Need help repairing MXR pedal, reverse polarity?

[Zben3129]

I just bought a pedal, broken, and all the information I was provided was that "the battery gets hot and the led doesnt light up"

Problem is it is a SMT pedal  Not what I was expecting


There are some through-hole components (4 transistors, tant capacitors, and 2 diodes right near the battery connection. I have a feeling these are the polarity protection diodes, which is why they would be easily replaced. I checked for continuity between + and -, and there isnt any. However, the voltage drop across one of these diodes is about .03v, and if I remember correctly this holds true in both directions.

So, are there any diodes that would feasably give .03 voltage drop? Or was reverse polarity power applied, shorting this diode out?

Thanks

Zach

[GibsonGM]

If I had to take a guess, I'd say the diodes are junk...no idea why they're reading .03V, I've never heard of a Vf that low!!  Try replacing them with 1N914 small signal diodes, can't hurt.  Make sure they go in the same way the old ones are facing (cathode bands the same way).
The battery probably is getting hot bc something fused and are acting like a short circuit. If you're lucky, they may have saved the board.....don't keep applying the battery til you replace them, LOL. 

[Zben3129]

Alright, I have found the problem to be the zener diode. When I remove it from the circuit, the LED lights up. Out of the circuit, it still measures .03v drop in either direction, but essentially 0 ohms resistance...it is shorted. This was the connection from + to - I was looking to find.

The problem is it appears to be part number 1n4744a, which is a 15v zener. All phase 90 schematics I can find have a 5.1v zener  I doubt it is a purpousely mislabeled part, so maybe the EVH version uses a 15v zener diode? This doesn't seem to make sense considering the maximum voltage for the circuit is 9v...


Zach

[Zben3129]

Fixed!

Just replaced the zener with a 9.1 and all is well 

Now, how does a pedal not burn its polarity protection, but burn just a zener diode (a 15v 1w zener diode )

schematic : http://tonepad.com/getFile.asp?id=42

Zach