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

author: hackerb9
author_rank: Newbie
timestamp: November 16, 2020, 01:44:11 am
content: [ This post is in reference to:https://www.eevblog.com/forum/projects/flyback-transformer-cracked-epoxy-repair/by @JDW ]Flyback transformers are something I knowverylittle about, but I'm in the same boat as @JDW: My Macintosh Plus's flyback transformer died. As JDW pointed out, nobody makes these flyback transformers anymore. Even the supply of old stock seems to have dried up (or become prohibitively expensive).So, what are the possible solutions for people who want to keep "antique" computers running? While the responses to JDW in the original thread were quite educational, the upshot, at least for a hobbyist, was that it is not feasible to repair a flyback due to the complexity, materials, and equipment requirements.I presume it'd be even more difficult to design and build a complete replacement flyback transformer than it is to repair an old one, so the only other solution I can come up with is:Is it possible to modify a common and cheap flyback transformer so that it will be suitable for the Macintosh Plus (and similar compact Macs of that era)?

author: S. Petrukhin
author_rank: Super Contributor
timestamp: November 16, 2020, 02:10:07 am
content: In any transformer, only the winding can fail. The difference in flyback transformer is the presence of a gap to prevent the core from being magnetized or the presence of a demagnetizing winding. The transformer can be rewound.

author: hackerb9
author_rank: Newbie
timestamp: November 16, 2020, 03:04:34 am
content: Quote from: JDW on March 11, 2020, 10:36:36 amBy the way, I'm now installing as a replacement a flyback that I took from a deceased Macintosh SE. I suspect it, too, will have problems as the epoxy has browned with age and looks like it is peeling off in places. Fortunately, unlike the epoxy on the known bad flyback transformer, my multimeter shows the replacement having infinite resistance. (On the bad one, I read about 10 MΩ to the frame). Perhaps not all brown epoxy is bad?The epoxy that shows conductivity is on a LIFON 157-0042B (date code 8837). The epoxy which may be good (or at least insulates beyond the ability of my multimeter to detect) is a Monitron 157-0026-C (date code 8703).[Side note: The part ending in '26 came with Macintosh Pluses and '42 with Macintosh SEs. My Plus had a '42 in it because I had replaced the flyback transformer decades ago. The junker SE I got for its flyback transformer must have had a similar operation done, but in reverse! It amuses me that when I put this '26 in my Macintosh Plus it will become more like an original factory Macintosh than it was during most of its working life.]I can't find any proper data on these flyback transformers. They are listed as equivalent to "HR42031", but even the "HR Bible" doesn't list any specs.

author: james_s
author_rank: Super Contributor
timestamp: November 16, 2020, 03:19:29 am
content: Do you mean the glue that keeps the core from vibrating or coming loose from the coil? That shouldn't matter if it cracks, but if epoxy that the HV secondary winding is potted in cracks then that is a problem. These are relatively simple flybacks, it might be possible to adapt a similarly sized surplus flyback, possibly winding new primary windings, usually only a few turns. If there is enough market for them it's possible a Chinese company could reproduce them. There have been reproduction flybacks made for a number of common arcade monitors. I'm hoping the flyback in my SE/30 holds up, I think it's the same one. I remember buying a new flyback for a Plus I had for something like $14 but that was pushing on 25 years ago

author: hackerb9
author_rank: Newbie
timestamp: November 16, 2020, 03:55:41 am
content: Thank you. Is rewinding a flyback transformer something people recommend? Remember I'm an amateur with such things and do not have special skills or equipment.I was hoping for a simple fix like the guy on YouTube who used Gorilla "100% Silicone" Sealant to fix an arcade CRT:I'm not exactly sure what would be wrong with doing that, but it seems like the consensus in the previous thread was that simple fixes wouldn't work.I was imagining there might be some way to make an adapter so I could use some other flyback transformer that is easily available and functionally equivalent. But, I don't know if (a) such a conversion is even possible and (b) what parameters need to match for a flyback transformer to be "functionally equivalent".I apologize for my ignorance; that's why I'm asking for help.

