Wednesday 16 July 2014

Self switching channel FENDER TWIN AMP 94 - the "EVIL TWIN"

I know this is a issue about this guitar amp. I've done some research on the Internet to find the problem before I proceed with the investigation of the amplifier.

Unfortunately I couldn't find something to give me an exact localization and solution of the problem. Again, I hope this post will be useful for someone.

CAUTION!!! Inside the amplifier there are VERY high voltages even after unplugging it from the outlet. If you're not sure you know how to handle an open amplifier, let a professional do it.

Subject: FENDER TWIN AMP '94 "EVIL TWIN" guitar amplifier

Symptoms: It switch itself from normal to drive channel for 1 second or less at very loud peaks.
This is one of those guitar amplifier that sounds the best. The problem is that it is too loud and heavy. Being so loud and because it's a combo format, the big vibrations of the loudspeaker affect all the components inside the amplifiers. So many old amplifiers have problems because of that.
This amplifier works normally except that at some loud peaks the normal (clean) channel switches itself to the drive channel.

Problem solved: YES
The first thing to do was to test it, to see how it was switching. As in 90% of situations, when I arrived at the owner's place, the guitar amplifier didn't want to reveal the problem. No matter how loud it was, he performed without the issue. It's like when you have a bad tooth ache and the pain stops just when you arrive to the dentist's.
 So I decided to open the amp and simulate the vibration with the wood stick test mentioned in the previous post. 

But first, with the help of the schematics I locate the component that was doing the job of switching the channels. I measured the voltages with the DMM to is if are as the schematic indicates.

Voltages were very good. As you can see in the video, taping the all the PCB was very sensible for switching the problem.  My guess was that could be a cold / broken soldering connection. It was a good guess :D.
Because there was no visible connection problem, I reinforced all the soldering on that PCB, the amplifier stops switching itself and the job was done.
Conclusion: When servicing, and not only electronics, it is good to have a detective's eye. It's very useful to know some history of the product and the conditions of use. Many combo guitar amplifiers that I know have some problems derived from vibrations. Vibrations have two sources: transportation and loudspeakers' use.

Those vibrations affect two main components over time: low quality soldering and the vacuum tube filament.

Hope this is useful for someone that encountered the same problem.

Good luck!


  1. Thanks SO much for this post. Just got one with this exact issue and your post fixed it. Much appreciated!

  2. This has now helped 2 people, my '94 twin has the same issue. After spending some time with my daughters boyfriend gigging a few time it came back with a bad tube. Once I replaced them the lead channel popped in for a second or 2 twice. Thank you, now I just have to find someone to fix or try it myself.

    1. I managed to find\fix my issue using the same method of tapping, my issue was not the dual op amp but the 9 pin ribbon cable connector coming from the pots to the board. There were several cracked pins that could only be seen with a magnifying glass. The board is difficult to position without desoldering it, but I managed and now have my amp at 100%. Thanks again.

  3. This method worked for me, however my issue was in the power on lamp.... the connections were loose. Almost like a loose ground connection. Once I tightened it, all the static and switching issues went away and the light itself was a brighter and solid. It really doesn't make sense to me unless the grounding was somehow tied into this.... anyway played the heck out of it and works 100% now. Thanks

  4. Hello, a question... I know you have to wait 30 minutes unpluggede the amp so you can work with it... but if I want to reinforce the soldering... I have to do anything special? because people tell me that into the amp maybe we have some high voltage.

    Thanks for your help. I have same problem and I'd like to solve.

    Greetings from Spain

    1. Miguel, just be very careful. The big capacitors can hold high voltage for a relative long time. They need to be discharged prior to work inside the amp. There are a plenty of videos on youtube on how to do that. If you are uncertain, just go to a experienced technician. Don't risk your life for this.
      Thake care.


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