Solid State 300 - Setting Bias current

[Bert Beukema]

Hi all,

This machine is on my bench, it's now playing well again. I have the original service manual and schematics but neither tells me what the procedure is for setting the Bias / Idle current. The manual refers to the 4 trimpots on the driver board as 'AC Balance Adjustment' and 'DC Bias Current Adjustment'. I think the bias should be set to around 15mV. It is a cap-coupled receiver, so it should normally be set over the power resistors or the power transistors.

I think this machine is basically the same topology-wise as many other Sansui receivers from that same era. But I don't want to mess around with it as the driver board is crap to get at with any feelers. Anyone want to have a guess at the correct points to set the trimmers from?

Thanks!

Bert

 

[Bert Beukema]

I just saw there are speaker fuses on there. So the bias should likely be set over these.
Aim for 15mA the service manual says (+/- 3 mA).

Now to think about the AC balance.

 

[Bert Beukema]

Ok this is a quick approach. To set the AC balance you should use an oscilloscope until it clips, if you have one.
If you don't clip a multimeter (+) grabber on the positive leg of the output capacitors; C815/C816.
Measure the voltage on the main capacitors (+) to chassis. In mine it was 46v. Set to half amount using VR801/VR802.
For bias remove the back panel. Then remove the fuses.
Clip the positive grabber and the negative grabber of a multimeter on the fuse holder (check the schema to see where the + and - go).
Then use VR803/VR804 to set the bias at 15mA. If your multimeter only has volts, set for 30mV.

 

[Bert Beukema]

So case closed.
Procedure above is only for those in a hurry and/or not in possession of an oscilloscope + function generator.