On Thu, 1 Mar 2018 at 17:21 Francesco <madowax@...> wrote:
Thanks Siggi, I have just tested every resistor you listed out of circuit,
they all test good and they are still in spec even the 1% tolerance ones, I
replaced R1878 when I received the scope, the previous owner butchered it,
probably the grid bias problem was there when he sold the scope, I also
replaced c1991, just to be sure it wasn't leaking. The only thing I found a
bit strange is the waveform from pin 7 of high voltage trasformer, the
sinewave seems a bit asymmetrical, however -300v test point is exactly at
-300V. Thanks again.
Regards
Francesco.
Hey Francesco,
well, now that you know the resistor values for sure, the next step would
be to measure the voltage drop over them in-circuit. I'd suggest leaving
the scope running for 10-15 minutes before you do so, as it's easier to get
consistent measurements that way. Alternatively you could short the
slow-start capacitor to get a consistent measurement in the stone-cold
state.
If you measure and report the voltage drops over R1994, R1881 and R1992, we
can do a bit of math and see whether the currents make sensible values.
Is it reasonable for you to do this with the grid bias pot at min and max
settings?
Note that resistors can go wonky in weird ways. In my 485, I found a
resistor that measured in spec on my DMM, but had voltage-dependent
resistance in circuit. A bit of a head-scratcher that one :).
Siggi
