Bass Guitar Pre-Amp - Assistance required from local geniuses please!

[SteveNielsen]

Re: Bass Guitar Pre-Amp - Assistance required from local geniuses please!

I would think anything capable of dissolving epoxy or acrylic potting is likely going to dissolve the the circuit board some too.

[stj]

Re: Bass Guitar Pre-Amp - Assistance required from local geniuses please!

it's slow,
and i'd only do it on the solder side at first.

you can also destroy resin with a heat gun,
but not on this - it may contain electrolytics.

[SteveNielsen]

Re: Bass Guitar Pre-Amp - Assistance required from local geniuses please!

Yeah, I recall trying to un-pot a small board with heat and metal tools to pull and chip away at it back in the late 70s in a city radio shop. That was for a thing called a "canary", a hydrocarbon gas detector for use in sewer systems by the shitheads, er, I mean sewer maintenance guys. It didn't go well for that board but we were able to make a copy of the circuit as a result and that did work out well as we had a LOT of the devices to maintain.

I also worked for a company that used to pot the boards for ion generators and that was done for two reasons; 1 to keep the high voltage from arcing across the board, and 2 to keep people from seeing how incredibly simple the design was.

[stj]

Re: Bass Guitar Pre-Amp - Assistance required from local geniuses please!

what was the design, one transistor, one resistor, one ceramic cap and a coil with a tap to kill the charge in the cap?

[SteveNielsen]

Re: Bass Guitar Pre-Amp - Assistance required from local geniuses please!

No transistor. It had a diode, a resistor, a cap, a custom wound ferrite core tubular transformer, a high value (like 10meg) resistor on the output to limit current and a pot to adjust the input side of the transformer to the desired level on the output. Some of them got up close to 10Kv output. Most hovered around 8Kv. I've still got a working unit here but alas, it is potted in epoxy. When building them I'd immerse the board in Fluorinert to test them and that kept the sucker from arcing all over the place. We started to cover the boards and components with high voltage RTV to inhibit arcing instead of potting them at one point (it was cheaper and faster) and that worked out fairly well but I don't have one of those around.

[Ozyris]

Re: Bass Guitar Pre-Amp - Assistance required from local geniuses please!

Just thought I'd tidy this up.

I didn't complete this fix as it only cost $4.95 to get another unit from China. A little annoyed with myself at not sorting it.

The new one had the same number of connections and was wired the same way as I described above. No colour coding on the wires to copy though.

I guess I could have tried dozens of combinations until I got one to work (provided the opamp wasn't blown) but honestly I couldn't spare the time. Especially as I had a different one to fix. One where I could see and get at the whole PCB.

Thanks all for your kind input. Till next time!