Help debugging a vintage Micro Amp

Started by opuspocus83, August 27, 2013, 04:33:02 AM

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opuspocus83

Hey guys,
I recently purchased an old MXR Micro Amp and initially it worked great.  I have been running it off of battery power as there is no connection present for a wall adapter.  Last night I plugged it into my pedal board that has a 9V battery clip adapter and now I have no sound when the pedal is engaged.  I run the Micro Amp in front of the amp, and the pedal board was also running a delay pedal placed in the FX loop.

It was working very well until this and it's kind of a bummer since I just got a new guitar yesterday and would really like to try it out with my usual signal path.  It seems as though I burned out one of the components.
Apparently old MXR pedals are reverse polarity; would this be related to the problem?

I'm pretty newb-ish when it comes to this stuff but can follow directions if anyone could offer advice on where to start.

GibsonGM

Hmmm...well, a 9v clip is a 9v clip, and if that's the only way you hooked this puppy up, it should've been the same as using a battery!  So, something popped....you need to debug and see what's wrong!

Read this:  http://www.diystompboxes.com/smfforum/index.php?topic=29816.0

If you can post a schematic of the microamp, that would be VERY helpful!   

Check the voltage at the clip you used...perhaps it is defective?  Try the battery again?   Could be as simple as a broken battery clip wire in the unit, or a blown diode or something.   Dunno if it switches at the jack ("turns on"), but there's another area to look at before tearing it apart.  Mechanical stuff usually breaks first...in/out jacks....bad patch cord...y'know.

Start at the beginning :o) If it worked yesterday, chances are you can get it working again if you keep your patience.

For now, make any test measurements only with a battery!   
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opuspocus83

Hey thanks for helping

This is the schematic:
http://www.generalguitargadgets.com/pdf/ggg_mamp_sc.pdf?phpMyAdmin=78482479fd7e7fc3768044a841b3e85a

I will be in and out of class until 9:50 tonight but will sift through that material you directed me to and respond based on that

opuspocus83

I found this and it sounds like what has happened:

QUESTION
I've got one of those SKB powered pedalboards. Is it possible to lop the tip off one of the power cords and rewire it with reverse polarity to power a PNP pedal?
Power Supply - How can I drop 9V to 5V easily?

ANSWER
Only if it's the only pedal you are powering with the power supply. if you are also using that same supply normally with other effects, you'll be shorting +to - at the ground connection when you attach one pedal to another...think about it. the plus side is ground on the PNP box, and minus is ground on the others, then you connect the grounds together...fizzle!
the only way to do it is to connect a second supply that allows you to make the negative connection on your PNP pedal BELOW groundlevel. so you can lop off a connector and reverse it to powera positive ground (PNP fuzz face) pedal, but it has to be connected to a different power supply.

What components would be most likely to have blown?

induction

Have you tried it again with a battery?

The schematic you posted is negative ground, but it sounds like you are claiming that your pedal is positive ground. Do you know that, or are you just guessing?

The purpose of D1 is to prevent reverse polarity damage. If you reverse your power connections, it won't work, but nothing should break.

Try it with a battery. If it doesn't work, post some pictures.