Defeated by a Commodore 1950 Monitor

[Brent Hilpert]

On 2014-Dec-02, at 2:07 PM, Mark J. Blair wrote:
>> On Dec 2, 2014, at 11:50 , tony duell <ard at p850ug1.demon.co.uk> wrote:
> ...
> 
>> 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.


A couple comments:

- The supply for the vertical drive section comes via a rectifier and filter from the pins-11/12 secondary of the multi-coil transformer down the page (part of some switching power supply). I was going to suggest you should probably find around 25V on that supply line, looks like it's OK as you found 24V at IC402.

- Q402, Q403 form a complementary driver for the 'supply' side of the V coil, controlled by the vertical centering pot. It looks like they just form a DC driver for the coil. They would form a sort-of H bridge with the output transistors of IC402.The V around the junction of the Q402,Q403 emitters (see also R414,R413,C422) should probably be around the mid-point of the supply, +12V or so, and should vary with the V centering control.
(The objective is so the IC402 output can drive current in both directions through the V coil so you get both up & down deflection.)

If, for example, Q402, R414 or R413 failed open there would be no current supply for the V coil. You said you were getting some variation with the V centering, so, yes, the problem may well be elsewhere.