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Sansui 5500 No Stereo, Clear Mono

Richard P

Richard P

Now Many Reel to Reels is Too Many
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I'm working on a Sansui 5500 receiver and am having an issue with the FM Multiplex board. I get good FM reception but no stereo. I have a good 19 kHz, 3 Vrms signal after TR402 and a 38 kHz, 1Vrms signal at the base of TR403 but no 38 kHz on the collector of TR403. DC voltages around the transistors on the MPX board look OK (marked in green) and the transistors pass a diode test, but maybe TR403 is not passing the 38 kHz signal properly. Please have a look and let me know what you think. The full MPx board schematic is attached for reference.

1760384756813.png
 

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Did you check TR414? Drives the stereo lamp. Emitter/Base short would ground out the signal.
 
Thanks for the suggestion, @Punker X . I did test all the transistors and they all check out fine with a diode test.

I re-ran the alignment procedure a few times to make sure everything was optimized, which is a challenge with this receiver since the alignment instructions are somewhat abbreviated and don't cover anything except the basics. I was able to finally get the stereo light to come on by turning VR402, the stereo indicator adjustment pot, all the way down and feeding a moderately strong signal from my leader 3216 - 40+ dBu. Separation adjusts well using L401 and VR401 but the threshold is pretty high to get stereo to kick in. Same with the FM muting. I wonder if one of the amplifier transistors in the RF MPX or IF path has lost gain and isn't amplifying as much as it should . . . Sensitivity is good on the tuner, so i'm still focused on the MPX board for now. Caps all test good for ESR and capacity in-circuit. Maybe a leaky transistor or cap pulling the signal down.

19 kHz Pilot at the collector of TR402:

20251013_143200.jpg

38 kHz doubled pilot at the base of TR403:

20251013_143050.jpg
 
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At what level to you lose the stereo lamp. 40dbu is not all that bad. I would expect to loose stereo in the lower 20 to upper teens. You seem to have decent signal there. Rough adjustments without looking at the alignment procedure. I would peak the 19Khz with L401 and LC401a. I would adjust LC401b for highest separation. I typically make these with the generator set at around 60dbu.
 
I was losing it around 38 dbu. Not terrible, but none of the local stations I normally pick up in stereo were triggering.

I saw that the stereo light and muting circuit receive power from a rectified tap off the IF board that is amplified by TR202 and TR203. I peaked both the coils for maximum DC voltage after the 1N60 diode that feeds the MPX board at 4B. That helped a little. I also redid the full alignment with a weaker signal than I usually use and gained a little more from that. I played with L401 and VR401 (which are both adjusted for separation in the SM - detail below) and was able to get the stereo light to turn on around 28-29 dbu. I still can't get the stereo light with the local stations except for 1 station that's right in town, but it's better. I'll take a look at what peaking the 19 kHz signal with L401 does for me then adjusting separation with VR401. LC401 is a sealed unit so neither of the coils in it are adjustable, unfortunately.

Seems like an overly complex MPX circuit with the muting function muddling things up . . . Thanks for the help.


Sansui 5500 IF and MPX Schematic.JPG

1760484269796.png

1760484319679.png
 
Peaking 19kHz with L401 didn't change anything . . . I'm kind of stuck where I was. I can get a stereo indication and stereo separation at around 28-30 dBu from my SG but strong local stations struggle to get stereo reception, particularly in the upper FM range. I wonder if one of the transistors in the MPX circuit has low gain causing the issue. TR402 or TR403 maybe? The tuner is very sensitive, pulling in weak stations (in mono) very easily so the IF board seems to be doing it's job. The section of the IF board feeding the MPX switching circuits also seems to be working OK, producing about 2.9 VDC at 4B on the MPX board. . . any thoughts?

I also noticed that VR402 (stereo indicator adjustment) and VR403 (muting adjustment) don't seem to have much effect . . . probably related.
 
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Assuming you peaked T202 and T204 for maximum signal strength and adjusted VR202 for on scale max reading. Probably does appear to be down in the muting/stereo threshold circuits. Taking both to minimum I would expect a stereo operation down to into the low teens if not lower.

Do you have a cheap tester?


Much better than trying to use a multimeter and the forward/reverse bias method. Also great for checking lead orientation before installing. Are you testing the transistors in circuit? I would probably just put in new devices in that part of the circuit and check the resistors for drift.
 
