[TROUBLESHOOTING - Fender Evil Twin]

Discussion in 'The Squawk Box' started by Kuroyama, Aug 16, 2009.

  1. Splattle101

    Splattle101 V.I.P. Member

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    I am wondering if the scratchy / static noise you're hearing is not DC leakage through failing coupling capacitors in the preamp?

    As far as the clean channel sort of works / dirty channel doesn't at all is concerned, I'd try doing valve swaps between the preamps. This amp has two separate preamps with their own preamp valve and tonestacks. If the clean channel is (nominally) working, you could take that working valve out of V1 (preamp for clean), and put it in V2 (first preamp for dirty) and see if your dirty channel comes to life.

    I don't know the rest of the layout in your amp, but that's how it would work in a Blackface Fender. After a quick look at the schematic, the only valves that are unique to the dirty channel are V2 and the first half of V3. All the rest are in both channels. So I'd try swapping V1 and V2, and then V1 and V3. Not that I think there'll be a miracle, but it might help narrow the problems a little.

    [EDIT]In fact, I'd buy two new 12AX7s. Good ones. GTs or something. You will need them anyway (if nothing else one of them will replace your old phase inverter, which will need it after 13 years), and it will give you a sure fire test. Put one of them in V3, which is half in the dirty channel and half in both, and the other in V2 which is preamp for the dirty channel, then see if there's any life there.[END EDIT]
     
  2. Cygnus X1

    Cygnus X1 Senior Member

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    I'm not offended!
    :D
    Do the tube swap as suggested as well as the caps.
    Take your time, you'll come out much better in the end.
     
  3. bad261

    bad261 Senior Member

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    For what it's worth, this is a great thread.
     
  4. SatelliteAmps

    SatelliteAmps Senior Member

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    If the noise follow's the volume pot, then the noise issue is before the volume pot.

    You can try moving the tubes around in the amp, if you don't have any spares. If the issue changes, then it is tubal. If it doesn't, then it probably isn't tube related.

    Staticky noise (crackle) can be a number of things. Bad tubes, dirty sockets, dirty or worn out control pots, cold solder joints, intermittent filter cap, or possibly a failing bypass cap. Fix the stuff you know is wrong, and clean everything, and then if the problem is still there, start checking solder joints. (Anything made with a solder bath has this potential. It's a pain in the butt to track down. To be honest, the easiest thing is to just go through and reflow every solder joint on the board, just to make sure they are all good).

    Channels not switching are usually a bad transistor. There is a transistor (or possibly an OpAmp IC), that controls the switching. These will occasionally go bad, and stick in one position. You can check the voltage coming out of it when you try and switch channels. If it doesn't change, then there is the problem.
     
  5. Liam

    Liam V.I.P. Member

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    Just a thought. Staticky noises can also be bad relays, and I notice from the schematics that in common with some other Fender channel switchers the Evil Twin uses relays K1 and K2 to do the switching. I wonder if this could be the same reason as channel 2 being dead....

    If it were me I'd pull the relays and hard wire around them, or even just bridge them while I was troubleshooting, just to be sure.

    Another a good thing to note. The Fender schematics with all the test point voltages on them are great. The dc voltages (in the square boxes) you can just check with a multimeter and learn quite a bit. To do anything with the ac voltages in the oval boxes you need to inject a 1 kHz sine wave at the specified level. If you can find a signal generator with which to do this you will also want a dummy load, as it will be deafening otherwise.

    From the schematic that is a VERY cool looking amp, and the last thing I'd do would be to turn it into anything else. Reminiscent of the Tonemaster, one of my all time favourites. But the switching system is a right old can of worms waiting to be opened. Might take some troubleshooting if it's anything to do with that.

    Liam
     
  6. randelli

    randelli Senior Member

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    +1000 I like your attitude and agree with you.

    I am sorry, but it appears as though I left out the crucial sentence in my post. I meant to offer that it may not be a component(s) causing the issue; but it could be a burnt or shorted trace on the PCB. This would be very diffucult to track down; but might be less expensive than replacing multiple parts to find it later.

    GOOD LUCK with the repair!
     
  7. Kuroyama

    Kuroyama V.I.P. Member

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    Randelli

    I apologize if I came off as indignant with my reply. I meant what I said, but I dont mean to infer any sort of negative tone. I should go on to say that I appreciate the warning about carelessness inside a tube amp. Cyg, and Splat were VERY clear in their warnings early on. While Ive always had a healthy respect for firearms, my earliest experiences with rentals, then my own taught me that not everyone has that level-of-respect-for-things-that-can-kill-you. So while I am far from "experienced" at this, I suppose it would be wise in my writings here to be clear about the dangers involved. I wouldnt want anyone reading through this in the future and thinking: "Hey, if HE can do it, so can I!"

    A person should know their strengths. From doing design work, building model aircraft, working on cars and motorcycles, to never understanding whats so difficult about surgery... I understand that Ive got fairly steady hands which are a good thing to have here. The remainder lies in knowing what and how to touch.

    So as a warning to anyone else who might be thinking to buy a "junked" amp, and try to restore it themselves... just think about the resources and skills that will be involved. If you are willing to bet your life that you have them, then this may be for you. The other end of that is that sending it to a pro might only cost a few hundred dollars. What is your LIFE worth?

    [ame=http://www.youtube.com/watch?v=1lozkT1zrIU]YouTube - Kendrick Amp guy electrocutes himself in demo video[/ame]
     
  8. SatelliteAmps

    SatelliteAmps Senior Member

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    Sounds like you have the right attitude about this.
     

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