On Wed, 28 Feb 2018 at 12:22 Francesco <madowax@...> wrote:
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)
If you have no control range to speak of on the bias pot, then I'd guess
Q1980 has gone bad.
Looking at the schematic, what the service manual states makes sense.
Discounting Q1980s base current, its collector voltage will be 42.7V +
150kOhm * (I_R1881 + I_R1992). The two control currents are then in the
range of I_R1881=~0-~440uA and I_R1992=~180-~120uA.
In simulation, I see the collector voltage range the service manual
describes, but when I disconnect Q1980s collector, I see the control
voltage rising from ~30V through to ~125V as C1990 charges, pretty much
irrespective the setting of the bias pot.