I would also check or just replace all the electrolytic caps that the signals pass through, starting at the JFET transistor after the detector out, TR207. i.e. C401, C406, C407, C411, etc. These small value ~10uF caps can be problematic over time, I've seen a lot of bad ones in units this age. Also check TR207 which precedes the MPX board.
As a min check AC voltage levels on both sides of the caps, i.e. use AC coupled scope with stereo signals. There should be zero attenuation of AC signals across them.
 
Thanks for the suggestions @Punker X and @Bob@FM.

VR202 only affects the signal strength meter, so I left that where it is for now - if I max it out, the signal meter pegs.

I have a couple of the M328 transistor testers to check hfe. I also have a Sencore TF46 Super Cricket and a couple of curve tracers, but I'll start with the M328. I normally try to repair before restoring, but in this case, I'll go ahead with the recap on both the tuner and MPX board and shotgun the transistors in the muting & stereo threshold section of the MPX board.

Do you think it makes sense to do all the transistors on the MPX board?

For subs, I'm thinking:

2SC711 --> KSC1845
2SC871 --> KSC1845
2SC634A --> KSC1815
2SA678 --> KSA992 (Ic only 50 mA OK?)
2SC735 --> probably OK, it's just a lamp switch and seems to work but I can drop a KSC2383 in if needed.
 
Finished the recap and it helped a little but VR402 (stereo indicator adjustment) and VR403 (muting adjustment) don't seem to have much effect. All but 4 of the caps had excessive leakage ranging from 15uA to 400+ uA . . . capacitance and ESR were good for all of them (using Sencore LC102). Next, the transistors. Any thoughts on my subs listed above?
 
I don't get it. TS403 is cut off, see the base/emitter voltages in your first post. Before you embark on a blanket swap of transistors replace this one first. 2SA970 would be my choice but 2SA992 is fine too. Oh, and check why TR414 is pulling TR403's emitter low, cutting it off. The schematic doesn't say but it looks like it shows voltages in mono mode (or no signal).

Edit: Of course it is TR413 pulling TR403's emitter low cutting it off when conducting.
 
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I don't get it either . . . probably my lack of info and understanding. Here are some more measurements for TR403 to help:

........................................B....................C..................E
No Sig..........................13.1...............0..................7.47
40dBu no pilot..........13.37............0..................7.43
40dBu w/pilot............13.84...........1.54.............14.5

So it appears that TR403 is being turned on as the signal w/pilot is increased. At 40dBu input (75% modulation, 7.5 kHz, EDIT - was % - pilot) i get a solid 38 kHz output from the collector of TR403. As i decrease the input signal, I lose the 38 kHz signal at the collector of TR403 around 26 dBu.

So I think the 19 kHz signal from TR401 collector feeds the circuit below (TR409, D407/D408, TR410/TR419 etc.) to switch the MPX on and off. When it's on, TR413 allows TR403 to conduct. The point it turns on at should be adjustable by VR402 and muting should be adjustable by VR403, but neither of those seem to have much effect.

4B input to MPX board/2D output from the IF board is 2.47VDC at 40 dBu input and 1.84VDC at 26 dBu input.

TR205 base has 80 mV P-P 10.7MHz signal and TR205 collector has 2.3V P-P. TR206 base has 0.57V P-P and TR206 collecotr has 7V P-P . . . seem reasonable?

Thanks for your continued help . . . I'm starting to understand IF & MPX, but it's a slow process.


1760799683416.png
 
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PE9ZZ said:
I don't get it. TS403 is cut off, see the base/emitter voltages in your first post. Before you embark on a blanket swap of transistors replace this one first. 2SA970 would be my choice but 2SA992 is fine too. Oh, and check why TR414 is pulling TR403's emitter low, cutting it off. The schematic doesn't say but it looks like it shows voltages in mono mode (or no signal).
Click to expand...
Ding ding ding!

You mean TR413 not 414 of course
 
Take your pilot to 10%. That is a standard. May be part of your threshold problem
 
Sorry. That was a typo. I meant 7.5 kHz not 7.5% . . . . edited above.
 
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Richard P said:
TR205 base has 80 mV P-P 10.7MHz signal and TR205 collector has 2.3V P-P. TR206 base has 0.57V P-P and TR206 collecotr has 7V P-P . . . seem reasonable?
Click to expand...
Based on these numbers, TR205 amplifies voltage by a factor of 28.75 (2.3/0.080) while TR206 only amplifies voltage by a factor of 12.28 (7/0.57) . . . maybe that's not the correct conclusion from those measurements.
 
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