Defeated by a Commodore 1950 Monitor

[Mark J. Blair]

> On Dec 1, 2014, at 21:48 , tony duell <ard at p850ug1.demon.co.uk> 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:

http://elektrotanya.com/aoc_adc_commodore_1950_cm314_monitor.pdf/download.html

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 nf6x.net>
http://www.nf6x.net/