Help debugging a vintage Micro Amp

[sadwings75]

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.

[jof006]

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.

Does it work when you go back to powering it from a battery again?

It seems as though I burned out one of the components.

What are you basing this on? Can you see/smell a burnt component?

Apparently old MXR pedals are reverse polarity

Really?

I can't find a schematic of an original, only reissues, so if you have a schematic or link to one - please post it.
Can you post photos of the components - both sides of the PCB?

[sadwings75]

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.

Does it work when you go back to powering it from a battery again?

No. When I put the battery back in it only works when in bypass. I took the battery out and wasn't powering it with the pedal board, and it was reacting the same way.

It seems as though I burned out one of the components.

What are you basing this on? Can you see/smell a burnt component?

Inside are the remains of that old foam they used to use so it's pretty dirty, but it doesn't appear that anything is obviously burnt out upon visual inspection. I only suggested that because it had been working without issue until I plugged it into the board.

Apparently old MXR pedals are reverse polarity

Really?
Here is a schematic - http://www.generalguitargadgets.com/pdf ... a841b3e85a" onclick="window.open(this.href);return false;
Like I said I'm fairly new to this, and was basing the reverse polarity idea on something I had read in another forum so it may not be correct
I can't find a schematic of an original, only reissues, so if you have a schematic or link to one - please post it.
Can you post photos of the components - both sides of the PCB?

PIctures to come later in the day

Thanks for your help on this

[sadwings75]

Here are the pictures:

http://s974.photobucket.com/user/opuspo ... e.jpg.html

http://s974.photobucket.com/user/opuspo ... f.jpg.html

[sadwings75]

Found this:
"""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 been destroyed by this?

[jof006]

From the photos I can see that the red wire (assuming from the battery's +ve) goes to the Vcc+ of the op-amp, so the pedal itself does not look like it's "reverse polarity" (from what I can see).
I can only assume that your pedal board's polarity was not set correctly for the battery clip adapter. Is that a possibility?
Do you have a multimeter to check it?

What components would be most likely to have been destroyed by this?

If the pedal board powered the MXR the wrong way around, the most likely component to suffer would be the op-amp IC. The TLO61 does not have any inherent reverse polarity protection.
After that, the capacitors MAY have suffered some damage, but they're probably OK.
On the schematic you linked (later circuit design) a reverse polarity protection diode (D1) has been added. Unfortunately, your pedal does not have this (at least not on the PCB photos).

Are you going to try and fix this yourself?

[sadwings75]

From the photos I can see that the red wire (assuming from the battery's +ve) goes to the Vcc+ of the op-amp, so the pedal itself does not look like it's "reverse polarity" (from what I can see).
I can only assume that your pedal board's polarity was not set correctly for the battery clip adapter. Is that a possibility?
Do you have a multimeter to check it?

It is a possibiity. I got this used on Craigslist for $30. I played it and inspected the guts before purchasing. A few days later I noticed that it looked like someone had modified it to be able to change the battery without opening the enclosure. The battery connector has been made longer by clipping the original wires and adding another battery clip. There is also a small hole drilled on the side to run the now longer battery connection wires through. Everything else appears to be original. I do have a multimeter.
The reason I suspected it was a reverse polarity issue was the fact that the pedal worked flawlessly until I plugged it into the board and the limited time I have had to research the issue has revealed several different sources suggesting that. How would I use the multimeter to determine this?

What components would be most likely to have been destroyed by this?

If the pedal board powered the MXR the wrong way around, the most likely component to suffer would be the op-amp IC. The TLO61 does not have any inherent reverse polarity protection.
After that, the capacitors MAY have suffered some damage, but they're probably OK.
On the schematic you linked (later circuit design) a reverse polarity protection diode (D1) has been added. Unfortunately, your pedal does not have this (at least not on the PCB photos).
When I started reading into reverse polarity protection via diodes I checked the schematic and noticed that the "older" spec still lists diodes. If I read the pot correctly it was manufactured in 1979 so this Micro Amp is pretty old at around 30 years give or take. Haven't seen a schematic that is truly representative of this older version

Other sources have sugegested the op-amp is the first place to start. How would I use the multimeter to check this? Place one probe at one of the jacks and put the other probe at one of the contact points to measure the voltage?
Are you going to try and fix this yourself?

