MPX 100 - Help

Discussion in 'Fisher' started by PaulWC, Feb 9, 2019.

  1. PaulWC

    PaulWC Active Member

    Messages:
    111
    Location:
    Northern Virginia
    I recently purchased a wood cabinet for my Fisher MPX 100 and before installing it in its new home I needed to work on the unit since it was not consistently recognizing the stereo signal. You also needed to tune the tuner slightly off it's highest best tuned position to get the MPX 100 to get some stereo indication. Wouldn't hold on to it consistently.

    I had previously worked on the unit several years back to recap, clean contacts/controls, tube check and bin cleaning, etc. I also changed out the selenium rectifier for a modern full diode bridge. If memory serves, it seemed to have worked better then than now, although it was never very stable on stereo. Haven't used it much since we don't listen to FM much and the Executive's mono is glorious.

    Opened up the unit to check all caps, resistors, etc, and all are fine, except for the voltages to the first plate on the MPX signal V1 tube is way off. The schematic states it should be 150 volts, it's 104. The Schematic also shows the second plate to be 60V but I believe that is incorrect and should have shown 160 V (based on review of lower serial number schematics). I checked the resistors in that power line and all are OK. Something is going on that I can't figure out. Is the replacement rectifier bridge at fault? I needed to change out the resisters in the power supply to get the schematic's voltages.

    I could just change out the resistors to V1's first plate and add a resister to V2 to get the voltages in-line. But I wanted to check with you all to see if I missed something. See the pics and any help would be welcome.

    Cab and Unit.jpg VI Closeup.jpg Power Supply Closeup.jpg VI Closeup.jpg
     

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  2. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Paul --

    1. Check the voltage at pin 8 of V1. If notably lower than 8.5 volts then check R4, R6, R44, R47, R48, and R50 for value. One or more has likely risen substantially in value.

    2. When changing to silicon, R50 should be changed to a 1.8K at 5 watts. The original piece is waaaay under-rated, and if still in place has likely gone high in value.

    3. 60 volts is correct for the second plate value. If voltage is high, check the values of R15, and R16.

    4. All voltages should be checked with no signal applied at all.

    5. Not unlikely that V1 has just gone south on you. Check by replacing with another known good tube.

    Other than R50, it's not a good idea to arbitrarily raise the values of any resistors to achieve target voltage levels. Most circuits operate at low impedance levels to maintain good frequency and phase response. Raising resistor values could easily compromise the unit's performance.

    7. How is the alignment of the tuner you are using. When max signal strength and max stereo lock don't agree, the problem is often in the alignment of the tuner itself.

    Good luck with it -- I hope this helps!

    Dave
     
  3. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    37,807
    Location:
    Southern NJ
    big +1 on the tuner alignment. If that isn't right, the MPX won't be either.

    agree with the rest of it too. If the voltages are off, check that the resistors aren't off tolerance and that the tubes aren't bad. Changing the resistors to force the voltages is not the way to go. It worked when it left the factory with those values, and will work again with them.
     
  4. PaulWC

    PaulWC Active Member

    Messages:
    111
    Location:
    Northern Virginia
    Thanks Dave for the tips.
    1. V1 pin 8 is 26V! All resistors check out fine.
    2. When I switched to a silicon bridge I installed a 1.5k 2 Watt resistor
    3. R15 and R16 are the correct values. Plate is 166V.
    4. No signal was applied when checking voltages.
    5. V1 checks out fine on my tube tester. I don't have a spare so I'll need to order one.
    6. I hooked it up to my 4000R in the '59 Executive and my TA800. Same results. Since the voltages on V1 are really wacky I suspect the MPX-100 over the other two tuners.

    Also, is is normal for the MPX-100 to default to stereo with no signal applied? The relay contacts are in the stereo mode but the schematic notes that reading are to be taken in the monophonic mode. Do I need to somehow force the unit into mono to do a voltage test?

    You confirmed that my first troubleshooting activities to to check for resister values and voltage readings. Can any of the small ceramic disc capacitors cause this type of problem?

