MF-300 & Remote Control Acquired

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

  1. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Scotts Mod-U-Tron in 1969-70 sounded like a good idea at 1st, but IIRC, it ended up being one of the factors in their subsequent closure. Too many problems with the actual boards.
     

     

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  2. gadget73

    gadget73 junk junkie Subscriber

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    Very cool that you've gotten this thing going. You may be the subject matter expert on these things now. I doubt many even exist anymore, let alone in complete working order and certainly not with the level of reverse-engineering and diagnostic work you've done on yours.
     
  3. audmod01

    audmod01 AK Subscriber Subscriber

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    Thanks guys! It has been a considerable learning project.

    I also downloaded the Admiral patent mentioned, and it describes some of the circuitry used in this unit in general terms but the main claim in that document was that the engineers mentioned had come up with custom relays that had two or more relays interconnected by a common armature in such a way that a fully energized coil would prevent the actuation of another relay due to momentary noise spikes. This particular remote control for Fisher does not utilize special relays with such an interconnecting armature on a fulcrum pivot. I still need to determine how the engagement of the mercury switch in the hand unit affects the output of the hand unit and consequently its detection in the RK-20 receiver.

    It was worthwhile to look at the patent, but unfortunately no useful new information was found that more fully describes the Fisher RK-20 design.

    Joe
     
  4. audmod01

    audmod01 AK Subscriber Subscriber

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    With two remote control hand units to try, I have learned that the OEM transducer used seems to be optimized to the oscillator circuit in the hand unit. The hand unit from Matt's system produces more than twice the output than mine does with its new transducer. I also learned that the channel or station tune output of the hand unit is momentary regardless whether the button is held down or pushed momentarily. By contrast the volume up or down function is continuous until the volume button is released by the user. The MF-300 only requires a momentary push of its front button or command from the RK-20 unit as it has its own hold-on circuitry inside the tuner that releases when a suitable station is tuned in on frequency.

    When the volume down or tune down function is activated by rotating the hand unit counterclockwise from level, you hear a brief 60Hz buzz (tune down function) or continuous buzz (volume down function) coming from the transducer. If you hang an oscilloscope probe on the base of the volume change circuit driver transistor Q4 you can see either a continuous sine wave or a sine wave modulated with 60Hz depending on whether the hand unit is horizontal or tilted counterclockwise.

    My next step is going to be measuring the two transducers to see what the characteristic capacitance of the OEM unit is versus the replacement I bought. If the one I bought is lower in capacitance, I may be able to bridge it with a suitable fixed capacitor to make it close enough to the OEM unit to work properly. Currently I appears to be only triggering the station tune function. It may be that its capacitance is too high compared to the OEM unit. That would likely reduce the output the higher the frequency the oscillator emits. Another possibility is that its impedance is too high or too low.

    Joe
     
  5. audmod01

    audmod01 AK Subscriber Subscriber

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    Sorry for delays, Spring is here and yard mowing has had to be done.

    This morning I decided to measure the capacitance of the OEM transducer in Matt's remote hand unit since I did not have to change it in order for the oscillator to work. His hand unit works both Matt's RK-20 receiver and my RK-20 receiver. The capacitance of the OEM transducer in Matt's hand unit measures about 88 to 90pFd. The Murata transducer I ordered for my unit measures 2500pFd. So there is a large discrepancy in internal capacitance of the two transducers. It appears that on my hand unit the frequency of the output is shifted to a much lower frequency and it does not operate the volume functions, only the channel change functions. Matt's unit does both well. I need to see if I can put another capacitor in series with the new transducer in my hand unit to get it to operate at the higher volume function frequency.

    Joe
     
  6. rufleruf

    rufleruf AK Subscriber Subscriber

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    Wow - this is quite the odyssey. Time to check in on my 1800 thread
     

     

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  7. audmod01

    audmod01 AK Subscriber Subscriber

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    I took a look at the parts that make up the ultrasonic transducer in this remote hand unit. There is a thin metal plate which the moving membrane is attached to. The metal plate and membrane is held close to a perforated circular metal plate which allows the sound to pass through the perforations. The circular metal plate is riveted to the beige phenolic base that all the transducer parts are mounted on. The rivets are extremely small and I removed them with some jewler's needle files. That allowed me to get at the thin metal plate the membrane is mounted on. The membrane is extremely thin - so much so that even a slight breath will cause it to flex. In my unit this membrane had worn away such that about 1/3 of the outer edge had come apart.