author: james_s
author_rank: Super Contributor
timestamp: November 16, 2020, 04:10:06 am
content: I'm still not sure exactly which glue you're referring to. If the transformer has already failed though glue is not going to fix it, and if it's just the glue between the core and the potted winding it doesn't matter, that cracking won't cause it to fail. Rewinding is not a project for someone not experienced with transformers and HV but there are companies out there that are capable of building replacements. Whether that's economical depends on demand.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: November 16, 2020, 11:34:59 am
content: Oh, I'm sorry I didn't understand the term correctly enough. I thought we were talking about a power supply transformer. We have such a transformer is called a string transformer in russian, not a flyback.

author: james_s
author_rank: Super Contributor
timestamp: November 16, 2020, 06:49:50 pm
content: Quote from: S. Petrukhin on November 16, 2020, 11:34:59 amIn Europe it's typically called the line-output transformer or LOPT.

author: hackerb9
author_rank: Newbie
timestamp: November 17, 2020, 03:36:04 am
content: Quote from: james_s on November 16, 2020, 04:10:06 amSorry, I was referring to the "epoxy" inthe post by JDW. @JDW called it that because that's what Larry Pina calls it in his book "Macintosh Repair & Upgrade Secrets". Pina had mentioned that regluing the seals at the top and bottom of the transformer was a possible fix for a failed flyback transformer, but didn't recommend it because replacements were so cheap back in 1990.People on JDW's thread had a lot of advice, but it sounded like the ultimate outcome is that regluing old flyback transformers is not something an amateur can do.So, while I'm curious about using epoxy as a preventative measure, what I'm actually trying to ask is a different question: Is it possible to create an adapter (perhaps simply swapping pins to the right position) so an easily available flyback transformer can be used in compact Macintoshes?

author: james_s
author_rank: Super Contributor
timestamp: November 17, 2020, 04:15:12 am
content: I suppose it could be some kind of epoxy, but the goop used on most flyback transformers doesn't look like epoxy to me, either way it doesn't really matter. I don't see how that adhesive could be an issue, it's just used to mechanically hold the core together and prevent vibration that would cause it to whistle. The actual HV winding and rectifier diode and underneath that bobbin the primary windings are potted in epoxy resin inside the plastic housing and if they have failed the really is not much you can do. The only time I've repaired a flyback with glue is when the casing had cracked and it was arcing out through the crack to something external. Once the arc has damaged the windings and it actually stops working it's done.This is a slightly newer color TV flyback in this xray but the internal construction is similar. The HV winding is wound on an outer bobbin that is split up into segments to reduce the voltage across any one segment. This one has multiple diodes dispersed between the three sections but the lower voltage monochrome flybacks are usually just one group with a single diode on the output. The low voltage windings are on the inside closest to the core, everything is put into a plastic housing and filled with epoxy resin.

author: akimpowerscr
author_rank: Regular Contributor
timestamp: November 17, 2020, 08:08:19 am
content: For a new flyback transformer, look here:http://www.hrdiemen.com/reparation/flyback/index

author: hackerb9
author_rank: Newbie
timestamp: November 17, 2020, 04:45:23 pm
content: Quote from: akimpowerscr on November 17, 2020, 08:08:19 amThanks, but I have searched through the HR site extensively and the equivalent part numberHR42031is not only no longer manufactured, it has no specs listed.

author: akimpowerscr
author_rank: Regular Contributor
timestamp: November 17, 2020, 05:41:33 pm
content: If you can't find what you are looking for, don't hesitate to contact them, they are very flexible and can even make a "custom" flyback.

author: james_s
author_rank: Super Contributor
timestamp: November 17, 2020, 06:42:10 pm
content: Quote from: hackerb9 on November 17, 2020, 04:45:23 pmI think those are just the pin names, maybe something like Main 1, Main 2, etc. Tip is clearly the HV anode, these names are just used for reference in connecting that testing instrument to measure the secondary voltage.I bought several HR flybacks back in the day, if they're still around I would agree it's worth contacting them. If they sold a replacement back in the day they may still have the specs required to make up a batch.

