So I have an old Planet Audio TT2150 150W x2 max power amp. I bought it used for $20 about 3 years ago. It's worked well until recently. I notice I have no bass. Eventually I get around to checking and sure enough, the fuse is blown. It only had a 10 amp fuse cause I don't exactly work it that hard, so I put another 10 in. I have bass again. So I drive it maybe 20 minutes and no more bass. Fuse again. So I put another fuse in, this time a 15. Now the speaker hums very loudly, slowly moves up and down maxing out the speaker and sounds terrible like a witch screaming or something. Then blows another fuse.
Amp's torn apart on my desk right now. I've fixed a few amps before. Once I found a broken bridge, once a broken resistor, once a component had a piece of solder that fell and welded itself shorted. Usually simple stuff. So far, no such luck. I do notice one area of the board appears heat damaged possibly
. Also, someone added two resistors to the amp. No idea why, but it's definitely the work of someone that can't solder. However it's worked for 3 years flawlessly up until now.
I've checked all the capacitors so far and they're all fine and measure what they should. Checked all the diodes and they are all checking fine. Trying to check the transistors right now. Exactly how would I do this? What should I check next?
This pic shows the added resistors. No idea why these were added, but it worked fine for 3 years.
Here's a pic of the board. From center, to the right a bit you'll see the discolored area I think has overheated.
Zoomed in on discolored area