    I spent several hours today looking on the internet for ultrasonic transducers again and there are simply none that have a capacitance around 90pF. All of them are about 1800pF to 2400pF which is apparently too much to allow the transducer to reproduce the required frequencies with adequate amplitude and also be modulated with a low frequency such as 60Hz or 120Hz (the buzz that the Fisher manual mentions when the hand unit is rotated vertically while a button is pressed). I am thinking of looking for some metalized Mylar film that might be thin enough to replace the damaged film on the thin metal mounting plate I described. I might get lucky enough to repair the transducer and get it to work correctly again.

    In the meantime, for anyone who has one of these hand units, I suggest you be very careful with it. Do not blow air inside the front of the unit to try to clean it. A small breath of air could even be enough to damage the transducer beyond hope. If you want to clean the unit, take the covers off. The aluminum grill at the front of the case will lift out. Avoid touching the front of the actual transducer element. You can clean the circuit board with cotton Q-tips moistened with isopropyl alcohol. Do not allow the liquid to get on the actual transducer. Clean the battery terminals using the Q-tips and follow that with fine sandpaper. I used an old typewriter eraser which comes in a pencil form that can be sharpened in a pencil sharpener. It may be necessary to re-tin the metal clips using some solder and some rosin flux. DO NOT USE ANY ACID FLUX! Acid flux is used by plumbers and leaves residue behind that will continue to corrode forever. Excess rosin flux can be removed with alcohol. Get a new 4.5V battery made up (see instructions on type of button cells used on previous posts in this thread). Install the battery making sure the + terminal is against the forward metal clip. Carefully reassemble the case and try the remote. You may get some action from your RK-20 remote receiver. Listen for the volume motor to turn on and rotate the volume control or for relays inside to click. If you do, you may have restored function to your remote system.

    Joe
     
  8. rufleruf

    rufleruf AK Subscriber Subscriber

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    Joe,

    Have a look at the Fisher WT-50 remote - I can't imagine they reinvented the wheel a few years later. Maybe the same inside?
     
  9. audmod01

    audmod01 AK Subscriber Subscriber

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    Matt;

    So far I have not been able to find information on the Fisher WT-50 remote control.

    I did discover that Admiral made a "clicker" type ultrasonic remote control hand unit for their televisions.
    Admiral hand unit 01.jpg Admiral hand unit 02.jpg Admiral hand unit 03.jpg Admiral hand unit 04.jpg Admiral hand unit 05.jpg Admiral hand unit 06.jpg Admiral hand unit 07.jpg Admiral hand unit 08.jpg
    You can see the similarities to the case used for the Fisher remote hand unit. Unfortunately inside it totally different, using tuning rods similar to those used by Zenith for some of their TV remote controls. This remote control is currently on the auction site.

    Continuing my investigation of ultrasonic transducers, I began to compare the ease with which the membrane of the remote hand unit transducer would wrinkle and I believe it is at least as thin as the typical Saran (or similar) wrap used on food items in our homes. The thickness of that type of wrap is about 8, 9 or 10 μm thick, so, if I can find some metalized Mylar or similar material in that thickness range it should work. There were two very small hollow brass rivets that held the Mylar film against the other metal parts in the transducer. I need to find some replacement rivets or find some analog clock/watch screws and nuts that would be the right size to fit the hardware of the transducer mounting phenolic bracket.

    No current transducers really fall into the capacitance range the OEM unit presented as a load to the oscillator circuit in the hand units. I believe that rebuilding the existing transducer will be the best chance for success.

    Joe
     
  10. rufleruf

    rufleruf AK Subscriber Subscriber

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    Look at the 801 manual on hifi engine - the WT-50 is meant to work with it.

