SVT-CL R59 failure

Discussion in 'Amps [BG]' started by UncleFluffy, Feb 15, 2014.


  1. UncleFluffy

    UncleFluffy Gold Supporting Member

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    It looks like my dead-ish SVT was down to R59 in the power supply failing spectacularly. This is the resistor that feeds the 380V rail for the final stage on the main board that drives the output tubes.

    [​IMG]

    My immediate thought is to replace it with something that has twice the wattage rating, but before I did that I'd like to check the collective TB wisdom as my expertise is more in the 20nm sub-1-volt range than big hairy tube gear.

    First question: is the higher-wattage replacement a smart thing to do? I know that's generally regarded as a bad thing to do when replacing screen grid resistors but this looks like just a dropper resistor on a power rail.

    Second question: is this more likely to be "just one of those things" or a symptom of a problem somewhere else that is likely to come back to bite at an inconvenient time?

    Thanks in anticipation for the wisdom.

    edit: I just noticed - you can see it in the pic - that C22 has also failed.
  2. nashvillebill

    nashvillebill

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    It could be indicative of something else drawing too much current. So if you replace the failed resistor with a higher power rating resistor, the next failure will be even more spectacular (and more expensive).

    The capacitor to the right of the failed resistor, a 47uF, appears to be bulging by the way. Could just be the cardboard end but it should be checked.

    More detective work is necessary. When I find burnt resistors, it's my belief they should not merely be replaced without finding out why.
  3. jeffmaxwell

    jeffmaxwell Gold Supporting Member

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    Find out what caused the failure and repair that problem and then put the correct resistor back in for R59. A higher wattage R59 will likely cause a spectacular failure of some other component on that 380v line. Something drawing too much current caused that resistor to fail. A higher wattage resistor wont solve the problem
  4. beans-on-toast

    beans-on-toast

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    Aug 7, 2008
    I would replace it with the same valued resistor, 2.2K, ½W.

    Bill is right about that cap looking bulging on the top. C21 and C22 are 47uF/450V caps that are on either side of the blown resistor. D26, a 1N4007 diode also connects there. Check em all.
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  6. UncleFluffy

    UncleFluffy Gold Supporting Member

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    Thanks guys. Definitely looks like that cap died. D26 tests good. I'll replace both C21 and C22, put the 2k2 1/2w back in and check the drop across it is in range.

    btw, it's the 365v rail that feeds the first stage after the phase inverter, not the 380v one. My mistake.

    edit: In fact, I'll replace all the 47/450 caps. They're almost certainly all from the same batch.
  7. UncleFluffy

    UncleFluffy Gold Supporting Member

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    Update:

    Just ran it flat out into a dummy load with a 40-4k sweep input for an hour solid. Held up fine.

    Thanks again for the wise words.
  8. beans-on-toast

    beans-on-toast

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    Good to hear that you got the amp back in the saddle again. Did you replace the caps with the same product or did you go with a different make?
  9. UncleFluffy

    UncleFluffy Gold Supporting Member

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    Different make, industrial spec. The extra couple of dollars seemed worth it.

    edit: I can now confirm that an SVT-CL with bass/mid/treb/freq at 12 o' clock is actually pretty flat from 40Hz to 4KHz. That's a surprise.
  10. beans-on-toast

    beans-on-toast

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    I use Panasonic industrial caps when I can. Very good products.
  11. Fluid Power

    Fluid Power

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    If the #22 Cap failed {dumped it's load, through a short) wouldn't that blow the resistor? So maybe it's just a bad Cap?
  12. UncleFluffy

    UncleFluffy Gold Supporting Member

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    Yeah, that's what it looks like.

    I'm guessing that it's either just one of those things or the caps were from a crappy batch. Just in case it was the latter I replaced them all with better-grade stuff while it was on the bench.

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