Crate PA-8 repairs

[shinychrome0]

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!

[Enzo]

First, isolate the problem.

Since all 8 channels act the same, we can assume they are not the cause, at least not directly.

Plug a signal into the Line In jack on the back.  Music from a CD player, a guitar, whatever.   Does it still distort?  That tests the power amp by itself.

Now plug the signal into the EFX return or AUX return jack out front.  Distorted?

Now plug the test signal into one of the eight channel inputs and set the controls as you would normally.  Run a cord from Line Out on the rear panel to the input of some other amp and speaker.  How does the signal sound coming out over there?  Distorted or clear?

And no circuit works right without good power supply.   So are the ICs all getting a solid clean +/-15VDC?

[shinychrome0]

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.

[shinychrome0]

All the voltages that i can find coming off of the power supply section (all 6) and voltages on the power transistors are right where they should be.  I can't leave it powered up for long because the same two diodes start heating up majorly.  I just can't figure out why.  There is no DC on the speaker terminals though.  

One thing that might be a hint is the two transistors just before the power transistors.  The base and emmiter appear to be shorted, unless something else in the circuit is throwing the reading off.  they are mje15030 and mje15031 tansistors.  One read 70 both directions between B and E in the diode test setting on my meter, the other read 120 both ways.  possible short?

[Enzo]

D7, D8 are 5w zeners and they normally get pretty hot.   Hard pressed to think of any other diodes that would get hot.

Q4 has only 122 ohms between B and E in circuit.  Same with Q9.

It's a long shot, but just remove Q12.  it is a mute at the input.  If it is bad, it can mess with you, otherwise we don't need it until we are done here.

Q8, Q3 are limiters.  Try removing them.

Is Q1 shorted?

What is the nature of the distortion?  Do you have a scope?

[shinychrome0]

Its tough to describe, but its not extreme.  About like a blown speaker probably would, but that's obviously not the problem.  And no i don't have a scope.  I have a scope program on my computer but i don't think i should plug the speaker out into the line in on my computer.  I'll try messing with those other transistors and get back to you.

[shinychrome0]

Should those zeners really be getting hot enough to make it smell like they're burning?  That really doesn't seem normal and the board is kinda charred underneath 'em.

[shinychrome0]

q12 measures 1100 and infinite between gate and drain, and about 40 both ways between drain and source and 1090 and infinite between gate and source.  Sounds like a shorted transistor to me.  Am i right?

[Enzo]

40 what?  Ohms?  Millivolts on diode test?

That is a JFET, it is ON until you turn it off with voltage at the gate.  SOurce and drain are interchangable, and have a low resistance between them.  I usually expect maybe 100 ohms, but who knows.  Since the source and drain are more or less connected together, the gate will act like a diode to them.  SO until I know what you were measuring, so far it sounds OK.

But the bottom line is this, with Q12 removed, does the amp now work correctly?

Yes, those zeners can get pretty darn hot.

[shinychrome0]

ohms sorry i don't know why i didn't mention that.  And i wasn't sure about powering it up without that transistor, but i just checked it and no it doesn't work yet.  I checked the datasheet for that transistor too.  Its only supposed to read about 50 ohms between source and drain anyway so i guess thats probably within range.  Q1 appears to be functioning normally.

And there's the problem. It was either Q3 or 8.  I removed both and the amp works.  One of them reads as being shorted.  So i guess i'll just order those.  Is there anything else i might need to check that might have been the root cause of the transistor shorting?  or was it probably just a bad transistor?

[shinychrome0]

So i guess this is not as fixed as i thought.  It worked fine with the transistors removed, but with the replacement transistors installed, i've generated a new problem.  There is now some sort of oscillation that starts a few seconds after powering up, and slowly works its way up in pitch.  I'v double checked all my soldering and everything seems to be clean and installed correctly.  I get sound throught the amp  but now the oscillation is there as well.  Any thoughts on what i might have bumped, touched, changed, screwed up, etc?  The original distortion prblem is gone at least.

[shinychrome0]

One thing i forgot to mention.  I originally installed the new transistors backwards. The pinout is reversed from the original transistors.  The first time i powered it up( with the transistors backward, it oscillated like this, and nothing changed when i put the transistors in the correct way. 

[shinychrome0]

Enzo, you have any other ideas here?  I can't find anything else wrong with the dang thing.

[Enzo]

Do not assume transistors you already installed are OK.  ANy time ther is a circuit fault, the new transistors may have been damaged - and wiring them backwards certainly counts.  All around all those transistorws are various resistors.  Many times when transistors short out, they take out resistors with them.  SO check all the resistors in the output section for opens.

[shinychrome0]

Should i measure them in circuit or life one end and measure them that way?

P.S. i used new transistors the second time.  I ordered 15 of each so i had plenty to spare.  So i wouldn't think the new transistors would be the problem.