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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:
    1,042
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    Dave,

    I’m pumped to have this running again.

    Now all that is remaining is to install the three can caps from Hayseed Hamfest. The one at the back of he cabinet looks a little tricky. One last challenge and I can enjoy my Fisher for years to come.

    Could not have done it without your help.

    Thanks again
    Rick
     

     

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  2. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

    Messages:
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    Dave,

    Just finished with the can caps!!!! Recap and basic upgrades complete!!

    The old 500c is singing up a storm. I am going to call this job done for now. Maybe some day I'll feel compelled to continue with upgrades, but right now I just want to listen for a while.

    Thanks again for you advice and counsel. There is no way I could have done this on my own.

    Cheers,
    Rick
     
  3. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

    Messages:
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    New issue.

    Measured 99 vac between the receiver case and the ground at the wall outlet.

    I have not flipped the unit over yet to investigate yet. Thoughts?

    My shop floor is all wood and elevated from the ground on piers so the resistance to ground is high. I thought I detected a little juice when I had my finger on the pointed corner of the faceplate which is what prompted me to a meter on it.

    This is pretty confusing as the unit seems to working ok.
     
    Last edited: Sep 14, 2018
  4. Tim D

    Tim D AK Subscriber Subscriber

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    I wouldn't know how to guide you in the hot ground area. Sounds like a job for "Super Dave"!

    Do you trust the wiring in your house?
     
  5. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
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    Location:
    Ball Ground, GA
    And it likely is working just fine. If you notice, where the AC power line enters the unit, one side of the AC line is connected to the chassis through a parallel path of an 820K resistor, and a .01 uF cap. These components work to provide a virtual chassis ground (for minimum hum), and a static charge bleed off since the unit could also be connected to an outside antenna. If the AC plug is inserted so that the side of the line connected to these components is the hot side, then the chassis will in fact generate a little tingle.

    To eliminate such concerns, I mark which side of the power cord plug is the hot side (NOT the side going to these components!), so that I always know to insert the marked side into the short slot of the AC plug. Or, you can retrofit the unit with a TWO prong polarized cord set -- wired in appropriately of course -- to completely eliminate the issue. Most folks like to keep the original Fisher power cord in tack however, so the marked plug works great in that scenario.

    I hope this helps!

    Dave
     
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  6. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

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    Thanks Dave!!
     

     

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  7. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

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    Rotated the plug and voltage dropped to 8.8 vac. Cool!!!
     
  8. knockbill

    knockbill Addicted Member

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    The small (shorter) slot in your wall receptacle should be Hot,,, if it isn't, the circuit is wired backwards...
     
  9. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

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    He outlets are wired correctly.

    The old Fisher plugs weren’t polarized with the wide neutral blade so you can insert either way.
     
  10. knockbill

    knockbill Addicted Member

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    Understood,,, but you never know and theres always that 50-50 chance of flipping the plug,,, I always change to at least a polarized cord set if its two wire, three wire if it can work... Save the original for the next owner...
     
  11. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

    Messages:
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    Being fairly lazy, I’ll probably just put a black sharpie dot on the hot side.
     
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  12. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

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    Need the Fisher experts again.

    Starting to hear a soft crackling sound between songs. Hear it on radio and Aux inputs. Too soft to be heard when music is playing. Sounds like record static and comes through both speakers, but you can hear it individually from each speaker (not like a mono sound, of that makes any sense).

    Anybody ever had this problem?

    Could it be as simple at cleaning the volume pot again?

    Possible tube issue?

    Thanks in advance,
    Rick
     
  13. notdigital

    notdigital AK Subscriber Subscriber

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    That'll be my guess. Between songs, when you hear the crackling, put finger on tip of 12ax7's and roll the tube/tap it in the socket. See if that ID's the culprit. If so, clean the tube pins and socket. May have to re-tension the socket as well. Maybe it's just a noisy tube which will have to be changed If that doesn't track it down, then I'd flip er over and tap around the guts with a non-conductive instrument to see if you can localize the cause.
     
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  14. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

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    I appreciate the help notdigital. I'll try to look at it today.

    Thanks,
    Rick
     
  15. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

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    Well that didn't go as well as I had hoped.

    Was doing the tube wiggle on V15 and V14. On V14 (I think, could have been V15) got a pop from the back of the unit and lost the left channel..:(

    Nothing obviously burnt or damaged from a look underneath.

    Swapped V10,11 with V8,9. No change, left channel still gone.

    Swapped V14and 15. No change.

    Thoughts on what I messed up? Any suggestions on how to start tracking this down?

    Thanks in advance,
    Rick
     
  16. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Make sure that the Tape Monitor switch contacts are clean. Both channels pass through it, so if it's contacts are dirty, a channel can drop out. If that gives no joy, then next stop, turn the Tape Monitor switch on and connect an input to the Monitor input to see if the left channel produces sound from that input. This input uniquely only uses 1/2 of V14 and V15 -- not both halves like the rest of the inputs do -- so it can be a good troubleshooting aid in helping to determine where the problem might lie.

    Also remember that both channels pass through V14, and both channels pass through V15 -- these tubes do not each service a specific channel. Therefore, if one half of either of these tubes goes dead (the 12AX7 is effectively two tubes in one envelope), you will lose whichever channel that the dead section serviced, while the other channel will still play fine.

    Let us know!

    Dave
     

     

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  17. notdigital

    notdigital AK Subscriber Subscriber

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    If messing with V-14, 15 caused the pop, I'd zero in right there. Juggling the power tubes did not affect the present condition so I'd say it is not related to those tubes.....

    As I was typing Dave jumped in to mention that dastardly Tape Mon switch! Yup, and make sure it is not riding half-assed as that will cause a lot of noise in a channel (but not the sort which you originally complained of.) Also, if you heard a "pop" and if you installed "fusistors" at pin 5 of the power tubes, check them to make sure they're still good.

    I'd focus on the v-14/15 tubes as the cause. If not the tubes themselves, then check the integrity of the components/wiring/joints related to the tube you believe you wiggled that caused the loss of the channel. A comparative voltage check of those tubes will also move the diagnosis along.

    Hopefully it's simply a tube. A ballsy check is to sub one of the MPX tubes into the suspect location. If it is the tube, I'll guess you'd get music right away. However, if it is wiring/component/socket related, then you run the risk of losing that tube as well.

    Good luck!




    .
     
  18. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

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    Haven't found the problem yet.

    Cleaned tape monitor switch. No change.

    Looked at and poked at the V14 and 15 terminations. All looks ok to my untrained eye. Nothing loose or broken.

    I-pod feed to Tape Monitor. Right channel only.
    Swapped V14 and V15. Still right channel only.

    Checked V8-11 pin 5 resistors. All about 10 ohm.

    Pin V14 V15
    1. 188 V 184.2 V
    2. 1.2 VAC? 0.44 VAC?
    3. 0.98 V. 1.2 V
    4. -25.3 V. -12.7 V
    5. -12 V. 0.001 V
    6. 187 V 187 V
    7. Hum. 0.52 VAC?
    8. 1.02 V. 1.1 V

    I cant find pin 4 or 5 on the schematic.

    Any clues in this information?

    Sorry for my ignorance on how a tube amp works.

    Cheers
    Rick
     
  19. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

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    Sorry about the table. I tried to fix in edit but it still is hard to read.
     
  20. bhamham

    bhamham AK Subscriber Subscriber

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    Something's fishy with pin 5 on V15. Have you tried tightening the socket pins with a dental pick or really small jeweler's screwdriver? Give the tube pins a hit of deoxit and work them up and down the socket a few times.
     

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