Quick and Easy 400 Phase Inverter Adjust.

Discussion in 'Fisher Forum Stickies' started by larryderouin, Dec 1, 2013.

  1. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Alan; Don't confuse the transformer/speaker grounding with the phase inverter. WITH POWER OFF(pull the plug and bag it) Ground capacitor C-27A, then do the adjustments. Pin 3 is static, and pin 1 is adjustable. AGAIN NO POWER!
     

     

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  2. RUmad

    RUmad AK Subscriber Subscriber

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    Larry, I LOVE your enthusiasm! LMAO! Thanks for the info. I'm most obliged. To be clear, the power will most definitely be off. Thank you sir!

    Alan
     
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  3. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    I just hate seeing guys trying to become part of a A.E.D. (Automatic External Defibbrillator) either intentionally or by accident. It's not really enthusiasm, more emphasis if you will. 40 years in Emergency and preventive medicine will do that to you.
     
  4. RUmad

    RUmad AK Subscriber Subscriber

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    I understand and completely agree. I got zapped, when I first started tinkering, with about 1400 volts. Luckily it was very low current and I only had one hand in the chassis. It was a Bell & Howell scope transformer I used for my first preamp build. Let's say I've been extremely cautious since then. :yikes:
     
  5. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    OUCH!!! That'll definately put a kink in yer hair!!
     
  6. RUmad

    RUmad AK Subscriber Subscriber

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    Yes sir, I was totally reevaluating my idea to work with tubes. Lol. Glad I stuck it out though!
     

     

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  7. RUmad

    RUmad AK Subscriber Subscriber

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    HOLY $H!T was that easy!! Thank you again Larry! I'm eternally grateful that we've not only got real experts on here, but experts willing, even eager, to help us little people. Not only would I NEVER have touched those PI pots, for fear of throwing everything out of whack, but they already were way out of adjustment. Now both sides are perfect. Y'all rock.

    Alan
     
  8. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    I got nailed by a TV Flyback transformer when I was 7. It had been sitting on Dad's bench for a couple of hours and I grabbed it wrong (never grab one by the H.V. terminal end and ALWAYS wear thick rubber gloves!!!) Literally knocked me on my a__ across the room. Wasn't fun, but Dad laughed about it after all was said and done (and I had been checked out by the ER doc's.) Wasn't the 1st time getting zapped, but the last for a Flyback transformer.

    Don't thank me, (SHORT VERSION) I was just the instigator with the original question to Dave. He answered back, with his usual 6 page dissertation on phase inverters, dumbed down for 3 celled moron's like me. So then it snowballed into the thread it is. I did the rest of the research on the receivers and Dave vetted it. He's got more than enough on his plate without having to run down minor stuff like adjusting P.I.'s without an analyzer. Plus I learned a few things along the way. Now I've got to look into the amps and how they are done. Luckily part or most of the answer is in Dave's previous posts.

    The more and better that we all help each other the stronger the mind pool becomes. No piece of information here is useless. Even the most esoteric piece of dope from 1946 can be just the thing that get's another FISHER back on the road and running.
     
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  9. RUmad

    RUmad AK Subscriber Subscriber

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    I agree, and I do thank Dave, for this particular thread, and helping me with other issues on my first Fisher rebuild. I thanked you specifically for walking me through this process. I can't say it enough, I love it here at AK. It's one thing to build something and have the satisfaction of it working, it's taking it to the next level to be able to say, it works, AND I know it's done CORRECTLY. So if I were to list everyone in order to thank them, I'd more than likely accidentally forget some names, so thank you everyone. I've got a few very minor details to work out, but within the week id say I'll have this in my system, and I can't wait. Albeit missing ONE dang knob on the tuner. It might've worked without help, but it never would've been 'right'.
    And after this? Two more boxes of Fisher gear to rebuild! Yes!!
     
  10. Kelshar

    Kelshar New Member

    Messages:
    49
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    can someone help me located the C525C in this photo? I found it on the schematic but am unable to physically. sorry if someone has already pointed this out but I'm unable to locate it on a search.
    I recently purchased this 400 at the end of May. My first venture in tubes. everything seems to work but I just want to restore it to its full potential, make it last and be protected.
    thanks!
     

