Author Topic: LeCroy 9350AM - Inverted CRT Image  (Read 1156 times)

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Offline nsternie

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LeCroy 9350AM - Inverted CRT Image
« on: July 18, 2018, 03:40:55 am »
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
 

Offline Circlotron

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Re: LeCroy 9350AM - Inverted CRT Image
« Reply #1 on: July 18, 2018, 03:59:21 am »
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?
 

Online floobydust

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Re: LeCroy 9350AM - Inverted CRT Image
« Reply #2 on: July 18, 2018, 04:27:26 am »
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.
 

Offline coromonadalix

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Re: LeCroy 9350AM - Inverted CRT Image
« Reply #3 on: July 18, 2018, 10:50:58 am »
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 ... ???
 

Offline nsternie

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Re: LeCroy 9350AM - Inverted CRT Image
« Reply #4 on: July 18, 2018, 02:09:11 pm »
Quote
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.

Quote
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.

Quote
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?)

Quote
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"  ;).
 


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