Fender Acoustasonic sfx shorted? Ugh!

[Indyryder02]

I’ll apologize ahead for my lack of forum use knowledge.. mostly only ever been a reader.. I’ve seen this topic on here years ago.. not sure how to add to it or if it’d be seen.. so starting this one.. welcome to advice for future posts..

I have one of these on the bench currently.. and it has me stumped. Customer originally had it apart.. trying to see if he could replace the master vol. he bolted the main board down with the bolts going the opposite way through board.. so it wasn’t actually threaded into the chassis.. said it worked a minute then went dead..

i first of course found a blown 4a fuse.
then shorted output trannies on one side.
replaced with a new set of 1294 3263’s
r275 r279 failed...
brought it up slowly on variac.. seemed to have a bit much amp draw.. but not pegging.. maybe an amp or so..
by accident, checking some voltages, I clearly had shorted out r249 with a nice pop! output trannies failed on that side again..

ordered a new set. Replaced failed r249

nothing else testing suspect..

now amp meter pegs immediately slowly bringing up to even 20v... I’ve been through the board multiple times and can’t seem to find any short or anything to cause this instant high draw...

Checked c149.. and main caps etc..
only thing I’ve found is the leads to c151 seem shorted.. same at c141. Neither reads this once removed from circuit.. yet Thir pads still do.. and I can’t seem to find anything else that would cause this.. I’m clearly missing something I’m sure pretty obvious.. I’m just wore out on this one and can’t think straight anymore!!!
many help mucho appreciated!! TIA

[Indyryder02]

P.s. if I pull the two red wires From the PT, is the only way it stops.. What is my dumb &$$ missing? Haha!

[g1]

the leads to c151 seem shorted.. same at c141. Neither reads this once removed from circuit.. yet Thir pads still do.

Did you mean C151 & C152 ?
Have you checked the diodes D50, D51, D52, & D53 ?

[Indyryder02]

Just C151 and also further down at C141 all of D50-D53 test ok. Those were all my first guesses and C149 across. But all seem to be ok... I’m sure it’s something obvious I’m just missing.. need a break from it! Haha

[Jon Snell]

The first thing to consider is why did the output transistors blow and just replacing them is not fixing the reason, as you have found out.
Check the driver stage; remove the output transistor drive first, R273, 275, 277 and 279 will stop stray base current.
Check the voltage across R249, should be 1.5 ish. Any more then check the stage prior; Q15 will have about 2.7v C E if not find out why not.
Check the constant current source from Q19.
Keep methodical and the fault will easily be found.

[g1]

Just C151 and also further down at C141

Then C142 probably measures short across it as well? Did you check D42 and D43 at the amp outputs?

Do you have the speakers or any load connected? You should not, until you get it so there is no DC on either amp output.

[Indyryder02]

Can’t really check voltages... as It’s almost 2 amps by 20 volts.. and don’t want to kill another set of outputs..

the main heat sink bar the trannies are bolted to. Was loose from the chassis due to being bolted in backwards.. after it had a master volume replaced..
suspect the problems started with the loose board.

C142 does also read short across like 141.
the D42 & D43 seem to test fine.

[Indyryder02]

No load or speakers hooked up.
seems to peg amp meter immediately when bringing it up.
so I’d expect to find a dead short of some kind..

[g1]

Well, you have a short from minus rail to ground that measures with your meter set to ohms. Track it down, you don't have to power up anymore til you locate the short. You may just have to go along and lift one leg of everything connected to that rail.
As someone was in there before, it's also possible there is a solder bridge or a wire connected in error. With this kind of previous repair attempt, you can't take anything for granted.

[Indyryder02]

I’ll keep looking. Really appreciate the advice!! Thanks!

[Indyryder02]

K.. with both C151 and C152 removed.. one leg of c139 C140 C141 C142 C143 C149 and C155 all removed..
it sill pulls high current immediately.. only when I lifted legs of D52 and D53.. it stops drawing. I’ve checked everything a few times it seems.. any other cause for this I’m not thinking of?
Get .0000 ohms between pads of C151. But resistance on C152
same at pads of .0000 C141 and C142. I still can’t find a cause though anywhere.. all trannies and diodes have tested ok best I’ve been able to tell.
ugh. Need to get away from it again for a bit. Going in circles!

[The Dude]

Don't forget that the negative rail is supply for the output section(s). The short doesn't have to be in the supply part of the schematic. You could have a shorted output transistor, shorted D42 or D43, etc. - anything that connects to that rail and has a path to ground if shorted. I think maybe you need to climb out of the tree and look whole forest for a second.

[g1]

You don't need to check if it pulls current anymore. As long as that 0 ohms is showing up across the pads for C151 the problem is still there, and it will pull excess current. Trying to power it when you know the short is still there can cause more damage.
Do you have the output transistors on the minus rail removed? If not, remove them. (Q32,36,31,35)

[Indyryder02]

That’s what I was thinking! I’m second guessing myself now!

So, it must have been a problem with one of the OT’s insulators. Which is one of the first things I checked, but still must have fooled me. I removed them as suggested. Retested them.. they all still tested fine..
checked pads at 151. Ok now.
made a slightly larger insulator for one that was just barely large enough apparently. Reinstalled. No more short.

another case of not double checking the obvious suspects!
got out in the weeds instead and lost sight of the big pic.

any suggestions on protecting ribbon cables? One coat hanger’d it’s way broken from the board.. had to surgically repair. Ok now. Not fun though.

Hopefully will go back together without issues this time. Anything I should be ware of when trying to power up again? I’m still fairly new to solid state. I have a much better grasp on tube tech.. and the proper life signs to look for when bringing to life.

Really appreciate the advice.
First time I’ve reached out to anyone else for any help with this stuff, as the techs in my area aren’t willing to help really.

So again, thank you! Very kind to share your expertise and wisdom to us guys still trying to figure it out!

[Indyryder02]

K. Got it back together.
brought it up slowly..
amperage came up slowly to about .75 amps.. hovered there for a min.. but very slowly kept moving.. kept my on it to see what it was gonna do.. then it just out of nowhere tried to peg.
i shut it down fast. So far no obvious causes. What would make it idle for a bit with steady current... like normal... then start climbing like that? Very strange.