SP-600-JX-6   sn 7092,   (R-274B sn 965)

   This receiver had been stored in an unheated shed for 5+- yrs, and never powered up during that time.  A previous owner had made some modifications, which appear to be the addition of a product detector and some changes to the AVC circuit.  There is an added "fast/med/slow" switch added in place of the dial lock.  It was received for inspection in late May, 2007, and was examined initially on June 22, 2007.  Before examination it was removed from its cabinet and both were put out in the hot sun for a day to dry out and hopefully remove some of the mildewy smell.

    Upon general inspection, the following was noted:  It had a very thick dust layer covering everything on the top of the receiver.  The front panel is in good condition, minor scarring at the rack mount holes, probably a nicotine stain on much of it.  Several toggle switches are not original, including the added one in the dial lock location.  The BFO knob is not correct, it is the same as an Audio Gain knob, rather than having a zero center.  All tubes and shields are in place, the tube hold-downs for the HV rectifier and the audio output tubes are missing.  The hole plugs in the RF deck are all in place, and there is a right angle SO-239 type adapter in the antenna jack.  The spare fuse cover is in place, but cracked, and 1 hold-down for the crystals in the FCU has been broken off.

    The audio output terminals are marked "8 ohm" and there is a small extra transformer under the chassis, connected to the audio output tube's plate circuit, the original audio output transformer has been disconnected, and it's condition is presently unchecked due to the interference of the added transformer.  Under the chassis there is a small board with a 6BE6 tube and some associated circuitry which maybe be an added product detector.  Wires to the "Mod/CW" switch, and a nearby terminal strip with it's lugs identified by a label lead to that conclusion. 

    There is date codes of 6/52 and 7/52 on the power supply filter capacitor, and the bathtub capacitors.  The P.S. filter cap. sections check to have an ESR of 1.0, 1.2, and 1.3, which is acceptable, but due to it's age, it should be replaced.  There is a 16-3 grounding AC line cord installed, the 2 amp AC line fuse is blown, the HV fuse (maybe 1 amp) is intact.  One power supply filter choke, L-51, is open, and must be replaced.  There is no evidence of any of the "black beauty" capacitors having been replaced. 

   click on thumbnails for full size pics.

panel1r.jpg (498687 bytes) topoa.jpg (946251 bytes) top2.jpg (865718 bytes) top3.jpg (958085 bytes) labelHr.jpg (632946 bytes)

general look at top, overall.  can see the dirt film, which should clean up fairly well. 

botoar.jpg (651007 bytes)

bottom overall.  at top center, you can see the bottom of the filter choke, L-51, that has a red mark stripe on it, by a previous owner, may have known it was bad.

backoar.jpg (385136 bytes) audoutr.jpg (487854 bytes) bot2.jpg (1047246 bytes)

back panel, audio output terminals, small added audio transformer inside


proddet1r.jpg (733912 bytes) proddet2r.jpg (869322 bytes)

what may be a product detector

much later - I've cleaned it up and done some inspection.

Before cleaning  TopOAB4wash.jpg (974760 bytes) FrPnlB4wash2.jpg (730067 bytes) 


After cleaning  TopOAaft3.jpg (905280 bytes) FrtPnlAft4.jpg (663998 bytes) Some polishing compound and wax should clean up the panel a lot.  Circular scratches around the large knobs may be able to cleaned up some, but should be fairly well hidden by the skirts of the knobs, which made them.


There is corrosion on much of the chassis, and the steel parts such as control shafts.  The top of the chassis is pitted and won't look real pretty (but is clean).  Brass parts for the band change mechanism and the dial drive are badly tarnished and will be cleaned up where necessary for proper functioning.

Ph_Sel_shafts2.jpg (629762 bytes) FCUshafts.jpg (649929 bytes)  dialdrive.jpg (634294 bytes) The shafts can be cleaned up, and probably must be to remove all the hardware. 

