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Old 9th October 2010, 08:05 AM   #1
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Default My Fender Twin Amp Not Working - get your guesses in

Let's play a game.

Click the image to open in full size.

I was practicing tonight for a gig tomorrow at a local festival. About an hour into the practice (not running very hard and not playing constantly), my Fender Pro Tube Twin Amp stopped working. I don't have time (or really the knowledge) to troubleshoot and fix it myself, so I'm taking it in tomorrow morning in hopes that my local tech can get it working. Let me tell you about it and tell you what I've checked so far. Let's see who can guess the problem.

Seriously, the real problem is that I will have to use a solid state MG series Marshall instead of my tube amp. I will try to make it out alive.

For a while now, the amp has had an intermittent hum that isn't loud, but annoying at times. The hum volume was consistent regardless of the position of any volume/gain/tone/reverb/etc knob on the amp. Nothing made the volume of the hum change. The hum would show up when the amp was cold or hot. It would go a way whenever it felt like it.

When the amp stopped working, the hum was there, but no signal from my guitar could be heard. After a little troubleshooting, the hum is no longer there. I can not detect any sound at all coming from the speakers at this point.

I pulled the three fuses (main fuse and two under the balance & bias plate) and checked for continuity, but all three were good. I swapped every tube in the amp with old tubes I have that were still working just fine when I swapped in the TAD power tubes and JAN Phillips preamp tubes I now have.

I pulled the chassis out of the enclosure and visually inspected things... lots of capacitors and resistors in there. There was no obvious damage. I screwed everything back together.

The power tubes are about 4 years old and the preamp tubes are about 2 years old. I play somewhere between 6 and 8 hours a week. I drive the amp hard enough to sometimes need the full power setting, but I might be able to get away with the 1/4 power setting most of the time if I tried. I definitely don't run hot enough to overdrive the clean channel. I use both clean and gain channels. I use the vibrato occasionally. I never use the reverb. I use a few pedals from time to time to either get a different distortion sound or boost the level for lead work. Those are in line on the input. I have a multi-effects pedal I run through the effects loop, but rarely use it for anything other than a tuner.

That's about all I can tell you that might matter. So get your guesses in and see if you can call the busted part(s).

Here's a schematic of the amp if anyone wants to see it.
http://www.schematicx.com/view-schem...ier-schematic/
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Old 10th October 2010, 03:09 AM   #2
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I took the amp in today. The electronics tech guys were not there today. The guys that were there did some simple troubleshooting, like plugging an external speaker cab to see if that would work. That didn't do it.
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Old 10th October 2010, 04:09 AM   #3
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If the tubes and fuses were truly good, my next guess will be an electrolytic capacitor fried.
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Old 10th October 2010, 08:26 AM   #4
Arnulf is offline Arnulf  Europe
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Why guess ? Take a multimeter or an oscilloscope and measure along the signal path, working from the input onwards. First determine operating conditions for all the tubes, then trace the input signal (instead of strumming your guitar or using a signal generator you can use some other signal source, say powerline voltage stepped down with transformer at least 200-times assuming ~115V mains voltage, or computer/CD player etc. playing at very low volume - say at 1 or 2 out of 10).
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Old 10th October 2010, 02:00 PM   #5
DF96 is offline DF96  England
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Given your fault description, it was obvious that it could not be the speaker so I hope the techies there are better than their non-tech colleagues!

Diagnosing faults remotely is not easy. Guessing would only be useful if it were known that there is a common fault like this on that particular amp. Looking at resistors/capacitors can sometimes find a fault, but usually not.

You didn't tell us what troubleshooting you did to get rid of the hum, after the music faded. That might give a clue. Of course, you could have introduced an extra fault!
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Old 10th October 2010, 06:16 PM   #6
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Arnulf: Because you can't play a guessing game without guessing.

DF96: The non-tech guys answer the phones, run the register, and sell guitars. Well, one of them is the owner, so he usually just talks about fishing. The hum went away shortly after all the amp stopped working. It wasn't actually fixed. It may or may not have anything to do with the other issue.
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Old 10th October 2010, 06:28 PM   #7
49 - for the 18th time
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About da hum - was it at 60 cycles or something higher? If higher take a guess as to about what - say 500 hz - or sumpt'n like that.
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Old 10th October 2010, 06:32 PM   #8
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Here is a link to a schematic - is it the right one?
http://www.fender.com/support/amp_sc...ReissueE73.pdf
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Old 10th October 2010, 06:43 PM   #9
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The hum was pretty low. It might have been 60 cycle. It wasn't consistent at all and kind of sounded like it was boiling in and out at times.

That schematic is not the right one. I've got a link to the correct schematic in the first post in this thread. It's the newer Pro Tube series of amp, not the old Twin Reverb.
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Old 10th October 2010, 07:13 PM   #10
49 - for the 18th time
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Without a scope and signal source it's gonna be hard to find.
The most common problem I found with ax amps was the darn phone jacks getting broken down due to wear and tear. Usually the contacts wind up getting mauled and will no longer make or break connections properly - so check your pins and insulators on the phone jacks. (I do not have much experience with ax amps - mostly PA stuff and home stereo stuff). (Unless you wanna include a buncha Military and Government radar kinda stuff).
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