Had a Stupid Moment PC2150

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OK I finally got around to buying the Mjl21193's and 94's for my 2150.. I pulled all the 35 and 36's and pre bent all the leads on the 93 and 94"s... needless to say someone distracted me and I didnt know it at the time but I installed the 94's in the 36's place and the 93's in the 35's place.. so my dumb self reversed them.. i powered it up and ofcourse no sound.. by then I realized what I had done.. I took the 93's and 94's and put them in there right place...
Powered it up and the right channel works perfect. Left channel has distortion at low volume.. Took out the 93's and 94's and checked them.. they where good.. so i replaced the drivers.. still the same noise.. so the guy down the street from me that used to work at ppi told me it was probably the driver boards.. So i replaced the big surface mounts on the board with no improvement... and I replaced the smaller parts above the larger ones.. still no luck..
So where do I go from here??? .. nothing on the driver boards looks burnt..
Anybody have any ideas? Or does anybody have a driver board?
 
Compare the signals on the input of both driver boards. If they're essentially the same, look at the speaker output signal on the scope. Is it missing the top half or bottom half?

If not, how is the signal different, compared to the other channel?

Have a speaker or some other load on the output. Be careful. The outputs may overheat when testing. This could cause them to fail. Monitor their temperature the entire time it's powered up (on both channels).

Also, the regulators in those amps tend to heat up quickly. They're likely 2N6488/91s (Q16, 17, 68, 69?). If they overheat and short, it could destroy all of the op-amps in the amp.
 
The waves on the good channel are completely different than the hurt channel.. I dont know a whole lot about wave forms but I can tell there is something very wrong with the left channel.. And the driver board gets very hot very quick.. and it looks like it may be missing the top half of the wave form..
 
With no power applied to the amp, measure the resistance from the pads where the overlaid resistors make contact. You should read 100 ohms. Compare to the good driver board. Do you read 100 ohms for each resistor?

http://www.bcae1.com/temp/ppidriverboardresistoroverlay01.swf

The overlaid resistors are not actually going to be there. The 100 ohm resistors are printed under the large SMD resistors. The ones on the photo show where to make the connections if the ones on the board are open or well out of tolerance.
 
Well... Ideally, you should cut the 300 ohm resistor so it's completely open but the boards are easily damaged (don't recommend removing SMD transistor to cut the resistor) so I'd suggest simply installing a 100 ohm resistor between the two points to see if the problem goes away. You 'could' use a higher value resistor to make it 100 ohms with both the printed and new resistor in parallel but the printed one probably isn't reliable. A slightly lower overall resistance shouldn't hurt anything.
 
Ok perry I found the driver boards.. I put it in and still the same problem.. Had my ppi guy look at it and he said that with the signal generator on it everything looks good on the scope.. I tried different head units, rca's, tested in and out of the car and the distortion still persists.. Any ideas?
 
The resistors are good. It looks ok when a tone is injected but with music I cant really tell besides for how it sounds...Rca jacks are good too.. The ppi guy told my that he checked the switches and the Ic on the input side and they seemed ok.. He told me to just start replacing parts but I really just dont want to shot gun parts.. I really want to know exactly what it is..
 
There could be a power supply problem that's causing excessive power supply noise/ripple that only shows up at high power. If that's the case, you'll probably have to drive it hard while monitoring the ±15v, ±rail and ±driver board supplies. You'll have to have the transistors clamped tightly to the sink if you have to drive it hard.

Do you have a way to clamp the transistors to the heatsink and also allow access to the circuit board? If not, you may have to make something like the clamps in the attached image.
 

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Check for excessive ripple/noise on the following points.

* the end pins on the driver boards

* the power supply pins of the op-amps

* the collectors of the output transistors

The noise/ripple on those points should remain relatively constant no matter how hard the amp is driven. You're looking for high frequency noise. Low frequency ripple is likely to be from your DC power supply and probably wouldn't cause the distortion your hearing (unless it's really bad).
 
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