Defeated by a Commodore 1950 Monitor

Mark J. Blair nf6x at
Tue Dec 2 13:33:52 CST 2014

> On Dec 1, 2014, at 21:48 , tony duell <ard at> wrote:
> It is not uncommon for the vertical shift control to apply a DC bias to the yoke, and thus the output amplifier
> stage could still be the problem.

Thanks, that's good to know. I don't have a lot of experience with analog monitor circuits yet.

> Do you have a URL for this manual? I might give it a look and see if I can deduce anything.

I found it here:

I'll appreciate any insight you might offer, because I'm not very good at making sense of monitor circuitry yet. My first suspect is IC401, followed by possibly IC402 or IC553.

>> have been designed to be slapped together without much consideration of service accessibility, and the service 
> That is quite normal, alas.

Yes, and it's a big part of the reason that I haven't been motivated to get much experience fixing monitors so far! I had previous bad experience trying to fix a 19" Hitachi monitor for my Sun 3/60 20ish years ago (thought I had it narrowed down to a $70 transistor; replacement blew, too; ended up taking it to a shop and paying real money for somebody with a clue to fix it).

I did have much better experience with a DEC VR201 recently. Its circuitry was a lot easier to get to for maintenance, and it had a simple failure mode: Blew its fuse immediately. So I powered it through a current-limited supply and started looking for hot spots with my IR thermometer. Hottest spot was a diode feeding power to a subcircuit, and second-hottest spot was a capacitor from that power rail to ground. Replaced the cap, monitor became happy.

> Older Commodore monitors were Philips inside. I have an idea this 1950 is an AOC chassis, but I might
> be wrong.

You're right: It's an AOC CM314. I'll try calling AOC shortly on the slim chance that some old-timer might have a dusty pile of documentation in his or her office. If not, one of the notable surplus dealers on our list has contacted me about possibly supplying a working monitor. Finally, there's a 1950 on eBay, but I'll look for cheaper options first.

Mark J. Blair, NF6X <nf6x at>

More information about the cctech mailing list