author: wraper
author_rank: Unknown
timestamp: November 17, 2020, 06:56:58 pm
content: Quote from: hackerb9 on November 17, 2020, 04:45:23 pmEquivalent part number is HR 42032http://www.hrdiemen.com/reparation/flyback/model/42032And can be purchased according to this shophttp://www.fontanacomponenti.it/categoria-vm/trasformatori-eat-monitor/hr-42032-1570042a-trasf-eat-monitor?vmcchk=1

author: jonpaul
author_rank: Super Contributor
timestamp: November 17, 2020, 07:52:46 pm
content: Flybacks are all custom manufactured.Rewind is impractical as the winding is a special low capacitance technique and needs the correct windong machine."Repair" of cracked epoxy: The HV will find a way to the air, it will not work over time.Jon

author: james_s
author_rank: Super Contributor
timestamp: November 17, 2020, 08:15:57 pm
content: Quote from: jonpaul on November 17, 2020, 07:52:46 pmLargely true, however HR and at one time other companies sold replacements that were close enough usually that one part number would cross to several original parts. Sometimes minor tweaks to the circuit were needed to make it work properly. Many CRT displays have enough similarities that the flybacks are not that different between some models of similar types. The biggest challenge is determining which parts may work as specs are not normally published for these custom components.Winding HV transformers is challenging but not impossible to DIY, I have wound several of them myself, usually my goal is just to develop as much voltage as possible though rather than to match some existing circuit.About 25 years ago I found a 25" TV by the dumpster at the apartment we lived in. I dragged it home and tried it out, found a bad capacitor in the horizontal section and replaced it, the TV worked but every few seconds make a loud snap which I discovered with a little lightning bolt shooting out of the side of the flyback to a metal bracket a couple inches away. Being a broke teenager I used what I had on hand and after cleaning the area around the crack I glopped on a blob of hot melt glue. My brother used that TV for about 6 years like that before we moved and I gave it to someone else. The key is to repair it before the arcing causes carbon which is conductive or damages something else.

author: hackerb9
author_rank: Newbie
timestamp: November 18, 2020, 02:44:25 am
content: Quote from: james_s on November 17, 2020, 04:15:12 amOkay, that's good to know and makes sense with what Larry Pina said in his book. If the "epoxy" seal is broken, it can cause a "ringing noise".Quote from: james_s on November 17, 2020, 04:15:12 amI think the reports I'm seeing online about people repairing their flyback transformers using epoxy must be exactly this. Perhaps the confusion is that the arcing seems to happens exactly where the "goop" at the top has turned brown and peeled off. (See the images in @JDW's thread). Since applying epoxy at that spot has fixed the problem for other people, maybe the presumption is that the brown stuff that is crumbling away must have been epoxy.But, as you say, it doesn't matter what kind of goop it was. The important questions for me are:ⓐIs it crazy to try to make an adapter that will swap around pins so I can use a different black-and-white TV flyback transformer to replace the one in the Macintosh Plus?andⓑSince the top of the flyback transformer seems to be a weak point, is there any harm in preventively applying silicone epoxy?Thanks!

author: james_s
author_rank: Super Contributor
timestamp: November 18, 2020, 06:33:02 pm
content: No, it's not crazy, although it will be quite a crapshoot to find one that happens to work. At the very least I would start by carefully examining a working flyback, measure the inductance of each of the windings, and then drive one of them with a low voltage AC signal of around 15kHz from a function generator and measure the voltage on each of the other windings, that will let you calculate the turns ratio. Adapting a different flyback may require adding one or more windings to the exposed core. BUT if HR can indeed still supply a suitable replacement then messing with adapting something else is silly, just buy the correct part.As far as I know there is no such thing as silicone epoxy, there are silicone compounds such as the stuff commonly used as caulking, and there are expoxies, which are two part polymer adhesives. Both often have good electrical insulating properties and both could be used to repair an arcing flyback. If that glue is turning conductive then it's probably not a bad idea to scrape it off and replace it with something else, either epoxy or silicone will probably be fine.