Scrambled Screen: Monitor or Machine?

[Roscoe]

I am having display problems with my PCjr and I am hoping that the collective wisdom here can point me in the right direction.

Machine boots and runs fine - for a while. After a period of time (approx 30 to 45 minutes - not exactly timed) a reboot will produce a scrambled screen as shown:


This is the IBM Logo. You can kinda guess what is going on.

This is what I have observed so far:

• Failure only occurs on reboot.
• Sometimes it recovers by itself on a mode change (for example, 40 to 80 columns).
• Sometimes it can be resolved by turning the monitor off and on again. But this is not always reliable.
• Reboots do not seem to help.
• Turning the machine and monitor off and waiting 5-10 minutes resolves the problem.
• Turning the monitor off (and leaving the PCjr running) for about 5 minutes has been tried 1 time and was successful.
• The machine continues to boot despite the display problems. You can guess what is happening behind the scenes if you are familiar with the boot process.

My guess is that the monitor is having a heat related horizontal sync problem.

There is an ebay listing showing a monitor with the the same behavior here: https://www.ebay.com/itm/ibm-pc-jr-disp ... SwQzJb7c5A so I don't think I am unique in having this happen. (To clarify, the ebay link is not my monitor.)

I am hoping that I am not the first to see this problem, and that the fix is easy.

Has anyone seen this failure mode before or am I just lucky?

If it has been seen before, is the known cause the monitor or machine?

Is there a known fix?

[KenG]

I would say it is almost certainly the monitor. If it was the computer, turning the monitor off and back on would have no affect. I think there is horizontal sync adjustment knob inside the case.

[Brutman]

I think you have your answer already - the monitor works fine when the monitor is cold, and degrades as it warms up. So it's definitely something that is now marginal that is affected by heat.

I don't know enough about CRTs to fix them; you might try asking over at vcfed.org on their forum or on the classiccmp mailing list. For some reason "capacitors" come to mind, as they are a common component to fail, but I wouldn't start just replacing the capacitors ("recapping") without doing more background research.

As a work-around you could try some forced ventilation through the back of the case; a small PC-style fan on the outside of the case temporarily should be able to help you prove if it is a heat related problem or not. (The monitor I always use has one built in - a nicely executed hack by the person who owned it prior to me. Except for the amount of dust it's probably depositing in the monitor.)

[Roscoe]

The monitor was always my working assumption.

Which is better than the PCjr itself, I think.

I was hoping the answer was going to be more along the lines of: "We've seen this before. Replace cap C102 in the monitor and all will will be well." One can always hope, right?

[jmetal88]

Does the picture come back if you smack the side of the monitor?