Blown output tranny?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by slider313, Jul 22, 2006.


  1. slider313

    slider313 Silver Supporting Member

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    A friends Univox head has no output. The voltage on the power tubes(4 -6L6's) and preamp tubes read correctly. Is there a way to test for a blown output tranny without the use of a scope?
     
  2. WaltC

    WaltC Member

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    Yes, but it's not particularly easy. You can disconnect the primaries and secondaries (label them carefully if you're not sure what gets connected where), inject a known AC signal into the primaries and measure what you get out on the secondaries (or vice versa) to see if the coils are performing correctly.

    But first....

    Did you try connecting the amp to a known good speaker? Easier to do and quicker.

    keep us posted...

    Walt
     
  3. hasserl

    hasserl Member

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    It seems everyone want to jump to the worst conclusion right away, and suspect teh trannys have failed. Do the basics first, trannies are actually prety stout and will not typically be the failed part. They can and do fail, but when troubleshooting use the process of going from the simple to the more complex. Like checking the speaker connections and the speaker itself. If it's a single speaker they can and do fail like your symptom (I had a Tonker do that in my test cabinet).

    For a good read on output trannies and how to test them check out Geofex.com, clikc on Tube Amp faq's in the top left corner and scroll down to till you get to that section. http://geofex.com/
     
  4. slider313

    slider313 Silver Supporting Member

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    I connected the head to a good 4x12 cabinet and got nothing.I probed with the amp on for cold solder joints and checked voltages. I tried changing the phase inverter tube as well. Then all the pre-amp tubes. I checked the ground connections and input connections.
     
  5. ES350

    ES350 Member

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    ...OBVIOUSLY, this is a test that you should ONLY attempt if you know what you're doing and have the proper meter, if not on either point, it's time to take it to a competent repair shop and technician.

    Yes---dont mess with plate voltages or any of the high potential stuff unless you know exactly what you are doing---some of those big Univox amps have very high B+ (600v or more, IIRC).
     
  6. slider313

    slider313 Silver Supporting Member

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    Thanks Old Tele Man. I have worked on "live" amps but was unsure of the procedure using a DMM. I don't own a scope to trace the point of where the signal stops.
     
  7. Wakarusa

    Wakarusa Member

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    There's an easier/safer way to test transformers and other coils (like chokes). It looks like this:

    [​IMG]

    It's a filament transformer with a 60W light bulb in the primary circuit. This particular setup has the ability to switch a floating ground for uses other than transformer testing, but that's not strictly necessary.

    To use: With the amplifier turned off remove the power tubes. With your DMM measure resistance pin 3 to pin 3 on the power tube sockets. If it reads open you've got an open winding. If it reads shorted connect the magic tester pin 3 to pin 3 (again with the amplifier turned off). If the bulb lights you've got a shorted winding. Result: OT primary tested with the highest voltage encountered = 12.6VAC. Cost: Maybe $15-$20?

    With the exception of DMMs, o-scopes and signal generators, my best test equipment is all made from scratch :)
     
  8. Wakarusa

    Wakarusa Member

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    I've found it's also handy with customers. You can show 'em the live voltage/current calculations, you can show 'em the reading on the LCR meter, but understanding dawns almost instantly with "If the bulb lights up, even a little bit, your transformer is dead..."
     

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