Sansui Eight Bias and DC offset

Leestereo

Super Member
I've been restoring a Sansui Eight of late and after studying the schematic and some experimentation I've figured out the how to adjust the DC offset and the bias. To adjust the DC offset, set VR802 to give 2.04v on R804 and then adjust VR804 for 0V on the output. The bias is adjusted by VR803; monitor voltage drop across R832 or R852 (use whichever is more easily accessed). These are the emitter resistors of the output transistors. I set the bias at 30 mA which is 9 mV across the 0.3 ohm emitter resistors. At this setting the cross-over notch should no longer be an issue and the cooling fins barely get warm. Note that I found it necessary to reapply heat sink compound to the final driver transistors and the output transistors since the original used had all but disappeared over the course of 32 years.
 
Nice work...

As an aside, most meters are not so accurate as the voltage drops to barely-perceptable levels in the low mV regions, so for adjusting bias, I'd recommend putting the probes across both emitter resistors and adjusting for 18mV.
 
Thanks for the hint Echowars, I'll give it a try. Still struggling with the FM though; might be a bad cap somewhere since the voltages are off on the FM front end as well as the IF board.

Read about the work you did on that Pioneer 780, now that was especially nice work!
 
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