Fisher 500C Right Channel Distortion

Discussion in 'Fisher' started by 365nut, Sep 7, 2017.

  1. 365nut

    365nut New Member

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    I know there are many posts on this & I've read as many as I can find. I learned a lot from these--but still unable to solve my problem.

    Just completed Metalbone kit restoration following his very good instructions & includes C97 replacement. Have also replaced C92 & C98. C91 OK. Checked all voltages and as far as I can tell they are where they should be. Playing MP3 player through AUX inputs. Left channel sounds perfect. Right channel distorted pretty badly especially at high volume. Have replaced or swapped 12AX7s in audio path without any change. Have swapped R & L pairs of output tubes without any change. Have thoroughly cleaned tube pins, sockets, switches and controls. Have checked all resistors in audio path (comparing L & R) & all check OK & very close. Don’t have a scope but am using a newly restored T-1 Heathkit signal tracer with a speaker which is a pretty decent trouble shooting tool with some limitations. Sound on L & R channels at volume control is clear. On V-14 C52 and C53 sound clear at outputs. When signal reaches V-13 (12AX7 right channel driver), sound is distorted at C79 & C80 and seems quite over modulated compared to left channel at same points. And of course it is bad at the two right channel output tubes.

    Would appreciate any ideas on what I may have missed in my attempts to trouble shoot this. Anything obvious--a resistor that checks good--but is bad, a bad tube that appears to be good, a bad solder joint----?? Sure hope it isn't the output transformer. Thanks.
     
  2. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

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    Welcome to the Asylum;

    V14 and V15 (L & R High Filter Amp is V15, and the Low Filter Amp is V14.) and they amplify both channels. V13 and V12 are the Driver (1 section) and Phase Inverter (1 section). If the distortion is absent at the volume control and is present at the output coupling caps, I suspect the Phase inverter pots are mis-adjusted. There are a couple of threads in the STICKIES that pertain to this.

    The 1st thread is the Quick an easy 400 Phase Inverter check and adjustment. http://audiokarma.org/forums/index.php?threads/quick-and-easy-400-phase-inverter-adjust.559415/
    I know, I know, the title says 400. It started out as a 400 only thread and quickly went to a "will it work on this model or that model". The 500c adjustment is in post #1. But you can read all the way thru if you want.

    If you want to go further and remove the "noose" from the Phase inverter, remove R113 and R115 and redo the adjustments. It's not as constrained as the PI "noose" on the 400's but it's still cramping the 500c's style a bit. This is covered in this thread although you'll need to dig a little.

    Both of these are in the FISHER FORUM STICKIES at the top of the main page. The 1st is in the main stickie page, and the 2nd is in the Reference Technical Threads.


    Do the ADJUSTMENT part 1st and then test.
     
  3. 365nut

    365nut New Member

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    So, here’s what happened. Followed the “New and Improved” instructions. Shorted the B+ supply point, etc. Measured and recorded he resistance value appearing at #1 pins of L & R phase inverter tubes. Then connected my ohm meter to #3 pins. When I tried to adjust the phase inverter pots, there was no change in the resistance. Would appreciate any additional thoughts. Thanks.
     
  4. 365nut

    365nut New Member

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    Haven't replaced C91 but planned to replace it. Will go ahead with that & see if it makes any difference.
     
  5. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Nut -- That's because the phase inverter adjustment procedure was originally written for the 400 receiver. The procedure still works, but with the 500C, you have to do it backwards:

    1. With the unit turned off and unplugged, short C91C to ground.

    2. Connect the negative side of your Ohmmeter to the chassis, and measure the resistance at pin #3. Record this value.

    3. Now move your probe to pin #1, and adjust the Phase Inverter control for the same reading at pin #1 as was measured at pin #3.

    I hope this helps!

    Dave
     
  6. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

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    Dave beat me to it.
     
  7. 365nut

    365nut New Member

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    First, I continue to be grateful for your responses--Audiokarma is a great forum. Now to the nitty gritty. Good news is with your latest advice I am able to adjust the L & R phase inverters and have done so. Thanks. Bad news is I still have distortion on the right side. So, any "next step" suggestions?
     
    Last edited: Sep 10, 2017
  8. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    OK. Can you give us the voltage readings for all the pins on V13?

