Fisher 400 My next project with a lot to learn

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

  1. curtsad17

    curtsad17 New Member

    Messages:
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    Location:
    Media, PA
    Thanks for all the help on my first project SA300. It is sounding good and only caught fire once.

    I had a Fisher 400 given to me. All fisher branded tubes. The fuse was blown, I believe that was a good thing. I slowly brought it up on a variac with dim bulb. Deoxit. Both channels played with distortion on FM. Amazingly with CD input it sounds pretty good.

    I installed the 10 Ohm Cathode resisters and rear lugs to test it. .377 .284 .288 .285.

    My power supply voltage tests look pretty good . see photos.

    I plan to do Dave Gillespie's mods one at a time starting with the EFB mod. Later IBAM, and Power supply modification.

    I have read the "Improving the Fisher 400" thread numerous times, and will be using it as my guide.

    Any suggestions are surely welcome and thanks in advance for all the help I will need.

    Curt
     

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    Last edited: Jul 12, 2018

     

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  2. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Curt; Good Job so far. Where did you get the test point charts. Can you post them up for others to use? If so we can make them a sticky, and also modify them for 500 and 800c or the 500/800B. If you run into problems or have a question, ASK! No DUMB QUESTIONS HERE (well, maybe a couple of really off the wall ones:D).

    Larry
     
  3. gadget73

    gadget73 junk junkie Subscriber

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    Just a suggestion, but I'd probably start with the power supply caps, then the IBAM or bias/balance (my preference) before doing EFB.
     
  4. curtsad17

    curtsad17 New Member

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    Location:
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    I built my EFB board today. My first attempt at anything like this. It took quite a while, I am glad I do not need to make a living building these. I am open to constructive criticism, please don't hold back. I need to learn.

    I would love to test this some way before installing. I remember some discussion about that. I will need to find that discussion and impalement it.

    Gadget, Thanks for the advice. Even though I just built the EFB board I can hold of installing it. I was a little anxious to try and build it.

    IMG_7542.JPG IMG_7543.JPG
     
  5. gadget73

    gadget73 junk junkie Subscriber

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    If you have a couple of power supplies you can mock it up. I've done it using a 0-160v supply to sub for the B+ feed and a 15v for the bias side. Basically I just vary the voltage on the B+ supply to see what the screen and bias output does. Reducing B+ should shift both. I never trusted my work enough to just put it in and give it hell with a few hundred volts without a test run first.
     
  6. grindfix

    grindfix Full time hobby Subscriber

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    I'm not sure Mosfet heatsink is sufficient. It may not get very hot at idle but will at higher loads. Most people here mount it directly to the receiver's chassis.
     

     

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  7. curtsad17

    curtsad17 New Member

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    The heat sink is also the mounting bracket. It makes a 90 and continues underneath the board where It will be mounted to the chassis and then makes another 90 and will be against the bracket for the tuning flywheel. I did not plan on bolting it to the flywheel bracket, I thought attaching to the chassis with the existing sheet metal screw would be sufficient.
     
  8. curtsad17

    curtsad17 New Member

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    Location:
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    Exactly my thoughts. When I though I was done building the board I had a 22k resister left, I did not realize I had omitted it.

    Is it possible to use the 400's B+ power supply and 115 ac? I could vary the B+ and the AC with a variac, or leave the ac fixed at line voltage.
    .
     
  9. grindfix

    grindfix Full time hobby Subscriber

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    It may be sufficient. The way I see it though, bracket may be the bottleneck in heat transfer. Let's see if anyone else has opinion on it.
     
  10. gadget73

    gadget73 junk junkie Subscriber

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    Don't see why not. I'd probably stick a current limit resistor in the mix just in case something goes badly. 10K should do it, its not like the circuit will be using any amount of current just to output to a voltmeter.
     
  11. fred soop

    fred soop Super Member

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    Put some thermal compound on all metal to metal joints to maximize the heat transfer.
     

     

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  12. Dave451

    Dave451 AK Subscriber Subscriber

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    +1 on the thermal compound. I had a smallish heat sink on mine, but Dave G told me the semiconductor only generated about 2W of heat at full load and that would probably be sufficient, and it has been, so I expect you're OK if you use the thermal compound between the semiconductor and the sink and the sink and the chassis. Nice looking build on the board!
     
  13. curtsad17

    curtsad17 New Member

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    Here is what I have, is it correct? I read somewhere that 100% zinc oxide also works .

    I was happy with my first build of a board util I saw this one by member tekuhn. I have a goal to strive for.
     

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  14. curtsad17

    curtsad17 New Member

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    Location:
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    Today I added the current limiting thermister. That was easy . I used a CL 60

    I also added the 100 Ohm screen stability resistors. I know they are not required with the EFB mod, but I will be doing the power supply upgrade before I install the EFB board so I guess it gives me some protection.

    The 10 Ohm cathode resistors I installed are 1/2 watt. Would I have been better off using 1/4 watt for the protection ability? I think I have seen both used.

    Another question. After installing the screen stability resistors I powered it up, with fingers crossed, all good, maybe better. Here is the question:

    My measurements across the cathode resistors before installing screen res. in VDC .377 .284 .288 .285.
    My measurements after screen res. .248 .234 .233 .244

    Any ideas why the changes?
     

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  15. curtsad17

    curtsad17 New Member

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    Location:
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    My next step is the power supply. I think I want to do the improvements developed by sony 6060. It seems that the schematics mentioned in his thread are not available. I will ultimately be doing most or all of what is described in the "Improving the Fisher 400" thread. I can see the schematics drawn by Louis on that thread that I believe they would be the same. The problem is when I try and copy and paste a section of the schematic (so it is enlarged) to a word document , it copies compressed vertically. I was able to do that with other schematics and it worked fine. I would like to have hard copies of the original design and the proposed so I can study them side by side while also looking at the actual wiring. If I print the schematic in its entirety it is too small for my eyes to see.
     
  16. gadget73

    gadget73 junk junkie Subscriber

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    Double check those 100 ohm resistors. They should not have changed voltages enough to affect the current flow through the tubes.

    If you're using Adobe to read the pdf, see if it has a poster option on the print page. That will slice and dice the page into multiple sheets that you can tape together in order to get a full size print.
     

     

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  17. curtsad17

    curtsad17 New Member

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    The 100 Ohm resistors are 1% and all measure within .5% Could it be that some old components are getting broken in after being idle for so long.
     
  18. gadget73

    gadget73 junk junkie Subscriber

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    Who knows. I was just thinking maybe you ended up with 1K instead of 100. Sometimes I have a hard time with colors on new resistors, and that 5 band color thing constantly screws me up.
     
  19. curtsad17

    curtsad17 New Member

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    Location:
    Media, PA

    I know what you mean. It takes me longer to figure out the color code than checking them with the DVM
    Thanks for the adobe info. I found the poster function and it worked. Never too old as they say.
     
  20. fred soop

    fred soop Super Member

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    1,931
    With todays smaller resistors and colors not as good as they were with vintage Ohmite, all resistors should be checked with a meter before using. I keep the 5 band 1% in the bags with the distributors labels showing resistance, then check them anyway.
     

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