Help with 500c Phase Inverter Noose Removal

Discussion in 'Fisher' started by buffdriver, Jul 19, 2018.

  1. buffdriver

    buffdriver AK Subscriber Subscriber

    Messages:
    102
    Location:
    Seguin, Texas
    Hi Everyone,

    I've lurked here many hours lately, but haven't posted since my 500T rebuild thread a few years ago. The tube bug bit me hard recently, and I was able to find a very nice stock 500c. I set about to service it and install some mods. To date, I have completed the following:

    1) Replaced power supply cans with Hayseed Hamfest cans
    2) Replaced all electrolytic caps
    3) Replaced selenium rectifier with silicon bridge from Mouser
    4) Installed 10 ohm cathode resistors
    5) Installed 100 ohm screen stabilization resistors
    6) Replaced 330k control grid resistors with 220k
    7) Replaced 0.047uF coupling caps with 0.068uF/630v film caps
    8) Installed EFB Modification
    9) Installed IBBA Modification
    10) Replaced the tired output tubes with a matched set of Tung Sol 7591As from Jim McShane

    At this point everything was working great and I was nearly meeting factory spec for output/distortion (27W/channel with both channels driven, 0.7% THD). But the sound seemed a bit thin...

    In search of better sound, I read up on Dave Gillespie's Phase Inverter Noose Removal modification in this thread:
    http://audiokarma.org/forums/index.php?threads/fisher-400-service-bulletin.671281/

    I removed the 180K noose resistors and replaced the 33k resistors with 56K to allow the phase inverter to be properly adjusted. All proceeded without issue, but upon powering up, my distortion was through the roof:

    0.5% at 1W
    1.0% at 4W
    1.9% at 12W
    3.0% at 27.8W where clipping was noted

    Then I read this thread

    http://audiokarma.org/forums/index.php?threads/newly-acquired-fisher-400.736364/page-3#post-10081856

    about the need to reduce the B+ supply to the AF Amplifier stage, but I'm really confused about how to do this. The thread is written for a 400, though Dave Gillespie references a 500c twice.

    Sorry to be so long winded with my question. I know it's a lot to digest, but if anyone has accomplished this mod, I would be very grateful for any advice that you might have. Please ask if you need pics or readings.

    Regards,

    BuffDriver
     

     

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  2. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Buff -- Congrats on all the improvements you've made to your 500C!

    When the phase inverter noose is removed, it means that all current flowing through the inverter cathode resistor now passes through the inverter tube itself. But to pass that much current, the inverter section self biases itself to virtually zero bias voltage, causing all the distortion you noted. The answer is to lower the plate voltage on the AF Amplifier stage that is direct coupled into the inverter stage. The way that is done, is by disconnecting the plate resistor of the AF Amplifier stage from the B+ source feeding the inverter section, and connecting it to the next lower B+ voltage source -- that is, the B+ voltage source feeding the Filter and Tone Control stage tubes. That simple change will lower the plate voltage of the AF Amplifier stage enough to allow the inverter section to properly bias itself at an appropriate point on its load line, with the phase inverter noose removed. The result is that the phase inverter stage then has much greater output at lower distortion since it no longer has the noose choking down its output capability.

    I hope this helps!

    Dave
     
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  3. Catmanboo

    Catmanboo AK Subscriber Subscriber

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    Location:
    Atwater, ohio
    Dave, as usual, outstanding! Jerry :thumbsup:
     
  4. buffdriver

    buffdriver AK Subscriber Subscriber

    Messages:
    102
    Location:
    Seguin, Texas
    Dave,

    I can't thank you enough for that! Your explanation was just the ticket. I had been substituting the lower voltage B+ for the entire tube. Once it sunk in that the tube is a twin triode and we are only wanting lower voltage to the AF amplifier half, I saw what you had been trying to say. My receiver is back to normal distortion levels. The right channel still has a bit more distortion than the left, so I may try rolling some tubes to find what's causing that, but it's still a half order of magnitude less than before.