I removed the left side panel to get to the area under the selectivity switch for replacement of capacitors, and to get a better look at the circuit installed there on a PC board.  That ckt has a 6BE6 and appears to be a product detector.  Wiring for that modification and for the added switch for AVC timing has some connections hanging in mid-air and un-insulated.  These should be cleaned up, as well as any other instances of questionable workmanship.  Several capacitors have split open, I'm sure there are more.  Resistances were checked from all tube pins to ground, and compared with the manual's chart.  Several readings pointed towards possible failed capacitors and resistors which will be followed up and replaced as required. 

AVCcaps.jpg (1021352 bytes) AMCWswitch.jpg (769826 bytes)   Extra product detector wiring at the MOD-CW switch.

Under chassis pix show further affects of long term storage.  IFareaOA.jpg (1198393 bytes)

Biasbathtubsr.jpg (312837 bytes) I would recommend removal & replacement of these 4 bias supply "bathtub" capacitors, and the other 3 bathtubs, with discrete components mounted on terminal strips, at a cost savings over 're-stuffing" the bathtubs.  (that was done)

Audioxfmrr.jpg (348206 bytes)  The original audio output transformer must be checked to determine if it is usable.  This small transformer has been wired in in place of the original, and may be usable if the original isn't.  The large green cables, and the orange disc ceramics are part of the product detector modification.  

    The choke I hoped would fit won't, so I'll either find one that will do the job (somewhere), or put in a direct replacement.  I may be able to "borrow" one from one an SP-600 have. 

  Audioxfmrr.jpg (348206 bytes)  The audio xfmr has one open secondary, but the primary is still connected for some reason, and the replacement is in parallel.  Kinda strange, I'll hold judgment until I put power to it.

  biasnewcaps.jpg (363929 bytes)  I have replaced all the bathtub caps with discrete components, looks good.  

  SidepnlClean.jpg (909160 bytes)  I have polished up the side panels, chassis and the IF cans.   IFareaClean.jpg (880278 bytes)   Looks much cleaner than in the first pix at the top of the page..  

Panel yet to be done.  

  IFcansB4.jpg (747716 bytes)      IFcanCrack.jpg (616933 bytes)      BBOD's ("Black Beauty of Death", paper capacitors) in the IF cans have been replaced.  IFCansAft.jpg (691399 bytes) 
                            note the cracked one, above

 IFareaAft1.jpg (1093638 bytes)   I've finished all underchassis work except the B+ filter cap & installing a new line AC cord, which will wait until I'm ready for power, just so it won't be in the way.  

    Work remaining -- pull RF deck & replace BBOD's in there, about 22 or them.  That's the biggest task remaining.  The FCU, xtal controlled freq. unit, must be pulled & 2 BBOD's in it replaced.  It's a pain in the neck to get out & back in.  FCUconx.jpg (837630 bytes)  Black sleeve goes to FCU. 

    Then the front panel cleanup or repaint, and the cleanup of the gearing behind it.  Then I'll get power on it, check it out & align it.

FCU3r.jpg (258961 bytes)  The FCU is out, it's 2 BBOD's have been replaced (after the pic was taken).  With cleanup, it's about an hours work for just 2 capacitors.  It will be re-installed after the RF deck is done.

PODarea1.jpg (825876 bytes)  The right side area is considerably cleaner now, and the 1 BBOD in the "pod" has been replaced.  The power supply chokes came out of the large holes, the BFO can is open at the left.  The FCU was above the forward choke.

RFDeck1.jpg (848242 bytes)  The RF deck is removed, sitting cross-wise for a pic.  It has 4 of it's "finger" contacts broken, 1 is not used, but the other 3 must be replaced.  22 wires must be un-soldered to remove the RF deck, and there are about that many BBOD's in it to be replaced, almost half of all that are in the receiver.   

   I've been asked to do more cosmetic cleanup to make this receiver look as good as possible, so I will strip, repaint, and fill the engraved lettering of the front panel, and take extra pains to clean up the RF deck which was badly pitted and tarnished. 

RFDecktop93.jpg (744187 bytes)  Progress being made, slowly, in brightening up the top of the RF deck.

  The RF Deck has been cleaned,  recapped and re-installed.  The 3 broken female connectors for the turret modules have been replaced.  Didn't get a pic showing the approx. 20 capacitors that were replaced inside it.

