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?
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?