I asked this elsewhere but got no feedback. Maybe my questions are unclear,
or just plain too dumb! This AL572B had 2 bad tubes, a blown grid resistor,
and a failed 3055 bias transistor.
I figured out from the rather badly drawn schematic how the automatic bias
switching is supposed to work, but it's still giving me a lot of pain.
Rather than rely on RF to operate the bias I forced the bias circuit into
operation for testing by grounding the collector of Q101 in the bias circuit
to simulate it being turned on. This gives me about 9 volts on the collector
of Q103 / bias line to the centre tap of the filament transformer. Amp idle
current is about 5mA! Shorting the zener D103 drops the collector of Q103 to
about 0.7v as it gets turned on more and the bias current rises to, what I
think seems a more reasonable 125mA. Pulling the bias line to ground
entirely just causes the grid current meter to go hard negative!
With RF applied I see values of plate current that translate to what I am
seeing for power out, when factoring the plate voltage and 60% efficiency so
I doubt the meter is wrong. So why the very low 5mA with the stock bias
circuit switched on?
Something else bothering me, When the T/R/bias relay is open the bias line
coming from the centre tap of the filament transformer has about 23 volts on
it. This goes to 9v (or0.7v) as above when the relay is energised. Where
does the 23 volts come from?
What is the function of D117 and C140? (C140 is a tantalum polarised
capacitor though the parts list might suggest otherwise.)
Okay, enlighten me.
Thanks
Martin, HS0ZED
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