[Amps] Hercules II With SD 1405 Bias Current Setting

[Mark Robinson]

I have my Hercules II amp on the bench for a repair and I decided to 
check the power supply voltages and the PA transistor bias settings.

The power supplies with the undriven amp keyed was 14.03V,  13.98V, 
14.05V and 13.92V


I then lifted the 4 red Collector jumpers from each pair of PA 
transistors.  I put a DC ammeter across one collector jumper and 
measured the collector current on that pair of transistors with the amp 
keyed and a dummy load connected to the input of the amplifier and a 
second dummy load connected to the output of the amplifier. I measured 
the collector current on each of the four PA transistor pairs.

I was expecting to see a bias current of 250 or maybe 400 mA but I measured

Top Left       1.795A
Top Right     1.760A
Bottom Left     1.460A
Bottom Right   1.486A


My amplifier doesn't have the MRF 458 transistors but is fitted with  SD 
1405 transistors.

I tried turning down the Bias adjustment pot on the 81405 Bias Board but 
the lowest I could see was around 750mA with the pot set at minimum

I checked D2,  the 1N751A  5.1V reference zener on the bias board and it 
was reading 5.05V.  The DC supply to the bias board is 12.7V

What bias current should the SD 1405 transistor pair be set to?  Is it 
more than the bias setting for the original MRF458 transistor pairs ?

If I want to get the bias voltage down more then I probably need to 
lower the reference zener voltage.


73 Mark N1UK