Stage 112se amp issues - revisited?

[musiclover50s60s]

Hello All,

I've read many hits of the threads about a Fender Stage 112SE amp and it's many woes and repairs. I found all the info insightful and the techs helping out - willing to lend a hand.

I initially presented this issue to a member (Enzo) privately, he asked I post publicly, so all can learn. So - here I am.

I acquired the Amp aforementioned after it shut down. I was "told" it started humming loud, no controls worked, etc.. - then it shut down. This after a microphone was laid in front of it or something to cause feedback from music playing through it or some such hook up. Once opened - the fuse was not blown. I double checked it with the schematic to be sure it was the proper fuse, it was. Then I seen R75/76 fried - a huge burn mark under them - almost through the board. The PC traces which connect the resistors on both ends - cooked.
I took the suspect capacitor out - C49 .1uF and tested it both with my DVM which showed .104 on the 4 uF range and being I like to double test - I did an ohms test - it was definitely not shorted. I'm wondering - I know Enzo said it was because the amp was oscillating. The transistors "seem" to be testing ok. I wonder - if I simply replace the two resistors and pads/traces, if that may be the only repair - or should I be looking for other parts that "may" have gotten hit as well? I've never run into "oscillation" issues before. Always been goofy stuff - tubes going bad, transformer giving up the ghost, etc...

I'm curious - I read somewhere that "feedback" doesn't hurt an amp. True or false? Because that is basically what I am told was going on before the smoke session. Would "that" have caused the oscillation?

Any and all info is greatly appreciated. Thank you in advance to all who pitch in with replies.

[Enzo]

Feedback is a generic term, like "speeding" in a car. In a car both 5 miles over the speed limit and 100 miles over the speed limit are both speeding, but one is dangerous and the other not so much. We get feedback all the time in guitar amps, but if you plugged a mic into it and placed it in front of the speaker, you'd likely drive the amp to its max and leave it there. Not the best thing for an amp.

When I was thinking of oscillation, I was thinking more in terms of the amp being unstable for some reason and cranking out RF for all it was worth. Those resistors and cap are there to dampen off any such signal, or die trying. Such oscillation is technically feedback, but not in the sense of leaving the amp and going back into its input. The frequency is way above hearing, and the speaker won't reproduce it anyway.

All we can do is replace the burned resistors and see what the amp does then. I'd run the amp without a load until I knew it was stable, then with a load for testing. Without a scope it will not be as easy to see RF oscillation if present. You could look up "RF Probe" and see various examples of this very simple circuit to stick in front of a volt meter. But a scope is better. Lower wattage resistors would work for a demo to see if the amp is unstable, but as soon as they have to do any protecting, they can be taxed. If they are 2w and you have none, 5w would be fine if you have them.

When the amp is idling, we also want to watch the heat sink. If it starts to get quite warm, turn it off so we can suggest further action there. A current meter to watch the mains draw will also expose that.

[musiclover50s60s]

OK, I replaced the two resistors (R75/R76) and reinstalled all, including the transistor heatsink bar (all secured). I went to test the amp at idle. But the fuse blew before I turned the amp on. I had it plugged into a multi-strip which was off as was the amp. Once I checked all, I switched the strip on, and before I could switch the amp on by it's own switch, the fuse blew.

From my knowledge, it appears the Transformer is shot (Primary winding).

Initially, when I got the amp, the issue was thought to be a possible feedback issue - to fry the two resistors. I didn't plug the amp in to test further with it already damaged. When looking around to find the trouble of the amp to begin with, I replaced the resistors, checked the transistors (all tested ok - no shorts). I tested some other parts too while looking, and all were ok. I didn't disassemble the switch wiring or anything else not necessary to pull the board out enough to work on it. Of the wires I did pull, I labeled every wire as it was removed. I double checked it with the schematic as well. So - I know the wiring is correct. The night the amp went out, though initially thought to be a possible "feedback issue", there was a storm, a couple other devices on the so-called surge strip that it was plugged in to also went out. I doubt, given the age of this thing - a "92", that a transformer is to be found.

Any ideas on a replacement transformer? I doubt with this being a 92 model, there's one out there. I feel I've hit the end of the trail for this job. I will try contacting Fender - but my initial email has yet to be replied to. Thanks for the help in any case!

