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    Transformer gone bad?

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    jrethorst

    Posts : 23
    Join date : 2009-02-01

    Transformer gone bad?

    Post by jrethorst on Thu Dec 28, 2017 2:12 pm

    I have a beautiful pair of Mark 3 amps, that sat for a while in a house near the ocean and developed some rust. Firing them up again, one did not work. A technician said the output transformer was bad and needed to be rewound.

    This tech mostly works on flat-screen TV's and didn't seem familiar with tube gear (but is the best-recommended tech in this small town).

    Do transformers go bad often? I'd thought they were so basic and heavy that not much went wrong with them, and wonder whether this is a mis-diagnosis.

    If the transformer is in fact bad, will rewinding fix it? Would I do better to replace it? If so, what kind of price should I look for?

    Thank you,
    John
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    j beede

    Posts : 425
    Join date : 2011-02-07
    Location : California

    Re: Transformer gone bad?

    Post by j beede on Thu Dec 28, 2017 3:04 pm

    Beautiful and rusty?

    A couple thoughts.

    If you are a DIY'er you could:

    -Swap the left and right channel OTs and see if the problem follows the "bad" transformer
    -Pull the output tubes and apply an ac signal to the HV side of the OTs and compare Vout, left versus right. 12VAC across the BL and BL-WHT wires should produce a small ac Vout measurable at the 4/8/16 Ohm taps. Left and right should measure the same VAC.

    My guess is that the flat screen TV guy already did this to diagnose your problem. www.Dynakitparts.com sells replacement transformers if needed.

    jrethorst

    Posts : 23
    Join date : 2009-02-01

    Re: Transformer gone bad?

    Post by jrethorst on Thu Dec 28, 2017 5:17 pm

    Well -- they sound beautiful.

    Dynakitparts has a replacement transformer for $150. Does rewinding fix a transformer? I didn't get a quote from this guy for that, but I'm sure he'd charge much less, proably half.

    Thank you.
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    j beede

    Posts : 425
    Join date : 2011-02-07
    Location : California

    Re: Transformer gone bad?

    Post by j beede on Thu Dec 28, 2017 7:36 pm

    I am a fan of the MkIII.

    If they were my amps I would do a bit of troubleshooting before replacing or re-winding any output transformers.

    There are many issues that could cause a MkIII to "not work". A failed OT would not be at the top of my list of suspects. I would eliminate the simple issues before tackling the complex or expensive ones.

    If you have corrosion on the transformers that suggests that you may have bad electrical connections interrupting your B+ or bias supplies.

    If you are a DIY'er here are a few things you could check in ascending order of difficulty:

    -Replace the 3A glass fuse
    -Remove the 5AR4 and power up the amp with all the other tubes in place. Confirm that all the tube filaments glow.
    -Power down the amp and let it sit for an hour.
    -Rotate the bias pots full CW and full CCW 10-20 times. Park them at the mid-point.
    -Replace the 5AR4, connect speakers, power up the amp. Confirm that all the tube filaments still glow.
    -If you are okay with working on HV circuits, measure the dc voltage at pin #8 of the 5AR4. Measure the dc voltage at pin #5 for each output tube. Report your findings.

    If you get this far and answer okay to each step above, there is more to be done before the OT is declared guilty. If this is already beyond your DIY comfort level, you would do better to take your MkIII to a guitar amp guy, not to a flat screen TV guy.
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    Peter W.

    Posts : 577
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: Transformer gone bad?

    Post by Peter W. on Fri Dec 29, 2017 8:56 am

    Before doing anything else, lets' check out the purportedly bad transformer with your trusty VOM/DVM.

    http://diyaudioprojects.com/Schematics/Dynaco-Dynakit-Mark-III-Tube-Amp-Schematic.htm

    In Ohms:

    Tubes OUT (but marked for location). Amp upside down with bottom plate off:

    a) Output side: Measure from C to 4, then 8, then 16. You should get progressively higher readings. If you do, the secondary winding is good. If it cuts off, then the secondary is bad.
    b) Measure from 16 to the chassis. This is the OPT ground. The reading should be near-0 ohms.
    c) Measure from Pin 3 to Pin 3 of the two KT88s. You should get a reading - this is across the entire primary winding.
    d) Measure from Pin 4 to Pin 4. This is the center section of the primary winding. You are verifying that all points are properly connected as you verified that the winding is good in c) Above.
    d) Measure from the choke secondary to either Pin 3. This is the source of B+ into the system.

    If all is well, it is not the OPT that is in question. Everything after that is cheap(er) assuming the power transformer is good. Confirming the initial diagnosis is key before chasing down any rabbit holes.

    Best of luck with it.
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    Peter W.

    Posts : 577
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: Transformer gone bad?

    Post by Peter W. on Tue Jan 02, 2018 5:31 pm

    Please update us: Did you check out the transformer? And, is it well-and-truly bad?

    Dogstar

    Posts : 293
    Join date : 2014-06-23

    Re: Transformer gone bad?

    Post by Dogstar on Thu Jan 04, 2018 8:25 am

    j beede wrote:Beautiful and rusty?

    A couple thoughts.

    If you are a DIY'er you could:

    -Swap the left and right channel OTs and see if the problem follows the "bad" transformer
    -Pull the output tubes and apply an ac signal to the HV side of the OTs and compare Vout, left versus right. 12VAC across the BL and BL-WHT wires should produce a small ac Vout measurable at the 4/8/16 Ohm taps. Left and right should measure the same VAC.

    My guess is that the flat screen TV guy already did this to diagnose your problem. www.Dynakitparts.com sells replacement transformers if needed.

    So you're saying the rusty amp now has a patina like they try to say is a bonus on an antique car?

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