Compaq Presario 425 CRT screen not working.

Discussion about old PC hardware.

Compaq Presario 425 CRT screen not working.

Postby northdale » 2019-2-10 @ 15:01

Hi.

The computer was sent to me, and got damaged during shipment.
The computer turns on, but the screen is black.
The video signal seams to get through to the crt part of the computer, we measured it with an oscilloscope.
There is some sparkling sound, so It gets power. There is some glow in the back part of the tube, but it's faint.
When I turn off the computer, there is a little flash in the monitor. Any ideas?

https://www.youtube.com/watch?v=fe6lVTJg1Pw&t=1s
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Re: Compaq Presario 425 CRT screen not working.

Postby dkarguth » 2019-2-11 @ 02:57

I'm going to warn you, unless you know exactly what you're doing, DO NOT MESS WITH CRTs. They are EXTREMELY dangerous when you don't know what you're doing. The voltage across a CRT is enough to kill you instantly if you are not careful. CRTs operate on thousands of volts, and they store that voltage in the tube like a capacitor even after they are turned off. You must first discharge them, by putting a resistor between the TUBE GROUND and the anode to bleed off the stored current. The earth ground is not a substitute for the tube ground. Since the tube is like a capacitor, just connecting one leg of it to the earth ground will not discharge it, you have to bleed the voltage across both leads of the CRT.
I'm not saying you don't know what you're doing, I just thought I'd say that just in case, as some people are not aware of the potential danger when servicing old CRT devices.

I'm not an expert in CRTs, but your CRT is definitely powering up. If the tube was vented, there would be sparks and a loud screeching sound coming from the back of the tube. It sounds like there is something wrong on the tube driver board, like maybe the board was damaged in shipping. I would advise against poking around it by yourself, as that board is the same board that has the high voltage components on it, such as the flyback transformer. I know you may not regard this as particularly helpful, but I would advise to find someone with more experience with CRT circuitry to help you in this project.
"And remember, this fix is only temporary, unless it works." -Red Green
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Re: Compaq Presario 425 CRT screen not working.

Postby northdale » 2019-2-11 @ 05:54

Yeah I know I can die. Thanks for the warning and the info :)
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Re: Compaq Presario 425 CRT screen not working.

Postby dkarguth » 2019-2-11 @ 18:42

lol with that said, I would see if you could introduce a test signal into the crt circuitry. I bet it's VGA. Maybe the video connector on the board is labeled?
"And remember, this fix is only temporary, unless it works." -Red Green
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Re: Compaq Presario 425 CRT screen not working.

Postby northdale » 2019-2-12 @ 06:04

I could try to do that,,I think I fried one resistor I need to replace first. A ground thing touched it and it smoked. I think so at least, I need to measure it. But we did find the rgb output from the motherboard and got a signal when we measured with an oscilloscope. And there was continuity on that cable connecting it to the crt. I was also wondering about reflowing everything. With soldering iron, not baking it.
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Re: Compaq Presario 425 CRT screen not working.

Postby northdale » 2019-2-16 @ 16:01

I treied to feed rgb signal. No change. I connected to another crt and I got a picture. So the problem is with the crt itself.
If there is a faint glow on the back of the tube, does that mean that, that part is probably working?
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