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No power to phono section of Fisher 500C?

Discussion in 'Fisher' started by Mikeorg, Oct 2, 2018.

  1. Mikeorg

    Mikeorg New Member

    Messages:
    42
    Location:
    OREGON
    The 12AX7's two of them are not lighting up in the phono section of the 500C I'm working on I'v replaced the C13&14 and C16&17 capacitors in the circuit but still no power to phono any ideas??? everything else is sounding wonderful.
     

     

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  2. Tom B

    Tom B AK Subscriber Subscriber

    Messages:
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    There are a lot of Fisher specialists (Experts) on the site who can probably help more, but I would suggest the following

    Not seeing those 12ax7 lighting up is not uncommon as it is very hard to see illumination on those small preamp tubes.
    But I would check and make sure you are getting the proper fillament voltage on those tubes. Pin 4 to pin 5 of each tube (V16 and V17) should read near 11vdc. If one of those tube filaments measure 22vdc, and the other one measures 0vdc, then the tube showing 22vdc has an open filament and needs to be replaced.
    If the filment voltages looks fine check voltage on the plates on tubes . Pin 1 on both V16 and V17 should read 105vdc, and pin 6 on both tubes should read about 85vdc. If these voltages are missing you probably have a power supply problem (bad 22k ohm resistor or bad filter cap c91A.)

    Also check and make sure the reverb jumpers are in place.
     
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  3. Tim D

    Tim D AK Subscriber Subscriber

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    Location:
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    +1 for Tom B's response.

    Also, if it is a power supply issue to the filaments then all four of V14, V15, V16 and V17 would not be getting the ~11VDC each. Did you check that the filaments were not lit in a darkened room or were you looking in ambient light? Are V14 and V15 lighting up? If they are then the filament power supply is probably good. Did you try re-seating the affected tubes and checking that the sockets are making connection?
     
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  4. Mikeorg

    Mikeorg New Member

    Messages:
    42
    Location:
    OREGON
    Just V16 & V17 are not lighting and yes I turned off the lights and took the bulbs out of the stereo light sockets as well.
     
  5. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,673
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    Once you find the problem (about the only thing it can be is a bad tube, dirty socket, or broken/bad connection), recheck the quiescent current of the output tubes if you have installed the means. If you have installed adjustable bias and it was set with these tubes not lighting, then the output stage current draw will rise notably now due to the increased drop across the DC heater/bias supply filter/dropping resistor when these tubes are brought on line.

    Dave
     
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  6. Mikeorg

    Mikeorg New Member

    Messages:
    42
    Location:
    OREGON
    I've rolled a lot of tubes & am positive the ones I have in the sockets are good, and can find no broken connection's, I don't know how to test if I am getting any power to the two 12AX7's in the phono stage I've followed every wire all the way and can see nothing obvious..
     

     

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  7. Tim D

    Tim D AK Subscriber Subscriber

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    Given that the V14 and V15 are apparently lighting up, Dave is spot on as to what the likely causes must be for the filaments not lighting up on V16 and V17 and the impacts to other circuits - particularly after you get it fixed.

    Some other things to consider....
    - Was it working for you and then it stopped? Or did you purchase it in this condition from someone else?
    - If you purchased it this way, does it look like someone was in there modifying any of the wiring around those sockets?
    - Could there be a short somewhere in that leg of the filament circuit? This is probably less likely because it would affect the tubes on the parallel circuit (V14 and V15) and possibly would have damaged the power supply for the filament circuit, but it is something to consider.

    Do you know how to use a multimeter to check the DC voltages and do you have a multimeter with a high enough DC Voltage range? Are you comfortable with measuring high voltages safely? If not, do you have access to someone that can do it for you (safely)?
     
  8. Mikeorg

    Mikeorg New Member

    Messages:
    42
    Location:
    OREGON
    Tim D I'm doing this piece for a friend, He gave it to some guy to work on years ago and it came back not working so he knowing that I like ~as a hobby~ to bring back to life tube gear " fairly successfully with a lot of research and questions & mostly help" I've learned to recap and a amount more, I have a multimeter and have not done any thing with it but check continuity but if I am to move forward in my hobby/past time/passion I will and am willing to learn to us it to it fullest! IMG_4081.JPG IMG_4082.JPG IMG_4084.JPG IMG_4085.JPG IMG_4086.JPG
     
  9. notdigital

    notdigital AK Subscriber Subscriber

    Messages:
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    NYNY
    If you have an alligator clip, ground the black probe to the chassis.

    BEFORE you start, make sure you have identified the pins; tube pins are sequentially numbered on the underside;

    Power up; if you have a variac set to 117vac;

    CAREFULLY, place tip of red probe to the tube pin # you want to check. Write them down and post. You can refer to the schematic for the ideal/lab values.
     
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  10. Tom B

    Tom B AK Subscriber Subscriber

    Messages:
    1,433
    Location:
    Bay Ridge, Brooklyn New York
    From looking at the pictures.
    Follow the white wire that goes from the 2 purple caps tied together on the - side to where it goes by the preamp tubes. It probably connects to pin 4 on either V15 tube, or V17 tube. I would assume that there would be a parallel wire connecting pin 4 of V15 and V17 tubes. There should then be a wire connecting pin 5 of V15 to pin 4 of V14. Also there should be a wire connecting pin 5 of V17, to pin 4 of V16. pin 5 of V14 and pin 5 of V16 should be connected to chassis ground.
    With this in mind using voltmeter as described above, test voltage at pin 4 of V17. It should read -22vdc.
    Now check pin 5 of V17, it should read -11v. If it reads 0v then the filiment of v17 is bad. If it reads -22v the problem is further out.

