Fender Acoustasonic sfx shorted? Ugh!

[Indyryder02]

Amperage climbs with voltage to about 1 amp.. then pulses to about 1.5 and back?
I’m not familiar with this type of problem..
is this an indication of a particular parts failure?
obviously I don’t know transistors and diodes as well as tubes!

[Indyryder02]

It appears Q31 and Q35 have shorted again..
however this time.. Q15 and Q29 seem to still be ok..
so may help narrow down what’s taking em out.
many more suggestions? TIA

[Indyryder02]

Ok.. more info.
looks as though only one of the 1294’s is shorted EC.
i believe it was Q31. I’ve checked everything around em in circuit.. in and out of the circuit. Such as R259 R275 and R279 Q27 Q23 and Q19 as well as all the other diodes etc.. checker C141 as well. Something in this area has to be amiss if it keeps failing I assume, correct? Thanks!

[Jazz P Bass]

Are you using an isolating pad behind the output transistors where it meets the heatsink?

[Indyryder02]

Yes.. one was turned a bit previously.. that was the cause of the previous short found on the minus rail.
I did make a new insulator for one, out of clear package plastic (does it have to be mica?) , as it seemed too small.
I reinstalled and retested to be sure the short was gone.
I think the current failed OT is the one I made the insulator for..

I’ve only ever used my Variac’s amp meter when bringing things up.. to check for shorts etc.. with tube tech it’s never been an issue.. and I always looked down on DBT’s kind of as hokey.. but I can ones value with solid state equip. more so. Since it takes so very little to kill these devices.

Seems I should be close.. I can only find the one shorted OT.
but nothing in line with it that’s failed.. And of course I want to find whatever caused it short the transistor before attempting another replacement and power up.

[Indyryder02]

Looks like I’ll be making a DBT tomorrow, and forging ahead.

Not sure what else to check at this point. I’ve tested everything in line with the failed Q31. Everything tests good or as expected.

I assume it’s ok to run solid state amps without the outputs in place? Like testing with power tubes removed would be? When needed.

thank you!!

[g1]

Yes, you can try it that way. Also you can add just one pair at a time til the DC is sorted out. Do the screws for the outputs have insulating washers?

[Indyryder02]

No insulating washers that I could see on any. Just the mica insulators for the back.

[Indyryder02]

installing a new Q31. I’ve checked everything Even remotely in line with it. Everything tests good.
But definitely don’t want to take out another one.

[Indyryder02]

Ok.. brought it up on the newly put together DBT.
part of the problem is, I’m not as familiar with how solid state amps take current compared to tubes. I’m used to watching them heat up. Stabilize. Most of the solid state stuff I’ve repaired over the years, has been pretty steady, and nothing too crazy amperage wise.

so..
took it off DBT. @120v it’s pulling .75 amps. And every so slightly seems to creep up very slowly.. to where I want to shit it down.. seems like it has done this before, came up slowly (normally I’d say, not immediately pegging meter or trying,)
to about .75a and hovered there for a min.. then out of no where. Whomp! Tried pegging meter and took out the OT.

(I have seen some larger power amps and bass amps take up to 1.5 amp or so at idle... and be ok. but again. Just not familiar enough with solid state. Really want to avoid blowing another one.. )

any clue what’s normal for this amp? Or a way to find out?
anything else I can do to prevent more issues? Thanks.

[Indyryder02]

Threw it back on DBT. As my main fear is it jumping from idle current before I can react. On the DBT.. the bulb is dimly lit.. and the current on the Variac holds just above .5 amp.

assuming it’s still getting what it needs... it may be steady now.. and not trying to yank current out of nowhere.

Prob do a couple of very careful voltage tests and see if everything is acting like it should.

Appreciate the help! Just need to bounce ideas sometimes even!

[Indyryder02]

Ugh!!! It sat steady on the DBT. For a while. Checked all voltages.. nothing seemed out of the ordinary.
Took off DBT.. brought up slowly again on Variac.
same exact thing.
Idles about .75. Hovers a min or two like it’s charging up, then. Bam! Took out Q31 and Q35 again...
I’m beside myself with this one. Going to grab some scotch to help cope I think!
What in the world is causing this to sit idle ok... then pow! Take those same two OT’s out. I’ve tested and checked every part on the board I think five times! Losing it!

I just don’t get it. I thought I finally had it this time..
Cap turned into a resistor somewhere? I’m at a loss at what would act like this..

Please Help! I’ve never been so jammed up on one amp!

[Helmholtz]

Confused, what is a DBT? Same as an LBL aka lamp ballast limiter?

[Indyryder02]

Yes. Dim Bulb Tester.
it actually did allow me to bring up to operating voltage and test. Everything tested as it should. Slightly lower of course through the current sharing device.
but nothing alarming or out the realm of where it should be..

holding steady idle around a half amp for maybe 20 mins or so, testing voltages. All ok.

feeling safe enough to throw it back just on the Variac.
the same thing occurred... it comes up normal. Then every so slowly starts gradually creeping up... not enough to think it’d be much out of the ordinary.. until it keeps going.. then almost immediately. It just pegs the meter and shorts the OT’s only on the one amp channel on the minus side.

I’ve repaired hundreds of amps, many solid state. Hifi receivers etc etc.. this one I’m tempted to buy to just get out from having to further work on it!!

really dislike this amp!!!

has anyone else experienced this action?
idling like that on the ampmeter? Then just pow!

All I can’t think of is a capacitor.. because of the way it’s charging up? Out in the weeds at this point again. Just stumped. Tested every part in that side of the circuit in and out of circuit.. just to be sure. (Of course tedious and time consuming, not good for parts either).
Just don’t get why I can’t find anything to test even suspect let alone bad... when clearly something is amiss if it keeps taking OT’s

[Helmholtz]

I did make a new insulator for one, out of clear package plastic (does it have to be mica?) , as it seemed too small.

Normal plastic might melt at elevated transistor temperatures. Also has poor heat conduction.