  A replacement filter choke was selected and tested.  The 2 chokes and power transformer were cleaned in preparation for painting.  They and the right side panel were re-installed after painting, and a new 3-wire line cord installed..

paint1.jpg (700772 bytes) SP600sn7092pix/paint2.jpg (703078 bytes) SP600sn7092pix/ironin1.jpg (816680 bytes) SP600sn7092pix/ironin2.jpg (753263 bytes)

  I have not re-installed the FCU yet, will wait until after power-up and testing, which is next after the tubes are re-installed.   Similarly, the front panel will be repainted after initial testing and troubleshooting (as reqd). 

   We're getting close to completion, it will be interesting to evaluate the product detector installation and AVC mods.

The receiver was powered up and checked out at this point.  I'll get back to that in a bit -- 

But first, let's look at the cosmetic work on the front dial area and front panel, which was done after determining that it was at least in fixable operating condition:

before --

dialsb401.jpg (1011608 bytes)  gearingb4.jpg (1062390 bytes) pnlb401.jpg (585097 bytes) pnlb403r.jpg (1331292 bytes) pnlb405.jpg (511074 bytes)

partial dial and band change disassembly in the second pic. 

after --

dialsaft05r.jpg (425526 bytes) pnlaft01r.jpg (2058833 bytes) pnldone01r.jpg (471067 bytes) pnldone02r.jpg (586303 bytes)

There was a lot of tarnish, not rust or corrosion, on the gearing, etc., behind the front panel.  It's been partially disassembled and cleaned up, but not completely disassembled and polished, as it's essentially hidden from view when the panel is in place.  Dials were just dirty, not scratched, dinged, etc.  Front panel was stripped and repainted with the same paint as used on the cabinet, then the engraving filled with acrylic latex paint.  The cabinet had been sent out to be sandblasted, primed and painted with 2 coats of gray.

All done --  

frtOAdone05.jpg (531783 bytes) frtOAdone03.jpg (591031 bytes) frtOAdone04.jpg (486354 bytes) topOAdone001r.jpg (658767 bytes) topOAdone002r.jpg (556635 bytes) topOAdone003.jpg (757991 bytes)

All back together, aligned, tweaked, etc., ready to go into the cabinet which was sandblasted and painted.

botdone001r.jpg (549815 bytes) botdone002r.jpg (679422 bytes) botdone004r.jpg (617578 bytes)

Underneath views, across the bottom of the center pic you can see the replacement audio transformer,  the added break-in relay, the small greenish electrolytic capacitors for the bias supply, and the round blue AC line bypass caps.  The vertical can is the 3 section re-stuffed B+ filter capacitor, with a bead of white epoxy at the cap. 

OK, circuitry details/problems -- 
   When powered up initially it was evident that the AVC wasn't doing it's job.  All tubes had been tested, several bad ones were found and replaced, in one instance an improper tube had been installed.  I worked on nailing down the AVC problem for several weeks.  Sections were isolated and checked with the bench megger.  The "downstream" sections had resistances of 500+ megohms to ground, indicating no leaky capacitors or misconnections.  Resistances thru the chain to each tube were checked, noting was un-connected.  Besides what seemed to be insufficient AVC voltage being developed, the S-meter was very stingy, and it's driven separately from the AVC line.  The S-meter was sticky, and was removed, opened, and the culprit fine thread or hair was removed.  

    It appeared that the controlled stages weren't loading down the line, perhaps the source was of insufficient impedance to drive even a small load. A number of minor changes were made in the AVC circuit to further reduce some high resistance paths at the source, and to help 'fast attack".  This finally has resulted in proper AVC action, but the S-meter, even after some changes, is still very stingy.  Another meter was temporarily substituted with no difference.

   The product detector had been installed be a long past owner.  It worked, but not well.  It overloaded easily, audio output was somewhat distorted.  Looking back, it would have been much quicker to have completely removed it and installed a circuit that is known to work well.  This one was similar, but there were at least 2 obvious mistakes in wiring or thinking that had to be found and corrected.  it's audio output is now clearer, but it is somewhat stronger than the audio output through the AM detector, so when switching from AM to SSB, the audio gain must be reduced.  I reduced it some, but could not go further without increasing distortion.  It has some feedthru when the input to the product detector is removed, there should be none, and that may be part of the distortion.  

