Sansui 771 - Relay

cecilk

Active Member
My friend dropped off a Sansui 771 that he said made "crackling noises" and didn't work properly. Unit is all original with no signs of recent service.

I tested it and found that connecting a speaker to the left channel resulted in good sound.

But when I connected a right channel speaker as well, the relay tripped and the unit would go into protection mode. It would try to turn itself back on every few seconds without success.

I'm not really understanding how the relay works and what causes it to trip. Why would it be fine with no speakers plugged in, fine when a speaker is plugged in on the left channel, but then trip when a speaker is plugged in on the right channel?

I'm thinking of doing a recap - but before doing that wanted to really isolate the problem to improve my troubleshooting skills.

Measurements
I downloaded the service manual and ran the DC offset and bias current tests. The DC offset on the left side is fine - I adjusted it down to ~5mV and it is stable. Bias current on both channels was successfully calibrated to 19mA.

However, the DC offset on the right channel is all over the map, constantly changing, fluctuating from -100mV to +250mV. That seems quite suspicious.

I measured voltages on the F-1500 power supply board and found the following:
Pin 25 measured 45.2v (spec = 33v) - This feeds the tone control board
Pin 26 measured 12.1v (spec = 13.6v) - This feeds the tuner boards

The +35v and -35v voltages on the output transistors (that also feed the driver board) are within 5% of spec and seem fine.

Service manual with schematic is at https://www.hifiengine.com/manual_library/sansui/771.shtml

Any thoughts on these results?
 
Sounds like bad differential amp transistors. (2SA640). They can cause popping, as well as protect initiation because of high DC balance. TR2 and TR4 are the ones for the right channel, I believe.
 
Sounds like bad differential amp transistors. (2SA640).

Yes - the trimpot for the DC offset is right near those on the driver board so that would make sense.

From my research, a KSA992 will sub for the 2SA640. I think I might have a few of those - I'll give it a go.

They can cause popping

My friend said that the unit would "crackle" - this might be the cause.

Thanks!
 
crackling many time comes from noisy differential transistors like these
 
UPDATE: replacing TR02 and TR04 with KSA992's solved the issue.

I am now able to set a stable DC offset < 10mV and relay has stopped sending the unit into protection mode.

Everything running smoothly now.

Thanks, Tom!
 
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