2 Dec
2014
2 Dec
'14
2:46 p.m.
On 2014-Dec-02, at 2:07 PM, Mark J. Blair wrote:
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.
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.