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    Buying a 50 year old power transformer is sometimes a roll of the dice - photo

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    Bob Latino
    Admin

    Posts: 1970
    Join date: 2008-11-26
    Location: Massachusetts

    Buying a 50 year old power transformer is sometimes a roll of the dice - photo

    Post by Bob Latino on Sat Jan 24, 2009 3:45 pm



    Kevin at Dynakitparts recently relayed the following story to me ..

    "Bob, Attached photo of a failed P-782 power transformer brought to me for evaluation. A recent Ebay purchase by a customer of mine looking to save a few dollars on his MK III amplifier project. This transformer caught on fire immediately upon powering up.

    Seller claimed this was removed from a good working amplifier although there were no return provisions listed in the Ad. I feel bad for this customer and many others who have gone this route in pursuit a good deal.

    Dynaco products for the most part were well made and a great value. Their power transformers were not designed to last 50 years. In addition, today's higher line voltages will place an additional burden on these vintage power transformers resulting in higher secondary voltages and increased temperature rise.

    The photo reveals the extent of failure to this unit. Take note of the burned windings and leads. This is a good example and reason not to install a used transformer of "unknown" origin on your next amplifier project.

    Regards, Kevin @ Dynakit"


    Understand that many Dynaco transformers are more than 50 years old and buying a used POWER (especially) transformer can sometimes be a "roll of the dice". If you are going to play a Dynaco amp on a daily basis buy a NEW power transformer.

    Bob

    Clik2media

    Posts: 40
    Join date: 2009-01-18

    Re: Buying a 50 year old power transformer is sometimes a roll of the dice - photo

    Post by Clik2media on Sat Jan 24, 2009 4:07 pm

    Great info BOB:
    Is there a way to test transformers BEFORE applying power to them, aside from some obvious leaks etc, is there anything else noobies can look for, test for etc to prevent a call to the fire deptartment. I actually have a set of transformers the guy told me came out of a Marantz, so I guess I am checking for a way to test those too.

    cheers

    Michael

    Bob Latino
    Admin

    Posts: 1970
    Join date: 2008-11-26
    Location: Massachusetts

    Checking a PA-782 power transformer

    Post by Bob Latino on Sat Jan 24, 2009 4:49 pm

    Clik2media wrote:Great info BOB:
    Is there a way to test transformers BEFORE applying power to them, aside from some obvious leaks etc, is there anything else noobies can look for, test for etc to prevent a call to the fire deptartment. I actually have a set of transformers the guy told me came out of a Marantz, so I guess I am checking for a way to test those too.

    cheers

    Michael

    Michael,

    The transformer should have been pretested first OUTSIDE of the amp by attaching the two BLACK (primary) wires to a 120 volt AC power source through a 1 or 2 amp fuse. Then you check for the proper voltages on the secondary side of the transformer.

    For a Dynaco Mark III PA-782 power transformer - If the fuse blows when you apply 120 volts AC to the two black wires then the transformer has dead short somewhere in one of the windings and the transformer is bad. If the fuse doesn't blow then get out your multimeter and check the AC voltages listed below ..

    1. The two GREEN wires should give you 6.3 - 7 volts AC or higher ACROSS the two GREEN wires.
    2. The two RED wires should give you about 820 - 840 volts AC ACROSS the two RED wires.
    3. The two YELLOW wires should give you 5 or 6 volts AC ACROSS the two YELLOW wires.

    If this checks out OK then you can put it in the amp but that still won't guarantee that it is OK. Once the power transformer is inside the amp and turned on the tubes draw current from the transformer and the current draw could cause an old power transformer to fail.

    On your Marantz power transformer the wire colors will be different so I can't say what voltages you will get on what wire color on the secondary side of the transformer - BUT - most power transformers have two black wires on the primary side. If you can get a Marantz schematic for the amp that these transformers went into then maybe you can figure out what the voltages should be on the power transformer that you have.

    Bob

    DynakitParts
    Admin

    Posts: 158
    Join date: 2008-11-30

    Re: Buying a 50 year old power transformer is sometimes a roll of the dice - photo

    Post by DynakitParts on Sat Jan 24, 2009 5:21 pm

    [b]Michael,
    When bench testing a power transformer (out of circuit) I would recommend using a variac and slowly increasing the primary side voltage in increments of 10 volts and checking the secondary voltages at each progressive increase. Typically, if the transformer is defective, you will know this before reaching the full operating voltage of 117 to 120 VAC."Buzzing, smoke and sparks" Keep in mind that the secondary high voltages on the Dynaco P-782 are Lethal as is the case with most tube amp power transformers. Keep all the secondary leads separated or better yet, attach all these leads to a screw type terminal strip. The variac method also provides a safe method of determing the correct secondary tap voltages when you have a transformer with faded color leads or no schematic for reference.
    Just simple math.

    Kevin[b]

    danf

    Posts: 58
    Join date: 2009-01-19

    Older Power Transformers

    Post by danf on Sat Jan 24, 2009 11:11 pm

    I agree that caution is wise in powering up older transformers, and that a new transformer may be the best option. For example, many of the original PA060's I see in ebay photos look cooked, so I would opt for a new transformer for an ST-70 rebuild. If you just want a nice Dynaco amp, buying a new kit is likely to be cheaper and smarter for most people than rebuilding old amps. You know what the project will really cost upfront.

    However, I confess, the fiddling with old stuff is what I crave. In practice, I have never come across a bad power transformer in the equipment that I've purchased. In many cases, the transformers are the only parts worth saving for a rebuild or new project, except perhaps the chassis.

    I carefully inspect the transformers before I use them and apply shrink tubing to dubious leads, or trim the leads back and splice on new wires. With a cloth leaded transformer like older Dynaco's, I start by going over the leads carefully, possibly taking off the end bells for inspection before applying any power, even with a Variac. The wire and shrink tube costs to properly repair an old transformer will wipe out some of the savings you think you're getting if you have to buy the materials for a single transformer.

    danf

    Posts: 58
    Join date: 2009-01-19

    Ok, I got a bad MkIII power transformer

    Post by danf on Tue Feb 10, 2009 9:28 am

    Just to close this story out, I just received a MkIII chassis with my first bad power transformer. The core was heat damaged and the leads underneath the end bells were cooked and cracked, waiting to produce the fried mess shown in the photo above. After corresponding with the seller, it appears that the filter cap had shorted, and that was apparently when the damage to the power transformer occured. Given that the stock filter caps on MkIIIs have a reputation for failure, this is something to look out for. Luckily, the seller was an honest person and gave me a refund.

      Current date/time is Sun Nov 23, 2014 4:40 pm