Fender Acoustasonic Jr

[srussell2]

good evening...I just purchased a Fender Acoustasonic Jr amp model # PR-334 (date on the board is 1997). I bought it right as the seller was having problems with it. 1) on the main channel at low volume the sound from the amp had a distinct click or snap when striking the strings on my Ovation CS-257...the problem worsened as the volume was increased. 2) using the inst/kine/mic channel as soon as I plugged in my instrument there was a substantial 120hz buzz which also worsened as the volume was increased. Overall the sound of this amp was crappy and thin.

I looked it over for bad connections and while powered on the bench, moved the power caps and voila, noise gone. Removed the board and it had a bad solder joint on one cap...resoldered and retested, No noises on either channel. Reassembled the amp and speakers into the cab, powered it up and played for about 30 seconds and it shut off. I removed the speakers and board again and discovered 2 things. I screwed up and had one set of woofer leads connected to the horn output and vice versa. I assume this caused some overloading of the horn circuit?? The fuse was also blown...replaced the fuse and reconnected everything correctly this time. Now when the unit powers up it strokes the right speaker cone all the way out and the voice coil gets hot fast. I measured 39V DC at the speaker terminals with the speaker disconnected and also no noise. Switched the other speaker to the right circuit and it strokes the speaker cone and instantly heats up...so it's not a shorted voice coil. Have I toasted a power transistor?? The amp works fine and is quiet with that right speaker disabled. one of the large blue resistors (collector resistor?) was discolored so i measured the resistance still on the board...0 ohms. I removed the resistor and it still reads 0.  I'm sure I caused whatever the new problem is, I just need poined in the right direction. I'm a master certified auto instructor and I'm fluent with meter and scope usage, I'm sure I can win this war with a little help...please? thanks!!!

[J M Fahey]

You burnt the right channel.
Do not connect any speaker there or you will kill it.
Ask Fender for the schematic and post it here.
They will either send it straight on or charge nominal $5 or thereabouts.
A "large blue resistor" means nothing, any coding on it?
What does the PCB say below it?
I *think* your combo has 2 chipamps, one per channel, if so, *maybe* you can replace the bad one and save your amp, but from now avoid turning it on.
Search here "lamp bulb limiter" and build one, you'll need it later.
Good luck.

[mexicanyella]

SR2,

If the Acoustasonic Jr. you bought is a pre-DSP, spring reverb one, here's the schematic, from a thread I posted a month or two back...

http://support.fender.com/schematics/guitar_amplifiers/Acoustasonic_Jr_schematic.pdf

Good luck with the repair; I think these are pretty cool amps, including for non-acoustic uses...but I'm kind of weird like that.

[srussell2]

Morning all,

Looking at the schematic and using the diode test function on my meter, I get 0 volts from the base/collector and collector/emitter so I guess Q29 is shorted?

[srussell2]

OOPS! I'm sorry...thank you both for your replies. Any recommendations as to where to purchase components? Thanks again...

SR

[Roly]

Morning all,

Looking at the schematic and using the diode test function on my meter, I get 0 volts from the base/collector and collector/emitter so I guess Q29 is shorted?

Yes; with it isolated you should read somewhere between "500" and "700" (mV) for a good device.  It should test as two diodes connected to the base, and infinity collector to emitter.

It is vitally important that you find all the damaged components before powering up (via a limiting lamp) or you will simply destroy the replacement components at well.

This particularly means the driver transistor associated with the dead transistor, and the output transistors partner to the other supply rail which possibly may now be open circuit rather than shorted.

If you can't be certain with the device still in circuit then you are going to have to disconnect it to be certain (and to replace it correctly, photos are handy for this).

You should also leave the speaker disconnected until you can see that the output (aka "half rail") is settling to within about 100mV of zero DC across the speaker terminals.  Anything more than about a tenth of volt out of balance needs to be investigated.

You also need to be very sure that you have corrected the original cause of the failure or you will be very rapidly back to square one.

[srussell2]

evening all,

just to close the loop on this one here's what happened. yes, i toasted the 2 mosfets on the right channel. also took out 3 zeners, 1 resistor and 1 driver transistor. also found on this unit were a couple other issues not related to my little screw up...the reverb tank was bad, it was missing 3 pot bushings. 1 pot was too damaged to repair so it was replaced, 2 were rebuilt. added new bushings and a new tank plus the component repair on the board and also resealed the cabinet. 175 bucks later and this thing sounds amazing...if anyone in the central va area needs a referral to an authorized fender guy, pm me. this guy is the shiz...guaranteed.

thanks to all the posters for your time and advice...you guys rock!

sr2