On Wed, 28 Feb 2018 at 06:53 Francesco <madowax@...> wrote:
Tested another U950, from a working scope of a friend, both U950 work on
his scope, both doesm't work on mine. It seems like grid bias is not
clamping the ac waveform at the correct amplitude. If I ground the
inverting input of U1890 lowering its output near 0 volts everything starts
to work perfectly (intensity pot can completly dim out the traces and I got
no artifacts or spots on display), when the RC delay starts to increase
again the output voltage of the op amp, however the same problems reappear.
I have already tried to swap U1980 with a good one but nothing changes,
same problem, (its behauviour was indeed correct, it follows the rc delay
as it should, so that was expected).
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
