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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    I bought a Fisher 500C at an estate sale. The guy there said if was working earlier in the day but was not when I arrived. It was plugged in.

    Got it home and brought it up slowly on a variac, but no sound at all. Approximately .7 amps at 120volts. No sound from aux, radio, or tape in. Did not test phono. The on/off does not shut off. Is it possible that it was working and someone jacked the volume knob around and killed it?

    I'd love to get this working someday. Any thoughts from the Fisher team would be appreciated.

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  2. rufleruf

    rufleruf AK Subscriber Subscriber

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    Dig that enclosure!

    Could be all sorts of things not happy making it not work - probably just a flat tire from cracked sidewalls. Post some underside pictures with the bottom removed and pictures of the deck and tubes.
     
  3. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

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    Another angle. Dual 1009 in the top.

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  4. larryderouin

    larryderouin You can be sure if it's Westinghouse??????? Subscriber

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    If no sound and voltage measurements are ok, I'd look 1st at the jumpers on the Reverb jacks.
    1.) Make sure they are actually there. They must be installed for operation.
    2.) If there, make sure they are in the CORRECT HOLES. OUT to IN (LEFT), and OUT to IN (RIGHT), Not out to out and in to in.
    3.) Make sure they aren't pushed all the way in so that they are contacting the outer ground connection on the RCA JACK. Pull them out 1/4" ensuring they are still tight in the jacks.

    4.) If they are missing totally, take a SHORT DUAL RCA CABLE and install it in place of the jumpers. RED RIGHT, LEFT WHITE.
     
  5. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Besides Larry's excellent suggestion, here's a couple more to check if the Reverb Jumpers are not the issue:

    1. See if you get sound using the Tape Monitor inputs. This feature injects the signal later in the circuit; whether you do or do not get sound when trying it this way can help to establish where the signal is lost in the circuit for the other inputs and tuner function.

    2. Check to make sure that the two 12AX7 tubes mounted closest to the front of the set in the middle of the chassis are lit up when the set is turned on and operating at full voltage. The heaters for these tubes are wired in series; if the heater in one tube goes open then neither tube will light up, and the set will appear dead to all inputs.

    Also try connecting your speakers directly to the output impedance terminals of the set. Connecting them this way will bypass any potential issues with the Speaker Selector switch.

    If none of these possibilities bring the problem to light, then you're likely into troubleshooting under the chassis. That's obviously more involved, but there's plenty of great folks here that can help you with that too. If it comes to that, please provide some good closeup pics of the underside (section by section) so we can get a better idea of the set's actual condition.

    Good luck with it!

    Dave
     
  6. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

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    Thanks guys. I'll try to take a look today.
     
  7. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

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    Looks like jumpers are in place. I pried them about an 1/8 inch and retested tape monitor, aux, and tapehead. No sound at all. Not even static. Tried with speakers connected to impedance terminal with same result.

    The 12AX7 tubes have a glow but less than the other small tubes.

    Larry where did you want me to check the voltage?

    The guy at the sale said it was working at one time during the day, but was not working when I got there. I had all ready told him I wanted when he told me this some he had no reason to lie.

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  8. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Next best guess then is the long green resistor near the bottom left corner in the second pic. If it has opened up, you'll get nothing from nothing. They have been known to fail just from heat and age, but failing power supply caps cab hasten their demise. It is a 1.2K 7W resistor, and is best to be replaced with a resistor of the same value, but of a 10W rating. Even with the resistor still in circuit, you should get some reading of resistance, but it if just keeps going higher and higher -- well past 1.2K, then it has likely gone open, which is not entirely uncommon.

    If that resistor is in fact bad, before checking it, it would be a good idea to momentarily short each side of the resistor in question to ground (after the unit has been turned off for at least an hour or so) -- this to make sure that any B+ voltage in the set has been discharged, so as not to damage your Ohmmeter if it hasn't.

    Let us know!

    Dave
     
  9. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

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    Wow! Not sure an hour is long enough to wait. That thing popped like a mo-fo when I grounded the inside leg. I'll need to get my nerve up to ground the outside leg.
     
