LeCroy 9350AM - Inverted CRT Image

[nsternie]

So I picked up an old LeCroy 9350AM from a lab at my university that was done with it. It seems to be in decent working order, with one rather annoying issue.

This what a normal (left) 9350 screen looks like, and what mine (right) looks like:

I hooked up the video line directly to my signal generator, and I figured out that the video signal being fed to it is inverted (High voltage == Dark pixel). I also looked at the waveform coming out from the board going to the CRT, and it looks like its just a pixel scan going across the screen, with voltage proportional to brightness (Or darkness in my situation)

Soo.... Im wondering if my issue is that the display board is outputting an inverted signal, or if there is something wrong with the CRT driving circuitry itself. Any ideas, pointers, opinions? I'm not really sure where to look next. Glad to provide more info if its helpful.

Thanks!
-Nick

[Circlotron]

IIRC most if not all the black and white TV sets I worked on back in the day had the video signal fed to the crt cathode. Higher voltage on the cathode meant darker trace, equivalent to lower voltage on the control grid. Does the board feed video to the crt grid or cathode?

[floobydust]

To me that is not inverted video- The CRT is just biased full on, with phosphor burns for the darker areas.
I don't see any modulation of the CRT beam or blanking, aside from the slightly brighter sawtooth lines.

If it's a video monitor, I would check at the brightness/contrast controls and see if they do anything.
A shorted video driver transistor (depends if CRT is cathode or grid-driven), or loss of DC bias for CRT cutoff is what I would look for.

[coromonadalix]

hum  does it have some color inversion of some kind, on my scope, the lcd display background can be inverted,  black / dark became white / pale "ish"  and the white is darker ...

[nsternie]

Higher voltage on the cathode meant darker trace, equivalent to lower voltage on the control grid. Does the board feed video to the crt grid or cathode?

The signal goes to the cathode, however, there is some additional circuitry between where I measured it and the tube itself.

If it's a video monitor, I would check at the brightness/contrast controls and see if they do anything.

I've played with every trim pot there is on the board, as well as following the factory adjustment procedure. Interesting note with that, there is a point where it says something like this:

1. Set gain pot to maximum gain.
2. Adjust "Threshold" pot until W303 (test-point) reads 2.0 +/- 0.1 volts.

When I did this, the test-point read 18V! No matter how I adjusted it, I couldn't get it below about 16V. I should note that this procedure was for the 9354A, but its my understanding that they're just about identical except for 2 more channels populated on the main sampling board.

A shorted video driver transistor (depends if CRT is cathode or grid-driven), or loss of DC bias for CRT cutoff is what I would look for.

Maybe this be causing ^that^? I've done a pretty visual inspection, but havent seen anything. Whats the best way to go about finding a shorted transistor otherwise? (i.e. can I just probe them with a meter, test the forward drop, what am I looking for?)

hum  does it have some color inversion of some kind, on my scope, the lcd display background can be inverted,  black / dark became white / pale "ish"  and the white is darker

Yeah this is exactly what happening. Ive looked through every menu and searched all around but I'm pretty sure this isn't a "Feature"  .

[kipp]

I know this thread is many years old, but a resolution hasn't been reported, so in case it helps I thought I'd mention that I've seen something similar to this that was the result of the scope (not this kind, a Yokogawa DL1200) having been dropped.  The symptom was a barely visible image against a bright orange background.  No blanking during vertical retrace, the horizontal sweeps during the vertical retrace were visible as a sawtooth like pattern on the screen.  The display unit was a standard NTSC monochrome monitor unit, with a flyback transformer for generating the CRT voltages.  A physical inspect showed the problem.  During the impact when the scope had been dropped the weight of the transformer had cracked the circuit board beneath it.  Several traces were cut through and at least one solder joint to a leg of the transformer itself had cracked open.  Some epoxy to stabilize the circuit board, some jumpers for the broken traces, and fresh solder on the transformer's pins fixed the problem.