Flyback Transformer, Cracked Epoxy Repair - Page 1
Forum section: electronics / projects

author: JDW
author_rank: Frequent Contributor
timestamp: March 11, 2020, 10:36:36 am
content: 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 Dope2. Epoxy (although what kind is not clear)3. Hot GlueI 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.

author: Gyro
author_rank: Super Contributor
timestamp: March 11, 2020, 11:43:27 am
content: 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.

author: Ian.M
author_rank: Super Contributor
timestamp: March 11, 2020, 11:56:45 am
content: 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 dopesEven 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.

author: JDW
author_rank: Frequent Contributor
timestamp: March 11, 2020, 01:31:30 pm
content: Gentlemen, thank you for the detailed feedback.Removing epoxy "without scratchingthe plastic " (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 112 times.)

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: March 11, 2020, 06:05:50 pm
content: 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.

author: wraper
author_rank: Unknown
timestamp: March 11, 2020, 06:11:13 pm
content: Quote from: JDW on March 11, 2020, 01:31:30 pmMost likely it becomes soft when heated and very easy to remove. For example you could simply heat it locally with hot air or whole transformer in oven.

author: Ian.M
author_rank: Super Contributor
timestamp: March 11, 2020, 06:22:05 pm
content: Its far more important to remove all conductive material from the surface, so I wouldn't worry about light open scratching, provided its going to be epoxy coated with sufficient film thickness to totally fill and cover the scratches. Pre-heat the surface with a hot air gun so the epoxy is as runny as possible at the substrate contact to minimise the risk of bubbles, as any voids become electrical stress concentration points leading to breakdown if large enough. That's also why its important to avoid cracking the shell, as there is a risk of a void between it and the internal potting compound, even if you manage to work epoxy into the full depth of the crack (which is near impossible to do successfully at the ends of a crack). When mixing the epoxy, stir carefully to minimise air entrainment and spread it carefully with a spatula or tongue depressor rather than a brush. If you've got the facilities to pull a partial vacuum to degas the epoxy, do so on the pot after mixing /before application, and then the whole flyback after application, before the coating starts to noticeably cure.Another key to avoiding a sticky layer is to respect the manufacturer's recommended environmental conditions for application, particularly avoiding high humidity or the lower end of the permissible temperature range, until its at least reached a full 'green' cure (i.e. can no longer be indented by a fingernail - test on a surface where insulation defects don't matter e.g also coat some of the core well away from the shell.)

author: JDW
author_rank: Frequent Contributor
timestamp: March 11, 2020, 11:46:36 pm
content: Quote from: wraper on March 11, 2020, 06:11:13 pmThis fly back was manufactured around 1990 and the epoxy has become brown over time due to the arcing. Therefore, it does not appear that this solid rock of epoxy would in any way become softer if I heated it.As such, I’m not sure of the best way to remove the existing, rock-hard epoxy. Do I use an X-Acto knife and risk inflicting deep gashes in the plastic? Do I use needle nose pliers? I’m not sure how to approach it.Quote from: Ian.M on March 11, 2020, 06:22:05 pmThe only kind of vacuum I have is the regular kind that cleans my rug. What kind of vacuum device are you recommending? More specifically, how do I suck out all the air from inside the flyback and then while the air is out apply epoxy?Lastly, you mentioned curing the applied epoxy at 100°C. My concern is that the insulation of the attached wire and the suction cup may not handle that high heat very well, especially considering it’s age. What do you think about that?