    More importantly, the sound just came alive! Great detail, crisp highs, and deep lows.

    Here are some pics of her up to this point. My next job is to work on the FM alignment. Again, thanks for the help, Dave!

    IMG_5855.jpg IMG_5857.jpg IMG_5846.jpg
     
  5. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,558
    Location:
    Ball Ground, GA
    Glad that corrected the issue!

    Is there any particular reason you feel the FM alignment is off? Normally, it is extremely rare that the alignment of any Fisher product will just "go out". By far all too often, the real scenario is that some component fails, which gets mis -diagnosed as an out of alignment condition -- prompting an alignment effort, professionally, or otherwise. The net result is that the receiver is then invariably made worse than it was before the alignment effort was started, because it wasn't the problem in the first place.

    I'm hardly trying to say that your receiver does or doesn't need an alignment. I'm simply saying that I would make absolutely certain it does before casually attempting that procedure. And if it does, be certain that the equipment used (if that is the chosen method) is of first class caliber. It can save a lot of grief in the long run!

    Dave
     
  6. buffdriver

    buffdriver AK Subscriber Subscriber

    Messages:
    102
    Location:
    Seguin, Texas
    I appreciate your help and advice. You're absolutely right--I didn't stop to consider why my reception might be poor. Now that I think of it, the FM reception was much better when I first tried it, before all of the amplifier work. So, I injected a 99MHz FM signal with 400 Hz into the antenna. Below is why the reception sounds so bad. At this point, I'm not sure if the problem is a tube or alignment. I've got decent alignment gear, but I'm a newbie. All of my work so far has been on late '70s solid state gear: ceramic filters, fewer IF stages, IC MPX. I would be very interested in your assessment.

    Kind Regards,

    BuffDriver

    SDS00001.png
     

     

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  7. gadget73

    gadget73 junk junkie Subscriber

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    just to be clear, it doesn't produce that same output when fed signal into one of the line level inputs does it? That looks a lot like crossover distortion from an improperly biased power amp.
     
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  8. buffdriver

    buffdriver AK Subscriber Subscriber

    Messages:
    102
    Location:
    Seguin, Texas
    Thanks for checking, gadget. No, the Fisher produces beautiful music (and test tones) with signals fed to the amplifier. This is a signal produced by my HP8656b RF signal generator's internal 400Hz oscillator modulating approximately 99MHz. The signal is fed from the tape monitor out directly to the oscilloscope--doesn't even go through the power amp.

    I've tested V1 - V6 minus the nuvistors. V1, the 6HA5 is not testing good on my Eico 666 at the moment.
     
  9. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    OK. You just answered some of my questions. So now, what does the signal look like right at the output of the Ratio Detector?
     
  10. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Also, how are you injecting the signal into the Fisher's front end?
     
  11. buffdriver

    buffdriver AK Subscriber Subscriber

    Messages:
    102
    Location:
    Seguin, Texas
    Dave, if I understand correctly, the ratio detector is Z5. Do you want the signal at test point 3? [Correction, looks like TP4 is the signal out of the FM Tuner and into the MPX. Is that where you would like the signal checked?]

    I injected the signal above at the antenna terminal using a 50ohm to 300 ohm balun. I constructed it myself, but it is the equivalent of the Sound Technology Model 100 matching transformer. I've used it on all of my alignments of solid state gear without issue. Is it a problem to use it here?

    For further background, I have previously (before I began having problems with the signal) injected a modulated MPX signal and observed the stereo separation. At that time, there was no issue with a distorted signal.
     
    Last edited: Jul 20, 2018

     

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  12. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    No. There won't be any AC signal present at test point 3. Check the signal appearing at test point 4. That is the output of the Ratio Detector Transformer.

    Just make sure that with direct injection as you are doing, that you don't overload the Front End with your RF signal. It's usually best to loosely couple the signal to test point 1 -- assuming your receiver is a later version with the nuvistor front end.

    Dave
     
  13. buffdriver

    buffdriver AK Subscriber Subscriber

    Messages:
    102
    Location:
    Seguin, Texas
    Will do. I will report back tomorrow with the results.
     
