The problem is definitely in the power amp section.  No distortion from the line out, but the same distortion shows up on all inputs, line in, channel, effects return etc.  I'll break out the test gear to open it up and check some voltages. I'll post what i find.

I have a Crate PA-8 8 channel powered mixer that is having some problems.  It was given to me for free because it needs repairs.  AT any volume, and on any of the eight channels, the output distorts.  I don't think the output transistors are blown or else there would probable be no sound coming out.  Two diodes in the power supply section have burnt the board, but they test fine.  But obviously something caused them to overheat.  I'm not entirely sure what to check next.  i can send a schematic to anyone who needs to look at it, but i'm not supposed to post it per crate's terms of use.  They were kind enough to provide it so i think i'll respect their rules.  Thanks guys!

I'm attempting to make a high-order low-pass filter that i run after all my effects in my signal chain.  I'm trying to figure out how to cascade several single pole filters. I know you can't just slap one after the other, but what if i stick a simple op amp buffer between each stage?  would that cascade correctly and give the desired effect?

Was the short just something temporary?  Because the transistor itself tested fine.  It didn't seem to be any worse for the wear.

So what could have caused this meltdown in the first place?  Just overheating on this transistor?  It has a really dinky heat sink.

Anyone had any experience with these?  They almost seem to simple to be true, but they're so darn cheap.

ok this isn't as practical as i hoped.  But what do the high priced IEC cables have that a standard shielded cable doesn't?  The shielding blocks the EMI,  and a heavy gauge wire makes sure that there is no current loss, so what else could it possibly do?  It seems the only two problems that a cable can fix/ prevent are already taken care of.

I'm doing some research to try and build my own high quality power cable.  I always like DIY projects, and the good cables are all way to expensive for me.  $1800 for a power cable?  Yeah, i'll pass on those.  In my research i came across a property of wires called the skin effect.  Its a little complicated, but i think i get the idea, which is that heavy gauge wire has a very low resistance to DC and low frequency AC signals (like the mains power frequency)  but they actually have a higher resistance to higher frequencies.  Correct me if i'm wrong.  The way i see it, a very heavy gauge cable could be used to filter out the type of high frequency noise on the AC line that expensive cables claim to get rid of. 

Is this a decent theory?  If so, what are the benefits of using different materials?  Is silver really any better than copper?  It sounds like silver is better at high frequncies, so would i actually be better off with something cheap like really heavy gauge copper?

Also, what kind of frequncy range is the noise on a power line usually at?  Mega of giga hertz?  Or much lower?

I will replace that diode.  Q19 was the problem.  I had to run short jumpers because the tracks were too badly damaged to repair.  It looked like a bad solder job from the factory.  But the amp appears to be working fine now.  No DC at the speaker output jacks, so i connected a speaker and everything seems to be back to normal.  I'll hook it up to its original speaker in a little while so i can get it a little louder.  My bench speaker is really low powered. 

So two questions:  one, what could have caused this to blow?  could it just have been a little heat from playing at full volume combined with the bad solder joint?

and two, is there a better way to repair the traces on the board?  Its lifted off the board for a good half inch on all three legs.

Ok so i found a definite problem.  the tracks connecting to Q19 are damaged, and one is completely broken.  attempting repairs now.

I'll check that again when i put the board back in a little later.  I've got it out right now to check some of the other parts of the power amp and filter section.  The measurement i took earlier was from the case of each transistor to the earth ground connection.  My multimeter was either on the 500v ac or 200v setting.  Probably the 200v.  The decimal point can be testy on this meter, so now that i think about it, its possible that it was 8.2 volts.  But it was exactly the same on both transistors, and completely steady.  I know there should be less than a volt of ac.  The filter caps seem to be fine though.  using the highest resistance setting on me meter they both show charging and discharging normally.  There is some heat discoloration around the diodes in the filter circuit though, mainly around D28.  It measures okay though as far as i can tell.  I tested it the same as the transistors earlier.  D28 read around 830 one way and infinity the other, D29 tested about 830 one way and 750 the other.  There are a few other spots that look like they've been overheated.  I'll figure out whats on the other side and test whatever i can.

I'm going to pull the board and inspect the filter caps.  There is a significant amount (about 82v) of ac on both transistors.  There obviously shouldn't be any.

bump.  Still there JM?

Alright, so i've got my current limiting desk lamp extension cord gadget.  Now what?

still working on getting a limiter together.  Haven't really had a whole lot of time on my hands lately.  What else are you going to have me test when i get it though?

BTW, from what i can figure out, i just need the light bulb in series with the hot wire in the extension cord correct?  Which color is this going to be?