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Diagnose Compaq 7500 Monitor No Video

J

jinkuspinkus

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Jul 16, 2025
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Good evening all,
A while back I acquired a Compaq 7500 monitor for cheap on facebook marketplace, and it ran great for a few months until the screen went dark and I stopped getting an output. I left it in storage for about 1 year but have decided to take a look at it and get it working again.
Before I put it in storage, when I powered it on, it would give a flickering image and there would be a clicking/crackling sound from the back along with the image appearing. Now there is no video, but the crackling sound can be heard in the video (Excuse the sniffles, I've got a cold, and the clicking in the background near the end is my oven lol) I took the shell off to see if I could see any arcing but its also kind of bright out. It is also worth noting that when the crackling sound occurs, some dust on the screen jumps up and falls again after, so I'm assuming it has to do with the high voltage somewhere.

View attachment IMG_5539.mov

Any Ideas what's going on? I've worked on electronics before, but never CRTs so any help is appreciated.
 
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Took a long exposure photo and managed to capture some arcing. Is this part of normal functioning or something I should look into?

Here is the arc, and note the glowing of the electron gun in the back, so cool.
IMG_5546.jpg

And here is the part of the tube with light.
IMG_5553.jpg
 
jinkuspinkus said:
https://www.manualslib.com/manual/418072/Compaq-Crt-Monitor-S7500.html?page=28#manual. Additionally I did find this manual online with a complete parts list and wiring diagram.
Since there is no video output I want to assume a bad flyback, but I am not sure. The part number of the flyback transformer is 6174T11003E.
Click to expand...

Don't jump to conclusions. Most these days initially blame a bad flyback transformer, regardless of what the fault is. If you actually had a bad flyback, there would be no corona discharge that you observed and photographed so very well, between the external Aquadag (the grey conductive coating/paint on the CRT's glass bulb) and the yoke's perimeter.

The external Aquadag is supposed to be grounded (by that I mean to the set's negative voltage rail or common, which is normally connected to ground if the VDU has a 3 wire line cord). This coating forms one plate of a capacitor. THe internal conductive coating(connected to the typical 20kV EHT via the anode cap) is the other capacitor plate and the glass bulb of the CRT is the dielectric. This capacitance, typically for a crt like this in the order of 1500pF helps filter the pulsed DC voltage, via the EHT rectifier, applied to the CRT's final anode under the big rubber cap.

You can see the bare braid connected to the external aquadag. In many color sets, rather than that being connected directly to the negative/common/ground system directly, it passes via a resistor to ground. The voltage across this resistor is sometimes sensed, and if it peaks too high the EHT system is inhibited. In any case, that fact you saw a spark there at all, suggests most likely that resistor has gone open circuit and the external dag is floating.

There are cases where an arc can perforate the glass wall of the CRT near the yoke, in that case the arc is coming from the internal conductive coating and through the glass wall. I doubt it is that, because when that happens the CRT loses its vacuum, and if that happens, with the heater running, it reacts with atmospheric Oxygen, the CRT fills with smoke and the heater is destroyed, in the same way a light bulb filament is, if air gets in.
 
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Thanks for the advice. So essentially there is a bad resistor somewhere along the common ground system on the tube that I need to replace? Would I be able to test this by jumping that resistor to see if i get an image? Also where should I look to find it? (I'm assuming on the neck board, since that's where most of the ground wires on the tube connect to)

Also since you brought up the aquadag on the back of the tube being grounded, there is a chance that at the old house I was staying at, the equipment ground in the wall outlets was bad lol. I wonder if that had anything to do with this.
 
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The thing is, if you were going to attempt to bypass a theoretical open circuit resistor on the pcb, you would ideally have to have the schematic and know exactly where the common/ground connection was too.

(It is unrelated to any ground system in the dwelling, it is about where the common is, inside the VDU)

I notice that the CRT's neck board is enclosed in a metal shell, and I can see some common wires there. You could assume that the metal shell of the neck board is likely at the set's common/negative supply connections. However I can also see that the braided connection wires also join that to the the CRT's external dag and a large heat sink, making it highly unlikely the dag has floated above the common. not impossible but it seems unlikely.

This raised the question of how it could be possible that the external dag has developed a voltage high enough to arc over to the yoke, if indeed that is what is happening as the photos seemed to suggest. It might be one of these cases that unless I could see exactly what is happening with that arc myself, it would be hard to predict the cause. I you could find the negative output of the main DC power supply, you could check for continuity between that and these other connections to the braided wire links.
 
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That arc might just be from static electricity that builds up on the surface of the CRT, and not related to the problem.

Considering that the grounding strap for the CRT is clearly connected to the chassis in several places, I don't think we're dealing with an ungrounded CRT. A quick resistance measurement would confirm that.
 
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