Sony STR-6050 - No FM stereo

[bluehills5]

Picked up a nice Sony 6050 receiver from a local thrift store ($60) and it is nice and clean and seems to work OK after a good dose of de-oxit.
But there is no FM stereo. The stereo lamp also does not light up and not sure if the bulb is also dead or not but flicking between mono and stereo it is clearly playing all stations in mono although it is picking up FM strong and clear.
Note (all?) the xx50 series do not use an IC for the FM modulation - it's all discrete components but I believe the xx55 series did use an IC for this function and I once replaced the HA1156 chip on a Pioneer SX to resolve a similar issue.
PS- I have the service manual downloaded.

 

[H/K crazy]

Either the FM section is out of alignment or the FM multiplex chip is no good.

 

[chazix]

Assuming noone comes along with "the answer", these hints might be helpful:

I once replaced the HA1156 chip on a Pioneer SX to resolve a similar issue

Please put any comparison along these lines out of your mind. Though the discrete circuit in the Sony is performing the same overall function as an HA1156, the way it does it isn't comparable at all.

If this unit were on my bench, I'd approach it by looking at sub-circuit functions. For instance, Q401's main function is to amplify the stereo "pilot", so I'd see if there was a strong 19KHz waveform on its collector. D401 and D402 are a "doubler", to derive a 38KHz signal from the pilot, so I'd see if there was a 38KHz waveform at, let's say, the base of Q402. Q402 is a 38KHz amplifier, so I'd see if there was a much stronger 38KHz waveform at its collector. Q404 is, I think, a "noise gate", whose job it is to kill the 38KHz signal if the overall reception is too noisy - one way of seeing if it was shutting the gate would be to remove it, or at least its collector connection. And just to be fairly complete, Q403 is the stereo lamp switch, but it is not your culprit, or at least not the only one.

If the above was all gobbledegook, and/or if looking at waveforms is a no-go, you could just try making DC voltage measurements around Q401, 402, and 404, and compare them with the voltages in the SM.

Either the FM section is out of alignment

I concur that this is a very good possibility.

Cheers,

chazix

 

[bluehills5]

I do not have easy access to a 'scope so the Hz measurements are not possible but I measured the transistors 401, 402, 404 and they all read about 20-30% high. So I measured the power supply and it is measuring all over the place but mostly about 30% high on all readings. The AC 59v supply is only reading about 43v AC so not sure what's going on here - other voltage shown as well. One thing I noticed after looking at the below thread is my power supply board uses one 1850R rectifier but this thread about a similar power supply issue but shows 2x 1850's.

http://audiokarma.org/forums/index.php?threads/sony-str-6050-advice-needed.540044/

Here are my supply voltages...

And a single 1850R rectifier and a soldered one from 2 diodes.

 

[schmikka]

These first generation Sony receivers are notorious for losing FM stereo. There are some threads here for repairing the 6060 FM stereo by swapping parts on the FM board, but apparently the 6050 isn't repairable with that same fix. I had a 6050 with the stereo out but thought it still sounded great in mono. And the rest of the receiver was fabulous and punched way above it's weight. Good luck with yours!

 

[bluehills5]

For now I'm trying to understand why my supply voltages are so out. How can I only have 43v AC from the transformer? Is this not fixed by the physical number of windings? Another clue - the power cord was covered in a black greasy film which extends to the cable going inside the unit so it was not some spilled crud like I first assumed. It's as if the cable overheated it started to de-plasticize.