    Now check pin 4 of V16, it should read -11v. If it reads ov and V17 was reading -22vdc, then the jumper wire from pin 5 of v17 to pin 4 of V16 is loose or missing. If pin 4 reads -22vdc then go to the next step

    Now check pin 5 of V16, it should read 0v. But a bad filiment of V16 would also result in a 0v reading. Remove V16 and do an ohms test of pin 4 to pin 5. It should read low ohms.
    If it reads -22v then the chassis ground connection from pin 5 of V16 to ground is loose or missing.

    It is apparent that someone did the solid state rectifier mod for this unit and replaced to original selenium rectifier as well as replacing the dual cap with the 2 purple axial caps on the bottom left. They may have been trying to measure voltage and increase or decrease the -22v and messed with the wiring. That -22vdc from that supply supplies filiment voltage for the preamp tubes, as well as control grid voltage for the output tubes.
     
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  11. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
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    Location:
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    On the phono preamp tube closest to the volume control, check the wiring for pin 5. There is a short jumper connecting this pin to the center ground barrel in the middle of the socket, but then the ground barrel itself needs to be grounded to the chassis as well, and does not appear to be in the photo provided. Check if this is so -- as if so, then the heaters of the phono preamp tubes will not light until a ground connection is made to the center ground barrel of this socket.

    Dave
     
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  12. Mikeorg

    Mikeorg New Member

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    It's grounded Dave. IMG_4097_LI.jpg
     
  13. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,673
    Location:
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    Good. That eliminates that possibility. It was hard to be certain with the original pic. OK. Make certain there are no internal breaks in the yellow wire going from pin 4 of this tube over to the other phono preamp tube. A closeup of the other phono preamp tube's wiring would help as well.

    Dave
     
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  14. Mikeorg

    Mikeorg New Member

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    42
    Location:
    OREGON
    OK the Continuity of the yellow wire from pin 4 on V16 to pin 5 on V17 is good, here is a photo of the V17 socket wiring. IMG_4100.JPG
     
  15. Mikeorg

    Mikeorg New Member

    Messages:
    42
    Location:
    OREGON
    I'v tested the 4 pin of V17 and got -25v & then tested pin 5 of V17 and got 0v next I tried 4 of V16 and got 0v????
     
  16. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    OK, it's narrowing down pretty quick then. The only things it can be are:

    1. The tube in V17 is bad, or

    2. The pins 4 and 5 of the tube or tube socket at V17 are dirty, or

    3. The pin 4 and/or 5 receptor terminals inside the V17 tube socket are broken.

    Based on the information supplied, it must be one of these three things.

    Dave
     
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  17. Mikeorg

    Mikeorg New Member

    Messages:
    42
    Location:
    OREGON
    I have checked the continuity of the pins in question and the V17 pins seem to be fine, I've rolled the 12AX7 with tubes I know to be good and still nothing...
     
  18. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
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    Location:
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    Mike -- What you have is conflicting evidence, which is not uncommon in the initial stages of troubleshooting. In the current example, you state that (relative to ground) there is -25 vdc at pin 4 of V17 while 0v at pin 5, and 0v at pin 4 of V16 with pin 5 of that socket confirmed as grounded. And, you've stated that neither tube is glowing. In taking these as absolute facts, then either:

    1. One of the items in my post 16 must be true, or if not, then

    2. One or more of the facts as given are in error.

    There can be no other possible outcomes here if the facts as stated are true. What happens so often however, is that something you think is true, is in fact not true.

    Think of it this way. If the product of a multiplication problem is 20, and one factor is 5, then the other factor must be 4. What's happening here however is that we're saying that the product is 20, one factor is 5, but the other factor is (say) 6, which cannot be true. Either the product is incorrect, or one or both of the factors are incorrect -- one or the other, but as is, something presented as true is clearly not true.

    For the case at hand, IF there actually is -25 vdc at pin 4 of V17 as stated, and IF there is 0V at pin 5 as stated, then either:

    1. The tube or tube socket's pins are dirty, or

    2. The tube is bad (as in an open heater), or

    3. The receptor pins in the socket are broken (as in breaking the circuit).

    The only other possible scenario if there is -25 vdc on pin 4 of V17 and 0 v on pin 5, would be the case of pin 5 of V17 somehow being shorted to ground. But if that were the case, and the tube's and socket's pins are all good, and tube is good, then its heater would be glowing wildly, since it would be receiving twice the normal working voltage. But you stated that the heaters are not glowing. So once again, the facts as offered don't fit. Maybe you have multiple bad tubes that you are assuming as good, but are not, or various other possibilities.

    It sounds rediculously elementary, but assume nothing, and retest everything. The answer lies in going back and finding out what you think is true that is not. Once you find that, then you can find the answer to the problem. That is the only way to solve problems that otherwise would seem to defy explanation, like the one you have in front of you.

    Dave

    Dave
     
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  19. Mikeorg

    Mikeorg New Member

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    Location:
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    I will retest and rethink, thank you Dave.
     
  20. baconbadge

    baconbadge New Member

    Messages:
    36
    Location:
    Queens, NY
    Before you test again, I would highly recommend a thorough cleaning of the sockets, a tightening of all the pins and maybe reflow the solder joints. Troubleshooting is about eliminating variables.
     
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