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I thought that might have been the case.
Originally Posted by Wildcat
I'd bet that ESD (static electricity discharge) found a sneak path from your instrument cable to destroy the ROM chip. They normally last "forever"... If the power amp is driven by the processor controlled by the ROM chip, that could also account for the blown speaker.
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08-11-2013 09:40 PM
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I too never had a problem plugging in or unplugging an amp. An amp that has a warning of this nature in the documents from the manufacturer I wouldn't buy. Originally Posted by docbop
Many amp techs don't want to diddle with SS because some SS schematics are not available, parts can be a pain to get or sub out and troubleshooting SS eats up more time than tubes. All a cost to the customer.
Add to that the possibility that the amp goes down again after repair for some unseen previous issue and guys like me don't want to deal with it.
For example I had an amp on my bench where the factory punched the holes in the heat sinks and did not grind off the little aluminum extrusions caused by the punch / drill. This caused the transistor to float above the heat sink relying mostly on thermal grease which did not work too well.
Even the best techs wouldn't pull every transistor to see if this type of manufacturing quality problem was there. So I don't generally repair solid state amps, unless it's something like a jack, speaker, or cabinet issue.
Unless the amp is very expensive I consider SS amps throw away.
Gary
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So perhaps a chain reaction of events ? How do you avoid ESD ? Originally Posted by TieDyedDevil
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Unless the amp is very expensive I consider SS amps throw away.
What price range, used, would you give for expensive? A friend has a Peavey Envoy that he bought new for $200 and worth $100 used. That would fall into the throw away category IMO.
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The JMUL is a great amp, worth the repair (imho).
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Originally Posted by jorgemg1984
I do agree and the purpose of my posting is to have record of what happened to the amp. Many times I see equipment being labeled problem prone and I do not want the JMUL to have a "perception" of being problem and repair prone.
My concern or to be noted to other owners is that the amp tech said that the stock speaker was built to Fender specifications by Jensen. He stated that several were only left in Mexico. Now I regularly use after market parts for my vehicles, but when it comes to speakers and a design like this I am not too sure that might be the case, but that is from my own ignorance for this type of thing.
My first thought was to replace the speaker myself. I have notice that the "odd" 2 ohms NEO speaker, 12 inch, are being made by Jensen and you can order it now. Almost several years ago when I first bought the amp I do not think that was the case as I had the thought in the back of my mine about needing another speaker down the road and recall not seeing the 2 ohm neo version.
To Fender, you folks got it right with this "non-typical design" for your product line. I would love to hear from or the story behind the team or person(s) that came up with this beauty. So one thought I leave behind for consideration on this product and that is, "resurrection".
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I still think Fender made some errors with the development and the marketing of this products... Quilter's success proves that, everyone wants a light Twin Reverb!!
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Perhaps. My theory is just that: a theory... No way to prove it without blowing up some more amps. Originally Posted by Wildcat
ESD is most prevalent when the humidity is low.
Certain kinds of fabrics and synthetic fibers are also prone to static buildup. Polyesters, for example...
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Not to dismiss the Quilter, which does look intriguing... (I have yet to see or hear one. Their dealer network is still very small.) Originally Posted by jorgemg1984
There are plenty of lightweight, high-powered amps from which to choose. At "polite" volumes, many of them are quite adequate. I'm not sure that many of them were created with the intent of replacing a Twin Reverb, though...
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With that intent I am just aware of the JMUL and the Quilter... the others I know are "jazz amps". The Evans also seem to be able to get that sound but are VERY expensive.
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Reviving an old thread, here; @jorgemg1984, do you by any chance know which part of the circuit your tech replaced the caps on, please? Ie, power supply, preamp boards, power amp board?
I've acquired a second-hand JMUL with the noise probs and fuzzy distortion on both channels is making me suspect a power supply issue.
Interestingly, the noise issue appears to be partial RFI sensitivity and varies depending on the physical orientation of the guitar to the amp; certain positions and angles, it's dead quiet. Others, it's noisy-as. This is with a shielded humbucker guitar that has no hum issues on other amps.
Also what wiring re-routing did he do (if you know), please? It's a very clean layout in there, I'm struggling to see what options, if any, there are in that sense.
Thanks heaps!
Paul
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Hi Paul,
I will PM you my tech's email and you two talk, I am not of much help here... hope you can sort it out!
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Legend; thank you so much for that! Will post findings if he's ok for me to do so (technically it would be taking potential work away from him). Originally Posted by jorgemg1984
Cheers, Paul : )
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Ok, here's an update: The amp is back up & running, thanks to assistance from jorgemg1984's technician and some subsequent intuition...
Turns out that the main distortion issue was actually a stuffed speaker; although it was still able to produce sound, somehow the coil former had tightened around the pole piece such that it was causing a physical distortion of the sound. Interestingly, the speaker has a neo magnet and its smaller diameter, combined with venting holes in the frame, allow a clear view of the coil & former, so I was able to confirm the problem. At very low volumes, it didn't cause a problem (presumably due to the minimal coil movement), but at higher volumes the problem became quite apparent.
Unfortunately I only worked this out AFTER putting some time & effort into replacing the electrolytic caps on the power supply as well as the one electrolytic (c19) on the clean preamp board; I suspect in hindsight that all of this may not have been necessary (!!). At least the caps are new now, though.
The clue for the speaker problem was that as the signal decayed, it didn't tail off into silence but instead cutoff abruptly, suggesting physical friction. I swapped the speaker out for an Eminence Canis Major that I'd been saving for a cabinet build and all was good from there (not to mention not having to buy/build the new cab!). Although some suggest that the 2ohm speaker is optimal, it still sounds pretty darn good to me (although I might look into costing a repair of the original for the heck of it).
In terms of hum: I full dismantled the amp and fully shielded the internal walls with copper shielding tape, as well as put in a shielding wall between the preamp & power supply circuitry using some copper clad pcb (see pics). This improved things somewhat (I no longer get hum using humbuckers (!!!), although the amp remains sensitive to single coil hum, although I suspect that my study is a pretty 'noisy' room to begin with. As per prior to the addition of shielding, playing with the physical orientation of the guitar to the amp continues to produce different results, albeit less sensitive; some spots are bad, others are null patches where there's virtually nothing.
Lastly, I also swapped out the existing TL072 op-amp's for OPA2134's (and put in sockets in the process); this made an improvement on the clean channel's clarity. Funnily, though, the oft-suggested improvement for swapping out the volume cap (C33) made no difference whatsoever; it's still as touchy as ever in the first few degrees of rotation. Oh well; still works, though.
Overall, this was a fun repair that had a happy ending. Even more so in that I got the amp cheap because of the problems. I'd also like to very much thank both jorge and his technician for their kind time and willingness to give their time and knowledge to help out a complete stranger. So, if you're in Portugal and need a guitar/amp tech, do look up Eduardo Krithinas of Quantik Repairing Service! I'm sure he'll look after you.
Happy playing!Last edited by swingingoodtime; 03-01-2015 at 09:21 AM.
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Hi man, been meaning to PM you because I was curious to know how it had all worked out. Glad to see you found a solution!! The OPA2134s are a clear improvement; weird about the volume mod... I found a profound difference and others have too.
Well I am glad everything worked out. Eduardo is the man, really!!
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