   Many extra hours have been spent with the AVC and Product Detector problems.  Overall, we now have a fine operating SP-600 in the AM mode, and it works acceptably well in SSB.  It's very sensitive, good selectivity with it's 3 xtal positions and 3 LC ckt positions.  I have spent many hours "band cruising" with it, both on ham bands and SW broadcast bands, while checking and tweaking the perfomance.  After 30 or so minutes of warmup, it can copy a SSB net on 20 meters for hours with very little adjustment.

And in the cabinet, ready for delivery --

in cab 03.jpg (603828 bytes) in cab 04.jpg (610712 bytes) in cab 07.jpg (614337 bytes) in cab 01.jpg (617380 bytes) in cab 08r.jpg (430255 bytes)

Later, a replacement audio output transformer was installed, the makeshift one removed. The audio output meter function now works.

Below is a summary of what has been done:

SP-600-JX-6 sn 7092, April 11. 2008

Circuit changes and modifications.

1. A previous owner had installed a product detector utilizing an added 6BE6 tube, mounted on a small PC board with a turret type tube socket. At least 3 problems with that installation were found:

The B+ circuit had several dropping resistors which resulted inonly about 58vdc ahead of the normal dropping resistors for the plate and the screen. This was responsible for some of the distortion coming out ot the prod. det. Some of the extra resistance was removed to put about 138vdc to the same place.

There was no “grid leak” resistor on grid 2, the IF input. A 1 megohm resistor was added.

A 0.47 bypass capacitor was across the 22k grid 1, BFO input, resistor. This was removed.

The B+ supply to the BFO buffer tube had been rerouted thru an added RF choke. This was retained, as it was necessary when the BFO signal was rerouted to the new prod. det.

2. AVC action was felt to be too little. Testing showed that the AVC controlled stages were in good condition after the re-capping job, with greater than 500 megohms to ground for the AVC buss, so attention was given to the AVC voltage generation circuit. To reduce load on it, 3 things were done:

R61, the primary load resistor, was changed form 1megohm to 11 megohms.

R60, the coupling resistor to the controlled stages was shunted with a diode to provide fast attack.

R97, at the AGC/MAN switch, which allows the RF gain control to have some affect upon the AVC when in AVC mode, was changed from 3.3 megohms to 10 megohms. Previously when the RF gain control was at full gain, the load to the circuit was 3.4megohms, now it is 10.1meg.

IF coupling capacitor C-138 to the AVC diode, and C-139 to the S-meter diode, were increased from 50pf to 100 pf to increase signal magnitude.

3. Perhaps because of the poor AVC action, a previous owner had placed a 10kohm resistor across the -51vdc bias line to reduce the max bias available. This was removed. After the re-capping and re-resistoring of the bias decoupling circuit, and with the re-tapping of the primary of the high voltage transformer from the 117 to 130 tap (reducing power supply voltages by ~10%) the -51vdc line actually measured -55vdc. R66 in the cathode of the AVC diode was decreased from 18k to 9k to encourage AVC action at a lower signal level.

4. Signal level out of the product detector was much greater than that out of the AM detector. The IF signal to the product detector was reduced by changing the input capacitor to only 5pf. The output level still is higher on SSB/CW vs AM.

5. The BFO injection control on the rear panel is operative, and should be set at ~50%. It's affect can be heard on SSB signals.

6. A relay was added to allow putting the receiver on standby from an external transmitter or transceiver. An RCA jack was installed on the rear panel for input of 12vdc to operate the relay.

7. A new 3-wire AC line cord was installed, with proper X-Y AC bypass capacitors.

8. One HV filter choke was shorted, it was replaced with a fully tested used one.


You will select upper or lower sideband by setting the front panel BFO frequency knob to between 1 and 1.5 khz above or below zero, respectively. 

There is a 3-position “AVC speed” switch in the hole previously used for the tuning lock.

A replacement non-standard audio output transformer had been installed by a previous owner, this was replaced by a good, used, original transformer. The audio output meter function now operates properly.

There were/are no hold-down clamps on the audio output tube or the HV rectifier tube.