Defeated by a Commodore 1950 Monitor

Mark J. Blair nf6x at
Tue Dec 2 16:07:28 CST 2014

> On Dec 2, 2014, at 11:50 , tony duell <ard at> wrote:
> OK, got it... What a confusing manual. It's roughly what I would expect a service manual to be
> (schematics, parts lists, some faultfinding info) but it is not well organised... Perhaps I am spoilt
> by HP and Barco manuals :-)

Good, it's not just me being dense then. :) I'd imagine it makes more sense to somebody who has seen a lot of monitor circuits, but I have a tough time making sense of it without some additional hand-holding.

> To me IC401 looks like a sync processor/oscillator. IC402 looks like the vertical output stage.

That's pretty much what my guess was. Some of the components south of IC401 look like they might be something like PLL filter components, and my guess was that IC401 is probably responsible for generating the vertical ramp. I didn't probe it beyond the DC voltage checks mentioned by the troubleshooting chart, though, because it's a bit hard to get to without major disassembly.

> I think pin 2 of IC402 is the output, pin 4 may well be the drive input (maybe a ramp voltage
> here?

I might as well tack on a test point wire or three while it's discharged and then poke around to see if anything ramp-like is coming out of IC401. I had assumed that the vertical drive must be fine since I can adjust the centering of the scan line, but I suppose that the bias circuit to the final drive transistors could still be good with the previous driver stage being bad. And especially since you have identified a possible cross for IC402 in your next message, so I might actually be able to replace it if it's bad!

> D404 and C410 may well be that boost circuit I mentioned and are worth checking first.
> Maybe then change IC402 if you can get one.

Sounds like a plan. I still have the fall-back plan of getting another monitor to play Frankenstein with from Cindy or eBay, but it'll be great if I can fix the monitor that I already have.

Page 37 of the manual recommended checking for 24VDC at pin 3 of IC402, which also connects to the junction of D404 and C110. I probed it with my DMM at C410 since that was easier to get to with the monitor assembled, and I did find 24V there. Next (maybe tonight if I'm not too tired after work) I'll try probing that probably drive input to see if IC401 is generating a ramp. If I see a ramp, then I'll try digging up a replacement for IC402.

If any experienced monitor experts can suggest possible identities for IC401, that would sure help! It wouldn't surprise me if it's a fairly common part monitor working incognito in this monitor.

If I do manage to fix this beast, then I should write up what I've learned on my blog so that maybe the next person trying to fix a similar monitor will stumble across my notes.

Mark J. Blair, NF6X <nf6x at>

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