MPX-100 SN 10001-19999 Series Multiplex Demodulators

Discussion in 'Fisher' started by audmod01, Oct 10, 2017.

  1. audmod01

    audmod01 AK Subscriber Subscriber

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    In looking at the different versions of the MPX-100 I note that the early serial number sequence is significantly different from the later serial number sequences. A specific difference is that the early series uses a Heptode/Triode tube an ECH84 (6JX8 is similar) as a stereo demodulator. The later serial number sequences use the double diode bridges similar to that used in the WX and MPX-65 and related multiplex demodulators that Fisher developed. I am wondering just how this early serial number sequence performs when compared to the later 20001-39999 and 40001-49999 sequence versions of the MPX-100. The ECH84 was a special tube that was developed for television sync separator application in Europe. This tube may not be in great supply these days. For that reason the early SN sequence MPX-100 may not be very desirable. If its actual performance is not as good as the later SN sequences that would be another reason for it not to be desirable.

    The MPX-200 beginning serial numbers 10001-19999 is quite similar to the last two serial number sequences of the MPX-100 and uses the same tubes.

    Joe
     
  2. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

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  3. audmod01

    audmod01 AK Subscriber Subscriber

    Messages:
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    Location:
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    Larry;

    That was my take on the early MPX-100 too - just not as good a design. The other issues that Dave mentions about the MPX-100 having very tight connections and components in the power supply pretty well steers me away from using any of the MPX-100 versions. I had thought about using one with my 202-R, which is much like the 200-T that Dave was working on. It uses the same ratio detector as the 200-T model does and may need to be changed. I think I will just stay with my MPX-65/WX approach instead. I spotted several MPX-100 units on ePay recently and was curious about them. That led to downloading each serial number sequence service manual on the MPX-100 series and taking close looks at their circuitry. The earliest version is so different and inspection of the circuitry shows that the "magic" takes place in the ECH84 tube. There are no demodulator diode pairs or bridges as in the later Fisher MPX demodulators.

    I made a bid on one of the MPX-100 units, but the price went up beyond what I was willing to pay. Now I am even more glad that I did not have the winning bid on that unit. I believe I would not have been satisfied with it if I had tried it with the 202-R.

    I am going to look for an inexpensive source for the later ratio detector transformer.

    Thanks very much for reminding me of that thread!

    Joe
     
  4. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

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    I've still got one MPX-100 I've got to send to Dave. Separation is very low on it, and I'm not confident about MPX'ers. However I have a Sherwood M3ax built in to my 610 consolette. Replaced the tubes on it and I'm getting about 25db on it with nothing but the tubes change to it. Separation is noticable and it's a lot better right now than the MPX-100. But I've got to recap the mpx-100 1st before I send to Dave. The MPX-100 is the last version IIRC.
     
  5. audmod01

    audmod01 AK Subscriber Subscriber

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    901
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    From what Dave mentioned about the MPX-100 it should deliver just as good separation as the later units used in receivers and factory stereo tuners. The relay sounds like it would be particularly troublesome. The usual culprits that appear in the later MPX units apply too of course. It may be that when you change capacitors, particularly the C6 electrolytic, that you may correct the poor separation issue. Let us know.

    Joe
     
  6. Rob Thomas

    Rob Thomas AK Subscriber Subscriber

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    I saw those on ePay too. They even have one of the newer MPX-100 models described as NOS. The seller claimed to have taken it out of the original packaging (included in the sale) for the first time to post pictures. They are certainly going for a premium these days.
     
  7. audmod01

    audmod01 AK Subscriber Subscriber

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    Rob;

    I agree about the premium price. I find it hard to justify paying what some are asking for just a multiplex decoder. Unless someone wants one as a period piece example the same or better performance can be obtained by using a MPX-65, WX or similar subchassis decoder. On Yahoo there is an FM Multiplex group which has a leader who provides circuit boards and ICs to implement an LM4500 multiplex decoder which is small enough to mount under the chassis of most any tuner and has outstanding performance with most any brand and model of tuner unless there is a real problem with the tuner. I installed one in my H-K F-50-XK tube type tuner that I build from a kit in the early 1960s. It delivers great sound. The original MPX circuit in that tuner was always flaky from the very beginning. The cost was under $65 and I supplied resistors and caps to populate the board.

    Joe
     
  8. gadget73

    gadget73 junk junkie Subscriber

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    If you don't mind solid state so much, some 1970s stuff should have an MPX board that is easy to repurpose. I have one salvaged from a Pioneer SX-990 that I intend to put in a box for that purpose. Feed it 12v and a signal straight off the detector and it spits out stereo. It can be easily switched to mono, and it can drive a stereo indicator lamp. Other gear from that era that had individual boards for each function can probably yield up something useful if you've got parts units or find something on ebay.

    I also have a couple of WX units. One is actually workable as a standalone, the other I need to put into a box and build a supply for but it does work.
     
  9. audmod01

    audmod01 AK Subscriber Subscriber

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    Location:
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    Gadget;

    Yes that is often an option. Years ago I added FM Multiplex decoder and audio output stages to an auto radio I had back when even having an FM receiver in a car was not common. I had just a tuner that converted the FM signal to an AM signal at the low end of the AM dial. I tapped the signal from the ratio detector in the tuner and fed it to a multiplex decoder I built up on a home made circuit board then to a repurposed amp that would operate from 12VDC. I used rear package shelf speakers in my sedan at the time. Many of the solid state MPX circuits are quite small, especially the ones that use an IC.

    Power for such a circuit can often be supplied from the filament circuit using either a voltage doubler or a bridge rectifier circuit depending on overall filament circuitry.

    Joe
     

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