[Test-Equipment] Tektronix 475 with problems, part 2

[d8lars@dtek.chalmers.se]

Hi!

First, thanks to all who have sent letters!

Some new findings. As I stated in my message the vertical unfocusness
where some 1 to 2 divisions high. I have now noticed that this is
dependent on which scale I use, at the lowest it is about 1-2
divisions, but if I go up in V/div the unfocusness gets lower.

This seemed strange so I took a closer look at the schematics of
the 475 and saw that different resistors where fed with 15 volts
to one of the vertical amplifiers, all depending on the V/div
setting. So it was not just a passive voltage divider that I first
thought. I also noticed that the unfocusness was a little higher in
channel 1, than channel 2.

As I do not have any more scopes, I used the 475 to look at the ripple
on the different voltage test points. I also checked the voltage
and resistance according to the manual. All was good, but once in
while I got a vertical unfocusness covering the whole screen, which
I never seen before, this with the lowest setting of V/div, if I
changed the V/div setting the unfocusness was lowered.

I could see this extra unfocusness on all regulated voltages, at
approximately the same value, full scale at the lowest V/div setting.

I also tested some of the things you all mentioned, the HV, the
astigmatism pot, and so on, but found nothing wrong there.

As all the voltages uses 50V as the reference, it seem as if most
of the problems comes from the 50V section. As this seem to be a problem
that comes and goes, and only stay on for a short while, could
it really be capacitor related? I tried too look at the voltage going
in to the 50 V section, but did not see that much added noise,
it had a quite large ripple, so I was unable to use the lowest V/div
range that I used on the lower and regulated 50V ranges, (anyone have
a number on the ripple on the 105V and 50V unregulated supply voltages
with good capacitors?) so I might have missed the extra noise. Could
it be a fault in the regulator circuit itself, and how to test that
in a good manor? The voltages was according to the schematic in the
service manual.

I'll see if I can get hold of some extra electrolytes and test with
new ones.

Any other idea?

/Lars

-- 
Lars E Pettersson        |  Chalmers University of Technology
d8lars@dtek.chalmers.se  |  Gothenburg, SWEDEN