I checked the voltage across R4419 and that measured 32mV. I'm pretty sure either something shorted or Q4420 acted badly before losing one of its junctions.
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Barry - N4BUQ
----- Original Message -----
From: "Roger Evans via groups.io" <email@example.com>
Sent: Saturday, September 25, 2021 11:08:01 AM
Subject: Re: [TekScopes] 7704A Vertical Amplifier Issues
I agree completely with Albert, if you look at the nominal voltages on the
schematic the output transistors of U4413 should have a collector current of
36mA ( (50-37)/360R ) so with any reasonable current gain they should not
draw more than a couple of mA of base current. Albert measures 4mA for the
total base current into both output transistors which is eminently
Neglecting the base current into Q4420, its base should sit at 14.88V ( (5k /
16.8k) * 50V) and its emitter should be around 0.6V-0.7V more positive so
15.5V - 15.6V and Albert measures 15.45V. The function of Q20 is to provide
a low source impedance to bias the output transistors of U4413 at 15.45V
rather than the 15V whichTek could have obtained by connecting R33 directly
to +15V. This also implies that R4419 should be a low inductance type to
maintain the frequency response of the amplifier.
I think it is safe to say that any junction transistor which measures a
forward Vbe of 2.6V is no longer a transistor! If you take Q4420 to be open
circuit from base to emitter then R4420 and R4419 supply the base current to
the output transistors without any voltage regulation which is why the bias
voltage shoots up to 17V.