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Hi folks. I am appealing to you Fender Warranty techs out there since this is a popular amp and by looking online it seems to be a common problem. I have checked the archive here and the problem is discussed but never resolved, or at least the fix was not mentioned. When you power the amp on it has a loud crackling noise and then louder pops (I mean like a gun shot, lol) at random intervals. Sometimes it will settle down. When it does the amp sounds fine, all functions. If you jumper the main/out pre/in jacks it still does it. It appears to be in the output amp or power supply section because if you plug into the main amp jack it still does it. The Enzo test has no effect nor does tapping on the board with a hard implement. It does not seem to be thermally related since it comes and goes. I have done the obvious things you usually do to this amp. Re- soldered the filter caps and any other iffy looking connections like the power amp emitter resistors and such. I am about to jig it to a dummy load (so it won't blow a speaker or my eardrum) and start some serious trouble shooting with some freeze and a scope. The thing is, it is intermittent. I was just curious if there is a bulletin out on this model or something. I was really surprised how good this amp sounds (when not acting up) when I was testing it. One of the best sounding SS amps I have played. Any response would be welcome. You are welcome to PM me if it is not acceptable to post proprietary information on an open forum. I am a very experienced (but retired) bench tech.
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Is that a difamp pair of transistors to the right of the main amp in just pass the op amp? I can't read the designations on this schematic. -
Maybe this will be more readable.Originally posted by Enzo
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Yeah, that is a diff pair after the TL072 opamp.
Starting at PA IN & working inward, I would see where the noise is starting at.
The TL072 output pin should be quiet.Comment
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Thanks for the schematics. Yeah looks like Q9 and Q10 are the diff pair DC coupling the power amp. And the symptom sure acts like a drifting diff amp. I'm going to just unplug the speakers and put the scope on it. I guess nowadays a new couple of 2SC2362Ks off the shelf would have close enough matched betas? You don't have to request or test a beta matched pair anymore?Comment
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There are no service bulletins on that amp, I believe that it is still on the do not repair list.Comment
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Thanks Bill. I really appreciate you taking time to research it. But what is the world coming to? From what I read, these units have a 5 year warranty. They are a 100 watt amp with a spring reverb and two 12" speakers, and it's on a no repair list? They just exchange them? For 5 years? The amp seems fairly easy to service. Single sided board. Guess I'm getting old!Originally posted by 52 Bill View PostComment
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DO NOT REPAIR list refers ONLY to whether Fender will pay the service center to fix it under warranty. It has ABSOLUTELY NOTHING to do with whether or not they can be repaired. Yes, they want to exchange them instead while under warranty. The vast majority of them, like any other amp, never need replacement/service.
Ease of service doesn't usually enter into it. I just repaired a guy's Mustang amp yesterday. I did it for free. To me it was easier to take the little chassis out, resolder the jack, and put it back together. Officially I would have had to keep the amp, call Fender and order a new one, then when it came, contact the customer again and then pack his old amp into the box, print out the call tag and take the thing to FedEx. And then file a warranty claim for handling the swap For the 6 or 7 minutes it took, I'd rather just fix the damn thing and be done with it.
The list is ALL about the value of the unit to Fender. (Or whatever company's list we are discussing)Education is what you're left with after you have forgotten what you have learned.Comment
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Wow... I haven't been a bench tech for a living in 10 years. Manufactures had no repair lists, but it was usually for headphones, remotes, sealed speakers and such. But a 100 watt combo amp? Must cost them nothing to make nowadays. Lol. I have to drop off my Onkyo receiver for warranty repair today at a little authorized shop. I hope it gets exchanged! Btw Enzo, do diff amp transistors have to be matched nowadays or are the tolerances now so close it doesn't matter?Comment
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I haven't seen matching diffy pairs in the last 25 years.
If I replace one, I usually replace both, "just because."Education is what you're left with after you have forgotten what you have learned.Comment
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Thanks Enzo. In a shop situation I usually would have a dozen or so transistors and use the 2 out of the pile that were the closest. Guess the tolerances are close enough it isn't necessary anymore. But yes replaced as a pair definitely. They're 80 cents each, lol. Thanks for the input.Originally posted by Enzo View PostComment
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I've got the same symptoms in the same model amp.
Some poking around has led me to suspect the differential input transistors too.
Replacement diff transistors don't appear to be available.
That's the 2SC2362K.
It's high voltage (120V max Vce) and fairly high hfe (min 160).
Nothing I've got quite matches that.
Closest I've got is the MPSA06, 80V max, 100 hfe min.
Would it be foolish to sub in 2 of those in this circuit?Comment
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I haven't ordered any yet because I will be on Kauai for the next 10 days and had to set it aside, but I found them at MCM for $.80 apiece. Seems to be a pretty common issue.Originally posted by dwmorrin View PostComment
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I always forget about MCM. Thanks!Comment
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Yeah, I would use an exact replacement for a diff pair. That's why I was asking our sage about tolerances nowadays. Even if you found a suitable sub it might be problematic if the tolerance wasn't as tight.Originally posted by dwmorrin View PostComment
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