Adcom GFA-5802 Biasing

[FootFungus]

I revived a beat-up Adcom GFA-5802. Now comes the final fun part of re-biasing without re-injury of the amp.

I have not been able to find the biasing procedure for the 5802. The 5800 service manual says to measure the current at idle on the B+ line. I assume the 5802 is somewhat similar.

Or, if I kind person can point me to a site that sells a PDF of the service manual, that will do as well. I have only found sellers of paper manuals.

 

[Nelson Pass]

Assuming that the output devices are matched - which they
are supposed to be, you can sense the current through each
by the voltage across the power resistor attached to its source.

I would start out with values about 100 mA or so and watch
where it goes.

 

[Hartonr]

Does anybody know what the correct biasing current should be for the GFA 5802? I will be measuring the voltage across the series source resistor, 0.33ohms, and adjust the bias with trim resistor R179, is this correct? What would be the optimum bias current for best sound quality?

 

[Hartonr]

Bias current, DC offset and modding input caps

OK last night I trimmed the bias the following way. Using trim resistor R61, trimmed the FET source current to around 90-100mA. I did not measure the source resistance so dont have a accurate current reading, might do this later. How critical is it to have an exact reading? Next using trim resistor R179, I trimmed to DC voltage at the output terminals to within 1mV of 0Vdc.

As far as modding the 5802, the first thing I notice is the poor quality input caps 0.22uF and 47uF parallel combination in series with the input. Any suggestions on a high quality replacement cap for these?

 

[lucpes]

As far as modding the 5802, the first thing I notice is the poor quality input caps 0.22uF and 47uF parallel combination in series with the input. Any suggestions on a high quality replacement cap for these?

I used WIMA MKS02 10uf, had to bend the legs to fit the 3.5mm pitch on the board but the sound improved imho.