Help wanted diagnosing no sound from Red Knob The Twin.

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Seanosaurus, Jun 10, 2019.

  1. Seanosaurus

    Seanosaurus Member

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    1993 Fender Red Knob The Twin--The sound just went completely at a gig, no smoke, no noise or bad smells. Powers up, (red light comes on) tubes all glow.... take it off standby..no sound whatsoever(nothing). HAVE checked all tubes (power and pre) and they all work in another amp, checked tube sockets for debris and cleaned best I could, tried various instrument cables in all 4 inputs,, checked speakers with another amp.. (bypassed preamp) plugging directly into power amp through and effects return... nothing. No reading change from multimeter in external bias test points when switching standby off. Checked external fuse, looks good... anyway to test with multimeter ? I remember reading the switches are junk on this amp, so was wondering if standby switch is faulty, but why would it cut out when on ? Is my next step is to check internal fuse(s)? Or is there anything I can do with a multimeter before removing the chassis? I'm willing to have a look inside and see if anything is obviously not looking good, but I will be taking it to a tech to get anything replaced. Any advice appreciated, hoping someone has the same amp and same problem... Have been running the amp at 50ish watts (only 2 6l6's) for more than 2 years with no prior problems.
    file:///C:/Users/Dell/Downloads/TheTwin.pdf
     
    Last edited: Jun 11, 2019
  2. VICOwner

    VICOwner Supporting Member

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    You are going to have to pull the chassis. I looked at a schematic, nothing really special about the circuits. Check voltage through the standby switch first. I don’t think it would fail while in operation either. I have more ideas that won’t be helpful until you get started.
     
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  3. GT100

    GT100 Member

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    I’ve heard that all the jacks are soldered directly to the PCBs on these.
    So it dosen’t take a lot to damage that connection and loose signal.

    Lloyd
     
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  4. Seanosaurus

    Seanosaurus Member

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    Thanks for your responses, here's what the insides look like, don't know how many times it's been worked on in it's 30 year life, says it was made in 1990. I'm guessing the two fuses on the right of the first picture are blown, so I will replace those and see what happens. I don't know how to test the standby switch with a multimeter, but it at least looks like the fuses are the problem ? Anything else I could check with no power while it's out ? Thanks again.[​IMG]
     
    Last edited: Jun 11, 2019
  5. Seanosaurus

    Seanosaurus Member

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    [​IMG]
    [​IMG]
     
    Last edited: Jun 11, 2019
  6. Seanosaurus

    Seanosaurus Member

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  7. Seanosaurus

    Seanosaurus Member

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    I tested the both the T2A 250v fuses (away from the amp) with a multimeter set to 200 ohms, reading never moved from .1 so they're dead for sure (maybe obvious to most by looking at them!). I couldn't find the F303 and F304 slots in the schematic notes, but they are listed- only for export models of this amp apparently. Thanks again for the help, hopefully replacing the fuses will solve the no sound issue... I have owned this for nearly ten years and looks like I'm going to keep it even though I gig with a 2x10 68 CVR mostly now, one day when I'm wealthy enough I will get all the recommended mods done (input jacks, switches, various mods to the od channel etc etc) and employing someone to carry it around, but getting the sound back is the first step.
     
  8. pdf64

    pdf64 Member

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    Obviously those fuses are blown but I don’t understand the reported 0.1 (ohm?) reading; perhaps they weren’t taken out of circuit before measuring?
    Something was drawing A LOT of current for T2A HT fuses to pop; to avoid subjecting the amp to a repeated fault current exposure, after replacing them, I suggest to power it up via a light bulb limiter
    https://el34world.com/charts/Schematics/files/Fender/Fender_the_twin.pdf
     
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  9. Seanosaurus

    Seanosaurus Member

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    SORRY- meant to say 1.0. They were taken out, I'm not going to poke around with a multimeter inside the amp,I don't like taking it out at all. Setting up a light bulb limiter is something I'd be uncomfortable doing, so I'll take it to a local tech as it's probably due a good look over. Thanks for pointing that out, not sure why it would draw so much current ? Possibly a symptom of something else wrong ?
     
  10. zenas

    zenas Member

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    Yeah blown fuses are almost always a symptom of something else gone wrong.
     
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  11. Chris Scott

    Chris Scott Silver Supporting Member

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    I would take it to a tech if I were you.
     
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  12. Kyle B

    Kyle B Supporting Member

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    Check the four diodes mounted near the fuses. If one fails short, it'd immediately blow those fuses. Easy check, easy fix. Cross your fingers. ;). They can be checked w/o removing them.
     
  13. Vanyu

    Vanyu Member

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    It’s been a while since I’ve poked around in one of these, but I believe these amps were all pretty notorious for popping either a big diode, or a big ceramic resistor mounted on the back output PCB where all of the speaker connections and effects loop are. That PCB would be the first place I’d look for an issue, all of the ones I worked on had issues that stemmed back there
     
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