemblies in order to access all of the parts that
need to be replaced (and others that need testing.) To successfully complete an
SP-600 rebuild you should be
experienced with complete mechanical disassembly and reassembly of
receivers. You must be able to perform quality component removal and
installation with a high quality soldering technique using first-class
soldering equipment and real SnPb solder. You must be able to keep track of your rework on several
different kinds of assemblies. You should have experience in troubleshooting and
aligning communications receivers.
That being said, when performing the actual rework the most
important thing to keep in mind is - don't be in a hurry and be
thorough in your inspection and rework. Even for
rebuilding an SP-600 is time consuming and exacting work but hurrying
through this kind of job will just lead to mistakes and shoddy workmanship.
Rebuilding the RF Platform is tedious work
and its band switching contacts can be easily damaged if that unit is not
handled carefully while it is out of the receiver. All of the turret
modules will have to be removed but this is easier than it sounds.
Again, these small units can be damaged by careless handling - be
careful. It can be helpful to take notes on how the removed units are
wired back into the circuit although there are excellent wiring drawings
in the SP-600 manuals. Since the entire receiver is thoroughly
disassembled to perform the rebuild a complete RF/IF alignment will be
required. How well your rebuilt SP-600
operates will depend on your technical skills, your quality of
workmanship, the type of soldering equipment you use and your rebuilding experience.
The quality of your alignment test gear is also very important.
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 (TM11-851) 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.
|What Needs to be Taken Apart?
- Here is
the list of assemblies that need to be removed from the receiver in
order to gain access inside the assembly to
replace molded capacitors. Quantities shown can vary depending on which
variation is being worked on. The quantity shown is for general
information - your receiver may be different.
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.
|Where to Start? - It might be a good idea to first get the receiver somewhat operational before starting disassembly. This will assure you that you won't encounter any really serious problems after you have gone through all of the work entailing the rebuild. Usually, you'll end up replacing a couple of capacitors and maybe a burned resistor to get the SP-600 running enough that you can check out all of the control functions to make sure the receiver is basically functional. Common sense should guide you in deciding whether or not to get the receiver entirely operational before starting the rework. When you are satisfied that nothing is seriously amiss with the receiver, you can begin disassembly which will require removing the front panel, the side panels, the tuning condenser cover and the bottom cover on the band switching turret. You can also remove the shield covers on all of the IF transformers, the shield cover on the BFO (to check parts) and the shield cover on the 3.5mc Crystal Conversion Oscillator. Everything will then be accessible for capacitor replacement and thoroughly checking all of the receiver circuitry.|
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)
|More on the RF Platform Rebuild
- Often times you will
find that the RF Platform was not carefully handled during a repair job that
was performed in the past. Reference the photo to the left and note that the
contact finger in the front row is broken. This is the 1RF section
and the Antenna Input connects to this contact finger. This is an old
break and was discovered when I removed the RF Platform from the SP-600
JX-21 receiver during its rebuild. This particular receiver showed evidence that it had been worked on many times during the past by a
technicians that were careless in their
workmanship. The fact that the contact finger was left unrepaired seems
to support this opinion. Repairing this broken contact finger is
possible because only five contact fingers are
used in the Local Oscillator, Mixer and 2RF sections but that Hammarlund made all of the
ceramic mounts the same - with six contact fingers. The Mixer and the
2RF sections use the extra contact finger as a tie point for components but the LO section will have one unused contact
finger present. It is possible to remove the unused contact finger
from the LO section and then reinstall it to replace this broken one in the
1RF section. The 1RF section on bands 1 and 2 is the only place where
all six contact fingers are used.
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.
|A Thorough Inspection of the RF Platform Reveals
More Problems - It is
possible that some of the "leaky" original capacitors in the RF Platform
circuit will have compromised some of the associated resistors in the
circuit. Thoroughly inspect the RF Platform carefully for burned
resistors and measure each of the resistor values just to be sure. As an
illustration of just what might be found, note the photo to the right.
This is the RF Platform from the SP-600-25C receiver. Note that R-15 is
burned. This is a 510 ohm resistor that now measures 30 ohms due to the
overheating that took place when in operation with the "leaky" original
capacitor, C-43. It can been seen in the photo that the
green-brown-brown color code has changed due to the heat - especially
the second digit designator band which now appears flat, pale tan rather
than brown. This suspicious appearance prompted physical measurement of
the resistor value which showed that the value had changed from 510 ohms
down to 30 ohms. Naturally, this resistor will have to be replaced along
with all of the original tubular capacitors.
Another interesting observation is that R-16 is missing from the circuit. This 1K ohm resistor is the plate load for the 2RF amplifier. Looking at the photograph to the right, R-16 should be connected to the terminal to the rear that R-15 is also connected to. It can be seen that no resistor is mounted there. During the initial check out of the receiver, operation appeared normal (for an original condition receiver.) So one can conclude that even though the 2RF amplifier was not operating to its full capabilities, it did pass enough of the signal through that stage that the defect was not noticed in the initial operation check out. This shows that a thorough physical inspection of the RF Platform is necessary to find all of the defects that might be present. The Connection Diagram for the "Tuning Unit" (RF Platform) is extremely helpful for the inspection.
photo right: Close-up of the Mixer section of the RF Platform which is where the plate circuitry is for the RF2 amplifier. Note the discoloration of R-15 and the missing R-16. (SP-600-25C)
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
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.
|More on the Front-end RF Modules - It's probably a
good idea to remove each one of the modules to closely check it over.
Though you can look through between the modules and see most of the
components, you can't see everything. As an example, look at the 2RF
module for the 3.45mc to 7.4mc band shown in the photo to the right. The
resistor is burned in two. It isn't really visible when the module is in
place and by looking between the modules. It was found because each
module was removed for close inspection. Besides the resistors, you also
should look at the trimmer capacitor and check for bent plates and check
the mica capacitors for cracks or other problems. The burned resistors
are generally caused by leaky molded capacitors located in the RF
Platform. Many times a rebuilder will recap the RF Platform and the six
modules that have molded caps but not check each of the RF modules for
damage. The end result is missing problems such as the one shown to the
right. It pays to be thorough.
Photo left: RF front-end module
with the original C-D molded capacitor installed (SP-600-25C)
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 3.955mc 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 3.955mc 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 3.955mc crystal.
The Selectable Crystal 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. There is also an ON-OFF function (labeled "VFO") that routes the B+ from the standard LO to the JX oscillator. A vernier frequency control was also provided. The VFO/crystal channel 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 data-type (RTTY usually) requirements would benefit from using the Crystal Controlled LO.
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Western Historic Radio Museum
Vintage Radio Communication Equipment Rebuilding & Restoration Articles,
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1909 - 1959
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