Hi folks, I just got an inquiry about replacing the power transistors in a Fender Acousasonic combo, user reports all four transistors are blown, and there is visible damage to the PCB. I am reluctant to take on the job, I don't have much experience cleaning carbon deposits from a PCB, if a tube socket gets carbon tracking on it I replace it without a second thought. Also, I'm not sure what kind of heat sink the output transistors have in the stock amp, but customer may want a heat sink upgrade. Any experience or tips would be very much appreciated!
Draw /photograph what's there before starting to remove the debris.
Cut and scrape, using scalpel and suitable scrapers (probably improvising) the damaged parts away, getting back to sound pcb.
Fashion replacements for the lost tracks from single core copper wire - for tracks connecting to output devices I use stuff taken from twin and earth mains cable. Fix these in place with epoxy resin so as to get good contact with the pcb tracks you're connecting to - don't leave gaps to be filled with solder! Use whatever clamps, weights, or blu-tak works.
Fit new devices and solder in.
Cut and scrape, using scalpel and suitable scrapers (probably improvising) the damaged parts away, getting back to sound pcb.
Fashion replacements for the lost tracks from single core copper wire - for tracks connecting to output devices I use stuff taken from twin and earth mains cable. Fix these in place with epoxy resin so as to get good contact with the pcb tracks you're connecting to - don't leave gaps to be filled with solder! Use whatever clamps, weights, or blu-tak works.
Fit new devices and solder in.
When a resistor burns up, it often covers the surrounding area with soot. It looks a LOT more dramatic than it really is. REmove the actual burnt parts first, then a Qtip dipped in isopropyl alcohol is my choice. Swab the area clean, soot usually doesn;t stick. The alcohol will evaporate quickly.
If the board is discolored, it may be OK, but as jcx mentions, char is bad. ANy charred area has turned to charcoal - it has carbonized, and carbon is conductive. SO any black, chared area must be removed. I do use a grinding bit in ym Dremel for that. You have to get it ALL,so if that leaves a hole, well, it leaves a hole.
SO once the soot is removed, look carefully to see if it got to char or if it just left a stain.
Upgrade, schmupgrade, find out before bending to the customer's whim. There is a huge difference between parts that fail and then burn up or overheat and circuits that lacked heat sinking. I am not aware of Acoustasonics having a reputation for overheating. There certainly are circuit failures than can cause an amp to run too hot, but that is not the same thing.
If the board is discolored, it may be OK, but as jcx mentions, char is bad. ANy charred area has turned to charcoal - it has carbonized, and carbon is conductive. SO any black, chared area must be removed. I do use a grinding bit in ym Dremel for that. You have to get it ALL,so if that leaves a hole, well, it leaves a hole.
SO once the soot is removed, look carefully to see if it got to char or if it just left a stain.
Upgrade, schmupgrade, find out before bending to the customer's whim. There is a huge difference between parts that fail and then burn up or overheat and circuits that lacked heat sinking. I am not aware of Acoustasonics having a reputation for overheating. There certainly are circuit failures than can cause an amp to run too hot, but that is not the same thing.
Enzo's quite right to re-emphasise cleaning - clean everything that can be cleaned before doing any scraping, none might be needed.
My apologies for diving straight into scraping, I seem to be developing an entirely unwanted reputation as a repair man of last resort for incinerated mixing desk power supplies. I don't know what I did in that previous life, but it must have been bad.....
My apologies for diving straight into scraping, I seem to be developing an entirely unwanted reputation as a repair man of last resort for incinerated mixing desk power supplies. I don't know what I did in that previous life, but it must have been bad.....
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