Punch 501s amp

[Itsme]

Hello, I am having problems with a 501s, The left channel was distorting and I found 3 burnt .1 ohm 2 watt resistors so I replaced them with 5watt .1 ohm that I had in stock and I tried it again and now it is pulling way to much current. Also I found a resistor minature that looks burnt but I can't tell for sure the board number is R155, On the other side R255 I think says 100 or 1001 on top. Very hard to read even with a magnifier.

[jol50]

Was it a gate resistor that is burnt?

[Perry Babin]

If the 0.1 ohm source resistors opened, it's almost certain that the output FETs have also failed.

On most Rockford amps, when the source resistors open, one of the two protection circuit resistors (in parallel with the 0.1 ohm resistors) opens. Generally, the one with the highest resistance opens.

If the layout is the same as the layout from the 301S (see attached photo) and the resistors around the burned one are the same, the value of R155 should be the same (1001 - 1k ohm). If the 501S is different, get the schematic from Rockford tomorrow.

[Itsme]

Yes, It's 3 of the big resistor's that are connected to the right 3rd leg of the IRF540's which I think are bad as well. How do I replace the fet's on a MEHSA board?

[Perry Babin]

You have to use a 'small' butane torch to remove the transistors.

You clean all of the old heatsink compound from the insulator (very important -- ask anyone who has had the hot compound fall onto their hand) and heat the insulator from behind. The transistor will release after ~20 seconds of heating (for the first transistor - the others will release more quickly because the insulator is already hot).

Wear safety glasses (<< very important). Keep the torch angled away from the board (important on the older amps where the insulator is parallel to the board).

I use the torch radio shack sells.

[Itsme]

I tested R155 on the board and it came out at .981 so I think its ok, But it look's like it got pretty hot because the number isn't visible on it. Here's a picture of the resistor.

Also Should I replace just the 3 bad IRF540's? Is there a better, stronger fet to go with? I do have the 540's but unforuntly I bought from Futurlec and every one of them has a differn't date code.

[Perry Babin]

It's best to go back with the original parts. They will hold up fine under normal use.

If R155 is burned too badly to read, it's probably open. You may be reading something else on the board. Since you'll probably have to order other resistors for the protection circuit, you should order the 1k resistors also.

You need to remove the new source resistors and check the IRF540s to confirm that they're shorted (~0 ohms between legs 2 and 3). While the resistors are out, find the protection circuit resistors and check them. If I'm not mistaken, resistors R172, R173 and R181 are the most likely to have failed. Also check R162. Q116 and Q118 should also be checked.

The resistors and transistors in the attached photo also need to be checked.