Sansui 7070 Mystery

[samgraysound]

Hi,

I'm working on a Sansui 7070 stereo receiver. At mid to high volumes of sound the protect relay is tripped. This occurs with or without a load. The mystery is why.

The two things that are supposed to trip the protect relay (according to the service manual) are DC on the outputs, or the thermal fuse going over 125C and closing.

The thermal fuse is not closing and there is no DC on the outputs.

As described in the service manual:
"(1)When exceeding over DC +2.5V at point A
1. TR03 turns on through D07 and TR04 off
2. By TR04 turning off, Relay RL01 turns off.
3. By the Relay RL01 off, it disconnects speakers terminals from power amplifier output."
(2) When appearing lower DC -2.5V at point A
1. Voltage level at point B drops down through D09 and TR04 turn off.
2. As same as above steps 1-2 and 1-3"

DC at point A is not changing at all. It sits at -15mv. However the relay still trips.

Protect circuit description and whole service manual attached.

Please help

-sam

 

[Bo Deadly]

My guess would be that one or more capacitors are bad and so you don't really have DC anymore. You need a scope to see how well the full bridge rectifier is performing. If you're using a conventional meter to measure DC, it will not see transients getting through. Or just replace the 3 electrolytics and see what happens.

 

[user 130681]

C15 if open circuit allows transients to flow into the base of TR03. There is also C18, that is part of the timer circuit.

 

[samgraysound]

C15 if open circuit allows transients to flow into the base of TR03. There is also C18, that is part of the timer circuit.

Took another look and lo and behold C15 is simply not there. There's a pad for it, just no cap. Kind of looks like it came from the factory that way tbh. Putting it in fixed the problem. Thanks!