On Q1980 collector with grid Bias at min I see 120 V and I have already
replaced C1991 and R1994 even if they tested good out of circuit (R1994 was
indeed a bit low: 1% out of tolerance on the low side)
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Both the grid bias pot and U1890A shunt current away from Q1980. By your
description it sounds like extra current is sneaking into Q1980s base -
maybe C1991 has gone leaky?
Page 3-37 has this description of this part of the circuitry:
"Transistor Q1980 is configured as a shunt-feedback amplifier, with C1991
and R1994 as teh feedback elements. The feedback current through R1994
develops a voltage across the resistor that is positive with respect to
the +42.6 V on the base of the transistor. The value of this additive
voltage plus the diode drop across CR1950 sets the upper clamping
threshold. Grid Bias potentiometer R1878 sinks varying amounts of current
away from the base node of the transistor and thus sets the feedback
current through R1994. The adjustment range of the pot can set the nominal
clamping level between +71 V and +133 V."
So the nominal adjustment range of the grid bias pot should result in 71V
to 133V on Q1980s collector. What are you seeing?
Siggi
