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?
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
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
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
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
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