Feedback is a generic term, like "speeding" in a car. In a car both 5 miles over the speed limit and 100 miles over the speed limit are both speeding, but one is dangerous and the other not so much. We get feedback all the time in guitar amps, but if you plugged a mic into it and placed it in front of the speaker, you'd likely drive the amp to its max and leave it there. Not the best thing for an amp.

When I was thinking of oscillation, I was thinking more in terms of the amp being unstable for some reason and cranking out RF for all it was worth. Those resistors and cap are there to dampen off any such signal, or die trying. Such oscillation is technically feedback, but not in the sense of leaving the amp and going back into its input. The frequency is way above hearing, and the speaker won't reproduce it anyway.

All we can do is replace the burned resistors and see what the amp does then. I'd run the amp without a load until I knew it was stable, then with a load for testing. Without a scope it will not be as easy to see RF oscillation if present. You could look up "RF Probe" and see various examples of this very simple circuit to stick in front of a volt meter. But a scope is better. Lower wattage resistors would work for a demo to see if the amp is unstable, but as soon as they have to do any protecting, they can be taxed. If they are 2w and you have none, 5w would be fine if you have them.

When the amp is idling, we also want to watch the heat sink. If it starts to get quite warm, turn it off so we can suggest further action there. A current meter to watch the mains draw will also expose that.

[g1]

If all the primary wiring is correct, and the power switch is off, there is no way the transformer can cause the fuse to blow. Blowing fuse with power switch turned off could be caused by a few different things:
defective power switch
AC cord wiring (or primary wiring) shorted to chassis
incorrect wiring

One very important point: you were given the amp in non-working order, you can not assume wiring is correct just because you made sure you put it back how you got it. Someone else may have worked on it, or you may have not heard the real/whole story of what happened.
Have you checked ALL the primary wiring against the schematic? If it is correct, and the power switch is turned off, then the power transformer is disconnected and cannot blow the fuse.

[musiclover50s60s]

I stand corrected.....

I was a bit rushed in my last post and didn't really get time to get back to the amp until today. Yes, all the wiring "was" correct. AND - the switch tested ok. Now for the real ringer.....

Most equipment I work on - the switches either are push button, turn one way or another or switch "UP" or to the side for "ON". Once I found a print out of this Amp's owners' manual and "really" looked at the schematic strictly at the AC input side, I see that "ON" - is "downward". The "ON" marking on the amp is worn off (the manual shows it to be silk screeded on the case). So, I just - yes that dirty word, "Assumed" down was "off". Thus, in reality, in my last post, when I said the amp was "off" and blowing fuses immediately, it was actually "on" when I hit the surge strip switch to on. So - I'm still thinking the AC Transfomer is fried. I checked all wiring on both sides, all is as it should be.

My apologies for the confusion! As they say, haste makes waste, and yes, I wasted my time posting in error (and yours for replying).

I contacted someone for a transformer, they said available to be ordered by certified dealer only, which I am not. So - I think I'm going to give this back to the party and let them take it to a Fender Dealer - to save him any more cost. Aside from 2 resistors and a couple fuses, I'm only into it for time (not big deal!).

If all the primary wiring is correct, and the power switch is off, there is no way the transformer can cause the fuse to blow. Blowing fuse with power switch turned off could be caused by a few different things:
defective power switch
AC cord wiring (or primary wiring) shorted to chassis
incorrect wiring

One very important point: you were given the amp in non-working order, you can not assume wiring is correct just because you made sure you put it back how you got it. Someone else may have worked on it, or you may have not heard the real/whole story of what happened.
Have you checked ALL the primary wiring against the schematic? If it is correct, and the power switch is turned off, then the power transformer is disconnected and cannot blow the fuse.

[g1]

Ok, that makes much more sense. Still to early to condemn the transformer though. It is one of the most expensive single parts, so you don't want to replace it unless you absolutely have to.
Also, when power transformer fail in amps such as this, they usually go open rather than short, resulting in no power rather than blown fuses.
So one more thing you could try before passing it on. Note where all the secondary wires connect to (make a map) and disconnect all secondary leads of the power transformer. Power it on and see if the fuse blows.
If the fuse does not blow with the secondary leads disconnected, you know it is NOT the transformer causing the fuse to blow.