fisher MPX decoder troubleshoot

Discussion in 'Fisher' started by vintelectra, Apr 21, 2013.

  1. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    The 1 uF electrolytic.
     

     

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  2. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    if you have a 1uf 400v Film, replace the lytic with a film. Panasonic ECQ or Cornell Dublier DME series are fairly small in physical size compared to the 56 caddy size of an Orange Drop. The smaller the physical size the less chance of it picking up stray RF interference.
     
  3. thornev

    thornev Active Member

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    I hope no one will hold it against me if I don't read all 20 pages of this thread, in case my questions were already answered. My MPX has been professionally aligned already and I found that the strength meter was not registering above 3 on strong signals. So I went forward with transformer adjustments. I'm just learning about electronics so please forgive the many and sometimes amateur questions.

    In the first page or so, Dave talked about adjusting the limiter transformer and the other 3 IF transformers. I've read the KM-60 "Adjusting without instruments" instructions and adjusted all 4 transformers to get my 500-C strength meter to behave more like it's supposed to. My questions...

    1) I was instructed to set the 500-C on FM MONO mode to perform the adjustments which I did. I found that only the limiter (which is Z4) and Z3 transformers had any effect on the meter. Z1 and Z2 did not move the meter. Is that normal?

    2) When performing these adjustments, do I do them while on a single FM station that initially reads between 1 and 2 on the meter?

    3) Dave mentioned aligning the "discriminator". What is the discriminator? Larry answered me privately. discriminator = Z5. I did not adjust this transformer. Guess I'll have to redo.

    4) Once I've done the adjustments, should I repeat the procedure on another mono station, or with the 500-C set to FM AUTOMATIC mode and find a low-registering stereo station? The KM-60 instructions say to redo the procedure on an FM static signal (that is, between stations).

    5) The KM-60 instructions say to do some adjustments from the bottom and the top of the transformers. Are there really 2 different adjustments, top and bottom? What's the difference?

    6) I couldn't get my meter to read above 4. I assume that's OK even though I know some towers are near (within 15 miles of) my location.

    7) In the same thread as above, Dave said that he "aligns the discriminator with a DC voltmeter". How exactly is that done? What I mean is where does one place the leads and using which DMM setting?

    Thanks, Thorne
     
    Last edited: Feb 8, 2018
  4. vintelectra

    vintelectra Active Member

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    Hi All.
    Dave:. Grounding coupling cap, the spurious signal disappears. disconnecting cap, signal thru cap is noise 20mV. Left output with a disconected cap shows residual signal +noise =20 mV. At the disconnected terminal
    to demodulate shows residual signal + noise 20 mV.
    (note: when checking the diodes, I noticed that the range reads 0.5 in the right diodes and 0.6 in the left diodes)
    Larry: Thanks I replace the cap for a new IEC same size as original.
    Thornev: Having Master Dave on the line , I deffer to him for your questions. Martin
     
  5. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    So what happens to the L and R outputs if you reconnect the composite coupling cap and ground the composite input to the sub-chassis?

    Thorne -- I'll get to your post in a little bit.

    Dave
     
  6. vintelectra

    vintelectra Active Member

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    That was the first step, the output signal resolved. ...?.
     

     

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  7. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    In the first step, I believe you said the input was open. I'm wondering if the output noise at the left audio output disappears when lug 1 is grounded.
     
  8. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Thorne --

    1. With the unit turned off, temporarily connect two 47K resistors (wattage uncritical, but low wattage works best. should be close in value) in series (one lead of one resistor to one lead of other resistor). Now, connect the remaining free lead of one resistor to Test Point #3, and the other resistor's free lead to ground.

    2. Using the schematic, determine where test point #4 is inside the set. It will be in the same general area as Test Point #3. Once Test Point #4 is found, for now, just note where it is.

    3. Turn the unit on, and let your 500C fully warm up -- 20 minutes minimum. Set for FM Mono. Carefully tune in a notably weak station for precisely maximum signal strength. Once achieved, DO NOT touch the tuning knob again. Leave the unit running like this throughout the entire adjustment process. Do not have any florescent lamps or even nearby LED lamps on, as these all emit gross amounts of RFI noise.

    4. Adjust in this order Z1, Z2, Z3 top and bottom and then Z4 (only one slug) for maximum signal strength. If the signal becomes very strong at all, wad up the antenna to reduce the signal strength reading. You want all the "IF" stages operating at maximum amplification, which only happens on weak signals. If the signal becomes too strong, then the adjustments won't have nearly as much impact on the signal strength reading. This is why it is important to start with, and maintain using an appropriately weak signal throughout the adjustment process. When the signal is appropriately weak, then each adjustment should produce a very definite and defined peak in the signal strength reading. If any one adjustment does not produce much change, or the peak produced is very broad in nature, that is typically indicating a problem in that IF transformer.

