MF-300 & Remote Control Acquired

Discussion in 'Fisher' started by audmod01, Nov 2, 2017.

  1. audmod01

    audmod01 AK Subscriber Subscriber

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    Gadget;

    I doubt that a cell phone could reproduce the 38 and 41kHz frequencies, let alone the 60Hz modulated frequencies at the same time. Most phones are not capable of much more than voice frequencies except through an earphone port which might be extended from that. One of the major issues would be the output level as well as frequencies. The output of the OEM hand unit is quite strong and the pickup microphone and internal receiver amplifier circuits have enough gain to work well with the output level from the hand unit. My thought is that if I am unable to get my OEM hand unit working properly again, I will look into designing a hand unit that uses 555 timers as mentioned before in this thread. The output levels might need to be amplified before being applied to say a tweeter or something similar of small size that would otherwise be considered a "full-range" speaker. 1.5" speakers can hardly be considered to be truly full-range although some are advertised that way. They might be capable of reproducing the desired frequencies with and without 60Hz modulation if paired with a step-up transformer in the hand unit. The trick may be in getting adequate output level to provide good distance capability in combination with the RK-20 receiver.

    Joe
     

     

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  2. gadget73

    gadget73 junk junkie Subscriber

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    You're probably right. Maybe a hackish pairing with a bluetooth speaker placed so that the receiver would control it would do. I was just thinking that with the internal sensors in a cell phone you might be able to duplicate the original tilt and roll functions. A dedicated purpose-built device is probably the better solution here.
     
  3. fred soop

    fred soop Super Member

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    Cell phone and computer based signal generators will generate signals in digital mode and this will typically be limited to 44 kHz sampling / 22 kHz output signal. Additionally, that high a frequency will be only a sine wave because no harmonics can be generated.
     
  4. audmod01

    audmod01 AK Subscriber Subscriber

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    Fred;
    Good information to know. Limitations of modern technology always needs to be checked before attempting any implementation of same. The frequencies of this system are close to the fundamental sampling rate, so that means a no-go.
    Joe
     
  5. audmod01

    audmod01 AK Subscriber Subscriber

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    Matt;

    Further investigation this morning indicates that the ultrasonic pick-up microphone is intermittent. After cleaning of the connector contacts, I was still getting erratic response, mainly from the volume up activation. I have both my RK-20 and yours side by side and when yours quits, mine is still working on that function. Looking at the microphone input to the first transistor amplifier input I can see the volume down coming from your unit's microphone just fine, but when I press the volume up function it sometime works and sometimes not. At the same time your hand unit works both functions just fine on my RK-20 receiver and its microphone always sees both the volume up and volume down signals from the hand unit.

    I finally took the microphone on your unit apart to see how it is connected inside. There appears to be a sandwiched double layer element suspended between two spring loaded plates that make contact with the sandwiched element. The center conductor of the microphone connector connects to the bottom or rear-most plate while the ground side of the microphone connector connects to the top or front plate. There are some tiny insulator buttons that surround hollow brass rivets at either end of the front and back plates. The center part of the top/front plate is diamond shaped while the bottom or rear plate has a circular cut-out in the middle of it. I was using a jeweler's loupe to look at the assembly very closely. I began to use a straight pin to clean around the brass rivet that held the center conductor brass rivet on the back side to its companion rear plate. After I had scraped enough with the straight pin it appears that I have gotten a better connection to that back plate. The unit seems to be responding well to the volume up function of you hand unit now.

    My prior efforts with cleaning the connector on the back of the microphone assembly evidently was just flexing the bottom/rear plate against its brass rivet and occasionally improving the connection temporarily. I am very hesitant to apply any solder and heat between that bottom/rear plate and its companion brass rivet. I think I will apply a very small area of silver conductive paste against the side of the brass rivet and the bottom/rear plate and see if that produces a reliable connection between the two. I could actually see the poor connection between that bottom/rear plate and the center conductor contact at the back of the microphone assembly with my ohm meter. You could sometimes see a good connection and then loose the connection.

    Apparently the vibrations coming from the hand unit hit the top/front plate and it has some very small folded over edge point contacts that touch the edge of the two-layer sandwich element between the front/rear plates. The very small vibrations transmit energy to the edge of that sandwich element and it produces signal output whenever an ultrasonic 38kHz or 41kHz signal is sent from the hand unit. I was very careful with handling of the microphone element as it has to be quite sensitive.

