Distorted tone, no sound, burning smell - Vox AC4TV Issue

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Bluesful, May 9, 2016.

  1. Bluesful

    Bluesful Supporting Member

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    Hi All.

    Wondering if you could help me diagnose an issue with my AC4TV, and please forgive my lack of technical knowledge here.

    I was playing the amp when all of a sudden the tone turned highly distorted for a minute and then the sound just cut out. This was then followed by an electrical burning smell.

    After I detected the smell I immediately shut off the amp. I left it for a few hours and then powered it up again to try it, and there was still no sound, and hasn’t been since that incident.

    So I was initially thinking given the burning smell that I may have fried the OT, but after Googling it seems it could potentially just be a tube giving out.

    If it is a tube I assume it’s more likely to be the EL84 rather than the 12AX7 in the amp, right? I don’t have a spare EL84 at home right now (other than pulling one from another amp) to try, but if I get a spare to try out the amp doesn’t need a bias does it? I can just swap the EL84 out?

    Any assistance or advice would be appreciated.
     
  2. Jeff Gehring

    Jeff Gehring Silver Supporting Member

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    Yep, the EL84 is a prime suspect, and you can just change it out. There is a possibility it may have taken a resistor with it, but you can try the tube first and see if that gets you going. Cathode biased, so no bias adjustment involved.
     
  3. Bluesful

    Bluesful Supporting Member

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    Thanks for that mate.

    I did read some info that I could have taken out a resistor, but I suppose the tube, or tube + resistor is a better result than the OT like I initially suspected.
     
  4. Jeff Gehring

    Jeff Gehring Silver Supporting Member

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    If you open it up you might look at this resistor, between the two large radial caps... (470 ohms)

    [​IMG]
     
  5. Bluesful

    Bluesful Supporting Member

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    Cool. I'll definitely check that, as I'll have to open it up to get to the tubes anyway.
     
  6. Bluesful

    Bluesful Supporting Member

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    So I've opened it up and taken a shot of that resistor as below. It looks ok to me, but what do you think?


    [​IMG]


    Could it also be one of these fuses? They also look ok to me though.


    [​IMG]
     
  7. racecarracecar

    racecarracecar Member

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    Resistor and fuses look good.
     
  8. Bluesful

    Bluesful Supporting Member

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    That's what I was thinking.

    It's looking more like that the tube is the issue, so hopefully it's not the OT. I won't be able to go grab a new one until the weekend unfortunately.
     
  9. Jeff Gehring

    Jeff Gehring Silver Supporting Member

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    I dunno, I think I'd pull this top one and check it with a meter...

    [​IMG]
     
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  10. damian1

    damian1 Member

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  11. Bluesful

    Bluesful Supporting Member

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    What do you think is suspect about it?
     
  12. UsableThought

    UsableThought Supporting Member

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    We learn by doing.
     
  13. Bluesful

    Bluesful Supporting Member

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    That's true, and I'll pull it as a part of the troubleshooting. I am however just interested in what Jeff is thinking.
     
  14. UsableThought

    UsableThought Supporting Member

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    That's my point. He has seen & pulled a lot of fuses and you haven't. Your education begins now. He could talk you to death & you'll still learn more by pulling the fuse.

    I'm not being flip, BTW. Teachers, psychologists, and philosophers commonly distinguish between "knowing that" and "knowing how." The former might help you pass a rote test in school but the latter is what helps you ride a bike or fix an amp. Hints are helpful & can be passed along in a forum but only you can develop your own fund of "knowing how" - a slow but reliable process.
     
    Last edited: May 11, 2016
  15. Kurzman

    Kurzman Supporting Member

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    These amps were known to have a run (including mine) where the rectifier diodes were sub par and failed. I had to end up replacing the EL84 and the diodes. A fuse may have gone also but I can't remember.
     
  16. Bluesful

    Bluesful Supporting Member

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    Interesting. Hopefully it's not that as I won't be able to replace that diode myself.
     
  17. BadHiwatt

    BadHiwatt Member

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    You need to check continuity with a Volt Ohm Meter (VOM).
     
  18. damian1

    damian1 Member

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    If you enlarge the pic it looks burnt to the right of center.
     
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  19. Bluesful

    Bluesful Supporting Member

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    I don't have a meter, but I can replace that fuse and see if that works. Just need to find where I put my damn bag of spare fuses.
     
  20. Bluesful

    Bluesful Supporting Member

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    What's interesting is that C2 and C6 aren't populated. Why is that?



    [​IMG]

    [​IMG]
     

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