Egnater Rebel-20 PCB Damage

[indyjosh1969]

Hi Folks,
I recently acquired a non functional Egnater Rebel-20 amp, and was told by the previous owner that it had simply "stopped working". Upon checking, the fuse on the PT primary side was found to be blown, but the fuse on the HT supply to the tubes was intact. After disassembling the amp and pulling the PCBs out, I noticed a dark patch on the PA PCB next to the first smoothening capacitor, C127. Desoldering and removing the capacitor shows the PCB was damaged when the capacitor blew. I cleaned the area using isopropyl alcohol and removed a good portion of the PCB around the capacitor lead as it had loose fibers. I have access to a megger in my office and could do a 500V test to verify the PCB isn't conductive.

I would like to request opinions on repairing the PCB. I was originally thinking I could use high temperature JBweld epoxy but found it is called "Steel Reinforced Epoxy Resin". Although JB weld's website FAQ indicated the epoxy is an insulator, I did not feel comfortable using it in the area that has the highest voltage of the amp.
PCB pictures before and after removal of C127 are attached along with schematics. I would like to mention, I have noticed differences between the schematics and this amp, which is a topic for another day!

Thanks for your advice in advance!

[indyjosh1969]

I apologize for the missing attachments, it appears I only have the ability to upload an additional 148.3 KB. I'm not sure how to get around the upload limitation. This is a link to the schematic https://music-electronics-forum.com/...7&d=1616450841

[Chuck H]

Well there's only the one picture which looks to be before removal. Evidence of damage only partly visible. I agree you should skip the JB Weld. No one ever expects the voltage we deal with in tube amps so you can't count on their endorsement.

If damage is such that you need substrate for mounting you could use clear epoxy as adhesive and just overlay a piece of FR4 or whatever. If there's enough matrix left to hold components I think just removing the burned material and testing for operation at voltage could be fine.

[Jon Snell]

Ensure the damage has been cleaned and is dry. Which you have.
Apply a thin film of clear conformal coating ( https://www.mouser.co.uk/ProductDeta...4s42AcNg%3D%3D ) to keep the board dry and fit a new capacitor.

There is no need to fill the void as that may cause issues a few years from now as the epoxy or whatever you use breaks down naturally.

[indyjosh1969]

Thanks for the response Chuck, I thought I had removed all the carbonized material but a closer inspection with higher magnification indicates the presence of more charred residue user the copper ground layer. At this stage, is appears that majority of the boards thickness in the immediate vicinity of the capacitor positive leg needs to be removed, material further away appears to be affected near the board top only and is cleaning up nicely with a bit of scraping.
The good news is that there are no components in the affected area other than the capacitor itself, so I am hoping cleaning of the charred material is going to be sufficient. To prepare for the worst, I will get clear epoxy and some FR4 material as well.
Thanks for your help once again!

[indyjosh1969]

Thanks for the response Jon, I will order the conformal coating material.

[The Dude]

......There is no need to fill the void as that may cause issues a few years from now as the epoxy or whatever you use breaks down naturally.

^^^^^^ Yes, that. There is no need to fill the void at all.

[indyjosh1969]

It appears, I now have enough posts to upload another ~500KB photo. This shows the PCB damage after the capacitor was removed and the area cleaned using isopropyl alcohol. I have since then removed most of the charred material but need to do one additional pass before applying the conformal coating I ordered last night. Hope to post pictures of the final condition towards the end of this week.
A big thank you to the folks who have shared their knowledge and provided advise!

[g1]

Be sure to get that little carbon track over to lug QC101.

[indyjosh1969]

Gents,
I had ordered a bottle of MG Chemicals 419D Premium Acrylic Conformal Coating on Amazon but received a bottle of MG Chemicals 4226A Clear Insulating Varnish instead. MG Chemicals website indicates this product is used to "....protect high voltage parts such as transformers, coils, motor windings, and various electric generator parts against arc, corona discharge, corrosion, and moisture fouling".

I was planning to return it and ask Amazon to ship the right item but would like to ask for an opinion, in case this product could be used for PCB protection.
Please advise, and my thanks in advance!

[g1]

I think that the one you received (4226A) should work great for high-voltage insulation. For repairing arc-over, it is actually preferable to the 419D.
(3000 vs 1000 volts per mil., 5500 vs 1500 breakdown voltage)

[indyjosh1969]

Thanks for taking the the time to respond g1, I hope to finish the repair over the weekend.
Have a great weekend

[indyjosh1969]

I purchased a new pair of magnifying glasses recently and noticed there were still some areas that were too dark compared to the surrounding areas, so i ended up removing more material from the PCB. I finally ended up gouging with an area about 3/8"(~8mm) in diameter at the surface. I then applied the clear insulating varnish but didn't realize it takes 10 hours to fully cure, so that's it for today!
BTW, this amp appears to have been manufactured sometime after August 2019 as that's the date on one of the PCBs, so it's a bit surprising the electrolytic capacitors crapped out in less than 5 years. Looking at the amount of damage done by one blown capacitor, I decided to replace all of the filter capacitors in the B+ supply, as they're the same brand. Removing the capacitors was easy after slicing the white RTV but now I've got to apply RTV to each capacitor. About a year ago, somebody recommended Permatex clear adhesive sealant 75151 RTV between the PCB and the electrolytics for another amp I was working on. Unfortunately, it arrived to late for that amp repair so I have a tube that has never been opened but about a year old. Does anybody have an opinion on this RTV? If not, may I request recommendations on a suitable RTV?
Many Thanks!

[Greg Robinson]

RTV stands for room temperature vulcanising. Most common silicone caulks/gap fillers/adhesives are RTV, though there is also HTV - high temperature vulcanising (for things like automotive gaskets). Other types such as HCR (high consistency/heat cured rubber) or LSR (liquid silicone rubber). Some silicones are two-part and require mixing. Regardless, all but RTV are specialty products.

Anyway, the important detail for electronics applications is that it is a neutral cure RTV. Many silicones use an acetic/acid cure, and will offgas acetic acid fumes during and even after curing, corroding and damaging components, circuit boards, and chassis.

Appropriate RTVs may be called neutral cure, platinum cure, tin cure, acid-free cure, etc.

While there are specialty RTVs for electronics with proper datasheets and specifications, these are normally quite expensive (at least here in Australia), usually by an order of magnitude or more. So I use simple hardware store neutral cure silicone. Costs a couple of dollars per tube, can go for a walk to the local store and grab another rather than needing to order and wait, always works just fine. YMMV.

EDIT: I just tried looking up Permatex 75151 (which appears to be a Home Depot product code for PERMATEX® CLEAR RTV SILICONE ADHESIVE SEALANT, 3 OZ. Home Depot is geolocked and won't allow access to their website from Australia, so I may not have perfectly accurate information), and it appears to be an acetoxy cure silicone - if so, it is NOT appropriate for this application.

[Jazz P Bass]

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