Acoustasonic SFX II Problem...Schematic?

[JoeC]

I'm having a problem with my Fender Acoustasonic SFX II. I have very limited electronics knowledgeable - but it seems like it may be an easy fix with the right guidance.

The amp works and sounds fine on both channels, but about every 3 or 4 minutes it will cut completely out for about 2 seconds, then come back on and function fine for another few minutes. This happens on both channels. I've tried different guitars and cables to eliminate those as the culprit - I'm sure it's the amp. The power light stays steady when this happens - I haven't opened it up yet, but I was wondering if anyone has heard of this problem before or know what could be causing the problem.

Thanks for any tips or advice.

Also, any idea where I could get a schematic for this amp?

Thanks
Joe

[JoeC]

Update - I found a schematic at the Fender website. The problem seems to appear only after the amp gets hot. My amp gets extremely hot - when I unplug the guitar cord, even the connector is hot to the touch - is this normal?

-Joe

[Enzo]

No not really. It might be thermalling on you. We'd want to look at why the power amp was overheating. Does it get real hot sitting there on but not playing through it?

[JoeC]

Thanks for the reply Enzo.

I left it on all night, I would say that it gets warm - even the guitar plug is warm - but not hot. It gets hot when I use it.

-Joe

[JoeC]

What does "thermalling" mean?

-Joe

[Mark Black]

"Thermalling": Amp repair talk for getting so hot it trips a thermistor or thermostatic based shut-down circuit. Some amps have 'em, some don't.

[JoeC]

Hmmm... Would a thermalling circuit shut it down for only 2 seconds at a time?
I played it again last night at a very low volume - and the amp starts out working fine - about 15 minutes later it's cutting out every couple of minutes for 2 seconds at a time.


I've heard that these amps run hot, but I'm not sure how hot is considered normal operation. If the heat of my amp is normal - then is it possible that something in the thermalling circuit is too sensitive? Is there a pot to adjust?

-Joe

[Enzo]

No, the thermal management is not adjustable.

Just for grins, monitor the DSP reset on page 18. Does it toggle when the amp cuts out?

On page 16 is the power amp curcuits. In the center is the power up circuit. Is there -34v at TP14? Does that voltage toggle as the amp cuts out?

Are the DC voltages around the drivers and predrivers pretty close to those on page 16? We might need to turn on Q15,16 a little harder.

On page 15, lower left is the mute circuit. Q41, is TP17 toggling when the amp cuts out?

[JoeC]

Enzo - My schematic only has 5 pages. Is yours a more detailed one? It seems as though it probably is...
If so, is it available online?

Last night I took the amp apart and touched up some solder points (even though they all looked good) and put fresh heatsink compound everywhere it could use it... I put it back together and left it on all night. Today I played for about an hour and it didn't cut out - but I'm not certain it's fixed. Sometimes it took over an hour of playing for the problem to start. If it starts acting up again, I'll do those tests you suggested.

Thanks for the help
Joe

[Enzo]

I took it from the factory issued service disc. Yours probably skips the several pages of parts lists and stuff. Just translate my page numbers to the DSP drawing for page 18, the power amp drawings for page 16, and the preamp drawing for page 15.

[Enzo]

I took it from the factory issued service disc. Yours probably skips the several pages of parts lists and stuff. Just translate my page numbers to the DSP drawing for page 18, the power amp drawings for page 16, and the preamp drawing for page 15.

[bobberino01]

I am having the exact same problem. Did you ever figure out how to fix this issue?

[RichT777]

Acoustasonic cut out!!!

Hello Gang,
I guess it was my turn! One guy in 2007, another in Nov. 2008, and now me. Any success for any of you in getting things going again? Mine is under warranty. Does anyone know if Fender is claiming to understand what the problem is? Thanks again to anyone out there.

Rich T.

[Enzo]

I doubt Fender even considers it a problem. I mean that in the sense that this sort of thing can happen to any amp model, we get such complaints about most any amp eventually. And of all the thousands of these amps, we have aparently seen three go down with it.

These amps are made with the same parts as all their other amps. An FX loop jack on this is the same one used on some other model. The same manufacturing processes are used for all their stuff.

If they brought out an amp and 10% of them all had the same thing go wrong, that would be serious. Those overheating resistors on the Hot Rod series come to mind.

SO the same thing might be wrong in all three amps, but chances are it is not really a systematic problem.

[Terrill]

I am now having the same issue. It is interesting because this never used to happen. Please post if anyone knows anything. Thanks