Help needed on 500C restoration

Discussion in 'Fisher' started by 365nut, Sep 28, 2017.

  1. 365nut

    365nut New Member

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    27
    Hi,

    I’m on my 2nd 500C restoration using a Metalbone kit. Before I began parts replacement, the set played—both R & L sides had excellent audio on original components. After each segment of cap replacement, I tested the set. Doesn’t appear I made any mistakes When I was all done, set sounded real good. Next step was replacement of C92, C98 & C91 can/electrolytic caps—in that order—caps from Hayseed. Each time I replaced a cap, I tested audio & everything seemed fine—except—after I was done with C91. When I turned the set on I heard a “pop” & saw a flash—happened so fast—couldn’t tell where it came from. Immediately turned set off. Didn't smell anything burned. Looked carefully for failed resistor—but found nothing. Later powered set up gradually with Variac & discovered that only the left side was playing--and playing normally. Everything else seemed normal (which told me the C91 hook up was OK). Using my Heathkit signal tracer, got good R & L audio both at volume control & at V15/12AX7. Got good audio at V12 (left phase inverter) 12AX7 but initially no audio at V13 (right phase inverter) 12AX7. Then again placed my signal tracer probe on the 1-6 (plate) connection of V13 & heard faint sound. At that very moment, smoke poured out of the right output transformer opening in the chassis & something smelled burnt. I Immediately turned set off. So, did I blow the OPT? And if so, what did I do wrong? Or, is there something else I should look for? And, how should I proceed?

    This probably gets old for you seasoned experts—but would appreciate any expertise any of you might bring to bear. Thanks. Learning the hard way, it would seem.
     
  2. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    8,985
    Location:
    Ball Ground, GA
    Hardly sounds like anything you did to to cause the transformer's venting -- but the venting doesn't sound good for the transformer. Time to take resistance readings between the various leads of the suspect transformer and compare to the other transformer. Would be best to disconnect the red lead from each transformer when taking the readings, and don't forget to check from the primary winding leads to the chassis as well.

    Let us know!

    Dave
     
  3. 365nut

    365nut New Member

    Messages:
    27
    Thanks, Dave. I'll try to get that done today and follow up with a post.
     
  4. 365nut

    365nut New Member

    Messages:
    27
    Follow up continuity test on output transformers keeping in mind that the RIGHT output transformer was the one that smoked. All readings in ohms. Red leads disconnected.


    Left Readings Right Readings

    V-8, V-9 Pins 3-3 .253 V10, V-11 Pins 3-3 .253

    Speaker Terminals (green & brown) 1.0 Speaker Terminals (green & brown) .8

    Red lead to ground open?? Red lead to ground 5.8

    Yellow lead to ground 1.0 Yellow lead to ground 1.0


    So, there is continuity on all except red on left--& left has not been the problem. Wonder why this is open and what purpose it serves. Hope I did everything correctly.
     
  5. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Location:
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    Nut -- There should be NO reading (as if the probes were not touching anything) from the red lead to ground -- so the left OPT is fine, but the right transformer has a short from the primary winding to the lams within the structure of the transformer. At the very least, it needs to be removed, and then if possible, the end bells taken off to examine where the leads connect to the winding wires. If the short is at the end bell, then possibly the transformer can be repaired. All too often however, the short is within the coil form, which means only a transformer re-winder can repair the transformer. So, it's decision time. You can either try and find one at auction from a junker, see if Trandendar is back in business of providing Fisher transformers, or look into getting that one rewound. The problem can be resolved, but it will take time for certain.

    Sorry to be the bearer of bad news, but the facts are quite clear in this case.

    Dave
     
  6. 365nut

    365nut New Member

    Messages:
    27
    Dave,

    Thanks for your analysis. Think Trandendar is now selling Fisher transformers. That's where I'll go. My main concern is determining the cause of the failure. Wouldn't want to blow out a new transformer. Is it your guess that my transformer was just ready to go or should I be looking for a cause.
    First time I've ever had a transformer blow. Have worked quite a few tube amps. Now also working on a Scott 299B. Which AK forum would you suggest to go for tips? There is no Scott section. For starters, one of the vertical porcelain resistors (80 ohm, 10 watt) that protrudes into the top side of the chassis is blown. Will have to replace with an under the chassis one. Should fit. Can epoxy the old one to maintain the original look. Thanks again. The real handle here is Paul. Take care.
     
  7. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Location:
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    Paul -- With any kind of modern manufacturing, transformers don't just "go" the way yours did. It failed for a reason, which had nothing to do with the circuit or your work, but everything to do with how the unit was stored over time. If a unit is stored for an extended period of time in a damp or moist environment, that moisture will work its way into the transformer and the paper used to separate the windings. If the high voltage is then applied to the transformer, current can flow through the moisture and ultimately form a carbon track, which allows more current to flow. Ultimately, the leakage current then burns through the insulation on the winding wire, and then either burns the winding open, causes an arc, or becomes conductive enough to effectively represent a short to the casing.

    One of the best things you can do to help prevent this type of damage if you suspect any kind of poor storage conditions at all, is to first remove the transformers before powering them up, and baking them in the oven at about 180F for a few hours to burn off any moisture the may have built up within the transformer.

    Sorry you're having to deal with this.

    Dave
     
  8. 365nut

    365nut New Member

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    Dave,

    I continue to be amazed and impressed with your knowledge and appreciative of your willingness to share it, not to mention the the time it takes you to do this. Guess I have to worry about the other transformer as well because it was stored under the same conditions. Have a new one on order. Turns out the company is not far from me. Thanks again.


    Paul
     
  9. 365nut

    365nut New Member

    Messages:
    27
    Dave,

    Back in business with a new transformer from Transendar Transformers which is located fairly close to me. For anybody thinking about a replacement transformer, I can tell you this Transcendar replacement is indistinguishable from an original Fisher sound wise--so there should be no hesitation in getting one if you need one. They do a very nice job. And the case is an exact replica of the Fisher.

    I hate to admit this, but I think I learned the cause of failure in my original transformer--and it was not moisture. I discovered that the impedance selector wire had a broken spade connector. The wire coming from the inside of the chassis had frayed loose from the connector and there was no connection to the right 8 ohm terminal. Guess one cannot be too careful when it comes to the basics--an expensive little thing to over look. But rest assured it won't happen again.

    Thanks again for your help.

    Paul
     
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  10. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
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    Location:
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    Paul -- Thanks for the info about Transendar -- nice to know they are back in business!!

    In a properly stabilized NFB amplifier as is used in the 500C/800C receiver, operating the amplifier without a load connected is hardly a death toll for the output transformer. Oh, if the unit were run at what is effectively full power output with no load, then things would start to get dicey pretty quickly -- but in that scenario, the first thing to go would be the output tubes, and the transformer would be next -- but even then it wouldn't be a slam dunk, taking multiple abuse events to ultimately destroy the transformer.

    It may not have been moisture. But ultimately, for it to fail in a blaze of glory the way it did, insulation within the transformer broke down, and allowed B+ voltage to flow directly to ground. The usual cause that gets that process started is moisture.

    Congrats on getting it running again, and so soon as well!

    Dave
     
  11. 365nut

    365nut New Member

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    27
    Thanks again for your help. Have learned a lot from your detailed explanation. Still have one more 500c to restore &
     
  12. 365nut

    365nut New Member

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    27
    a couple of Scott 299bs.
     
  13. 365nut

    365nut New Member

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    Want to again thank Dave, Larry and all of you who assisted me. The 500C has been humming along since I finished the restoration. Great to have you experts available. Couldn't have succeeded without you.
     

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