    Those front and rear plates appear to be very thinly plated silver over copper flashed brass. The surfaces of both have turned mostly black over the years from the tarnish that develops on silver exposed to the air. Silver tarnish itself is still a good conductor. I think the real problem is with the brass rivet-to-silver surface. The brass will corrode between it and the mating silver surface due to air and moisture. My cleaning effort, using a straight pin, was focused right along the edge of the brass rivet.

    I will eat lunch and then try again to see if the microphone is still working for volume up function. If any problem is observed then I will apply the silver conductive paste approach and run another set of tests.

    Joe
     
  6. audmod01

    audmod01 AK Subscriber Subscriber

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    My son is coming tomorrow and bringing his excellent digital camera with various lenses. He is used to taking pictures of very small things, so he is going to try capturing some views of the inside parts of the ultrasonic microphone. There are so few of these in existence any more that I think it needs to be well documented. My proposed solution for correcting the poor connection of the signal lead may be successful long term, but if not I may investigate obtaining an electret (condenser) type microphone to replace the original microphone.

    I will post the pictures here for posterity.

    The exact method that Admiral used in the construction of the ultrasonic microphone are not easy to determine. It would requite a really well equipped materials analysis lab to take this item apart and determine the device theory of operation. Some method is incorporated in the double layer sandwich area between the two metal plates which produces a small voltage output when vibration is applied from the front metal surface.

    Joe
     

     

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  7. audmod01

    audmod01 AK Subscriber Subscriber

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    Here are some pictures of the RK-20 Receiver Microphone Element from Matt's remote control system. I was fortunate to have my son David take these pictures with his professional level camera and lens. The microphone element sits inside an aluminum can which has a front protective grill. To remove the internal parts requires that four folded over areas on the back side of the aluminum case be bent away from the the back metal disc with its output RCA Jack. This allows the internal parts to be removed from the case. Once that is done you can view the internal parts. The electrical signal element appears to be a small square sandwich of some sort of piezo material. That sandwich piece is held at two corners by folded over tabs or ears which fit into machined grooves in the sandwich material. The front plate with its folded ears is at chassis ground potential. The back side of the sandwich piezo material makes contact with the back signal plate through two metal buttons that touch the two outside ends of the piezo sandwich material. It was not possible to show those metal buttons in the photographs.
    Microphone Element Front web.jpg
    Front View of Microphone Internal Assembly
    Here you can see the small brass rivets that hold the Front Plate with its folded ears to the support leads from the back side. You can see the machined groove that the top ear fits into of the black piezo element.
    Microphone Element Edge web 02.jpg
    Edge View of Microphone Internal Parts
    Here you can see how the signal lead connects through the brass rivet at the right while the chassis ground is connected through the rivet at the left end to the front plate. The orange items are plastic insulators. The one on the left end is slightly taller than the one at the right with a flange such that the Signal Back Plate does not touch chassis ground. The gap is so thin that it can only be seen with a microscope or Jeweler's Loupe. There was considerable corrosion between the back plate and the right brass rivet. I was able to clean that spot with a straight pin very carefully. That effort must have pushed just enough surface metal plating from the Signal Back Plate against the brass rivet until it established a good connection again. The front and back plates appear to be brass or berillium copper coated with silver (which have turned black from air contaminants and humidity exposure). Placing dis-similar metals against each other in compression without any solder just about guarantees corrosion will develop over time. In this case since the insulators at each end of the assembly are plastic, solder simply cannot be used without destroying the insulators! So, though I have established contact again between the signal lead, brass rivet and the Signal Back-plate, I plan to add a small amount of conductive silver paste at the area where I cleaned the contact connection to create a more permanent contact. The silver conductive paste is normally used to provide a good heat sink for CPU chips in computers.
    View attachment 1214102
    Another Oblique View of Microphone Internal Parts
    Here is a view that shows the internal parts so that one can have a better understanding of the parts stack-up.
    Microphone Element Edge web.jpg
    The strange thing about this particular microphone element and its failure mode was that it was intermittent. At times it would work very well and then it would just quit working altogether. It just goes to show how difficult diagnosis can be at times. I first thought the problem was with a poor connection right at the RCA output jack on the back of the microphone assembly. Just plugging and unplugging the cable from the circuit board would cause it to work at times and at times not. I cleaned the RCA jack and the cable RCA plug several times with erratic results. Almost anything I did to the microphone assembly would cause it to work for a while - then fail again.

    I am just glad to have finally found the cause and developed a cure for it! An alternative to the conductive silver paste is conductive silver paint. The paste should work though because it not only conducts but also keeps the air away from the freshly cleaned metal surfaces and thereby prevents corrosion there.

    Joe
     
    Last edited: Jun 17, 2018 at 8:09 AM

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