Mid 70's Univox Stage 400 Low End Flab

Discussion in 'Amp Central Station' started by Ripdoozer, Jan 16, 2020.

  1. Ripdoozer

    Ripdoozer Tele-Meister

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    Hello,
    I recently acquired an old mid 70's Univox Stage 400 (90 watt, 2x10) amp. I have cleaned it up and replaced both speakers, but there is a lingering low end issue that I cannot diagnose.

    Even with the settings all at "Noon" (Cilpping and Reverb set to zero), the low end of the amp is really farty, flabby and distorted. Even with the Bass set to 1 or 2 on the dial, it still distorts. I have cleaned all of the pots and dusted and instected the circuit board.

    I am less familiar with solid state amps than I am with solid state amps.
    What could the culprit be?

    Could a filter cap replacement help resolve the situation?

    Any advice would be appreciated.
     
  2. davidge1

    davidge1 Friend of Leo's

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    Sounds like the speakers are blown. I would try it with new speakers.
     
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  3. Ripdoozer

    Ripdoozer Tele-Meister

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    Thanks, but in my OP I said that I have already replaced both speakers. The distortion seems to somehow be circuit based.

    Thanks again.
     
  4. muchxs

    muchxs Doctor of Teleocity

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    Yes. If you think about it, that old amp is creeping up on 50 years old. Univox didn't use the best parts to begin with.

    Further, SS usually uses larger filter caps than tube. When they work, they work well. When they don't work...
     
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  5. Ripdoozer

    Ripdoozer Tele-Meister

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    I will give it a shot. I am not having any luck finding a schematic for this thing though.....
     
  6. muchxs

    muchxs Doctor of Teleocity

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    Bah. We don't need no stinkin' schematic!

    You seen one power supply, you seen 'em all.

    Just don't look in a Maven Peel Zeeta.

    :)

    Seriously, look for large capacitors. Replace 'em.

    Take notes and be careful. The schematic is usually there to CYA if you screw up.
     
  7. Speedy454

    Speedy454 Tele-Holic

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    If it is anything like the Univox U130 from the same era, there will be a large electrolytic capacitor in series with the output to block DC from the speaker.
    Those will dry out and cause issues.
    Check that out.
    All of the Univox U130L and U200L that I serviced had rock solid bass when they were working properly.
     
  8. Ripdoozer

    Ripdoozer Tele-Meister

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    Great. Thank you guys. I will check it out.
     
  9. Ripdoozer

    Ripdoozer Tele-Meister

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    So, is that large capacitor this this big guy? Also, How can I find the value so I am sure to order the correct replacement?
    Thanks!
     
    Last edited: Jan 17, 2020 at 2:51 PM
  10. muchxs

    muchxs Doctor of Teleocity

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  11. Ripdoozer

    Ripdoozer Tele-Meister

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    Ok. I think you should be able to see them now.
    I think the values are conveniently printed on the underside of the capacitor. I will go and check....
     
  12. Ripdoozer

    Ripdoozer Tele-Meister

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    So 2200 MicroFarad and 50V. Anything else I need to know in order to order the correct one?

    Thank you so much for your help.
     
  13. cometazzi

    cometazzi TDPRI Member Silver Supporter

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    Make sure to buy "axial lead" instead of "radial lead".

    Also, if that cap is bad, it might not be the worst idea to replace all the electrolytic caps on that board. You know, while you've got it separated from the chassis, and you're probably going to pay more for shipping than the parts anyways.


    :)
     
    Last edited: Jan 17, 2020 at 3:52 PM
  14. Ripdoozer

    Ripdoozer Tele-Meister

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    Ok, so based on this photo, how many caps would need replacement? I just don't know which ones are Electrolytic. Or is that all of the barrel caps? Axial and Radial....

     
  15. cometazzi

    cometazzi TDPRI Member Silver Supporter

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    It would be all those blue barrels. The little green lumps of chewing gum are mylar film capacitors and are probably fine.

    On second thought, start with just that large one and see if it solves the problem.
     
  16. Speedy454

    Speedy454 Tele-Holic

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    How about a better photo of those components under the circuit board?
    The transistors mounted to the chassis?
    Those will be your output transistors..
    One of those could be bad too. I had that once and it seems like it was only noticeable at lower frequencies.
    They can sometimes be difficult to find, and substitutes can be hit or miss.

    I did run across this...
    https://www.antiqueradios.com/forums/viewtopic.php?f=13&t=183499

    The 65 could be somewhat similar to the 400 or not.
    Univox did seem to re use designs whenever possible.

    If your output transistors are germanium like the 65, I'd be tempted to let it die rather than spend a bunch of time and money on it.
     
  17. Speedy454

    Speedy454 Tele-Holic

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    I also read that the 400 was a 40 watt available in a 1X12 or 2X10 configuration.

    It probably has a FET input transistor, and the clipping adjusts the gain of the FET.
    An overdriven FET is about as close to 'tube'overdrive as solid state could get back then.

    Some U130s had a a 'Pull PB' on the master volume. That would bypass/short the gain resistor on the FET and drive it to max gain. It was almost tolerable for an overdrive.
     
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