  14. buffdriver

    buffdriver AK Subscriber Subscriber

    Messages:
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    Location:
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    At test point 4, I captured scope output looking at the 400 Hz signal and the 10.7 MHz IF:

    SDS00002.png SDS00003.png SDS00004.png
     
  15. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    How did you modulate the 99 mHz signal -- with AM or FM modulation?
     
  16. buffdriver

    buffdriver AK Subscriber Subscriber

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    Location:
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    I used FM Modulation. Here is the output using the suggested technique of loosely coupling the input to TP1, 400 Hz signal at 10.7 MHz, 22kHz FM Modulation.

    SDS00005.png
     

     

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  17. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Hummm. Sure does seem to be a lot of IF artifacts in the output of the last scope shot. Maybe just noise from the front end. I've included for comparison, a pic of the signal produced at the equivalent of the 500C's test point 4, from a Fisher MPX tuner/preamp I recently aligned, using a 99 mHz signal injected at the antenna input that is FM modulated to 22 kHz by a 400 Hz sine wave (HP 8640B). Looks very much like the first scope shot you presented, so the tuner circuits proper would appear to be doing just fine at this point. How does the tuner sound in mono mode? If that is problematic as well, then with a good signal at the output of the Ratio Detector, your next stop is the diode matrix switching circuits........

    Dave
    SAM_2505.JPG
     
  18. buffdriver

    buffdriver AK Subscriber Subscriber

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    Location:
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    Dave, all of the shots that I sent you are in mono mode. Here is a shot under identical conditions that you listed above. I'm not experienced enough to recognize significant differences between the two.

    I'm going to spend some time cross-checking parts that I replaced during the restoration. In the IF section, it was only a few caps, and most were replaced with film. I will also check voltages on the diodes.

    As if I didn't have my hands full, it also appears that my power switch is dying. :( The unit initially failed to power up last time and then when it did, there was a pop from the area of the switch.

    Onward...

    IMG_5867.JPG
     
  19. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    The power switches are known failure points over the years -- particularly the ones in units with SS power supplies as the inrush current from the big voltage doubler power supply caps is large -- and this on top of the surge created by a whole bunch of cold heaters, too. Mark Oppat usually has the switches, but you should also insert a CL-80 current limiting device on one side of the AC line to help protect the new switch.

    So what test conditions were different between the first and last two scope shots you presented? Was the modulation level the same in each case? There just seems to be a lot of 10.7 mHz energy associated with the image in the last two shots. The p-p value of the 10.7 mHz envelope is greater than that of the 400 Hz modulating signal in the last two. That's why I'm wondering about the modulation level used between the first and last two pics. Notice how in the first pic, the p-p value of the 10.7 mHz envelope is far less than that of the 400 Hz modulating signal -- much like the image in the pic I presented.

    You might also try a scope shot at the mono input and Channel A and B output of the matrix network. They should be identical. If not, then you've found your problem. At either point, all 10.7 mHz artifacts should be gone.

    Dave
     
  20. buffdriver

    buffdriver AK Subscriber Subscriber

    Messages:
    102
    Location:
    Seguin, Texas
    While I digest the info that you've provided, let me clarify the conditions of the various pictures above.

    Post #14, Picture 1 is 400 Hz from TP4. Signal was injected at the antenna, 99MHz, 60dBf with 67kHz FM modulation
    Post #14, Pictures 2 & 3 are the same output with a smaller time base on the scope. Pic 3 is a single sweep.
    Post #16 Picture is 400 Hz from TP4. Signal was injected per step 1 of calibration instruction (10.7 MHz at TP1, 22kHz FM modulation)
    Post #18 was an attempt to match your parameters from post #17. 400 Hz from TP4 injected at the antenna, 99MHz, 71dBf with 22kHz FM modulation.

    I will also need to work out the power switch before I can safely turn on the receiver again. I did have a CL-80 installed. Perhaps the switch was close to dead already.

    BuffDriver
     

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