Are brightness fluctuations always a capacitor problem?

[elSchus]

I hope its okay that I start a new thread for this... it's the same machine as my prior post (Color Classic) but new symptoms.

History: A while back I was having a degauss problem (http://68kmla.org/forums/viewtopic.php?f=7&t=16895) that has cleared up ( ). I've been having flyback transformer issues as well (a whine), but those also are better than they were after I removed and re-soldered the flyback. There is still the occasional whine.

I'm now more concerned with a flicker/brightness fluctuation on the CC monitor (happens for ~3 seconds every 5 minutes or so). I had thought this was all part of the dying flyback, but with the whine lessened and the brightness fluctuation still there, I'm wondering if they are separate problems. I got a hold of Larry Pina's Macintosh Repair & Upgrade Secrets (1990). It's not new enough to cover the Color Classic, but the general theory should be the similar. He suggests that brightness control problems ("buckling and blooming") come from bad capacitors (p. 78).

Could a failing flyback affect brightness? What should my next steps be-- check the capacitors on the analog board, replace the flyback to rule it out? I'm not the best at diagnosis... I tend to just open the thing up and trying to fix multiple things at once (which is not very scientific).

 

[techknight]

mine does the same exact thing, and I havent traced down yet whats causing it.

as far as the flyback is concerned, you need to tighten up the core. Either wedge it to one side, and epoxy it in, or chisel away the old epoxy that has let go, and make a non-ferrous shim. itll kill the wine. I used to do this on old TVs back in the day. I believe the wine is cause by eddy currents/resonance occurring in the loose ferrite core.

Now keep in mind, raster blooming and brightness fluctuations are VERY DIFFERENT animals. raster blooming is caused by poor HV regulation/drop in HV or B+ going to the scan supply.

if the geometry is unaffected, and its "just" the brightness, the problem is in the video amplifier/driver circuits or supply. which is my case. Then again, it could be cuased by the motherboard as well, because i haven't had a chance to recap my CC yet and it just suddenly died and wont power up after putting it in storage for awhile. It worked before. lol. but thats ALWAYS the case these days.

I think the rule of thumb these days is this: Any aluminum electrolytic whether it be SMD or radial/axial through hole type, from the 80s/90s needs replaced by now. period. end of story. Some industrial grade capacitors are still holding up from ancient electronics rather well, but most consumer/commercial grade caps are shot by now. Of course everyone is going to have their own opinion on this matter because i do understand that some caps are just fine, while other shot, etc... but from a technicians standpoint, its just easier to restore your electronics and replace them all if you want it to last for another 20 years.

Oh, on a personal note, its been a long enough time now, that even the 5XXX machines are being affected. I just happened to pull out my old junker 5200 board i had laying around for parts, as i dont have a machine it goes to anymore. Noticed it has capacitor "goo" starting to flow.

 

[elSchus]

The epoxy fix is good news. Epoxy I can do. Are you talking about the yellow epoxy on the bottom (the PCB side around the pins), or at the top, where there doesn't seem to be any epoxy:



Or am I cementing it around the base on the outside?

This cap thing is too bad. I like tinkering with it, but when it's wholesale component failure at 100 solder points, it's discouraging. I'll recap it eventually. And I thought my SE- and SE/30-style problems were over with a computer 6-years newer...

 

[techknight]

Yea, right at the top. take a razor and get all that old glue/epoxy off, and make a wooden shim or some type of shim to tighten up the core. youll notice the whine go away. once done, re-epoxy it all back into place.

 

[Windman]

Hi guys,

I'm bumping this thread a bit...

I also have this exact same fluctuation problem on my CC.

It works perfect sometimes, with a great crisp clear high contrast picture.

And then for a few minutes, it gets very bright and fluctuates... and the brightness button does not seem to have any effect at that moment.

I haven't really tried to do anything about it yet.

I haven't replaced the caps on the motherboard yet. But I doubt that will solve this problem.

Next step for me is to replace all caps on the analog board. Little more expensive, but still worth it since it must be done sooner or later anyways.

If someone have any proposals on what to do, please speak out!

Thanks!

 

[James1095]

This doesn't sound like a capacitor issue unless there are SMT electrolytics that have leaked out on the board.

Does the Color Classic use the RPM control pin on the SWIM to control the monitor brightness like the Classic does? If so a good start would be monitoring brightness control line to the monitor board.