Earthquake 200dhc with hum in output

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Got this amp in a trade and although it appears to work properly it has an annoying low level hum that is being produced through the speaker outputs. It appears as if somebody has worked on this amp before . I'm guessing that at least the ps fets were replaced at one point and most likely the outputs went at that time as well . It looks as if somebody may have changed out a resistor near the speaker outputs as well. Every pic I've seen of the inside of this amp shows a 2-3 watt banded resistor (not sure of the value right now) right behind the speaker outputs which I believe I read somewhere is part of some sort of filter. My amp has a large white .1 ohm 5 watt wirewound resistor in place of that banded resistor. Here is a link to a pic of an amp similar to mine showing the original resistor. The resistor can be found in the lower left hand corner of the pic just to the left of the two output inductors. Google Image Result for

Could this reistor be part of the issue? Those color bands don't look like .1 ohm to me and that is what somebody put into my amp.Also was surprised to see a fuse soldered directly to the underside of the board across the positive speaker outputs. I know it was recommended to fuse the speaker line in these amps but it looks like somebody just cut the trace leading to the positive speaker output and soldered a 20 amp atc fuse right across it. Weird looking for sure. I added lengths of wire and made the fuse accessible from the top side of the board so you don't have to remove the board from the sink to change the fuse if needed.
Haven't had a chance to see if it hums with no rca's yet. I'm in the process of replacing all the electrolytic caps in this amp with some nicer panasonic FC caps.. As soon as I wrap that up and can get it back into the heat sink I will see if it hums with no rca's. Maybe the new caps will clean up that noise a bit. We will see shortly. Thanks for the suggestion on the rca's.
Well after replacing the caps it seems as though most if not all of the hum I was experiencing is gone. There is virtually none to speak of at this point. Just the slightest bit of hiss which more than likely would be inaudible in a car if it isn't being caused by my power supply. RCA's in or out of the amp seem to make no difference. Even though it works ok I am still not convinced there isn't a small issue somewhere in this thing. I noticed that if I touch the heatsink or rca shield with my bare hand it seems to alter the noise I am hearing. Touching the heatsink doesn't alter the sound as much as touching the rca shield does though . Touching the rca shield with no rcas attached and a sub hooked to the speaker outputs causes a nice pop of the speaker cone. Not sure if this is normal or not. I kind of accidentally touched the rca shield the first time I noticed it. Never noticed this on other amps.
There is about .6 volts of dc on the heatsink whether I use secondary or primary ground as a reference( there is no continuity between primary and secondary ground though). The voltage slowly trickles down while I am measuring it. Not sure if that is my meter stabilizing its measurement or if its really going down but it always starts right about .6 volts when the meter leads are first placed on the amp.
Well there is definitely either something wrong or a cap I put in was defective. I was playing the amp today for about ten minutes at a little more than half power when a huge puff of smoke came from the amp. I opened it up to reveal that the newly replaced 47 microfarad 100 volt cap near the speaker outputs had blown its top. Funny thing is the amp still works okay. I am still skeptical of that .1 ohm wirewound resistor that is right next to this cap but I am not sure how this circuit works. Only thing I am sure of is that originally this amp did not have a .1ohm resistor in that location. Not sure if the .1 ohm resistor is supposed to be some sort of upgrade or not. I am probably going to replace the cap again and see what happens.
I replaced the cap and also changed the .1 ohm resistor to a .2 ohm resistor as that is what it looked liked it originally had based on the pic of the other one . I ran a sine wave through the amp with no load to verify it was working and producing clean output and the new cap smoked after about 5 minutes(didn't play 5 minutes of sine wave, more like 4 minutes of music through a speaker at low volume and a minute or less of sine with no load attached at full unclipped output). I have a question. Is this cap supposed to be polarized? The original didn't have any clear cut markings indicating the positive or negative lead of the cap.Every other cap I replaced was clearly marked. The board however had markings indicating polarity in that location. If it is supposed to be bi-polar and I put in a polarized cap would it cause this failure I am experiencing ? I will check the terminals for noise a bit later when I get a chance to tear back into this amp. When you say noise though what exactly am I looking for on the scope? Should I look for this noise while driving signal into the amp or just while it is idling? Is there a section on your tutorial that covers this?
In the photos I have, it appears that there are no polarity markings. If this is the case, the cap is likely connected across the output of the amp and would have to be non-polarized.

The polarized capacitor would see reverse polarity half of the time and would fail.

Set the scope to 5v/div and 2mS. Drive a sine wave into it so that the waveform deflects the trace 1 division positive and negative. Do you see a clean sharp sine wave or is it thick/noisy?

Check both outputs terminals.

There's nothing that covers this particular problem on the DVD.
Well luckily I still have the original cap. I will slap that back in and see if she holds up. I will check for the noise as well. I had a feeling that the cap was supposed to be non polarized but I figured with the polarity markings on the amp it had to be a fluke that there were no markings on the cap. Live and learn I guess. Makes total sense that it would need to be non polarized being placed across the speaker outputs.
Well I put the original cap back in and so far it has held up. Ran the amp for about ten minutes with no load and the cap didn't get the least bit hot where before it would have already failed at this point. More extensive testing on that tomorrow. There appears to be noise on both of the output terminals Perry. Definitely not a clean sharp line. Very fuzzy looking . Output sounds normal though.
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