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  11. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    With 400 receivers, you need to specify whether you have an early or late model version (serial #48000 is the break point), as there are significant differences between the two units. From your pic, yours is a late model version.

    May I assume that you are really referring to C52C -- and not C525C? There is no such capacitor listing in any 400 receiver. If it is C52C you are referring to, it is located in the four section capacitor can closest to the RF tuner box, of the two rear capacitor cans.

    Since this thread is a sticky pertaining specifically to the phase inverter adjustment process, it would be best for you to ask these types of questions in the general Fisher forum, so that this thread can stay on its dedicated topic.

    Good luck with your restore!

    Dave
     

     

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  12. Kelshar

    Kelshar New Member

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    ooops, sorry. Yes, it's the late model version, serial number# 63067 and a typo on the *C52C*. I'll start another one in the general forum. thanks!
     
  13. vintelectra

    vintelectra Active Member

    Messages:
    455


    Hi PI fans: great thread! I use the THD method, but what about just equalizing signal voltage on both pwr grids?
    I got the "defibrillator risk". Martin
     
  14. Tubologic

    Tubologic Well-Known Member

    Messages:
    643
    Though the above described method(s) for adjusting the phase inverter is fast and easy it should only be used if you don't have access to the right equipment (dummy load,audio generator and distorsion meter) following Fisher's recommended procedure. Of course, by statically measuring and equalizing the values of the anode and cathode resistors in a split load phase inverter you'll get equal amplitude driving signals to the output tubes grids but this will not necessarily lead to minimum distorsion (the only purpose of the adjustment), due to variations in tube transconductance and dynamic characteristics, imperfect transformer balance and other circuit variables. FISHER could have easily used a pair of matched (5% or 1%) resistors (much cheaper than a pot) and dispense with the pot but they didn't. The pot is there to compensate for any unbalance in the output stage by slightly and purposely unbalancing the grid drive, in most situations perfect balance of the splitter will NOT correspond to the lower distorsion of the complete amplifier (it may even increase it), unless the output tubes are perfectly matched and the OPT perfectly balanced. Without a means for measuring the IMD at the amp output and adjusting the pot for minimizing it all the other methods are a hit or miss affair, and you could as well leave the pots in their mechanical center-position, which is even a faster and easier method.
     
    Last edited: Jan 27, 2018
  15. CORYK

    CORYK AK Subscriber Subscriber

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    131
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    BETHEL MN
    This procedure doesn't seem to work on the KX-200. Don't get any resistance change on pins 1 or 3 when turning pot. Any ideas?
     
  16. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    The KX-200 doesn't have a Phase Inverter Pot(s). It's not adjustable. If the pots' you were turning are on the top of the unit, I would suggest you redo the bias and balance procedure as outlined in the Service Manual.
     

     

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  17. CORYK

    CORYK AK Subscriber Subscriber

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    Ok, Thanks Larry. I think I've got it figured out by looking at the schematic.
     
  18. thornev

    thornev Active Member

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    Location:
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    Would someone please elaborate on this step in post #1? "Using an alligator clip lead, short the B+ supply point to the phase inverter tubes to ground. This would be the 320V source that appears across C52C. This is the 4 section CAN CAP. " On my 500-C I don't know where C52C is, and I don't know where the B+ supply point is. I believe my entire power supply module has been replaced with an SDS Labs board that is too well-attached to move. I looked on the schematic, but these terms are not listed. Thanks, Thorne

    PS - Found this post which should be a clue: "Make certainly you are grounding the correct B+ supply point. Based on the schematic, it is the point that feeds the center terminal of the phase inverter adjustment pots." Problem is I don't have a center terminal on my phase inverter tubes. It's just a hole. On my 500-C the schematic it says V12 and V13 are the phase inverter tubes, right?''

    Oh dummy me ! Center terminal of adjustment pots, not tubes !! Made the adjustments. The static value (pin 3) was 51K ohms. I was able to adjust the pots to 51.1K.
     
    Last edited: Feb 13, 2018
  19. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    POST #1 Edited to reflect use of the SDS board in the 500c/800c and a modification to the procedure.
     
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