Amstrad 486 - Attic find - Monitor fault \ VOGONS

Amstrad 486 - Attic find - Monitor fault

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First post, by Zurking

Posted on 2023-07-27, 17:34

Zurking Offline

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I've had great joy discovering these forums. I have lots of retro consoles and acorn/BBC PCs I'm trying to bring back to working order.

I recently found an Amstrad 486 in an attic. I've cleaned it all up, checked any battery leakage, to discover this uses a CMOS chip/battery.

It's powered on and I was able to access the BIOS, but after several minutes the power to the monitor will switch off. PC stays on, the monitor thenwon't come back on at the monitor power button.

If I switch the whole PC off and wait a few minutes to power on, the monitor will work again for several minutes. The power to the monitor comes from PC PSU.

I'd appreciate anyone's input of what to look into, so I can fix it. It really looks like a fun PC to keep and tinker with

Reply 1 of 4, by BitWrangler

Posted on 2023-07-27, 17:55

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Do you hear a "tick" kinda noise when the monitor goes out? Sometimes high voltage flashover trips them.

Unicorn herding operations are proceeding, but all the totes of hens teeth and barrels of rocking horse poop give them plenty of hiding spots.

Reply 2 of 4, by Deunan

Posted on 2023-07-27, 17:59

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Probably a capacitor issue. Testing electrolytic capacitors in-circuit requires decent ESR meter, so you probably will just replace anything suspicious. Or all of them just to be sure.
When you say monitor is powered via PC PSU I assume it's just the mains cable? I'd start with the PSU section in the monitor, recap, that is always a good idea to have less ripple and clean power delivery. Then the capacitors on the secondary side of the flyback, and also the cap in the primary drive of the flyback. Rest might be OK-ish, but if the problem presists also replace the small power bypass caps on the main PCB, this might be a logic issue due to spikes/ripple on the power rails.

Just keep in mind, if this is your first monitor job, CRTs are both fragile and dangerous. Always learn before you do, there's plenty of YT videos on the subject. I also like to clean the monitor inside but for that you need to know what can be hosed with water and what can't. If in doubt do only dry cleaning. Removing dust is more than just a nice touch, it can cause overheating and issues with high voltage arcing.

Reply 3 of 4, by Zurking

Posted on 2023-07-27, 18:59

Zurking Offline

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BitWrangler wrote on 2023-07-27, 17:55:

Do you hear a "tick" kinda noise when the monitor goes out? Sometimes high voltage flashover trips them.

Yeah, there's an audible "tick" like it's been tripped. I'll research high voltage flashovers. Thank you

Reply 4 of 4, by Zurking

Posted on 2023-07-27, 19:04

Zurking Offline

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Deunan wrote on 2023-07-27, 17:59:

Probably a capacitor issue. Testing electrolytic capacitors in-circuit requires decent ESR meter, so you probably will just replace anything suspicious. Or all of them just to be sure.
When you say monitor is powered via PC PSU I assume it's just the mains cable? I'd start with the PSU section in the monitor, recap, that is always a good idea to have less ripple and clean power delivery. Then the capacitors on the secondary side of the flyback, and also the cap in the primary drive of the flyback. Rest might be OK-ish, but if the problem presists also replace the small power bypass caps on the main PCB, this might be a logic issue due to spikes/ripple on the power rails.

Just keep in mind, if this is your first monitor job, CRTs are both fragile and dangerous. Always learn before you do, there's plenty of YT videos on the subject. I also like to clean the monitor inside but for that you need to know what can be hosed with water and what can't. If in doubt do only dry cleaning. Removing dust is more than just a nice touch, it can cause overheating and issues with high voltage arcing.

That's great info. Thank you, really appreciate it.

I've not repaired a CRT monitor before. I was about to replace some capacitors on a Acorn BBC micro, bought as a kit. I'll see how well I do on that job before I tinker with the monitor.

It would be great to get it in a working fit condition. I'll be sure to watch some YouTube videos 😁

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