Burned PCB of a tube amplifier (VOX AC15), help with track repair - Page 1
Forum section: electronics / repair

author: Yamin
author_rank: Frequent Contributor
timestamp: July 12, 2019, 07:27:18 am
content: Hi, I've got a VOX AC15 guitar amplifier. A conductive path between two terminals developed which caused the pcb to fry up and all the fuses were blown and also a high watt resistor was blown.The trace which is lifted just seems to be connected to the ground plane and I don't really understand the purpose of it, maybe saving copper during the etching process? Anyway my first question is can I just removed that piece of lifted trace?Second question is what do I do with the burned bit of pcb, its conductive. Should I cut out a hole or dig out the burned bit until its not conductive anymore. The pcb is single sided. The two terminals which are effected is TT5 and TT7.I have inlcuded a schemetic of the amplifier. I have checked the components near the vicinity, I couldn't find any components which are faulty other than the 82ohm resistor (next to terminal TT5 marked as R72) which was blown. Any more tips on what to look for would be really appreciated.The whole area was blackened and the pictures were taken after cleaning the are with IPA. I have included a pdf of the whole schematic.Thanks in advance for the help.

author: Yamin
author_rank: Frequent Contributor
timestamp: July 12, 2019, 07:28:36 am
content: Here is a close up picture and the pdf of the amplifier schematic.

author: Ysjoelfir
author_rank: Frequent Contributor
timestamp: July 12, 2019, 08:05:44 am
content: If i see that correctly they designed this PCB with a microscopic spacing between a 370V rail and ground. No wonder that there occured some arcing. Just slight amounts of moisture or condictive dirt should be enough to start carbonisation on that small spacing.I would grind away any blackened/carbonized residue from the PCB and also enlarge the isolation between the ground plane and high voltage points. It looks like someone didn't realy think about creepage distances while designing this PCB.

author: CJay
author_rank: Super Contributor
timestamp: July 12, 2019, 08:19:57 am
content: The copper bit could be there for a few different reasons, to provide some shielding between signals on the two adjacent pins, to act as a spark gap to 'catch' arcing between the pins or it could just be because it looked nicer or the PCB designed didn't bother to remove it from the drawing.Personally, I'd desolder the connector so I could drill or grind out the burned PCB material and then add a PCB lacquer to the hole to ensure the copper is insulated after re-fitting the connector

author: Zucca
author_rank: Unknown
timestamp: July 12, 2019, 08:28:56 am
content: Also some liquid e. tape could help

author: Ysjoelfir
author_rank: Frequent Contributor
timestamp: July 12, 2019, 08:32:22 am
content: I would doubt that it is for shielding reasons since the two adjacent pins are basically on the same potential from the mains transformer, just once with and once without rectification.I would also doubt a spark gap - for that reason the solder mask should be exposed on this pins and I would expect them to place spark gaps on all pins of the connector. It seems also not very logical to me to include a spark gap on a secondary HV winding of a tube power supply.I like your idea of desoldering and drilling out any burnt residue, it should be way faster and easier than grinding with the connector installed.

author: floobydust
author_rank: Super Contributor
timestamp: July 12, 2019, 06:15:16 pm
content: It's a terrible PCB design that does not have proper spacings for the high voltages present. Any humidity or dust/dirt, factory flux residue could make it arc there. Vox ain't what it used to be.The skinny trace running right along the PCB edge is a problem, right now that ground-pour shields it from the HV transformer voltages there, and if you remove the ground-pour copper where it arced, that little trace will pick up lots of AC hum.I would use an exacto knife and lift off the copper-pour and then see what the skinny trace is all about.You might have to kill er cut it and use a jumper wire to route it away from HV AC.

author: Yamin
author_rank: Frequent Contributor
timestamp: July 16, 2019, 04:05:14 pm
content: Hi guys, just an update and need some more advice. I removed the two terminals and removed the unnecessary exposed copper track (should I remove the whole middle copper track). I chipped out the burned PCB and now there is no conductivity between the terminals.I couldn't understand the point of the two thin tracks running down the bottom. But I have left it intact.I should add I found quite a bit of dust and dead insects in side the chassis when I first opened it up. Could these be the initial culprit?I need some help with figuring out what to use to fill the hole made. Would epoxy be ok? I couldn't find any good options from the local shops. All I have got is hot glue, but in my experience it doesn't give enough mechanical reinforcement. The pad on the left really needs it.I have included some epoxy mix available at some local shops.Thanks so much for the help everyone

author: Yamin
author_rank: Frequent Contributor
timestamp: July 16, 2019, 08:05:34 pm
content: Also I should add, is it a good idea to generally use Epoxy for PCB repair. I did a little experiment today I wanted to check if the epoxy was conductive. I poured some into a small cap (after it was set) and checked it using a Megaohm Meter with 500V it read as >2000 Mohms but with 1000V it was significantly lower.

author: cvanc
author_rank: Frequent Contributor
timestamp: July 16, 2019, 08:45:25 pm
content: This is a decent epoxy for minor board repairhttps://www.chemtronics.com/circuitworks-epoxy-overcoat-adhesive-syringe

author: CJay
author_rank: Super Contributor
timestamp: July 17, 2019, 08:58:16 am
content: You're going to have problems again if you leave that as is, there is no way you've got rid of all the carbon, drill out or grind out the section I've marked in red.