Sansui 400 receiver,,,

[knockbill]

Just an update,,,

Digi order finally came,, and I replaced the "temporary" transistors I used to get it running,, Then while biasing the amp, the Left DC pot failed and protection light came on,, and I killed the power... Replaced the bias pots with multi turn Bournes... Bias holds fine now....

TR503(indicator lamp board) replacement arrived but it had a rough trip!! The PO here damaged the package,,, and the transistor, doesn't read good,,,
I am still contemplating installing it to try,, but further testing and measuring values, shows there is no MPX, even tho FM comes in pretty strong... So the FM indicator light shouldn't be working!!!! So far I've replaced the 4 small transistors(TR404~407) on the MPX board,, and tested the Ge transistors (TR401~403) that are left and they test to schem voltage... Also tested all the diodes on the board, all are good....

I can read 19kHz after D401-2,, but can't get 38kHz after TR403,, or TP 4G-4H... I found that the borrowed scope I have has an iffy channel two, so I was taking frequency readings with channel one....

Bottom line,, there is no stereo coming thru FM/MPX, only mono, which doesn't change with stereo/mono switch...
AUX inputs with CDP have good, balanced stereo output, and tone controls have full range... Amp/preamp stages sound great!!

 

[knockbill]

Still trying to solve the no FM stereo problem,,,

I tested TR 402,, Schem 19.5V E,,,, 18.6V........ Reading .03V Schem C,,, .03 reading
TR403,, Schem 21V E and C ....... Reading 19.96V V both

I tested all the 4xx/5xxtransistors,, only TR503 is in question,,, Voltage matched all resistor pairs in circuit, all within 10%,,, all diodes measure right and no bulging/leaking caps,,,
VR 401,501 and 502 are the old crappy original ones, maybe they need replacing...

I think I may need to have a working transistor in TR503 position, tho... I heard on a video I watched something about the MPX(FM stereo) won't work if the indicator lamp is blown, (I found it blown last night, and replaced it...

I can't figure out the scope time frequency settings Math to confirm 38mHz, after TR403 and at T403,,,
Thats where I think the MPX signal starts and I can't read it... I can read 19mHz at D401/D402 and TP 4G/4H...

I just put the 2SB54 sub AC187 in for TR503,, the stereo indicator comes on, and stays on thru tuning the FM scale (the meter still reflects the station strength),,, and also stays on thru all the selector switch modes,,, But it did test leaky maybe shorted, as it didn't make it thru the PO too well...

If the receiver is on TR503 is on,,, but I did think it was shorted, or at least leaky!!! That makes the original look better, as there is no FM stereo so the lamp should be dark...

Whats next??

 

[knockbill]

I changed TR503 with a close NOS sub,,, and get the same result as the original TR503... So I think the problem is on the FM MPX board...
Not sure how to find that problem!

 

[knockbill]

Well as my usual MO,,, I put this guy aside for a while and recently pulled it off teh shelf to use the working preamp/amp sections,,,, but I just had to try the FM/stereo again,,, I found that when teh receiver is cold started, and I start it with the Tuner on FM stereo,,, the tuning MPX light comes on for a few minutes and then fades out,,,, but this is a new find for me,,, so my questions,,,,
1... Can I assume that replacing damaged transistors/caps on the MPX and stereo light driver board has been done right??
2... The working while cold thing makes me think that a component in teh MPX or FM tuning ckt boards could be failing intermittently or overheating??
3... Also I read that MPX falling out could be related to a failing silver mica cap in an IF can,,, the schem shows small caps in the IF ckt near teh cans, but not in them... I also read that lightly moving the IF can slightly in each direction could reveal that problem,,, I haven't tried it,,, want to see if anyone else heard that...
I have not attempted to make any adjustments to any can slugs in this guy either,,,
If anyone has experience with any of these symptoms,,, I 'd like to hear them!!! this is the last step in a very early SS Sansui a receiver rebuild from heading for scrap pile at a radio show, to where it is now!!!
Just turned it on to take pic,,, Stereo light is on, and it was working til I shut it down..
TIA