Sansui 8080db: no idling current, and no sound from speakers or preamp out

[EvanF]

First on the agenda: power amp. Using "main in", I found no signal. No idling current as well on either channel. Upon a recap of that section and testing of all semiconductors out of circuit on the left channel, no change in behavior. I assume both channels have the same underlying problem. I tested B+ and B-, looks good (-59 and +59 if my memory serves me well) because the caps in that section were all 63VDC and higher. I tested the sockets for the amp board, voltages there.

Preamp next!

Nothing on the preamp out. However, it appears the final stage is working, as pushing the filters in reduces noise in the ways they should. Tone boards appear to be out, they do not change the noise. I guess this is more of signal tracing solution problem,but just want to hear any thoughts from owners.

Passes protection, all fuses intact. All preamp boards recapped, as well as the power amp board and power supply. All voltages and currents measured with a 2016 calibrated Fluke 8840A multimeter. No pics for now, everything looked alright...

This amp is making me crazy .

And I have to replace the lamps when I'm done with the fix! Ugh....

 

[Tom B]

Did it work at all before the replacements??
Are the pre out/main in jumpers present?
Does the tuner work as far as signal and center tune?
Look for missing voltages.
On preamp board
Tone amp board
Tuner, IF
Check the function control on the DB board. This can get very intermittant, and well as many problems on the dolby board itself.

 

[EvanF]

Did it work at all before the replacements??
Are the pre out/main in jumpers present?
Does the tuner work as far as signal and center tune?
Look for missing voltages.
On preamp board
Tone amp board
Tuner, IF
Check the function control on the DB board. This can get very intermittant, and well as many problems on the dolby board itself.

Jumpers out, I tested main in and pre out separately. Bypassing the dolby board should free me from worrying about that section, right? I'll have to go probing with the schematic now, I guess...

 

[Tom B]

The outputs are fed + and - 44v
The drivers are fed + and - 54v
other boards are fed by +38v or +12 for tuner/IF

 

[EvanF]

The outputs are fed + and - 44v
The drivers are fed + and - 54v
other boards are fed by +38v or +12 for tuner/IF

Couldn't find the 44V, tracked it down to the two fuse 1Ω 1/2W resistors. Also found out that the power supply silkscreen is incorrect to a capacitor polarity, so it shorted. Got a new cap, put 2 1/4W foil 1Ω resistors in the place (figure lower power is good for fusing in this case, temperature coefficient may be an issue).

Left channel preamp out is great! 54V on each channel power amp out before relay. Output TRs are fine. Seems none of the semiconductors aon the driver board are shorted.

Right channel preamp out is dead. No signal after VR1 on right channel. The issue may be a dead ba312 IC, really not sure. I should try injecting a signal into the preamp to see if that section works. The issue may lie in the loudness section or dolby switch, both of which got DeOxited.

 

[LBPete]

Later in production Sansui replaced those 1 ohm resistors with fuses.

- Pete

 

[HankM]

this is Hank in Pueblo! Just picked up my 2nd 9090DBm and it has the same issues listed above. My first one 3 months ago also had issues with little or no sound at first test, but all was fine after extensive dissasenbly, cleaning, and deoxing, especially the Dolby board. Going to start on this one tomorrow. Any more info on your 8080 would be appreciated, thanks!