800-C Restoration

Discussion in 'Fisher' started by audmod01, Nov 12, 2018.

  1. audmod01

    audmod01 Super Member

    Messages:
    1,386
    Location:
    Tioga, TX
    The picture above is of the IBAM Board installed in the 800-C I am currently working on. Output tubes have not been installed yet. in my older 500-C install, I set tube currents to 30mA each. In this set I plan to set them the same until I get the EFB circuitry installed, then I will bump them up to 35mA each.

    Joe
     

     

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  2. audmod01

    audmod01 Super Member

    Messages:
    1,386
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    Today the 220K ohm 1/2 watt resistors and .068uF@630VDC capacitors came in from Mouser. I installed the grid isolation resistors and connected them to the IBAM yellow, orange, brown and black leads coming from the center tap of the variable resistors on the IBAM board. The .047 coupling capacitors from the phase inverter stages were all changed to the .068uF capacitors. Tomorrow if all goes well I may get a chance to check the adjustment range of the IBAM controls at the signal grid lugs of the output tube sockets.
    IBAM connected Caps changed web 01.jpg
    Joe
     
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  3. thornev

    thornev Well-Known Member

    Messages:
    770
    Location:
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    Joe - Were the .047uF caps bad, or did you decide just to upgrade because ______? Thorne
     
  4. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Thorne.

    With the Grid returns changed to 220K, the coupling caps needed to be changed to maintain approx the same R/C timing and .068 is the closest value commercially available. The old upgrade spec'ed .1uf (with the timing off a bit but not audible, and a slight risk of oscillation from the .1uf value) but with .068uf coming out in larger numbers it's now economically feasable to use them and get the timing closest to ideal.

    Larry
     
  5. audmod01

    audmod01 Super Member

    Messages:
    1,386
    Location:
    Tioga, TX
    Thorne;

    Yes, one .047uF ERO FOIL cap was very leaky and placed a positive voltage on a signal grid terminal even with no output tubes installed. Since one was so bad, I did not trust the others. That one bad cap would have had that output tube glowing cherry red on its plate structure and destroyed one of a matched quad of new TUNG-SOL output tubes. Larry is right about the time constant of the resistor-capacitor combination. .068uf and 220K ohms is so close to the original time constant that only a slight increase would make it equal to the .047uf/330 K ohm combination.

    Joe
     
  6. audmod01

    audmod01 Super Member

    Messages:
    1,386
    Location:
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    Today I did some testing of various tie-ins to the -27VDC negative bias source to the IBAM Board. Initially I had the 5.6K ohm resistor in series with the lead to the IBAM circuit. That dropped the available negative bias to signal grids too low to control output tube current well enough, so I tried a 2.2K ohm resistor and that was still not good enough. I decided to connect the -27VDC source direct to the board and now I have the trimmer resistors set to -26.2VDC. That is a good starting point to keep all the outputs throttled back to low conduction. Then I can install the tubes and adjust for about 30mA each and do some testing.

    Joe
     
    Last edited: Jan 18, 2019

     

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  7. vsat88

    vsat88 Like my Bluetooth ?

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  8. 1rebmem

    1rebmem AK Subscriber Subscriber

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    Last edited: Jan 18, 2019
  9. audmod01

    audmod01 Super Member

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    Location:
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    1rebmem;

    Yes, I am aware of the document as I created it with help from Dave Gillespie. The 500-C and 800-C are so similar that it will be easy to implement the EFB circuitry after I correct any other problems that might show themselves once I get to listen to any audio and radio functions of this unit. It has had its share of problems to correct already. I expect to have to change the 1uF cap in the MPX circuit and the 8uF in the Ratio Detector circuits.

    Joe
     
  10. mrphilco

    mrphilco AK Subscriber Subscriber

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  11. thornev

    thornev Well-Known Member

    Messages:
    770
    Location:
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    If someone could isolate just the EFB portion of the schematic, it would be helpful. (but not the hand-drawing because it conflicts with other copies of the schematic) Thanks. Thorne
     

     

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  12. audmod01

    audmod01 Super Member

    Messages:
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    Keep in mind that adding EFB also changes the output tube negative grid bias actively to response to AC power input fluctuations as well as to the screen grid of the output tubes. Heat dissipation in the output tubes is reduced to also help the tubes last longer. For those reasons there are changes to both circuits as to how they interconnect with the power supply circuitry. So expect further changes when EFB is implemented. I will see if I can produce a schematic of just the EFB circuitry. It may require some comments from Dave as it develops. Many people like to increase the negative bias to the output tubes to help the tubes last longer. However, in doing so without the addition of EFB, the distortion increases. EFB works to keep the tubes in ideal territory and reduces distortion while increasing power output even under increasing drive to the output stages by both channels simultaneously. Many specifications for equipment in the era when these were being produced and sold showed performance for only one channel driven at a time, and therefore were misleading to the average person. There was the introduction of "Music Power Rating" which was based on potential power that could be produced for an instant, but did not deal with sustained power output capability. The performance claims became so inflated and unrealistic that the government stepped in and advertising claims had to be throttled back.

    We are indeed fortunate that Dave has developed such beneficial circuitry to help our Fisher (and other brands of high-fidelity/stereo equipment) perform better and last longer in so many ways. Similarly, we are fortunate that there are suppliers that still offer tubes and other components for us to use. We need to be thankful to musicians who continue to like guitar and other amplifiers made for electronic modification of pure tones produced by musical instruments that please so many people. This has kept the vacuum tube industry going to our benefit.

