Sansui 9090 W/O Working Tuner/Dial Lamps/Meters

[PF Sound]

The muting circuit has it's own IF amp, TR04. Diode D08 rectifies the 70Mhz giving a varying DC level based on signal strength. That DC goes through VR03 to the base of TR05 turning it on or off based on the level. You can monitor the base voltage at TR05 to see what kind of levels you see when tuning. You can also monitor the collector of TR05 to see if it turning on & off.

You could have a bad IF amp IC which would give low levels. And typically even if one has failed it will still pass a signal.

I don't know what the typical levels would be. I have used a scope in the past to step through the IF strip looking for increased levels after each amp stage.

Could be a crappy transistor, carbon comp resistors that have drifted way out, IF amp IC, even a bad IF transformer (bad transformers are soemwhat rare)
Heck it could be that somebody has been twisting and turning pots and transformer cores.

View attachment 3571624

I'm chiming in on this old thread because I've got a 9090db with a similar issue in this area......stations are tuning in great but no FM stereo (can hear it if volume turned up over half way) or light (lamp is good), FM sounds good & loud in mono....muting switch has no effect.....I'm finding that pin 12 on the HA1196 doesn't change from .720v no matter what, therefore no change on Pin 9. The 76khz checks out good on pin 16. I tweaked VR03 & VR04 with no luck.
I was getting odd readings on TR04....voltage was fluctuating on the collector & at D08. I replaced it with a KSC945C & it seems stable now. I had tweaked T04 while troubleshooting but I noted that the reading at D08 was 1.80v when I started, I peaked it at 2.15v, & it now sits at 1.55v after replacing TR04. I don't fully understand the muting circuit & could really use some help sorting this. I started a thread in the tuner forum but that doesn't seem to be oft-visited, & this part of this thread is kind of where I'm at.

 

[JJCalvillo]

CLEAN THE DAYLIGHTS out of the Muting and Stereo/Mono switches. Then don't turn the volume up too much because the FM came back suddenly/loudly on the 9090 I was working on and took out a bench speaker. It didn't come back in stereo until I replaced C57. It could also be the Function Selector switch, those are pretty notorious for getting funky.

If you end up needing a new function selector switch there's a guy who sells a modern design that works quite well on the auction site.

 

[PF Sound]

Thanks for the reply. I've already cleaned the switches...removed & disassembled. I'm in the finishing stages of the restoration & this is kicking my butt. My C57 is measuring 379pF so it's in spec. I'm not seeing the collector of TR05 drop when moving tuner knob. The base voltage changes but nothing else. I'm trying to understand how it works.