Defeated by a Commodore 1950 Monitor

Mark J. Blair nf6x at
Mon Dec 1 17:44:09 CST 2014

I have a Commodore 1950 monitor that came along with an Amiga 3000, and I think it has defeated me. Its failure mode is that there's no vertical scan; just a single, horizontal line. Vertical drive is still working, because I can shift the line up and down a bit using the vertical centering control, but whatever circuitry generates the vertical scan doesn't appear to be doing it.

Trying to debug it has been challenging, even though I found a service manual online. The monitor appears to have been designed to be slapped together without much consideration of service accessibility, and the service manual does't describe the circuitry in much detail. It includes a block diagram, with no indication of which actual circuitry corresponds to which blocks. The schematic diagrams show connectivity between the parts, but don't identify the functionality of most of the ICs.

I think I've narrowed down to one IC that's a likely candidate for the failed component, but it's still just an educated guess that it's even the IC responsible for generating the vertical scan. And it's a no-name IC with the monitor manufacturer's internal part number (IC401, with part number 56A326-1), so I don't think I'm likely to find a replacement for it unless I find another model 1950 with a different failure.

I have an LCD monitor that I can use with the A3000, but the A3000 just doesn't look right with a flat panel on top of it. Now I wish I hadn't gotten rid of my old Viewsonic 17" CRT monitor all those years ago! I was sure happy to see it go at the time, but I failed to predict that I'd get interested in retrocomputing a decade later.

So, I guess I'll just keep my eyes open for a suitable CRT monitor that I like for the A3000. Maybe some older Viewsonic monitor in the 13"-17" range? If anybody near southern California might be able to use my failed 1950 for parts, or has a 1950 with a different failure that they'd like to dump on me, please let me know! Even though I don't anticipate being able to fix it, I don't want to throw it away, in case parts appear or some other collector can make use of it. So, I'll stash its carcass away for the time being.

Mark J. Blair, NF6X <nf6x at>

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