Fisher 500-TX recap

Discussion in 'Fisher' started by bwilzen, Jun 5, 2017.

  1. bwilzen

    bwilzen New Member

    Messages:
    9
    Location:
    Fair Lawn, NJ
    I bought a house last summer, and the previous owners left a few things behind including a Fisher 500-TX, one Fisher bookshelf speaker and a Dual 1214 turntable. At one point they had two Fisher bookshelf speakers downstairs and two XP 7B speakers wired upstairs, but only one bookshelf speaker was left behind. They were the original owners of house, which was built in 1960, and are now in their 90s.

    I turned on the 500-TX to test it out, everything seemed good -- radio, tune-o-matic etc. The turntable's speed switch was stuck, but I was able to take it apart and reassemble it to get it in working order. I hooked up the turntable to the receiver to test it out, and when I powered the receiver on I blew a couple of fuses. D'oh!

    The screw-on fuse on the back is fine, the two on the power supply board are both blown.

    I'm planning a recap, this thread has been helpful for my parts list:
    http://audiokarma.org/forums/index.php?threads/500-tx-sigh.541177/

    This thread has been helpful for assembling a dim bulb tester, which I'm planning to do before testing with live voltage again:
    http://audiokarma.org/forums/index.php?threads/compact-dim-bulb-tester-wiring.465423/

    Planning to do this slowly as I'm learning as I go. I have a couple brothers from out of town who have helped me get started.

    Any recommendations on what to check to make sure I take care of the blown fuse problem?

    What are the little tabbed inserts on each side of many of the connectors called? Should I pull them out with the components, or is there a way to keep them?
     

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  2. bwilzen

    bwilzen New Member

    Messages:
    9
    Location:
    Fair Lawn, NJ
    I found a couple of bad diodes, was planning on replacing those anyway. I may also have a bad transistor, will remove it from the board later to confirm. Hopefully those were the causes of the blown fuses...
     
  3. rufleruf

    rufleruf AK Subscriber Subscriber

    Messages:
    1,009
    neat acquisition story. Original owner of a vacation house I bought last year left a sort of working1959 Zenith console...

    I can't help with the rebuild of this, but will follow with interest.
     
  4. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
    21,694
    Location:
    Glen Burnie Md.
    "TABBED INSERTS" are a shim in the hole to keep the component lead tight. cut the lead off about 1" from the board and "J" hook the new component to the old pin. New components will probably have a smaller lead so the "J" Hook is probably the easiest and safest way to do this joint.
     
  5. bwilzen

    bwilzen New Member

    Messages:
    9
    Location:
    Fair Lawn, NJ
    Thanks Larry, that sounds doable. I checked out your 500tx recap thread from a while back while I was researching, nice work!

    I did some more hunting and found a bad output transistor in the power amp module, and several other issues on that board. Looks like this will take me a while, but I'm learning and enjoying the experience.
     
  6. bwilzen

    bwilzen New Member

    Messages:
    9
    Location:
    Fair Lawn, NJ
    The shorted output transistor was Q811 on the Power Amp board. I went ahead and replaced all four output transistors. When I turned the unit on when plugged into a dim bulb tester, the bulb stayed on.

    I should also mention that I recapped the unit and replaced resistors that I found out of tolerance, which I realize could potentially introduce a lot more variables. I regret not fixing the issue first, but nonetheless...

    I checked diodes and transistors on the power amp board. I found CR801, CR802, CR803 and CR804 were all bad. I replaced those, and still the bulb stays on.

    Diodes and transistors all check out on the power supply board. Any ideas on what else to look for? Is it unusual for an output transistor to take out all those diodes?
     
    Last edited: Feb 14, 2018

     

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  7. bwilzen

    bwilzen New Member

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    9
    Location:
    Fair Lawn, NJ
    So the problem ended up being that I had put the output transistors in the wrong slots...I had put PNP into NPN and vice versa. D'oh!

    I picked up a pair of Fisher XP-10 speakers on craigslist, can't wait to hook 'em up!
     
  8. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Location:
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    OUCH! Even if they test ok, they should be replaced as they've been exposed to reversed current and could very well be damaged.
     
  9. bwilzen

    bwilzen New Member

    Messages:
    9
    Location:
    Fair Lawn, NJ
    They tested fine, but speakers were quiet even at max volume. I ordered new ones, hopefully that’ll solve it.

    Output was good from headphones.
     
  10. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
    21,694
    Location:
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    The headphones are tapped off the output's before the speaker switch, and are padded down with a 2w 330 ohm resistor so the headphones are drawing down in the Milliamp range. I'm surprised you got headphones with the output's reversed. Before you hook up the XP-10's, put the 500-tx on a pair of disposable junk speakers 1st and play it for a few hours on them. If everything is fine after a few hours you should be fine at that point.
     
  11. bwilzen

    bwilzen New Member

    Messages:
    9
    Location:
    Fair Lawn, NJ
    Let me take that back about the headphones output being ok...it should measure at 1.9v and I'm getting 1.3v. Speakers through 8 ohm load resistor should get 21.9v and I'm getting 15.8v (about the same on both channels).

    I tried to do the Power Amplifier Idling Current Adjustment procedure in the manual, the output bias adjust is supposed to be adjusted to 15 mv, I'm only measuring 0.3 mv max.

    I'm also getting some very strange voltage readings on the power amplifier board, pic attached.

    Since my last post, I have tried putting new output transistors, and also tried replacing all transistors on this board, thinking they might have been damaged.

    I'm quite at a loss for what to try next, if you have any ideas, would greatly appreciate it!

    power-amp.jpg
     

     

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  12. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    What's with the twin voltages. Is that left and right sides, or what???
     
  13. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
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    Location:
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    Have you got a copy of the S.M. with the Service Bulletin in the back. Make sure there aren't any fuses on the emitter's of the output's. If they are there, Take them off and install jumpers, then do the fuse procedure in the S.B. if not already done.
    500tx bulletin.jpg
     
  14. fred soop

    fred soop Super Member

    Messages:
    1,943
    The feedback side of Q801 and Q802 is probably open. You show Q801 at -1.08v emitter and -9.5v base, a Vbe of 8.42v. Q802 shows -.76v emitter and -7.5v base, a Vbe of 6.74v. In both cases, this is probably enough Vbe to exceed the maximum rating, typically 5v for most transistors.
     

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