[Amps] Hercules II With SD 1405 Bias Current Setting

[Steve Thompson]

Mark,
Can I suggest you check some other voltages? Looking at the 
schematic, with the bias pot at minimum the base voltage on the 
SD1405 should be equal to the voltage across the bias reference 
diode. The reference diode is 1N4002 with 0.5mA going through it, 
I'm struggling to imagine that would be enough voltage to bias the 
SD1405 into conduction.

I've used pretty much the same bias circuit in commercial amps in 
the past except with a smaller reference diode running at higher 
current (so probably higher reference voltage) and there was zero 
current in the rf devices at minimum bias setting.

What are the diode and base voltages?

Steve G8GSQ

> 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