    5. Now connect your DC voltmeter red lead to Test Point 3 and black lead to chassis ground as you did once before. Reading will be negative. Adjust the BOTTOM of Z5 for maximum reading on your voltmeter. NOTE: With the voltmeter so connected, you could use it to adjust the slugs in Z1 - Z4 as well (always adjusting for maximum reading), but I have often found it is easier to adjust for a peak with an analog meter like the signal strength meter where possible. Your DVM reading will also indicate a peak as well, but the reading will not be absolutely stable, making it tougher to discern.

    6. With Z1 - Z4 done and the bottom of Z5 done, now disconnect the meter leads, and carefully connect the red meter lead to Test Point #4 found earlier, and the black meter lead to the junction of the two 47K resistors installed earlier. Adjust the TOP of Z5 for a zero (or nearly zero) indication.

    Your done. Now turn the set off, disconnect your meter leads, and remove the two 47K resistors temporarily connected earlier.

    Let us know how it goes!

    Dave
     
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  9. thornev

    thornev Active Member

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    Everybody says it and I want to hop on the bandwagon... Dave, You are the best. Friendly, responsive, knowledgeable, skilled, generous, kind, caring, and on and on. Thanks so much for your support.

    I have to say that everyone on this forum makes it pleasurable to participate on audiokarma. On other forums I can get thrown under the bus so easily for making a mistake. THANK YOU everyone !

    Thorne
     
  10. vintelectra

    vintelectra Active Member

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    Dave, the signal at the output disapears when grounding the cap on the demodulator side, which I think you suggested the noise would come from
    This cap on backward.But there is no signal other than 20mV noise in the neg terminal of the cap when disconecting it.
     
  11. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    I know. But nowhere can I find what happens to the noise at the Left channel output, when the composite input to the MPX sub-chassis (lug 1) is grounded. I know it's there when lug 1 is open, but what about when it's grounded?
     

     

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  12. thornev

    thornev Active Member

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    Dave - I'm sorry, but I'm not good enough yet at mapping schematic components to chassis components. I am unable to find test points 3 and 4. I thought I could do it by testing the surrounding components, but several of the resistors are registering 0 or out-of-scale when I have the DMM on auto mode. I tried 2 DMMs and I get the same results. Why is that? (Rats... I forgot I can read the color bands on the resistors to get the resistance that matches the schematic).

    Also I'm curious... You have me adjusting the IF transformers in the opposite order from the KM-60 instructions. It doesn't matter the order?
     
    Last edited: Feb 8, 2018
  13. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    In adjusting the IF transformers, order is not particularly important -- what is important is that the signal (station signal strength) is kept weak so that the last two IF stages -- known as "Limiters" -- don't in fact go into actual limiting, but operate at maximum amplification.

    At Z5 -- On the side of that transformer with three terminals, one of the outside terminals will have a 1.5K resistor (brown-green-red-silver) connected to it. The side of the resistor that is NOT connected to the transformer is Test Point #3.

    At Z5 -- On the side of that transformer with three terminals, the middle terminal connects to a 270Ω resistor (red-vilot-brown-silver). The other side of this resistor connects to one lead of a small RF choke coil (looks like a mid size resistor with wire coiled around its body, and a dab of paint on each end to keep the ends of the coil from coming lose). The other lead of the choke coil (not connected to the resistor) represents Test Point #4.

    Inside the Z5 are coils of wire and a couple of diodes. Depending on how you had your meter leads connected, the coils of wire could be shorting out the component you are trying to measure. Or, if the battery in the meter acts to reverse bias the diodes inside the transformer, you could get an open reading where you might be expecting continuity (diodes only conduct electricity in one direction).

    Dave
     
  14. gadget73

    gadget73 junk junkie Subscriber

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    If you happen to own a VTVM, this is the perfect time to use it. They also work great for setting a zero, most of them have a center-scale zero mark for exactly this kind of task.
     
  15. thornev

    thornev Active Member

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    I have only a digital multi meter. VTVM is for measuring vacuum tubes? I can't do that with a DMM?
     
  16. vintelectra

    vintelectra Active Member

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    Dave: Grounding lug one just cleans the spurious 38 K signal at left ch (now a clean sine of 1 volt ) Martin
     

     

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  17. vintelectra

    vintelectra Active Member

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    Should I replace the diode rings altogether???
     
  18. thornev

    thornev Active Member

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    Last edited: Feb 9, 2018
  19. vintelectra

    vintelectra Active Member

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    Thornev: This thread is meant for MPX decoder toubleshooting. Thank you
     
  20. gadget73

    gadget73 junk junkie Subscriber

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    Its not for measuring vacuum tubes, they have tubes inside them though. Basically its what you used before DMM's existed. Its an analog meter that doesn't load the circuit down. For most jobs they are functionally interchangeable, but sometimes its easier to look at a needle vs a digital display if you're just after a peak or a zero rather than a specific voltage.
     
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