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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 19th Mar 2015, 3:49 pm   #1
Dave Moll
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Default Colour monitor on IBM PC-AT - no display

As mentioned elsewhere, I had a problem with the monitor attached to my IBM PC-AT in that the display went blank after I had left it running for a while. It subsequently sprang back to life, so I put it down as a temporary fault as it had continued working until a couple of days ago, when it went blank again - again recovering after being left for a while. This time, however, the recovery has been short-lived, so it looks like a more permanent solution is required. At least I never got round to putting the case back on after opening it up when the initial problem occurred.

Interestingly, the (decaying) display appears briefly as the monitor is powered off - which may give a clue to someone with more in-depth knowledge than I have.

I would be grateful for any suggestions for checks I could make - such as voltages that should be present at various points on the CRT.
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Old 19th Mar 2015, 4:33 pm   #2
Alistair D
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Default Re: colour monitor on IBM PC-AT - no display

First port of call would to get your magnifying glass out and inspect the soldering on the PCB. Look closely around the legs of large components suck as transformers. You are looking specifically for a crack round the solder joint leaving some solder attached to the pad and some to the leg.

Al
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Old 19th Mar 2015, 5:38 pm   #3
Nickthedentist
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Default Re: colour monitor on IBM PC-AT - no display

Some gentle bashing with a plastic screwdriver handle while the unit's powered up (using a mirror to view the screen if need be) is never a bad idea, but be careful not to knock the pip off the back of the tube
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Old 19th Mar 2015, 5:52 pm   #4
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Default Re: colour monitor on IBM PC-AT - no display

I'd check the voltages on the tube - sounds as though it's being cut off except when the power rails fall at switch-off.
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Old 19th Mar 2015, 8:22 pm   #5
Dave Moll
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Default Re: Colour monitor on IBM PC-AT - no display

So far, I have measured the EHT at 20kV. I'm puzzling, however, how to access the connections on the base of the CRT, as the PCB has a plastic sheet over it held on by two pegs that give the impression they would break if I try to lever it off. I would prefer to remove the cover intact if at all possible.

In case it helps to identify the pinout of the tube, it appears to be designated M34JDU30X15.
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Old 20th Mar 2015, 10:40 am   #6
KeithsTV
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Default Re: Colour monitor on IBM PC-AT - no display

Are the heaters lit? I had a problem with a monitor where it faded out slowly. There were dry joints on the heater pins on the tube base.

Keith
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Old 20th Mar 2015, 12:55 pm   #7
Dave Moll
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Default Re: Colour monitor on IBM PC-AT - no display

Heaters definitely aglow. Besides, I wouldn't expect the brief display at power-off with cold heaters.
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Old 20th Mar 2015, 5:03 pm   #8
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Default Re: Colour monitor on IBM PC-AT - no display

Hi, I wonder if it could be a lack of ht voltage to one of the pins on the crt ? I seem to remember that if the first anode voltage wasn't present then it might give a similar fault, where the picture appears when the monitor is turned off.
regards Peter B
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Old 20th Mar 2015, 10:03 pm   #9
Dave Moll
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Default Re: Colour monitor on IBM PC-AT - no display

Thanks, Peter. That confirms the sort of possibility I was thinking of. Now I need to identify which pin is the first anode and devise a way of accessing it to measure the voltage on it.
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