Electronics > Projects, Designs, and Technical Stuff
Flyback Transformer, Cracked Epoxy Repair
JDW:
On the Analog/PSU board of a 1986 Macintosh Plus, I have a flyback transformer that works (CRT display looks fine) but it (1) makes a subtle yet noticeable high pitched whine, and (2) smells of ozone. Most of the advice I read is to discard such a flyback and replace it; however, these are hard to come by now day and so I am considering repair. I turned off the lights and flipped on power, and after my eyes adjusted I could barely see some arching around the epoxy. I made some photos below.
Composite Photo of Lights-on and off photos:
Photo taken with lights on:
16 minute Bulb photo, shot in the dark (arcing actually is very dim and quite hard to see with the naked eye):
I've searched online and read people report success with one of the following 3 fixes:
1. Super Corona Dope
2. Epoxy (although what kind is not clear)
3. Hot Glue
I live in Japan and can't purchase Super Corona Dope due to shipping restrictions. Epoxy is something I can buy, but I am not sure what kind is best. For now, I do have hot glue -- just the ordinary kind.
My question to you experienced in Flyback repair is this. Will hot glue fix this kind of problem? If you suspect it will not, what epoxy would work? Or would even that be a waste of time after seeing my photos?
Thanks.
Gyro:
It looks as if at least part of the problem is the brown 'gunk' used to secure the transformer housing to the core, it was often used for holding down heavy electrolytic caps etc. Unfortunately, with time, it goes brown and becomes conductive.
If you can safely remove enough of it from the end of the coil (without scratching the plastic!) you may find that this is sufficient to at least minimise the discharge, maybe eliminate it altogether.
Ian.M:
As Gyro has just pointed out, it looks like whatever goop that was used to bond the core in place has degraded and become slightly conductive.
To have any hope of fixing it, all the degraded bonding compound on top of the shell the winding is potted into must be removed without gouging or cracking the shell, (but don't dig down between the core and the inner tube of the shell), and the shell surface must be thoroughly cleaned to remove surface contamination. At that point, you can try photoing it running in the dark again - any 'hot-spotting' of any residual discharge is a really bad sig, see below. Then to have the best chance of a successful cure the flyback must be baked at close to 100 deg C for an extended period to drive out moisture before an attempt is made to seal the surface with epoxy or dope, following the manufacturer's exact application instructions, and once dried/set, I would recommend post-curing at an elevated temperature (less than the max permissible continuous working temperature of the coating) to ensure as complete a cure as possible for epoxies, and to drive out any solvent residue from dopes
Even so, it may be too late as if any carbonised paths have developed through the shell, there will be sufficient electrical stress at the point you attempt to seal over them to degrade the new insulating coating till it breaks down.
Corona dope, or any product sold as a sealing spray for EHT cables etc. e.g. spark plug cables in car ignition systems, *MAY* work.
A pro-grade laminating or electrical potting epoxy mixed precisely to manufacturer's instructions, should work. Contact local specialist epoxy suppliers to see if you can get a small trial pack of something suitable (rather than a 50 Gal. drum of it!) Any hardware store epoxy is likely to fail due to slight errors in the mix ratio leaving polar unreacted monomer, or due to conductive fillers. If that's all you can get your best bet would be a slow curing clear epoxy.
Hot glue absolutely wont fix it and will probably make it worse, as its prone to surface tracking and carbonisation that melts its way down into the mass till the bulk becomes a conductive goo.
JDW:
Gentlemen, thank you for the detailed feedback.
Removing epoxy "without scratching the plastic [shell]" (as Gyro said) would likely be an exercise in futility. I am not sure how to accomplish that. I might be able to get the brown epoxy (the part shown in my photo) off, but to be able to do that "without a scratch" is quite near impossible. I will give the epoxy removal more thought before I try anything.
Below is a high quality audio recording I made of the whine. My SONY PCM-D100 recorder's stereo mics were about 1 foot from the flyback, placed at the back of the computer with the back housing removed. You can hear the while shortly after the startup bong.
Bong_FlybackWhine.mp3 (765.19 kB - downloaded 97 times.)
Conrad Hoffman:
I'd think you need to get as much of the brown stuff off as possible. Then, heavily coat the area with epoxy and pull a vacuum on it. Release the vacuum and the epoxy should be forced into the cracks. Any clear epoxy should work, but the trick is mixing by weight and mixing about 5X longer than you think is enough. That will insure no sticky layer after it cures. Don't use a 5 minute epoxy.
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