350a No FM Stereo

[Sankaty]

Hi All - I'm very novice but here goes. Dumpster rescued a Sansui 350a the other day - it cleaned up well. Initial problem was the tape-mon switch was dirty. Once cleaned she came back. Needs new dial backlight bulbs. Otherwise has a wonderful warm sound.

Current issue is the fm side only mono. We live 16 miles from Chicago broadcast towers and have a good roof mount antenna. Tuning meter works - checked voltage on the tuning lamp at 6.5 volts. The fm stereo lamp just above the tuning meter lamp has no voltage and had a 8 volt lamp installed. Is this a problem with multiplex circuit? Did some previous tech install too much draw and blow MPX? Toggle of the stereo/mono switch or the mpx noise switch have no effect. I am just learning to read the schemes .... well trying anyway. Any suggestions would be great. I want to learn this stuff. Thanks.

 

[sregor]

If no stereo sound, the bulb is not the fault. "too much draw" does not make any sense - power is not cause mpx failure. I would start by checking DC voltages in the MPX board, to see if any of the transistors are bad. After this, it gets very difficult to trouble shoot and align without signal generator and scope. Some trouble shooting can be done with a strong signal and a meter which measures ac up to 76khz. VTVM, AC millivoltmeter, or scope would be required. FM mpx signal generator for most accurate alignment. (you may be able to get it working, but hard to get good performance or separation by using off air signal).

There should be 8 volts on the lamp Stereo signal should pull one side to ground - if no stereo should see 8 volts on both sides of lamp. 8 volts comes form D002 on the power supply, but if it were missing, the mpx circuit will still work, just no lamp.

 

[Sankaty]

Hi Steve. Thanks for your response. Glad the weekend has arrived!

Found 8.5V dc on both sides of the stereo indicator lamp & the lamp is ok. Cleaned all the switches again including the local/distance switch on the back plate. Also noticed tuning needle is off in different ways for both FM and AM. Was thinking about removing & testing the 6 radial caps on the mpx board? It would be the tightest soldering work I've attempted so far. That little board is FULL of components! Also thinking/admitting maybe I should shelve this FM issue until I've learned more.

Going to replace 4 transistors on the control board when parts are available as this unit does have a constant low level noise as described by Noah; "I do want to note, the 350A has some reported noise floor issues. The Pre-Amp/tone (F-1248 Assembly) utilizes X4 2SC458 transistors. The 458 is notorious for pre-mature failure and noise issues. These were swapped out with close HFe set of KSC1845 NPN transistors."

http://marantzhallo-fi.blogspot.com/2015/04/sansui-350a-receiver.html

 

[Tjet]

Hi Steve. Thanks for your response. Glad the weekend has arrived!

Found 8.5V dc on both sides of the stereo indicator lamp & the lamp is ok. Cleaned all the switches again including the local/distance switch on the back plate. Also noticed tuning needle is off in different ways for both FM and AM. Was thinking about removing & testing the 6 radial caps on the mpx board? It would be the tightest soldering work I've attempted so far. That little board is FULL of components! Also thinking/admitting maybe I should shelve this FM issue until I've learned more.

Going to replace 4 transistors on the control board when parts are available as this unit does have a constant low level noise as described by Noah; "I do want to note, the 350A has some reported noise floor issues. The Pre-Amp/tone (F-1248 Assembly) utilizes X4 2SC458 transistors. The 458 is notorious for pre-mature failure and noise issues. These were swapped out with close HFe set of KSC1845 NPN transistors."

http://marantzhallo-fi.blogspot.com/2015/04/sansui-350a-receiver.html

What ended up being your fix??