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Fender Evil Twin Red Knob help please

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  • Fender Evil Twin Red Knob help please

    Hi all,

    Anyone familiar with Evil Twin circa 1988 amps??

    I've been fixing some broken pots for a friend, have been inside one of these before but am a beginner with electronics (a brave one)....

    Anyway, reverb wasn't working and a few pots had got smashed off so I replaced the pots and found reverb connection problem easily enough, all fixed.

    While I was in there I noticed that the R128 ceramic 2.7k resistor had been replaced mistakenly by someone who had put three 8.2 resitors in parallel, yes, thats 8.2 ohm, so that must have been about 3ohms instead of 2.7k. I had the right one lying around from fixing an identical amp years ago, so I replaced that and re-biased the valves too. (spotting this resistor being wrong I don't know what it could have done to the amp but some squealing bezerk feedback had happened before with the guitar turned completely down - or so my friend told me, and he's wondered what that was.)

    Anyway, here's the problem, I put it all back together and hey presto 'nothing comes out of the clean channel'... the switching to the channel is all fine, thats happening and there's the familar 'background buzz' through the speakers just as on the dirty channel that this amp always has but nothing is coming out, turning the volume control up doesn't do anything.

    The clean channel volume pot wasn't one of the pots touched either

    Can anyone point me at anything I can look at that might be a likely cause of the problem, with instructions I'm pretty good at checking thing out and have a good multimeter to hand too.

    Help/advice appreciated.

    Cheers

    Mark

  • #2
    I'm also wondering whether it could be a pre-amp valve thats been disturbed?? All are glowing... I know there's two 12at7 and five 12ax7 but I don't know which ones are the exact ones involved in the clean channel, if I did know then I could swap them all around to elimiate that possibility very easily ???

    Its very difficult for me to tell from the schematic I have (its a fourth generation photocopy by the looks of it, and, eve if it wasn't I'm not sure I could work it out myself).

    Help would be greatly appreciated, I don't really want to have to drive this weighs-a-ton amp on a 120 mile round trip to the nearest amp repairers, probably twice if I have to leave it with them.

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    • #3
      Schematic
      http://www.schematicheaven.com/fenderamps/the_twin.pdf
      Not the best quality scan.
      Are you sure that the guitar signal is getting the first tube? The input switching arrangements are complex and the switches (built into the jack input sockets) might have failed. Check that you can measure a signal at pin6 V7B plate, the 1st tube in the clean channel. Then V7A plate pin1, second tube. That's a good start.
      Shorting out the 2k7 R128 might have overheated R129, the 30k bottom half of the potential divider, so check the power supply voltages against the schematic, A, B and C will be high if R129 has gone open circuit. Peter.
      My band:- http://www.youtube.com/user/RedwingBand

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      • #4
        Thanks for that, I'll have a look at the things you've pointed out later on and let you know how I got on. Cheers. Mark.

        Comment


        • #5
          A few tips

          Also if you've done some work in an amp, and something doesn't work afterwards that (sort of) did work before, then it was because of something you did.

          So retrace the work that you did. If you are a beginner, you might not have soldered the component properly to the traces. Hold the pc board upto a light, if you can see light coming through holes where the component go through the board to the trace on the other side, carefully flow some more solder on till you've closed up the holes

          Or you might have torn one/some of the pcb traces when taking an old component out (esp if you didn't de-solder them properly first before removing the components). The hole terminals on those little traces are delicate and can easily get damaged or torn if mis-handled, which will affect the circuit. Go through the (switched-off and unplugged!) amp with your R meter and check that you have connections from each component you worked on to the other nearby components via the traces you worked on. If you suspect a damaged trace, de-solder and remove the component, and inpsect the trace without any solder on it (testing it for continuity with your meter). If you can see the trace end around the hole is damaged of lifted off the board, or just plain dissappeared, then carefully scrape some of the silvery covering off the trace next to where the hole is, to expose a couple of mm of copper underneath, and when you re-solder the component back in, make sure to flow some solder over that bare bit of copper.

          Keep the hot tip of your soldering iron clean between soldering each component, by wiping the tip on a wetted towel. This helps keeps the solder joints good. Also, use some proper de-soldering braid to completely remove the existing solder from the traces/components with your hot iron, before you take the old components out.
          Building a better world (one tube amp at a time)

          "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo

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          • #6
            Thanks for the replies both of you. Both of you helped me find it quickly.

            I got underneath the front facing board with all the pots on it again and looked straight for the connections to the first valve, looked closer and that exact first wire where I had to check the signal as per instructions had broken from the pcb when I'd turned it back over putting it back together after all my 'new pots' soldering job. I only just noticed it as it was hanging in exactly the right place. Feel a bit thick for not looking for something so obvious!!

            Thanks to both anyway, I could have been looking for that for ages.... saved me a lot of petrol and mucking about for sure. Cheers.

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