    Have fun!

    Joe
     
  13. mrphilco

    mrphilco AK Subscriber Subscriber

    Messages:
    475
    Location:
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    Joe

    A schematic of just the EFB circuitry with comments on how to adapt it to the various Fisher models would be beneficial and greatly appreciated by all, I think.
     
  14. audmod01

    audmod01 Super Member

    Messages:
    1,386
    Location:
    Tioga, TX
    Good news on the 800-C! I was able to adjust current in each output tube to a stable 30mA. I then connected my test speakers and gave the unit a try. I sounded nice on FM, but there was no signal level indication and no stereo beacon lighting, although I could hear the relay click on and off. I was going to connect my meter across the meter to see if the signal was varying there and discovered the leads had been clipped off. I re-soldered leads and the level meter is working fine. The level shown is quite decent for my area and the reception is free from noise. Next step will be to clean switches and controls plus burnishing the contacts in the stereo beacon relay to see if that restores its action. I do not think I have stereo reception yet, but that could easily be the 1uF in the MPX circuit.

    More to come later.

    Joe
     
  15. audmod01

    audmod01 Super Member

    Messages:
    1,386
    Location:
    Tioga, TX
    I cleaned all the switches and controls. I replaced the 1uF coupling capacitor in the MPX-65 decoder which restored stereo sound on FM Stereo. The 1uF replacement is a metalized polypropylene capacitor and will likely never go bad. I also replaced the 8uF at the ratio detector with a 10uF@50VDC electrolytic. The old one showed some signs of leakage of electrolyte. The unit is playing happily in the study on my test speakers and sounds amazingly good for such cheap speakers (originally from a Panasonic boom-box).

    The stereo beacon responded to cleaning the contacts of the relay and is now operating correctly. The lamp was still good. To get the station indicator to read correctly just required sliding the indicator a little on the dial cord and it is working fine now.

    The volume control is well balanced until the sound level is turned nearly to zero, but I won't be operating it that way anyhow. Overall it is a great credit to its heritage now. I am using some Telefunken 12AX7 tubes in the driver/phase inverter stages.

    Joe
     
  16. rufleruf

    rufleruf Poor Impulse Control Subscriber

    Messages:
    1,203
    Good news Joe! Sounds like you now have a nice functional 800-C.

    I borrowed the good meter from this guy for a 500-C I was working on while I waited for a replacement to show up and I guess I forgot to solder the new one back in.
     

     

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  17. audmod01

    audmod01 Super Member

    Messages:
    1,386
    Location:
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    Matt;

    Yes, thanks to you and some parts & tubes, it is doing quite well. Don't worry about the meter not being soldered. It was easy enough to spot and take care of. I touched up the dial pointer by adding some flat black paint to the top of it which makes it appear to project only about 1/4 inch above the top of the numerals on the dial. That matches the factory dial pointers now. I have ordered some 41mm LED fuse lamps for illumination. The slide switches will get some new lubrication to free them up from dried and gummy oil.

    I need to add about a 33K ohm resistor across R204 to lower the MPX threshold. It is working as designed, but the station I like goes in and out of stereo due to aircraft signal bounces. That will take care of that. I will fabricate a bracket for the AM Loop Antenna and use a ferrite loop antenna from a Fisher parts chassis.

    Next I will begin working on adding EFB to the unit followed by the TL82 Preamp out circuit.

    Joe
     
  18. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
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    Location:
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    Joe - Watch out going much lower than about 47K as a shunt across R204. Doing so can promote false triggering, or triggering off of certain types of interference.

    By the time I published that fix, I had done a significant amount of research on different units to determine a good general value for installation in most units. What I found was that the value of the resistor (as you might expect) is dependent on the number of IF stages, and Limiter stages in particular. In general, units with a 4 stage IF strip can well tolerate a 47K shunt if the RF, IF, and MPX circuits are all ideally aligned. As those circuits deviate from optimum, lower values can be tolerated as potentially more noise creeps into the signal.

    In contrast, those units with a 5 stage IF strip work best with a 68K shunt. Since there is greater limiting action there is less noise, so less of a shunt is required to achieve stable stereo/mono switching action. When you determine what value is best for your unit, just beware and check for any false or lazy switching action. The lamp should stay out across the entire band when not tuned to a stereo station, and should extinguish promptly when off-tuning from the normal capture range of the pilot signal from a stereo station.

    So you pulled another out that had both feet firmly planted in the grave!! Congrats! I love it!!

    Dave
     
  19. thornev

    thornev Well-Known Member

    Messages:
    770
    Location:
    Mid Hudson Valley, NY
    Dave - That was one of the first suggestions you made to me - piggy-backing R204 across the cap. Last January as a matter of fact! It sure did make a difference in the ability for my 500-C to receive FM stereo broadcasts. I installed a 47K 1/4 watt. Thorne
     
  20. audmod01

    audmod01 Super Member

    Messages:
    1,386
    Location:
    Tioga, TX
    What I might do is install a variable control and adjust for best results over the entire band. Many units years ago did just that so that a receiver or tuner could be adjusted for best results in the situation it was used within.

    I am really pleased with how well this receiver is working.

    Joe
     

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