[Test-Equipment] Tektronix PS 503A

[Ray]

The current dialog above brings up a question for me.

I have a very nice SC-504 80 MHz scope mounted in the right hand 2 slots 
of a TM-504 .  It has worked well for me for years.

However, I just turned it on and the scope trace will not resolve to a 
fine line vertically as it should.  It seems to be internally modulated 
by about 3/4 of a division of vertical noise across the horizontal trace.

The focus control has no effect on the problem.  It does not appear to 
be any sort of an external RFI, etc., as inserting the internal filters 
or grounding the input, doesn't change the displayed noise. The noisy 
trace problem persists if connected to the cal test point.  In cal mode, 
the  top of the square wave has the same erroneous noise.   Changing the 
vertical sensitivity changes the amplitude square wave display as it 
should, but does not change the level of the problem noise on the trace. 
   The noise stays the same amount in all input settings.   I take this 
to indicate the problem is internal and not external.

The horizontal settings display and trigger OK.  Rotating the 
seconds/div knob seems to work as it should but does not give any clue 
as to any time interval or frequency, of the source of the problem.

Before I dig into it, does anyone have any experienced suggestions as to 
how to track the problem down and repair it?  As a guess, I would think 
it's a capacitor break down somewhere in the vertical output amplifier 
circuit, because it's in both vertical channels.

  I have the large extender for TM-500 modules and a photocopy of the 
manual.  I do not have a small extender board to match the small (other) 
connector on the back of the scope.  It may not be needed for 
troubleshooting.

Your attention is appreciated.
Ray, W4BYG