Re: Tek 2465a grid bias and brightness problems


Siggi
 

On Thu, 1 Mar 2018 at 02:00 Francesco <madowax@gmail.com> wrote:

Ok I have checked the Voltage at Q1980 collector, with r1878 full CW (max
grid bias) = 60V, with r1878 full CCW (min grid bias) = 120V, so there is
the correct range but shifted of 13V more or less. Q1980 test good out of
circuit.
Well, this shifting does seem to be your problem - especially if you're
measuring this at power-on, as the slow start should increase the nominal
voltages by ~10V initially. So if you're measuring this in the "cold"
state, you're more like 20V low.

I don't know where this is coming from, but there aren't very many
candidates.
If R1994 has drifted low, that'd do it.
If R1881 or R1992 have drifted high, that'd do it.
If R1878, the grid bias pot had high contact resistance or resistance to
ground, that'd do it.
If C1991 was leaky, that'd do it.

I think you'd best trace this by the currents to and from the Q1980 base
node. By measuring the values of R1881, R1992 and R1994, then measuring the
voltage drop over them, you should be able to to suss this out.

Good luck,
Siggi

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