Fisher 80-C Restoration

Discussion in 'Tube Audio' started by multichamp12, Feb 9, 2018.

  1. multichamp12

    multichamp12 AK Subscriber Subscriber

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    Hey everyone, I recently picked up a pair of Fisher 80-C mono preamplifiers and since there is not much info out there on them, I though I would post the rebuild. Both of these preamps have the much harder to find red lettering on the faceplate, instead of the black. I believe the red faceplates were earlier models, but I am not certain. Both serial numbers start with "15". The one with the cabinet was purchased here on AK, and the other was purchased locally.

    One unit is completely original, and the second one just has one of the multisection capacitors replaced (its taller than the original can). To my surprise, both of these units run great, no hum or buzz, but they will still be checked out so they are safe to use. Multi-section capacitors will be replaced, most likely with new cans from hayseed hamfest. The selenium rectifier and bridge rectifier will be replaced with modern 1n4007 diodes, and all resistors and capacitors will be checked. More to come soon!

    Link to manual: http://vintagevacuumaudio.com/schematics-manuals/fisher/fisher-80-c-service-manual-schematics.pdf

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    Last edited: Feb 9, 2018
    drtool likes this.
  2. AlTinkster92

    AlTinkster92 AK Subscriber Subscriber

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    Those are beauts! Would be a great idea to move this to the Fisher Forum where good help exists! Al
     
  3. s-petersen

    s-petersen Scott Subscriber

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    I had a pair of those also, and despite not having been rebuilt, they were also quiet as well
    P1010022.JPG P1010017.JPG P1010012.JPG
     
  4. multichamp12

    multichamp12 AK Subscriber Subscriber

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    If a moderator wants this in the fisher forum that's okay with me, I thought I would post it here for those who do not regularly check the fisher forum though.

    Yes these are very well built, with potted power transformers. Fisher made 2 different types of Serial Number IDs on the 80c. Mine both have a blue sticker on the back cover, the other type is printed onto the cover as s-petersens picture shows,
     
  5. drtool

    drtool It might get loud In Houston Subscriber

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    :lurk: Very nice. Love the slant face.
     
  6. DOBIEGUY

    DOBIEGUY AK Subscriber Subscriber

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    I'll be watching your restoration thread. I've got one but it's a 90-C. I think it's the same but with the addition of a rumble filter. A console pull, it sports all Fisher tubes and looks like it was made yesterday. Dead quiet,too.
    I can't justify changing parts even though they are fifty plus years old. It just sounds so damn good.
    Good luck with your pair. image.jpeg image.jpeg
     
  7. multichamp12

    multichamp12 AK Subscriber Subscriber

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    Sweet 90C! yes I believe it is the same besides the rumble filter and presence control
     
  8. multichamp12

    multichamp12 AK Subscriber Subscriber

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    I started on the first preamp yesterday. I am using film caps to replace the low value electrolytic capacitors, so the first cap off the full wave rectifier was replaced with a 10uf solen cap. It just barely fit! The full wave rectifier was replaced with 2 1n4007 diodes. I did not want to drill any holes in the chassis, so I used the existing hole from the selenium rectifier to mount the terminal strip for the diodes.

    I put a temporary jumper across the last 2 tabs on the terminal strip (going from the cathode end of the diodes to the 10uf cap) that can be removed and replaced with a 5w resistor to simulate the voltage drop of the original selenium rectifier.

    According to the sams photofact, the full wave rectifier is rated at .007a, and the voltage off it should be 250v. The voltage before I replaced anything was 243, and as expected, after putting in the diodes the voltage went up to 270.

    I am a bit confused as to how to calculate what resistor value I should use as a dropping resistor though. Can someone walk me through how the math works? Is using ohms law R = Voltage Drop/Ampere correct?

    Thanks!

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  9. gadget73

    gadget73 junk junkie Subscriber

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    You'd need to know what the current draw is to work something out.

    Try a 47 ohm resistor between the diodes and the capacitor and see what you get. Measure the voltage across it so we can figure out the current, and see what the output is.

    at that point its R = needed voltage drop / known current

    to work out the wattage of the resistor, its voltage drop * current. Double that and round up to the nearest common size.

    also I think I gave you some bad advice via PM. I hadn't seen the pics, and when you asked about a bridge rectifier I was assuming one of those flat pack jobs like the later receivers use. Those are 4 diodes in a pack. This is a full wave center tap setup, it only uses 2 diodes. For this, I would probably go with individual diodes mounted to a terminal strip, probably attached where the original selenium rectifier was mounted.
     
