Sansui 350 Bias and offset

[Stereoboy70]

Does anybody have the adjustment procedure for this receiver ?? I have a service manual but it doesn’t show how to set the offset or bias current. Any help would be appreciated. I just got this receiver and fixed a couple of issues. The headphone jack was giving me issues with the L channel output. Both channels run through the jack before going to the speakers. Also replaced the lights. Cleaned all pots and switches as well as replace some out of spec caps and a couple of resistors. Right now it’s sounding great.
Thanks for everyone’s help on troubleshooting issues I have had with my vintage equipment.

 

[petehall347]

its cap coupled output no dc offset but there is centre voltage . best set with a scope .

 

[Stereoboy70]

its cap coupled output no dc offset but there is centre voltage . best set with a scope .

Thanks for the reply. I’ll see what I can do.

 

[stereofun]

Took a quick peek at the schematic. I can see you do have trim pots available for center voltage and bias.

On my 800, I did not have the settings either - so here is what you can do.

Centre voltage: measure VDC from main supply cap plus to chassis. You should see around 50v per schematics. Now measure at each output cap (1000 uF) from plus to chassis. You should see around 24v per schematics. Next adjust Vr801/802 for each channel to set output cap at half voltage of main cap. NB: do verify that vr801/802 are indeed the right set of trim pots by verifying your voltage reading responds ! (Normally you'd use a scope for this setting as @petehall347 says, but this will be good enough for now)

Bias: remove both channel fuses ( not sure where they are located on the 350) - check service manual if in doubt. Probe across the empty fuse holders one at a time for mA!! - you will likely see somewhere around the mid to high teens. Whatever those numbers are you can pick a number in between the two readings. On my 800 I ended rounding slightly up and picked 20mA, also a common setting on amps from this generation and circuitry.

Again verify which set of trim pots "moves the needle" - for the intended setting. Hope this helps, Happy new year.

 

[Stereoboy70]

Took a quick peek at the schematic. I can see you do have trim pots available for center voltage and bias.

On my 800, I did not have the settings either - so here is what you can do.

Centre voltage: measure VDC from main supply cap plus to chassis. You should see around 50v per schematics. Now measure at each output cap (1000 uF) from plus to chassis. You should see around 24v per schematics. Next adjust Vr801/802 for each channel to set output cap at half voltage of main cap. NB: do verify that vr801/802 are indeed the right set of trim pots by verifying your voltage reading responds ! (Normally you'd use a scope for this setting as @petehall347 says, but this will be good enough for now)

Bias: remove both channel fuses ( not sure where they are located on the 350) - check service manual if in doubt. Probe across the empty fuse holders one at a time for mA!! - you will likely see somewhere around the mid to high teens. Whatever those numbers are you can pick a number in between the two readings. On my 800 I ended rounding slightly up and picked 20mA, also a common setting on amps from this generation and circuitry.

Again verify which set of trim pots "moves the needle" - for the intended setting. Hope this helps, Happy new year.

Super. Thanks for your help. Will try doing this today. I’ll let you know the results.

 

[skippy124]

Re the bias current value, the first page of the schematic available from HiFi engine lists specifications including the bias current, 17ma +/- 2ma. The fuses for the rail voltages for each channel are located on the rear panel (panel mounted fuse holders), F002 left channel and F003 right channel.

Cheers
John