    Dave
     
  9. 365nut

    365nut New Member

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    Powered at 115 VAC, here are V-13 voltage readings. Also measured V-12 for comparison.


    Pin # V-13 V-12

    1 285 285

    2 126.9 126.3

    3 129.7 129.4

    4 0 0

    5 0 0

    6 127 127

    7 0 0

    8 .99 1.01

    9 0 0
     
  10. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Hummm. Nothing wrong there. Let us know the output tube voltages as well -- although I don't necessarily expect any issues there, either. However, carefully inspect for any short or partial short across the output terminals -- either at the speaker terminal board, or the impedance terminal board.

    Will the right channel get quite loud although distorted? Or is it distorted and notably limited in power level?

    Dave
     
  11. 365nut

    365nut New Member

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    Dave,

    Will get you the output tube voltages. Right channel gets plenty loud, even though distorted--possibly a bit louder than the left side.
    Will do further inspection, as you suggest. Thanks again.
     
  12. craig25

    craig25 Member

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    Location:
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    I had a similar issue and discovered one of my cathode to ground "safety" resistors had failed on one of the output tubes. I went from 1/4 watt to 1/2 watt ones now.
     
  13. 365nut

    365nut New Member

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    Here are the output tube voltages:

    Pin # V10 V11 V8 V9

    2 0 0 0 0

    3 384 382 383 382

    4 356 356 356 356

    5 12.9 157 106 .209

    6 -18.4 -18.4 -18.4 -18.4

    7 & 8 all 0

    Off hand, the great disparities on #5 pins raises questions to me & my limited knowledge but this situation is also present on left side tubes which as we know is functioning fine.
     
  14. 365nut

    365nut New Member

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    Craig,

    Appreciate your comment.

    365nut
    (Paul)
     
  15. 365nut

    365nut New Member

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    This has a good ending--but I feel foolish. Craig nailed it. Honestly, I thought I had checked the "safety" resistors--but obviously missed one. Apparently, the one from the kit I installed was bad. In any event, the problem was a bad resistor on Pin 5 of right output tube V-10. All fixed & sounds great. Hope it is some consolation to Dave & Larry who spent time on this--that my learning curve has expanded as a result of your efforts. I have 2 more of these to restore that I've had in my basement for a few years. Now, I have a lot more confidence. Thanks again to all of you who weighed in.
     
  16. 365nut

    365nut New Member

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    While you guys are in the groove, I wonder if Larry can explain the need for the change he recommends in a January 7, 2016 post about removing the pin 5 side of 220 ohm resistor on output tube & soldering straight to a chassis ground point.

    And also your "one more change" to the screen supply--installation of a 100 ohm between pins 1 & 4, etc.

    How do these changes affect performance? Are these changes imperative? Thanks.
     
  17. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

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    Short Version: The 220 ohm resistor is on the heater (filament)circuit and uses pin 5 as a ground point. On the 500C they are R85 and R120. When you change the circuit by adding the 10ohm cathode resistors, you can induce a hum (AC 60Hz IIRC) that is quite hard to pin point if you do not remove the 220ohm resistors from pin 5. Changing the one end of the 220 ohm resistors to a hard ground, prevents the hum will allow a good reading on the cathode resistor.

    Dave can explain in technical terms as he's the Ramblin Wreck from Ga. Tech., and a hell of an engineer.
     
  18. 365nut

    365nut New Member

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    Thanks, Larry.
     
  19. craig25

    craig25 Member

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    Location:
    Sarasota, Florida
    Hi Paul,
    Glad I could help. I'd love to take all the credit, but Mike Samra is the one who deserves it. He talked me through the troubleshooting with my 400 when it developed a similar issue. He really has a sixth sense about this stuff.
     
  20. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

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    Paul. To finish your questions as to the screen stabilization resistors, they 1.) DO NOT affect performance in a negative sense. They WILL prevent arcing of the tube, which in the 7591/7868 tubes of today is a constant threat. See Dave G.'s article( Maximize Power Tube Life) on it, here..... http://www.tronola.com/html/maximize_tube_life.html

    Both changes are imperative, unless you have a bottomless wallet for power tubes. Especially the Russian Tubes, which are about 90% as good as OLD STOCK. Stay away from the JJ 7591's.

    http://www.tronola.com/html/dave_s_lab.html
    http://www.tronola.com/html/7591a_tubes.html
     

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