  10. multichamp12

    multichamp12 AK Subscriber Subscriber

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    Okay thanks. I will try a 47 ohm 5 watt resistor and post my findings.

    There are 2 rectifiers in the 80c, one is a full wave (M1) and one is a bridge (M2) unless I'm mistaken.
     
    Last edited: Feb 11, 2018
  11. gadget73

    gadget73 junk junkie Subscriber

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    looking at the Sams does indeed show a bridge for M2. Looks like a standard 2 diode full wave selenium jobbie though. How many lugs are on it? 3 says its just 2 diodes. 4 is a full wave bridge. I can't quite tell from the pics.
     
  12. multichamp12

    multichamp12 AK Subscriber Subscriber

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    M2 has 4 lugs. I was going to use 1n4007 diodes to replace it, but Dave recommended using 1n5408 which are rated at 3a so I will have to order some since I don't have any on hand.
     
  13. gadget73

    gadget73 junk junkie Subscriber

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    gotcha, bridge then.

    Its running tube heaters, so the current capacity is a good thing. If the voltages end up too high you should be able to bump that 15 ohm resistor after the resistor a smidge to compensate. I wouldn't be surprised if it comes out fine though. Low voltage selenium doesn't require a lot of sections, so its not as lossy.
     
  14. multichamp12

    multichamp12 AK Subscriber Subscriber

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    I put a 5w 50ohm resistor between the diodes and the cap, and it dropped the voltage about 5v, from 270 to 265. The voltage across the resistor is 375.

    Is current calculated by V / R = I?
     
  15. gadget73

    gadget73 junk junkie Subscriber

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    Voltage across the resistor is 375 volts? That doesn't make sense, also that would be dissipating such an insane amount of heat you'd probably be able to read by it. It should be 5 volts if thats what it dropped.

    I=5/50 so 100ma.

    Need to drop 20v so R=20/.1 or 200 ohms should get it done in theory.
     
  16. multichamp12

    multichamp12 AK Subscriber Subscriber

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    I believe my multimeter is starting to act weird, so I am using a different one now. I put in a 200 ohm resistor though, but am getting some odd results.

    With 200 ohms I am getting 1.5v across it, and around 265-268v volts at the 10uf cap (my power fluctuates) which is not making sense to me, since the voltage didnt drop after going from 50 to 200 ohms. At the 2nd cap (40uf) I am getting around 240v.

    Although, after checking all my other voltages, everything seems to be pretty much spot on. I am no longer going off the voltage readings in the sams photofact, since when comparing it to the original fisher manual, there are some differences in regards to the voltage readings. http://www.vintagevacuumaudio.com/schematics-manuals/fisher/fisher-80c-owner-manual-schematics.pdf
     
  17. gadget73

    gadget73 junk junkie Subscriber

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    might be reading AC + DC, which may throw off the reading across the resistor.

    doesn't make a lot of sense that going from 50 ohms to 200 would not alter the voltage, but if the voltages at the tubes end up right, I wouldn't worry too much about it.
     
  18. multichamp12

    multichamp12 AK Subscriber Subscriber

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    it's strange, but yea I'll keep it as is for now and compare it to the other preanp when I start on that one.
     
  19. multichamp12

    multichamp12 AK Subscriber Subscriber

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    I got some more work done on the preamp today. 3 capacitors on the terminal board were replaced with illinois film capacitors, and a sprague atom for the 25uf cap. The rest were replaced with NOS russian pio K40Y-9Bs, except for the 1uf 250v electrolytic cap which was changed for a Solen film capacitor, just like the 10uf cap in the power supply. The caps for the tone controls that dont see much voltage were left alone so I can preserve as much of the original parts as possible.

    All of the resistors are within 10%, so they will be left alone, with the exception of the resistors in the power supply which will be replaced with military vishay dale CMF60's. I am just waiting on my can capacitors, and a bridge rectifier to arrive, then this preamp should be ready to go!

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  20. s-petersen

    s-petersen Scott Subscriber

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