MTX amp smoked, now there is a burn mark, but the amp still works

[hamz01]

About two years ago, I briefly shorted-out my amplifier. I don't remember exactly how, but it may have been from my metallic screwdriver touching the positive terminal and the casing at the same time. In any case, the amp smoked slightly, but after plugging it in, found that it still played music normally.

I'm re-examining this problem because I have an issue with interference in my system (coming from my GSM phone) and I'm trying to figure out which component is picking up on the interference.

I've attached a picture of the burn... it's right there in the middle of the picture.

Any ideas on what effect the burn could have on the amp would be great. Thanks.

[Perry Babin]

That diode and a 1k ohm resistor (R23?) are connected between the input RCA shield and the chassis ground. This damage is generally done when 12v contacts the RCA shields.

If the diode shorts, it prevents the shield ground from floating and can cause ground loop noise. If it's shorted and no traces have opened, you'll measure ~0 ohms between the input RCA shields and chassis ground (with no RCAs plugged in). If it opens, it generally won't cause any problems. If the diode shorts and a trace opens, there could be other problems.

If you want to eliminate this as a problem, you should:

Remove the diode
Clean the area thoroughly
Inspect the board for open traces
Check the 1k ohm resistor
Replace the diode

The 1k resistor will probably be marked 1001. Its ends will be directly connected to the ends of the burned diode and the pads for D1 (not used).

Some phones cause interference and even if the amp is in perfect working order, it may not solve the noise problem.