  10. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Lo -- That is a strong indicator that the resistor identified is bad. It supplies power to virtually all of the set except the output tube plates -- and they won't draw any current if that resistor is bad, that then naturally allows the caps in the set to discharge at turn-off. I'm sorry it scared you, but it is precisely because I didn't want you to damage your meter (or hurt yourself!) in checking that resistor if the caps were not discharged. So, it was a stop gap safety measure to protect both you and your meter.

    Normally, you want to discharge caps through an appropriate resistor, which prevents such a spark. But not knowing your skill level or stash of any parts on hand, I just had you do it a sure-fire way to get the job done. But again, the fact that such a charge was still available indicates that the set is largely not drawing any current from the power supply. That can generally only happen if either:

    1. The tubes are not lit up, or

    2. The power resistor identified is open.

    If you have (say) a 10K 2W resistor, it is always best to discharge caps through a resistor like this. But failing that, getting the caps discharged one way or another is always preferable to not discharging them at all!

    There will likely be little if any charge on the other side of the resistor. Once your sure both sides of the resistor are completely discharged (short each side to ground (through a resistor if possible) at the same time for about 60 seconds), then remove the discharge leads and check the resistance across the resistor.

    Let us know!

    Dave
     
  11. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

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    Dave,

    Awesome stuff!

    I used a10W 600 ohm resistor to ground the outside leg. Not ideal, but all I have sitting here. Anyway this one was drama free.

    Resistor has no conductivity! Good call.

    Does anything else get damaged when this fails?

    image.jpg
    Check out the burn shadow on the center of case edge.
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  12. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Yep, if anything, it shows just how much energy the voltage doubler caps in the set can hold -- and it's a bunch! All of which shows why making sure the caps are discharged before working on a set is so important -- AND PARTICULARLY SO with a disfunctional set! You may think it's dead, but it can play dead like the best Possum ever! Anyway, you're here to tell the tale, which is the important thing!

    The resistor opening up did not damage anything else in the set. The fact that it went open can be indicator that some of the other power supply caps are beginning to fail, but the much greater likelihood is that the resistor just gave up the ghost from hundreds of heating and cooling cycles. As I said, it is a part that is known to fail over a significant amount of time use in these units.

    If you've got a second 600Ω 10W resistor, you could connect them in series (one lead of one resistor to one lead of the other resistor) making a 1.2K 20W resistor, and then connect the other two free ends across the open resistor in the set, and that will likely get you back in business -- at least so you can make various voltage checks to make a proper assessment of the unit.

    In any event, it sounds like you're on your way! The 500c is a very beloved Fisher receiver and capable of very fine performance indeed. Lots of wonderful folks here can help you with just about any issue you have with it.

    Good luck, and keep us posted!!

    Dave
     
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  13. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

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    Many thanks Dave!

    I'll order a new resistor along with some caps for the KLH 6s in the photo above, and a nice resistor for bleeding off caps!

    Any suggestion on the resistor? Got a part number or supplier handy? If not I'll do some homework on Mouser.

    As to The Fisher 500C, it is my only tube gear and I am pretty happy to have gotten it. When the estate sale crew gave me a good price and threw in the KLHs it was a no brainer. The power switch is shot, but if I can get it working, then a new power switch and recap are soon to follow. Did a bunch of reading here and elsewhere, and I really like what I read about the 500.

    Did you think 0.7 amp draw was ok?

    Rick
     
    Last edited: Dec 16, 2017
  14. larryderouin

    larryderouin You can be sure if it's Westinghouse??????? Subscriber

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  15. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

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    Thank you!
     
  16. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    The .70 Amp draw was appropriate for the condition the unit was in. Once it is repaired, the current draw will increase notably (likely double) due to the current draw from the B+ power supply that was not there when you took your initial reading.

    Dave
     
  17. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

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    Ok, so we are not seeing inappropriate current draw. That's great news.
     
  18. notdigital

    notdigital AK Subscriber Subscriber

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    Lo: Not to hijack your thread but what serial number is your unit??
     
  19. larryderouin

    larryderouin You can be sure if it's Westinghouse??????? Subscriber

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    Dang! I've never seen one overheated from end to end. Usually maybe a 1/4 of the length and it's still working fine. I would strongly suggest using a DBT on the 1st power up with the new resistor and using the meter to check the current draw .
     
  20. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

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    You bet!
     

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