Picked up a non-working Fisher 500C

Discussion in 'Fisher' started by Lo-Fidelity, Dec 15, 2017.

  1. larryderouin

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

    Messages:
    20,459
    Location:
    Glen Burnie Md.
    Yeah. Check your plate and screen voltages on your output tubes 1st tho. You may need Filter can caps. Also check the voltages coming off the big can (bias cap) underneath. DC VOLTS and the highest setting you have (600V?). If the voltages are 10%-15% high you may need new output tubes. We'll deal with that later. But for now get the metalbone kit. Plates are pin 3 (420VDC), and Screens are Pin 4-8 (375VDC) (There should be a wire lead (Bare) running across all 4 tubes for the screens. Just check at the 1st tube where the supply wire (usually orange) hooks to the 1st tube.) Post the voltages .

    The Metalbone kit is basic and addresses the basic stuff (coupling caps and such.) but the documentation is great! You get a manual that is specific for the range your serial # falls into. And i think he throws in a single sheet schematic. (Check the site).

    Main Filter caps you'd have to get elsewhere. www.hayseedhamfest.com has can cap kits that include the cardboard covered one, and the bias can underneath. I would suggest getting the the single big "Dual can" vs. the individual caps as you are just starting out and it's a simpler installation.
     
  2. Tim D

    Tim D AK Subscriber Subscriber

    Messages:
    732
    Location:
    Melbourne, FL
    Yeah. Sorry about that Larry. Got distracted as more hordings keep coming in. The most recent is a set of AR-4X's I'm working on for my daughters birthday. I think I needed that push. Time to get out the KM-60!

    And now back to our regularly scheduled Fisher 500C.
     
  3. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

    Messages:
    668
    Location:
    Houston
    All measurements at 117v from variac.

    check the voltages coming off the big can (bias cap) underneath.
    -18.9v from neg side of rectifier to cap
    3.96v from plus side of rectifier to cap
    0.0 mv to ground

    Plates are pin 3 (420VDC)
    V11. 425v
    V10. 427v
    V9. 425v
    V8. 428v

    Screens are Pin 4-8 (375VDC)
    374.7
     
  4. larryderouin

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

    Messages:
    20,459
    Location:
    Glen Burnie Md.
    Sorry. I gave you some BUM DOPE. The Plate Voltages should actually be 430V not 420V. These are off by at most 1% so call it good. Screens are pretty much dead on. Bias voltage is ok.

    Now with those ok. Check the heater voltages on tubes V14-15-16-17. They are on a -22V buss off the rectifier and are wired in series parallel. follow the heater wire from the bias cap, and take voltage on the 1st tube on pins 4 & 5. One should show about -22V and the other about -11V. Trace the lead to the next tube and do the measurements again, This time the voltages should be about -11v and 0V(ground). Do for the other pair. The voltages may or not be balanced. But they should be fairly close (within a volt or so).

    If the voltages are lower than -10.5v you'll need to adjust the input voltage on the heater line with a resistor and a cap (R/C network). To get the voltage right, Put a 50K pot inline set up as a variable resistor (connect pins 1&2 to one side and pin 3 to the other). Now adjust the voltage to about -11.5V. Turn off the unit and remove the pot without jarring the pot. Take a reading across the pins 1&3. Get a 1w resistor that is as close as you can get, and a 100uf 50V cap. As the voltage is NEGATIVE, the cap is grounded with the POSITIVE lead to Ground and the Negative lead to the line. Whether the cap is before or after I don't know as I've never done this as my heaters are usually right on. One of the other guys would probably know.
     
  5. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

    Messages:
    668
    Location:
    Houston
    Tomorrow's fun. Thanks!
     
  6. audmod01

    audmod01 AK Subscriber Subscriber

    Messages:
    1,031
    Location:
    Tioga, TX
    Lo-Fidelity;

    Good score! You will love your 500-C! These are hard to beat and include a lot of functionality in one package. This group has all the necessary expertise to get your unit to peak performance. Good luck with the rest of the restoration!

    Joe
     
  7. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

    Messages:
    668
    Location:
    Houston
    Check the heater voltages on tubes V14-15-16-17.
    Pin 4 Pin 5
    V14 -18.6v -9.3v
    V15 -9.3v. -0.1mv
    V16 -9.3v -0.2mv
    V17 -18.6v -9.3v

    Larry, are these values lower than expected due to the rectifier putting out 18.9v instead of 22v? The 9.3 would be 10.8 using a linear 22/18.9 ration.

    Should I turn the variac up to line voltage (125v)?
     
  8. larryderouin

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

    Messages:
    20,459
    Location:
    Glen Burnie Md.
    Don't turn up the variac to 125. 125 is too high for extended use on the 500c. Those heaters are freezing. Which accounts for extended warm up time to run, and possibly more distortion.

    What is the voltage coming off the NEGATIVE(-) post of the rectifier? If it's between -22 and -24v, You should be able to tap off that and put a 100uf 50v cap in the line (positive lead to ground). You will probably either have to use a variac or a bucking transformer to drop the line voltage down to between 115 and 120.
     
    Last edited: Jan 3, 2018
  9. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

    Messages:
    668
    Location:
    Houston
    I'll leave the variac set for 117v.

    Voltage from the old selenium rectifier is -18.9v.

    Guess I need to order a new rectifier and other parts.

    I haven't heard any distortion at lower to medium volume on KLH Sixes. The unit sounds great for the limited amount of time I have had it running.
     
  10. larryderouin

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

    Messages:
    20,459
    Location:
    Glen Burnie Md.
  11. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

    Messages:
    668
    Location:
    Houston
    Well I am further considering my options relative to kits and caps. Hope to order something in a few days.
     

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