Fender Acoustasonic SFX ii Repair

[jjones8710]

I recently went to an estate sale and picked up the aforementioned amp, a Fender Acoustasonic SFX ii, for $50. I originally had paid more for the unit, but when I got it home I realized it didn't power on so I went back and they gave me $125 back. I have an academic background in EE so I can read a schematic and do some basic troubleshooting. After downloading the schematic and removing the electronics from the cabinet, I discovered two things:

1. The fuse labeled F1 is blowing as soon as power is applied
2. The resistor labeled R257 shows signs that it has been scorched but I can't see any other components that show damage

Thanks for any assistance running down the problem. I'd love to get this up and working because it's a steal at $50. Also, if anyone has the technician's service manual I would much appreciate a copy of it. I can't find it anywhere online.

[52 Bill]

Welcome to the place.

1. The fuse labeled F1 is blowing as soon as power is applied
2. The resistor labeled R257 shows signs that it has been scorched but I can't see any other components that show damage

F1 is the main power fuse, if it is blowing, there is a short somewhere in the amp. If you have a light bulb limiter now is the time to use it or to build one.

R257 is part of the sensing network for the side speaker power amp. If it shows signs of heating, then I would start by testing the power transistors in that amp for shorts. Those would be Q29, Q31, Q33 and Q35.

A fast test would be to pull the two red wires that connect the power transformer to the pc board. That would disconnect the power amp power supplies from the circuit. Then if the power fuse holds, you will know that the problem is in the power output section.

[g1]

schematic here:
http://support.fender.com/schematics..._schematic.pdf

[Enzo]

The service manual has the schematics and layout g1 posted, and the rest of it is a parts list, the exploded view drawing of the cabinet and chassis, and a very rudimentary circuit description.

You are blowing the main fuse. The main reasons for that in any solid state amp are:
shorted output transistors on one or both channels (more likely just one of them)
shorted main rectifier (D50-53)

Less likely:
shorted filter cap/caps
loose piece of metal hardware wedged under a board causing short to ground

And absolutely the least likely:
shorted power transformer

[Indyryder02]

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

[Enzo]

You already started a new thread for your amp, which is how we prefer to do it, so let this thread return to sleep.

[Jazz P Bass]

Fender Acoustasonic sfx shorted? Ugh! - Music Electronics Forum (music-electronics-forum.com)