    Has a 22.8 kHz transducer
     
  11. audmod01

    audmod01 AK Subscriber Subscriber

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    Matt;

    I did find the Fisher 801, which was a Quad channel receiver of 1972 model lines. Unfortunately Fisher under the management at that time decided on a single function tuning only remote control. Not only that, but the operating frequency of the ultrasonic transmitter/receiver is at 22.8kHz so it operates at just a little over 1/2 the operating frequency of the RK-20 system which is centered around 40kHz. The transducer would not even come close to that of the RK-20 system.

    Joe
     

     

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  12. rufleruf

    rufleruf AK Subscriber Subscriber

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    Ah well, I was ready to send you my WT-50 if it would have worked.

    What else do they use ultrasonic transducers for? Garage door openers?
     
  13. audmod01

    audmod01 AK Subscriber Subscriber

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    Matt;

    Most if not all garage door openers use RF signals. Early ones were VHF or UHF frequencies. I remember some were activated by the passage of the Russian Sputnik satellite passing over. Eventually they introduced digitally encoded RF transmitters.

    Joe
     
  14. audmod01

    audmod01 AK Subscriber Subscriber

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    Today I was provided with a link to a company which makes metalized film materials for use in X-ray equipment and various other applications. They have several kits they offer of samples which I plan to investigate. Some of these are reasonable and I can afford to order a couple to evaluate for use in repairing existing defective transducers. I might ask if they could send a smaller amount, say 5 or 10 of a couple of thicknesses of metalized Mylar film using their sample cups and thin-film support windows. See:
    http://www.chemplex.com/thin-film/thin-film-selection-guide-1328/thin-film-selection-guide
    I need to also find a source for the extremely tiny rivets that hold the membrane and mounting ring plus perforated grill cover together on the phenolic mounting bracket. I plan to draw an illustration of how the assembly goes together for posterity.

    Joe
     
  15. rufleruf

    rufleruf AK Subscriber Subscriber

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    If you need any help paying for any of this let me know - fantastic that you are going to such lengths to get it working.
     
  16. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    HO & N gauge Train Collectors might use rivets that small, especially the kit builders. I'd check with a Hobby shop and see if they can come up with a distributor.
     

     

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  17. audmod01

    audmod01 AK Subscriber Subscriber

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    Larry;

    Thanks for the suggestion. I had not thought of that as a possible source. Two heads are better than one! One more possibility might be some very small screws and nuts similar to those seen in watches and clocks.

    Matt;

    I was very fortunate to have your receiver and hand unit to compare to. It turned out that there was not much wrong with either unit other than the electrolytics going south. Being able to measure the signal level to the transducer on a working hand unit is a real plus. No official service data really went into that much detail, so we are breaking new ground on this. If I can duplicate the internal capacitance of the original transducer in my hand unit we will know for sure that a thorough repair of these units is still possible even in current times. I plan to start with a 6um thickness first. Then I might try a 12um thickness of the Mylar metalized film.

    Joe
     
  18. audmod01

    audmod01 AK Subscriber Subscriber

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    As things turned out, I contacted Chemplex about the Mylar film and explained what I intended to use the film for. I received a response today and it was not what I was expecting to learn. The film samples that they offer are not metallized! The person who had sent me that link had not noticed this fact and neither did I when I decided to contact Chemplex. So, today I did a search again and did find some 0.5mil thick metallized Mylar sheets on Amazon and have ordered some. eBay was no help, nor was Alibaba, the Chinese source. Alibaba sources do have some metallized Mylar film in the range I could use, but their minimum orders are astronomical as to quantity and the cost is far beyond practical for this type of endeavor.

    The order is supposed to arrive May 29th. and consists of 50 sheets of 18" X 30" metalized Mylar of 0.5mil thickness which is the same as 12.5um. It is about double the thickness I wanted but appears to be all I can get for now.

    Joe
     
  19. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Damn Joe. Yer like a pitbull clamped down on a Mail Carrier's leg. I would have given up on the 1st page!:bigok:
     
  20. rufleruf

    rufleruf AK Subscriber Subscriber

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    Joe - is it possible to modify the receiver portion of the system to be responsive to the new transducer, instead of trying to modify something in the hand unit that works with the old component in the receiver?
     

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