Fisher 400 My next project with a lot to learn

Discussion in 'Fisher' started by curtsad17, Jul 11, 2018.

  1. curtsad17

    curtsad17 Member

    Messages:
    72
    Location:
    Media, PA

    Mystery solved... I brought it up on the variac after the install. Only brought it up to 105 ac. Interesting how the one tube increases current draw more than the other three as ac line voltage rises. I was able to repeat it. Amp sounds equally as good at 105 ac.
     

     

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  2. curtsad17

    curtsad17 Member

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    72
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    Media, PA
    double post
     
  3. curtsad17

    curtsad17 Member

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    72
    Location:
    Media, PA
    I finished up with the b+ power supply modification. I took my time and was careful. I powered it up slowly on the variac just in case, checking voltages along the way. It all looked good, so I installed the output tubes." Music" to my ears. which are not so good , so I can't say I detected any difference in sound quality. I have been reading about failing capacitors and how they will generate heat. I wanted to feel how cool my new top mounted caps were running. When I got to c56 ( newer schematic) I felt a tingle, a 199 volt tingle. The aluminum top to ground. Do I have a bad cap or , I have been doing some reading , I found this from the Nichcon site:



    Aluminum electrolytic capacitors must be electrically isolated as follows: The aluminum case and the cathode foil are connected by the unstable resistance of a naturally formed oxide layer inside the aluminum case and the electrolyte.

    The capacitor I use is a 680uf snap in cap. I made the assumption that because of the two terminals, the case was isolated. I guess I was wrong. The original 100uf Cap's case was the cathode and was isolated with the black cardboard and phenolic insulator.

    Does anyone know of an easy way to get the black cardboard cover off the old capacitor. I think I need to reuse it.
     

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  4. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
    21,696
    Location:
    Glen Burnie Md.
    You can remove the cardboard cover with a hairdryer set to med or hi. The adhesive is TAR Based.

    Now as to your install of C56. As it's not actually grounded to anything and it's part of the voltage doubler with C 57 c (200uf) up front, the case carries roughly 200VDC which if it touches the chassis, is a BIG DEAL BREAKER for it running. If you insist on keeping the cap there,you'll need to insulate the whole cap, as the cover will be too big. You can wrap the cap in electrical tape but it looks tacky! The best option is to open the bottom of the OLD CAP, remove the internal contents, and drill two holes in the bottom for the leads from a NEW cap (100uf is fine as the 400 is designed for that) and solder the leads to the lugs on the can cap bottom. Id the lugs and attach the top with an Epoxy. Let sit over night then install in unit. Use the cardboard cover, or you WILL get NAILED! This isolates the new cap electrically and physically.
     
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  5. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    36,097
    Location:
    Southern NJ
    I've been lit up by voltage doubler cans before. It sucks. The one in my Bogen runs +350vdc above chassis. That hurt.
     
  6. curtsad17

    curtsad17 Member

    Messages:
    72
    Location:
    Media, PA
    Thanks Larry for the info on the tar adhesive. I did what you said and I was able to slip the cardboard cover off. I did the power supply mod developed by sony 6060 . It uses 2- 680 uf 400 volt caps in place of the 110-200 uf originals.and a choke. My caps are 35 mm OD so the cardboard cover slipped on like a glove. I was able to enlarge the clamp enough by adding a longer machine screw so the clamp holds the cover in place.
     

     

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  7. curtsad17

    curtsad17 Member

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    Problem with EFB mod. I think I need help. I installed the board as designed by Dave G. I removed the OP tubes and slowly brought it up on the variac Voltages were rising nicely until around 100 vac R138 (15 ohm 5 watt) going to ground in the bias power supply started to get excessively hot and give of a little smoke.
    I believe my EFB board is correct and connected correctly. Any ideas?
     
  8. grindfix

    grindfix Full time hobby Subscriber

    Messages:
    441
    Location:
    New Albany, OH
    Is original rectifier still connected to it?
    I’d check your capacitors polarity first.
    60a positive to ground and 60b positive
    to original bias supply rectifier bridge.
     

