HP A4576A Trinitron Monitor Repair

[T3sl4co1l]

My old baby's finally kicking it, it seems.

Symptoms:
Horizontal width has been funny for a while; but it always went away after about 10 min (something warming up?).  No sensitivity to vibration, and too consistent, to be a cold solder joint or something.

The effect was about 30% less width, with screwed up pincushion: what looks like oscillation (feedback loop problem?) in the width, making the width sort of crawl, chaotically, every 50 or so lines (at 1600x1200x85Hz).

Last week, it seems a power outage or surge caused a complete failure.  At least the HOT and pincushion switch were shorted, and the fuse supplying them, blown.  I ordered replacements (2SJ449 --> FQPF9P25, 2SC5047 --> FJL6920TU -- a surprisingly close transistor still in current production).  The behavior is different (I get varying degrees of startup, it seems) but it's not working.

I have the service manual, N3 chassis; but it contains no troubleshooting info (full schematics are fantastic, but nothing saying what the MCU does).  I also have WinDAS, which communicates, but doesn't seem to offer any further illumination.

Tim

[T3sl4co1l]

Hah, nevermind, forgot to check the power supply, R674 was dead.  So the deflection blowout took out the HOT, pincushion switch, fuse on the deflection board, and the fusible 0.47Ω resistor on the supply board.

Pincushion was still briefly weird at first, and it's still doing this weird wheeze cycle before turning on (or, it did the first time, we'll see about subsequent use?), which I wonder if either one is an underlying issue that may be causative.

Tim

[shakalnokturn]

How does it behave without video input and at different resolutions?
Check C907 (33µF 200V).
Could you point to where you found a working copy of WinDAS please?

[T3sl4co1l]

http://oldcomputer.info/hacks/monitor_su/index.htm

The caps are old enough to be suspect, but none are bulging, and there are enough of them throughout the design that, I'd really rather not go down that recap rabbit-hole...

It's funky at other resolutions, like 1280x960x60Hz (and while it's still warming up), I can adjust HSIZE up and down, and down below "42" it pops straight, but above "64" it pops into negative pincushion (hourglass figure, more curvature towards the top, some wobble in the width).  As it heats up (or generally just acclimates to the setting?), the thresholds rise; now after a minute it's "71" rising and "51" falling.  Setting it to "100", the hourglass width is obviously growing with time.  Yeah, really looks like something has a nasty tempco in the pincushion circuits.

Heh, 2048x1536x60Hz doesn't look bad at all, no way I'm putting up with the flicker though...

Tim

[T3sl4co1l]

And here's kinda what the wobbling width looks like.  Usually it fills most of the screen, changing as things warm up, then *pop* it's normal and straight.  Now it seems to vary with HSIZE, triggering near the top and bottom as the upper threshold is approached.

Tim

[andy2000]

While it probably doesn't need a total recap, I would definitely look at the caps.  Bad electrolytics usually get better as they warm up.  Caps from that era rarely bulge when they fail, they just go high in ESR. 

If you don't have an ESR meter, I would start by replacing any caps in hot areas (look for dark areas on the PCBs).  Pay particular attention to the smaller caps since they tend to dry out faster than big ones.  I have an IBM branded Sony that's just a few years older and it had a bunch of bad caps near the MCU, and a few in other areas.

You could also try selectively heating or cooling certain caps to see if the symptoms change. 

[SoundTech-LG]

Still have a few of these N3 chassis main boards around (for some reason) if you need parts.

[T3sl4co1l]

Noticed it seemed to be running unusually hot.  Picture was alright though.

Put the enclosure on (it had cracked in handling earlier, the delay was gluing it back together), lasted about 8 hours, then, blink out, click, blinking amber light.  Yep, deflection's failed again.  Not sure if that's the inferior output transistor, or literally everything else being in need of replacement, but it's not looking good, in any case.

August 1999 - September 2019

I hope its replacement lasts even half as long!

Tim

[Zucca]

T3sl4co1l respect to you, I would have never attempted to repair a CRT monitor.
Usually I cut/keep the main cord and trash the rest.

[shakalnokturn]

Usually I cut/keep the main cord and trash the rest.

I usually cut/trash the cords and dismantle the rest. With just a bit more regret when it's a Trinitron or Diamondtron.