Ok, enough with the comic German, shall we?
It's been a little slow in the repair business from the shop lately so it has given me a chance to get caught up on a lot of projects that have been hanging fire for one reason or another.
Last week, I managed to get four of them out the door. I got my Fender Bassman knockoff in reasonably serviceable condition, finished up a long term Super Reverb, a Vox AC15CC with a blown screen resistor and something else that I can't remember.
So this AC15 Custom Classic-you know, the first series of Chinese reissues from Vox that were made in the IAG factory under the aegis of Steve Grindrod?
Anyway it had open dropping resistors and a scorched board, so it sat for a while until I was ready to tackle it and fight it to a finish. Because I'd sort of forgotten where I was when I put it down I started by rebuilding the power supply yesterday.
When I threw the switch this morning it was more or less the same thing-a flash of light from under the circuit board and some smells.
This time I jury rigged it so I could observe the underside of the board when powering it up, turned off all the lights, and threw the switch.
And there it was. One of the traces that runs between one of the dropping resistors and the power supply diode was sparking because the substrate of the circuit board became conductive when it got carbonized.
I removed everything that was in the way and commenced drilling out the damaged substrate until I got to solid unburned material. I slotted it out, cleaned it up with a file, painted it with clear nail polish, and laid in a bead of silicone. The dropping resistor was already there with a sleeve of spaghetti tubing around the trailing end.
The silicone will cure overnight and I expect it to be pretty solid by tomorrow, when I will take some images, reassemble everything and road test the amp. If it succeeds it will mean another long term project off the waiting list.
One issue has been the particularly nasty lead free solder that was used in this amp. Cleaning it up required a lot of flux, desoldering braid, and the liberal use of the ZD915 desoldering pump.
In retrospect it was the best $125 purchase I've made in the last year or two and I wouldn't attempt a repair like this without it. Marlin P. Jones has them now and then, but similar devices can be found on the internet. You can read about it here and there is also a great video on youtube on the EEV blog. https://www.youtube.com/watch?v=Ft50m8UU5WQ
UPDATE: The stunt where you turn off the lights and power it up? I used that again on a Crate BX100 solid state bass amp. It's as simple a solid state amp as you can get so I arranged to contribute the labor so as to get some signal tracing in. The first thing I noticed was a crackle from the circuit board and some bad solder joints on one of the power transistors. Turning the lights out and the power on showed me exactly what was happening.
As I was working signal tracing I noticed the briefest wisp of vapor coming off one of the resistors that couples the amp to the drain gates. It was open, and I did the pinky test and found an op amp I think was bad because it got hot. So I shall find out tomorrow if my instincts were correct.
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