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  9. curtsad17

    curtsad17 Member

    Messages:
    72
    Location:
    Media, PA
    Yes original rectifier is still hooked to it. I did not change anything in the original bias supply except disconnect the original white lead going to the bias network.
     
  10. grindfix

    grindfix Full time hobby Subscriber

    Messages:
    441
    Location:
    New Albany, OH
    Someone with experience will help soon. I wonder if that cardboard bias cap is shorted. I see from your pictures it is original. Did you replace bridge rectifier?
    Edit. I missed that you said original rectifier is still connected to resistor.
    I’d check your rectifier.
     
  11. turnhig

    turnhig New Member

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    Eddington, ME
    curtsad17 - Where did you get the test point charts. Can you post them up for others to use? Thanks.
     

     

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  12. curtsad17

    curtsad17 Member

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    Everything was working fine until I cut the white lead. I do not know how hot the resistor was getting before,but it is very hot now. Rectifier voltage checked OK prior to this.
     
  13. curtsad17

    curtsad17 Member

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    72
    Location:
    Media, PA
    I did not think about it when I took the photos of the charts , they are copyrighted. I do not believe I should post them.
     
  14. grindfix

    grindfix Full time hobby Subscriber

    Messages:
    441
    Location:
    New Albany, OH
    Do you get negative voltage out of EFB bias regulator?
    Have you tried to disconnect EFB bias supply from transformer windings and variac without tubes?
    Resistor could have been stressed before and finally failed. If it is open now, EFB bias regulator has no reference to ground without R138 completing the circuit.
     
  15. curtsad17

    curtsad17 Member

    Messages:
    72
    Location:
    Media, PA
    Output tubes are removed.
    I removed tubes 8-11 and brought the variac up to 67vac. R 138 starts to get hot at that voltage.
    My B+ is 239 volts
    Output to screen grids from EFB board is 170 volts
    Output to bias network from EFB board -1volt. It does not change while adjusting the 10k trimmer on the EFB board.
     
    Last edited: Jul 22, 2018
  16. curtsad17

    curtsad17 Member

    Messages:
    72
    Location:
    Media, PA
    Disconnected EFB board from transformer.
    Still have output from board to screen grids.
    Slight +.2 voltage at bias output, output still connected 330k grid resistors.
    R138 stays at room temperature.
     

     

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  17. grindfix

    grindfix Full time hobby Subscriber

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    441
    Location:
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    In my current build I initially mis-wired 22k resistor on EFB and blew one of the bias regulator transistors. This amp is not Fisher product and did not originally have 15 ohm resistor connecting transformer windings through rectifier to ground. I installed 2 diodes and connected junction to ground. I bet resistor would have saved my overloaded transistor. Check your EFB connections. Positive voltage while disconnected from transformer and not responding to trimmer is very suspicious. Check your transistor orientation carefully. Some transistor part numbers have a suffix that may mean that EBC orientation may be reversed or be different from traditional orientation printed in datasheets. Good luck!
     
  18. curtsad17

    curtsad17 Member

    Messages:
    72
    Location:
    Media, PA

    Thanks for the advice . Luck I will need. I will pull the board and check the build. My bet is I made a mistake somewhere. I will check the transistor orientation. I will disconnect the bis lead from the 330 ohm resistors and see if in fact there is any output with the led disconnected.
     
  19. grindfix

    grindfix Full time hobby Subscriber

    Messages:
    441
    Location:
    New Albany, OH
    Post better picture of EFB connections side. Try to take square shot so all connections are easier to see. Someone will spot an error if there is one.
     
  20. curtsad17

    curtsad17 Member

    Messages:
    72
    Location:
    Media, PA
    Ok here are some shots. If someone can spot an error I will be amazed. Not because I do not think there is one, there most likely is. I takes me quite some time to trace it looking at the schematic and flipping the board back and forth. I will start now. 3:36 PM
     

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