    Thanks for input.
     
    Last edited: Feb 10, 2019
  5. PaulWC

    PaulWC Active Member

    Messages:
    111
    Location:
    Northern Virginia
    See my response to Dave above. There must be an explanation. Will continue to troubleshoot.
    Thanks for your comment.
     
  6. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    If pin 8 has that much voltage present, then the tube is pulling one heck of a lot of current -- if the two cathode resistors are still correct in value. So:

    1. The bias for V1's triode section is largely provided by contact bias. So, check to make sure that R2 has not opened or gone way off value (low, or, open).

    2. Check R5, R7 and R8.

    3. If the resistors are good, then about the only thing left is the tube itself.

    Once the triode section is correct, then we can move on to the pentode section.

    Dave
     

     

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  7. PaulWC

    PaulWC Active Member

    Messages:
    111
    Location:
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    Hi Dave
    1. R2 was a little high at 5.3M (4.7 M called for) so it has been changed
    2. R5, R7, and R8 are in spec but I changed out all 3 just in case they were affected by the applied voltages. V1 pin8 still reads at 26v .
    3. Replacement tube on order .
    Stay tuned .

    Thanks again .
     
  8. PaulWC

    PaulWC Active Member

    Messages:
    111
    Location:
    Northern Virginia
    The replacement tube (V1) went in and there is some improvement as follows:

    Pin 1: 124v (was 103v), still not 150v
    Pin 6: 95v (was 166v), still not 60v
    Pin 8: 25v (was 26v), still not 8.5v
    I tested the grid volage on pin 9 and it is 7.4v. I don't believe the grid should see this type of voltage, if any.

    The only component not checked in this circuit is the cermic disc capacitor C4 - 470 pF. Can this be the cause if it is defective? Fresh out of ceramic disc caps so they are on thier way.
     
    Last edited: Feb 15, 2019 at 3:07 PM
  9. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    What is the voltage at the junction of R2, R7, and R8? It is also around 7.4 volts, then R7 must be goofy.

    The V1 triode section is "bootstrapped" so as to create a very high impedance -- this to no load down the final detector stage in the tuner. The stage accomplishes this by operation with 50% NFB (at audio frequencies), as set by the values of R6 and R8, the DC voltage drop across which should be equal. The Selector switch position should be in the Stereo/Mono Automatic mode, with no input applied when taking voltage readings.

    What is very interesting is that schematic shows a voltage drop across R6 of 25 volts, which would strongly imply that the indicated schematic voltage at pin 8 is flat out wrong......... You are likely in fact measuring the correct voltage at pin 8 with your 25 volt reading. It wouldn't be the first time that schematics were wrong!

    Dave
     
  10. PaulWC

    PaulWC Active Member

    Messages:
    111
    Location:
    Northern Virginia
    Hi Dave,
    The voltage at the junction of R2, R7, and R8 is 9 volts.
    The voltage drop across R6 and R8 is the same and 9 volts.
    I hooked it up to my TA800 and it is doing the job of pulling in the stereo signal on the strongest stations but not on all stations, but then again I don't have much of an antenna on the TA800. I'm about to call it a victory and the problem was the tube.

    Regards.
     
  11. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    But that would mean that the voltage across R7 is (about) 16 vdc (assuming 25 volts on pin 8 ov V1), which is nuts. Is the value of R7 verified?

    FM MPX Stereo signals always require a decent antenna, so that will affect greatly the ability for the unit to trigger into Stereo mode.

    Good luck with it!

    Dave
     

     

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  12. PaulWC

    PaulWC Active Member

    Messages:
    111
    Location:
    Northern Virginia
    R7 was replaced, so it's new and verified. I hooked the MPX 100 up to the Executive's 4000R and it behaving correctly, pulling in the hard to tune classical station in stereo. So I'm calling it done. Thanks for your help. I learned quite a bit on this adventure.
    Regards
    Done and operating.jpg
     

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