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Picked up a non-working Fisher 500C

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

  1. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
    21,791
    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.
     

     

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  2. Tim D

    Tim D AK Subscriber Subscriber

    Messages:
    1,237
    Location:
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    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:
    957
    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 Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
    21,791
    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:
    957
    Location:
    Houston
    Tomorrow's fun. Thanks!
     
  6. audmod01

    audmod01 Super Member

    Messages:
    1,219
    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
     

     

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  7. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

    Messages:
    957
    Location:
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    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 Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
    21,791
    Location:
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    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:
    957
    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 Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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  11. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

    Messages:
    957
    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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  12. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

    Messages:
    957
    Location:
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    Metalbone kit in hand and can caps order.

    Hope to replace the rectifier and bias cap this weekend and recheck voltages.

    Ran the unit for about an hour tonight and measured voltages as below. Sounded great through recapped KLH 6 speakers. Line power coming through variac at 117v.
    Negative lead from rectifier. -18.6v measured versus -22v target
    V8-11 pin 6. -13v measured versus -17v target

    Do you think the new rectifier will improve / increase these voltages?
     
  13. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Absolutely. Selenium Rectifiers (especially those Siemens rectumfriers) will drop voltages over the years. Rebuild the Bias supply per the Metalbone kit including the rectumfrier, and the dual bias cap(s).
     
  14. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

    Messages:
    957
    Location:
    Houston
    Upgraded power supply with new C97a&b, rectifier CR6, and capacitor C132.

    Negative lead from rectifier measures -24.7v versus -22v target.
    V8-11 at pin 6 measure -17.35v versus-17 target
    V8-11 at pins 4&8 measure 395v versus 375v target
    Voltage upstream R143 measures 440v versus 430v target

    Larry, Dave, what do you think? Ok to move to the B+ diodes and C102&103?

    Unit still sounds great!

    Dave, you will be glad to know that I can now bleed the cap voltage without welding the alligator clip to the chassis.
     
  15. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,673
    Location:
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    So you dropped out of arc welding class eh? That's a very good thing!

    The voltages to the output tubes are fine for now if you're still using the original (or vintage American) output tubes. You won't be able to fine tune the output stage operation until you get the 10Ω cathode resistors and 100Ω Screen Stability resistors installed at each socket, so you can know how the tubes are actually performing individually. Finish out the power supply work, and then also install the 100Ω Screen Stability resistors right away as well, so you don't risk increasing the chance of output tube arcing due to the installation of modern low ESR value power supply caps. Then, you can move on to the rest of the output stage work.

    Congrats on your progress!!

    Dave
     
  16. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

    Messages:
    957
    Location:
    Houston
    Thanks Dave. These are the original tubes.
     

     

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  17. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

    Messages:
    957
    Location:
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    Dave
    Do I need to pull the tubes before adding the 10 ohm resistors?
     
  18. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    In a word no. But for the sake of keeping the tubes in one piece when it's flipped over, YES. Do note that on one of each pair of output tubes there should be or is a 220 ohm resistor from pin 4 or pin 5 connected to the cathode ground. This needs to be removed from the cathode and grounded separately (R85 & R120). There is the possibility that they are located near the Main filter caps in the power supply so make real sure they are located and relocated off the tube sockets to separate grounds before power up after installation of the 10 ohm cathode resistors.

    Same deal for the Screen stability resistors. Make absolutely sure that the plug is pulled from the wall, variac, DBT etc, before sticking your fingers inside the unit.
     
  19. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

    Messages:
    957
    Location:
    Houston
    Larry, I put in the 10 ohm to ground and fired it up. Sounds good and I didn't notice anything unusual.

    Here's a picture before I put them. Do you see the resistor your talking about? I may walk out to the shop and look later. The risk is I'll end up staying out there too late.

    IMG_0968.JPG
     
  20. Lo-Fidelity

    Lo-Fidelity AK Subscriber Subscriber

    Messages:
    957
    Location:
    Houston
    Also, how do these tubes look to you guys? Is this blue tint normal?

    47EBA8F7-2B17-405F-9153-620C58A2D347.jpeg
     
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