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Picked up a non-working Fisher 500C

Discussion in 'Fisher' started by Lo-Fidelity, Dec 15, 2017.

  1. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

    Messages:
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    Did the deoxit thing. No change. Closer look at pin 5 I see that it is connected to ground so no surprise I guess.
     

     

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  2. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

    Messages:
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    Took a look at V8-11.

    Pin 3
    V11. 442V
    V10. 442V
    V9. -1.1V
    V8. Edit. -2.3V

    Transformer issue? Or transformer input issue? Something else?
     
    Last edited: Sep 30, 2018
  3. bhamham

    bhamham AK Subscriber Subscriber

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    OK, sorry just looked at the schematic. The pin 5 on V15 measurement should be to pin 4 (pin 5 to pin 4) using DCV to check for filament voltage.

    Wow, so V9 and V8 are at play too. Are the filaments lit? You can test that transformer with the unit off measuring resistance from pin 3 on V9/V8 to where the transformers tie each together - I believe it's two red wires, one each from each transformer that meet at a terminal. Should be a reading around 150 - 180 ohms or thereabouts.

    I'd look for a broken wire checking continuity for pin to terminal, etc. around those problem tubes.

    I'm sure Dave or NotD, or others will look in and have other/better ideas.

     
  4. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

    Messages:
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    Ok, so V15 pin 4 to 5 is 12.4 V. Approximately same as to ground in prior measure above.

    Now to the other. Doesn't look too good to me.

    V8-11 Pin 3 to the red wire common to both OPTs
    V11. Doesn't stabilize but ranges 195 to 400 ohm
    V10. 207 ohm. Stable on first reading. Second neasurement would not stabilize. 200 to 500 ohm range
    V9. 0.L open
    V8. 23.6 M ohm stable
     
  5. notdigital

    notdigital AK Subscriber Subscriber

    Messages:
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    It takes a lot to kill a transformer but not much if it was on its way out from a prior overload condition.

    Earlier in this thread you had replaced the long resistor at the power tubes. There is a long terminal strip there where one tab connects 2 red transformer wires, the long resistor and a thin red wire from C-98. Check the wiring to that tab. Of the fat wires at that tab, one is for the left output tranny, the other for the right. (With the back of the unit at your belly, the tranny on the right is the left output transformer.) Per your post #122, you're getting the proper voltage from C-98 to V-10 and 11; nothing at V-8,9. Try reflowing the tab.
     
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  6. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

    Messages:
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    Reflowed that connection and unfortunately there is still no voltage to V8,9 pin 3.

    I pulled the power tubes and tried to do some transfer ohm measurements, but the readings are very odd.

    Pin 3 to the red transformer wire junction.

    V11. 6.2-6.7 M ohm fluctuating
    V10. 210 ohm. Stable pretty quick
    V9. 0.L
    V8. 2.1 M ohm stable

    I am over my pay grade with this problem.

    Thanks,
    Rick
     

     

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  7. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

    Messages:
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    Edit. Double post
     
  8. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

    Messages:
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    Interestingly the OPT are different. The working channel has a bolted transformer and the non working channel is riveted. Wouldn’t think it would come that way from the factory but who knows?

    47DC0072-63B0-4C2F-B1E4-22802558F159.jpeg 41AE423F-C713-4793-81A2-9BB9AFC80BB1.jpeg
     
  9. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

    Messages:
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    Right OPT is a T991-116-2A and measured 6.8 M ohm (V10,11 pin3-3)

    I found some info from an old Dave thread that gave 6.365 M ohm for the T991-116-1A.

    Left OPT shows open on my cheap HF multi. Is there a trick to measuring high ohm stuff or is it really open?
     
  10. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Just out of curiousity, what's the # on the LEFT Transformer????? It should be a T991-116-1.
    The HF meter should be good enough to tell if bad or not. If it's indeed bad, Check with Buglegirl. She's got a parts 500c carcass that may have a good transformer. Also look for an 800c transformer, same #.
     
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  11. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

    Messages:
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    Location:
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    Larry,

    Hard to see down next to that Heyseed can, but looks like .....6 -1A.

    My T991-116-1A looks as police would say 10-54.

    Rick
     

     

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  12. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

    Messages:
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    Location:
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    Update.

    Replaced left channel OPT with a unit from Buglegirl, .

    The left channel is back in service!!

    There is now some intermittent static on the right channel. Not sure where it is coming from but it was on both channels before the left OPT let go. Hope that the right OPT isn’t the next shoe to fall.

    When the static is absent the unit sounds great. Dead quiet. Some further work to do, but one step at a time. I’ll celebrate the successes when I can get one.

    Thanks Team Fisher!!!
     

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