MF-300 & Remote Control Acquired

[audmod01]

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

 

[gadget73]

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.

 

[fred soop]

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.

 

[audmod01]

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

 

[audmod01]

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

 

[audmod01]

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

 

[audmod01]

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.

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.

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.

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. The microphone has an Admiral part number of 78B182-2B imprinted on its metal case.

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

 

[audmod01]

After several failed or less than ideal results to align Matt's RK-20 transmitter and receiver together, I realized that the main reason was my signal generator. On the frequency band of interest, very tiny fine tuning adjustments result in large frequency changes in its output. Cleaning of the control made no change in the results. I have ordered a new Feeltech FY6600 60mHz signal generator that allows computer control as well as front panel adjustment from 0.5Hz to 60mHz to within +/- 2Hz accuracy and provides Sine, Triangle and Square Wave output waveforms. It will also do linear and log type frequency sweeps for checking frequency response and sweep alignment of radio equipment. It holds promise as a real workhorse for many tasks and was less than $100 with shipping - made in China of course, but considering the features I could not pass it up.

Once I get the new signal generator in hand, I should be able to do a verified alignment of Matt's RK-20 remote control system and then do a much better job of working on my own unit.

Joe

 

[audmod01]

A friend in England has offered to send me a polypropylene capacitor which has film that is metalized on one side only. I am going to take him up on his offer. The more samples I can try the better for the transducer rebuild.

Joe

 

[audmod01]

I made a couple of discoveries recently:

1) When I said that the channel up/down tune button of the remote hand unit only output a short burst, the statement was only partly true. It turned out that my hand unit did that, but only because part of the spring leaf switch was deformed (bent). Matt's hand unit did not do that, so I had to re-form the leaf spring of my hand unit to output continuous CW or modulated signals for the tune function. It is a good thing that I have two complete systems to work with!

2) Both of the constructed batteries I bought from Batteries Plus turned out to exhibit a fault when installed in these hand units for over a week. The tough heat shrink that was used to keep the three button batteries in a straight stack deformed due to the pressure of the battery clip inside the hand units.

I am going to perform surgery on one of the battery packs and see if I can find a hard plastic sheath I can stack these inside of that will keep the negative end button battery from twisting crooked. I don't have an immediate solution for this. I will have to do some research. The physical space available inside the hand unit case has limits.

I am just glad that I discovered this before I concluded that I had completely solved the hand unit battery substitution issue.

Joe

 

[gadget73]

Is it possible to remove or de-tension the battery clip? Failing that maybe something like a pill bottle or similar hard plastic tube would work. Not sure how much space you have to work with in there.

 

[audmod01]

Gadget;

Thanks for the suggestions. I will try anything! It does need to be a hard plastic. I will try to reduce the tension on the battery clip, however I suspect it is made from Beryllium Copper, which can be quite brittle and crack easily. Another possibility would be to attach wires to the battery, remove the battery clips and solder the battery in circuit. There are bumps along the road of restoration no matter what kind of equipment is involved.

Joe

 

[gadget73]

Maybe you can fashion new clips out of standard copper or brass. Replacement ones for the Simpson 260 for example are just copper cut and bent into the shape it needs to be. Not quite as hard as the originals but when your choice is a rotted away original part or a new one you may need to tweak slightly at battery replacements, you tweak it.

 

[audmod01]

There are different alloys of copper, but brass might be more durable. I will have to investigate. Thanks for the suggestion.

Joe

 

[audmod01]

Yesterday I cut a piece of gray PVC 1/2 inch conduit to see if I could fit the 3-cell stack inside. The cells would not fit at least at first try. I need to try removing the thick heat shrink tubing and try again. If I can get the stack to fit inside the conduit sleeve it should be a good holder for the stack. If notk I will have to look for something else or see if I can re-form the existing clips in such a way that they do not exert so much force against the ends of the assembled 3-cell battery stack. I have done some searches for 3-cell battery holders but have not found anything that will work yet.

Joe

 

[audmod01]

After considerable searching I found a company that offers a set of 3 LR44 1.5V button cells in a battery holder and have ordered some to try. These battery holders are smaller than the area inside the remote hand unit case and the space between the existing battery clips. I think these will work well, but will know once I get the parts in hand to try. I may use wires to solder the battery holder to the circuit, but put some spacers to fill empty space and keep them from just floating about inside the case. I will take some pictures when I get the arrangement created.

Joe

 

[audmod01]

I finally received the three LR44 cells and battery holder I ordered through Amazon. These will replace the first battery pack I bought from Batteries Plus. The batteries in the plastic case are shorter than the OEM battery and the ones I had made up by Battery Plus. The battery stack after installment and being in place for a couple of weeks deformed so badly in both my hand unit and in Matt's unit that the voltage dropped to about 3.8V. It was not enough to operate the hand units. The replacements fit into an included battery case from DFRobot and are short enough that I can make a foam plastic block, cut out the center large enough to put the battery case in and have it fit snugly between the existing battery clips in the remote hand units. Here are some pictures that show what happened to the constructed button cell stacks I bought from Batteries Plus and the new LR44 3-cell case from DFRobot:

Here you can see how the lower unit has such deformation the negative pole piece has popped out of the outer sleeve of the right end cell.

This is a much clearer image showing how the pole piece at right has almost completely popped out of the cell case.

Here is one of the Batteries Plus constructed packs and the DFRobot battery case and LR44 cells. The contacts at the ends of the case can have wires soldered to the tabs on the outside of the case and then soldered into a circuit with short loops of wire.

This view shows the solder tabs on the outside of the LR44 battery case.

Now I can get back to restoring the two hand remote units and provide power to do the alignment.

Joe

 

[dcgillespie]

What an odyssey this restoration effort has been! Working with some of the earliest efforts of electronic wireless remote control systems surely presents more than its fair share of problems 50+ years later! No doubt your systematic approach to methodically tackling each hurdle will certainly allow you to prevail in the end. Fascinating to watch it all unfold, and the solutions developed along the way.....

Dave

 

[audmod01]

Dave;

Thanks indeed! We all love to see full functionality restored to our vintage audio equipment. Remote control systems are a unique challenge and especially these ultrasonic designs. Many of the piece parts are simply not available any more. As the work progresses, I may eventually need to develop a remote hand unit using more modern parts, but hopefully I can get both Matt's and my hand units operational again without any significant modifications. Matt's unit is the closest to being finished since his remote unit was operating after replacement of electrolytics and battery in the hand unit. His hand unit also works my RK-20 receiver as well as his, so even when the microphone in his RK-20 receiver was intermittent, it was still operating my RK-20 receiver. That pointed to the receive microphone in his receiver being intermittently open.

So, back to the workbench!

Joe

 

[audmod01]

I added wire leads to the battery cases and soldered the cases to the OEM battery clips. I took this picture of the two hand units with the battery cases and the plastic foam pad I mentioned surrounding them. Matt's remote hand unit is at the right and mine is on the left. As it turned out one LR44 battery in the three provided is bad and had leaked fluid out so that one will have to be replaced. I may order a package set of LR44 batteries if I don't find a local source that is reasonably priced.

Overall the result is good - small size and well enclosed batteries with good output voltage for the oscillator circuits. The battery cases open easily with a pen-knife inserted in the end-to-end seam on the sides.

Now on to alignment and testing.

Joe