Re: 7904 - Very Dim Trace


On 07 Feb 2016 19:15:56 -0800, you wrote:

Guys, this is my first post here. I've been tinkering at Tek scopes for along time - since I first bought a 531A in the late 70's. I'm looking for some tips on a 7904.

I picked this one up at an auction, and at first, it looked pretty good. Good enough that I've been scrounging plug-ins for it, and generally cleaning it up, and adjusting it. I switched it off one night, and the next day, I had no trace. Zilch. Beam finder, maxing the intensity controls - no help. Even in a darkened room, no light at all.

The filament is lit.

All LV supplies check out OK.

I don't have a HV meter to allow me to measure the anode supply. I know there's decently high voltage there, judging by the length of the spark when I discharge the electrostatic charge, however.

The 2950V measures high, about 3250V. Yet, curiously, the cathode supply regulator (U1635) isn't railed - in fact, based on my DC measurements in this area, I think the regulator is actually regulating - just at too high of a voltage. Weird. I wish I knew what the cathode supply measured before the issue popped up...
I assume you measured the output of U1635 and found it to be within a
couple volts of ground.

A high cathode voltage could be produced by leakage through CR1639
which protects the non-inverting input of U1635. U1635 could also be

On a whim, I tweaked the CRT grid bias (R1674) and can now get a faint trace. It's well focused, just dim. Pushing Beam Find does not h elp - it just gets blurry, reduces the sweep span, but it's still dim.
If the cathode voltage is high then the horizontal and vertical
deflection sensitivity should be proportionally lower. Is that the
case? You can check the calibration using the probe calibrator output

I would defocus the beam before conducting further tests in case you
fix the problem so that the CRT phosphor does not get burnt.

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