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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

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    Dave,
    In your AudioExpress article on tube life extension, you mentioned timers for staging power up and down. Are there applications for that with the Fisher 500c? You probably have covered this before. My apologies for either missing it or not following it when I saw it.
    Rick
     

     

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  2. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

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  3. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    I would prefer one without a Center tapped secondary.
     
  4. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    I think I would personally use a transformer rated for 6.3 vac at 3A. Under quiescent conditions, the receiver draws a little over an amp, so the voltage of the buck transformer will be higher than normal, and depending on how you wire it, could be subject to its own over voltage condition as well. I think a 6.3 volt transformer will be plenty for your bucking needs.

    Dave
     
  5. thornev

    thornev Well-Known Member

    Messages:
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    Dave... In my 500-C (see attached picture) that "big green resistor" to which you refer is orange. Is mine likely a replacement or is his resistor green because of the heat stress you mention in post 61? Thorne
     

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  6. notdigital

    notdigital AK Subscriber Subscriber

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    Your orange resistor and the green resistor in the other unit are the same except for the color. The heat stress he refers to is evident to the extent that both now show discoloration, not that it has changed colors. That resistor gets extremely hot while operating ergo the advice to raise the wattage slightly to make more durable.
     

     

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  7. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    In my experience, the earlier built units sourced a part that was green, while later units had the tan colored piece. The tan resistors seem to be more reliable in my experience -- but it could also be that they just aren't as old, or as much use before being stored.

    If you install the single resistor modification I developed to raise the MPX Stereo threshold, it will eliminate most of the relay switching on usable stations if everything is up to snuff.

    Dave
     
  8. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

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    Well damn...I screwed something up.

    Installed the 220k resistors on R121-124.

    When I brought the unit up something popped. Look as I might, I could not see anything amiss.

    Bought up with amp meter in line and current looked fine.

    From tape input everything is ok, but FM stereo lost the right channel. Mono plays through both speakers.

    I feel sick. Any idea what I did?
     
  9. thornev

    thornev Well-Known Member

    Messages:
    557
    Location:
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    Sanity check - You connected them to pin 6 of each power tube and to C77-C80 at their other end?
     
  10. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

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    This might be it.

    Just between V7 and the MPX section it looks like the white insulated wire shorted to the ground from V7 center post.

    Edit. Don’t think this is it after closer inspection.

    6C550998-28A9-4E7D-AC67-45A18E09655B.jpeg
     
    Last edited: Feb 4, 2018
  11. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

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    Yes.
     

     

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  12. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    If the Aux and Tape Monitor inputs and the FM Mono selector switch position all produce sound from both speakers, but FM Stereo functions only produce sound from one speaker, then that would tend to indicate a problem with either the selector switch, or the MPX decoder sub-chassis. First, check the cleanliness of the selector switch and use a good contact cleaner if necessary to get the job done. Then, if you recapped the MPX sub-chassis, check your work carefully -- particularly around V102, as that tube is really where the two stereo channels first come to life.

    I hope this helps!

    Dave
     
  13. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

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    Thanks as always Dave!

    Went back to the shop to follow up on your note.

    I think it is possible that R224 or R225 (I didn't trace the wiring enough to say for sure) may have shorted to L102 or L103 as it looked like the wire might have touched. I made some separation between them and came back up. In mono and then tried stereo. Stereo worked but the right channel tunes started getting really hot. I shut it down, but I might have ruined the right channel tubes as I didn't see them getting hot right away as the unit is on its side and tubes facing away. I also noticed the right channel transformer was warm while the left was cool.

    Not been a good day at the bench.

    Photo of possible short point (upper left of picture). I could have touched it steadying my hand soldering R121-124. I don't know.

    IMG_1044.JPG
     
  14. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    A short at the location shown would cause the Left Channel to drop out, not the right, so if it was the Right Channel that dropped out, then you may still have an intermittent connection or short somewhere to chase down.

    As a general point however, the coupling caps associated with V102 in Fisher MPX units should not be recapped using orange drop caps -- they're a fine cap, but they are simply too large physically for the allotted space, and therefore can cause collateral problems when used. I would highly recommend using a much smaller cap that is made by Panasonic, and available at Mouser. It is a very high performance .047 uF cap rated at 630 vdc, and most importantly, will properly fit where the original pieces did as well without causing any displacement of other components in the process. These caps therefore make for a very neat and tidy recap in the area, without adding any increased risk of component lead shorts. A pic of a recent recap I did using these caps is provided for reference. They are the two light brown looking caps located under the L102 and L103.
    SAM_2346.JPG

    As for output tube heat, I believe you installed 10Ω cathode sampling resistors at pin 5 of each output tube. Check the voltage at these terminals to see what the voltage is that's generated there. This will ultimately tell you how much current each output tube is drawing and therefore the amount of heat it is generating/dissipating. Let us know!