author: Ian.M
author_rank: Super Contributor
timestamp: March 12, 2020, 12:51:05 am
content: I would 'Dremel' away as much as possible without risking breaking through into the shell underneath, then see how easy it is to scrape it away with hand tools, taking care not to continue scraping areas that have been cleared down to the shell. Also if you are lucky its adhesion may already be degraded and you may be able to pop off a thin remnant by gentle prying with the edge of a sharp blade at the edge of the remnant.If you haven't got a suitable vacuum chamber set up for degassing you just haven't got one. e.g even if you had a high vacuum chamber, you wouldn't want to load up its interior surfaces with volatile contaminates, and you certainly wouldn't want them in the pumps.Also improvised vacuum chambers can implode far too easily, which can get excessively exciting if they aren't made of ductile materials, and the cost of doing it right would not be negligable.If you are concerned about the insulation's tolerance for 100 deg C temperatures, even 70 or 80 deg C would be better than not attempting to cure it. Low temperatures for baking out adsorbed moisture are more problematic unless you can also use a strong desiccant to minimise the ambient water vapour pressure, and tend to take much longer to be effective.

author: JDW
author_rank: Frequent Contributor
timestamp: March 12, 2020, 12:56:51 am
content: Quote from: Ian.M on March 12, 2020, 12:51:05 amI apologize for my many questions, but this is a learning process for me. Last question...Will a regular cooking oven be suitable for an 80°C bake/cure of epoxy, or would that release some toxic substances that may linger inside the oven, posing a health hazard? I have no means to buy Super Corona Dope here in Japan, but for those who can, it looks to be an incredibly toxic mix, so much so that they won't allow international shipping of it. Curious what that would do to the inside of one's home oven!

author: wraper
author_rank: Unknown
timestamp: March 12, 2020, 01:20:08 am
content: Quote from: JDW on March 11, 2020, 11:46:36 pmIt's not epoxy and generally such sort of adhesive becomes soft when heated. And it became brown not due to arching but simply due to age and heat. Now you are simply inventing conclusion out of nothing.

author: Ian.M
author_rank: Super Contributor
timestamp: March 12, 2020, 01:32:09 am
content: *DON'T* attempt to cure epoxy in any oven that will ever be used for food.You can improvise a low temperature curing or drying oven with a box and a variable temperature heatgun set really low, though some sort of temperature probe to check you aren't over-cooking the work is very much needed, and its hard on the heat gun to run it for an extended period. Remember, the heat gun motor needs room temperature airflow for cooling so don't make the box airtight or put the heatgun inside the box.

author: JDW
author_rank: Frequent Contributor
timestamp: March 12, 2020, 01:38:08 am
content: Quote from: wraper on March 12, 2020, 01:20:08 amTalk about "inventing"! How do you know for certain it is not "epoxy" (as it is clearly described in Larry Pina's excellentMacintosh Repair & Upgrade Secrets)? If it is not epoxy, why are we (at least, Ian and I) talking about an epoxy fix?

author: wraper
author_rank: Unknown
timestamp: March 12, 2020, 01:49:16 am
content: Quote from: JDW on March 12, 2020, 01:38:08 amHave you ever seen how epoxy looks like? Or even know what epoxy is? It certainly does not look like anything that have signs of drying.

author: Ian.M
author_rank: Super Contributor
timestamp: March 12, 2020, 02:03:24 am
content: Why epoxy fix? Epoxy is merely the most commonly available two part resin that (if the right grade is used) will have decent electrical insulation properties, will bond strongly to existing surfaces, is forgiving enough for a DIYer to work with and that has low enough toxicity components that inadvertent skin contact is unlikely to cause significant health issues unless you have already been sensitised by previous occupational exposure, (usually repeated prolonged and excessive skin contact or by breathing significant quantities of dust from sanding 'green' cured epoxy). There are other types of two part resin not in the general family of epoxy and modified epoxy chemistries, and various catalysed monomer resins, but they typically either are extremely nasty health hazards, or have poor adhesion or poor mechanical or electrical properties.

author: wraper
author_rank: Unknown
timestamp: March 12, 2020, 02:10:57 am
content: Quote from: Ian.M on March 12, 2020, 02:03:24 am

author: wraper
author_rank: Unknown
timestamp: March 12, 2020, 02:12:40 am
content: https://sound-au.com/articles/yellow-glue.htmFFS