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 recently 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 or astig controls have no effect on the noise. 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 on both channels even if connected to the cal test point. In the cal mode, the changes in vertical sensitivity is working correctly, but the top horizontal portion of the square wave has the same erroneous noise at any vertical sensitivity. The noise stays the same displayed amount in all input settings.
The horizontal triggers OK. Rotating the seconds/div knob works as it should, but does not give any clue as to any time interval or frequency, of the source of the noise. It appears to be random noise.
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 (the drawing is pretty small and a bit hard to read). I do not have a small extender board to match the small (other) connector on the back of the scope. It may or may not be needed for troubleshooting this. I have a 465 to troubleshoot with.
This is my first posting to this list. Don't know if I should also pose the question on TekScopes2@.... I don't yet know the distinctives of the two lists.
Your attention is appreciated.
Ray, W4BYG