    Dave
     
  15. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

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    Yeah, I hear you on the caps, but it was what came with the kit.

    I wasn't able to look at anything today, and will be working with an electrician the rest of the week on a remodel. I hope to look at it next weekend. Maybe by then some of the sting will have worn off.
     
  16. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

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    Well 6 months have gone by and it is time to restart this little project. Hope you guys have your thinking caps on because this old claw hammer needs some help.

    Quick recap. Back in January I recapped this unit with the Metabone kit and had installed the 220k ohm resistors on R121-124. When I brought the unit up there was a pop. I didn't know it at first as the tubes where facing away from me, but I also over heated the right channel output tubes V10&11. Don't know if they where damaged much or not.

    Fast forward to last night.

    Brought the unit up slowly on the Variac keeping a close eye on the tubes. Was using an iPod for audio source hooked into the Aux jacks.

    At 80 volts I had sound and keep the voltage here as the tubes looked ok. The unit sounded ok and all functions were working. Switched over to FM and the tuner worked in mono.

    I brought the voltage up to 90 volts and could see V10&11 starting to get warm (had the lights out and saw a slight but noticeable glow starting). Shutdown for the night.

    Today

    Repeated all of above and was pretty sure I was also getting FM stereo after moving the crappy little antenna wire around.

    Took voltage measurements across the V8-11 pin 5 10 ohm to ground resistors as shown below. Line voltage was ar 80V for these measurements.

    Voltage across 10 ohm resistor on pin 5 at 80V to unit
    V8 36.7mv
    V9 58.6mv
    V10 226mv
    V11 190mv

    Voltage to ground pin 6 at 80V to unit
    V8 -11.5V
    V9 -11.4V
    V10 -.4V to -1V. Unstable
    V11 -.4V to -1V. Unstable

    Raised voltage to unit up to 85V
    Voltage across 10 ohm resistor on pin 5 at 85V to unit
    V8 57.4mv
    V9 90.6mv
    V10 330mv
    V11 290mv

    Monitored voltage on the V10 resistor and raised voltage to 90-95. The mV over the 10 ohm resistor took off and I shut her down.

    Looks like a mess to me. Any ideas on where to go from here?

    Cheers,
    Rick
    Your brother in Fisherdom
     

     

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  17. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    There is a white wire that brings the negative bias voltage over from the power supply section to the power output tube section. It goes first to the center terminal of a 5 terminal T-Strip that services V8 and V9, and then goes from there by another white wire to a 1 terminal T-Strip that services V9 and V10. The bias voltage is clearly making it to the 5 terminal strip because V8 and V9 appear to be operating mormally, but it is not making it from there to the 1 terminal strip. Check for a broken wire, bad solder connection, or broken lead at one of the terminal connections for the white lead that brings the bias voltage over from the 5 terminal strip to the 1 terminal strip. Should be a rather simple problem to fix.

    I hope this helps!

    Dave
     
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  18. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

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    Dave,

    Your knowledge and skill at trouble shooting these units, with minimal information gather by knobs like me, continues to amaze me!!!!!

    See the picture below burned wire.

    Now the $10,000 question, did I damage the wire soldering in the 220k ohm resisters (very likely), or is something else going to blow when I fix this?

    IMG_1126.JPG
     
  19. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

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    Dave,

    Thanks a ton my friend!!

    The unit is sounding pretty sweet right now. Tubes look great and everything seems right on 115V line input.

    Rick
     
  20. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Location:
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    Glad you found the problem! -- and happy to help. As to your "$10,000 question", here's some thoughts that might help:

    I always like to use original wire when I can to help maintain the look of the original build. But I am always mindful that when you disconnect a lead from an original connection -- even carefully -- the unbending of a lead that has been wire wrapped for decades can really weaken it (I'm talking where the lead actually attaches to the terminal it connects to), allowing it to easily break off if later it is reconnected simply "as is". It looks like that may have been a possibility in the case here. When disconnection and reconnecting existing wiring, I always cut the exposed end back to a point just before the original wrapping point commenced, and then strip off a little more of the insulation to expose "new" wire, which then allows for a solid, worry free connection to be had when it is reconnected. Usually, there is plenty of lead to allow for this type of trimming if the lead is to be reused in its original application.

    Congrats on getting it going!

    Dave
     

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