I was interested in possibly modifying some values and just learning more about the circuit so I will probably purchase a Micro Amp kit from General Guitar Gadgets ($50), build that so that I can play, and work on debugging this to build experience. I have a SEL amp kit from AX84.com that has been sitting here for too long and I'm sure there will be some debugging associated with that experience so I might as well learn as much as I can before beginning that[/b]

[Haze13]

Just replace Ge diodes to the Silicon (4148/914). Vol pot to 100k and output cap 10uf to 1uf...

[stef]

Diodes

[Haze13]

D1 and D2.

[jof006]

Anyway...

Before we do anything else, can we check our theory that the pedal was powered the wrong way around?
I'm not sure what your skill level is regarding electronics, so I'll assume low until you tell me otherwise.

Take your LCD multimeter and check the pedal board's polarity - set it to read voltage (DCV)(40V if not autoranging) and place the black lead on the round connector and the red on the grippy connector. Paying close attention to any plus or minus signs that may appear on the meter, what's the reading?

Code: Select all

black --> O  {} <-- red

Doing the same thing with your battery, what's the reading?

How precious are you about this pedal? Do they go for stupid money? Do you just want it fixed quickly?

[spaceace76]

it seems pretty unlikely that you reversed the polarity. battery clips only attach one way, and after that the power from the board simply plugs in. this pedal is IC based and uses the same polarity as the rest of your pedals unless you have a germanium based fuzz or something similar.

more likely you jostled something that was already almost broken. there are tons of signs of this on your board, rotting foam and oxidizing solder joints. I would reflow those, or even replace the solder entirely. your components look intact, so most likely this is a wiring/continuity issue. take jof006's advice and check the power first, but follow the power connections up to the board to ensure the voltage is actually being delivered to where it should be.

In fact, I think I can see your problem from the underside of the board. The black wire soldered to the bottom of the board seems to have a lifted trace. Try bridging this connection or bypassing it with another wire to see if the pedal will come on. This is the ground wire for the entire pedal, so without it the IC would never bias properly and turn on to amplify. Give me a few minutes I'll point out the exact spot I'm talking about

[spaceace76]

brokentrace.png

(331.64 KiB) Downloaded 770 times

see the slight arching to one side? it looks like it's slightly off kilter, and there is enough grime from the rotten foam to have gotten in there and break the connection. I would check this after checking the power, and clean up this area as it will only cause more issues over the life of the pedal

[sadwings75]

I want to say a sincere thank you to all of those who have contributed to this thread so far. I was beginning to go through the debug process when I figured "why not try it one more time just for the hell of it?" and it it fired up as if nothing was wrong. This lends credence to the suggestion that the polarity of the pedal was not in play and that one of the components must have been the culprit.
You guys inspired me to go through and thoroughly clean all of the old foam and gunk out of it; looks brand new now.

I think I'm finally going to take the plunge and build a '68 Plexi kit with Valvestorm parts/Metroamp trannys so I'm glad to see the virtues of patience and willingness to help others is so strong in this forum. I was looking into getting a used Marshall 1959 reissue but have changed my mind.
I was thinking of breaking that process into 3 "chunks" so that I can pay it off over time. For anyone who is familar with the process do you have any suggestions as to how it should be broken up?
Valvestorm lists the parts in (roughly) the order that they will be needed so that helps.

Maybe the first chunk would be the chassis, transformers, choke, pots, tube sockets and components that will connect to them?
The second chunk would be the board, all components for the board, and wiring?

Not only will this allow me to afford it, but it will encourage me to take my time and make it perfect.