MF-300 & Remote Control Acquired

[audmod01]

Well fellow Fisher Fans, you know how sometimes you keep missing the obvious? Today I discovered that I was doing just that when dealing with my own RK-20 remote control hand transmitter unit. I could not get the volume function to tune up even though I could get the channel function to tune to the volume frequency and operate the volume circuits in the remote control receiver. I finally realized that when the channel button is pushed you add capacitance to the existing capacitance of C27 which is a 280pFd 5% silver mica capacitor that is always in circuit across the transducer and its internal capacitance (whatever it happens to be). Having to add capacitance to get the channel function to tune to the volume frequency with the adjustable inductor finally made me realize that I needed to add capacitance to that of C27 across the transducer to bring it into resonance at the volume frequency. I tried several capacitance values and finally settled at 110pFd across C27. With that amount of capacitance added, the inductor would tune the 41.805kHz frequency with a good solid high output AC voltage going into the transducer. I peaked the 41.805kHz output using the slug of the oscillator transformer while watching the detected signal at the base of Q4 in the receiver with my scope connected through a 470K ohm resistor to avoid loading down the base of the detector.. Then I tried the channel tune frequency and managed to peak its output at the 38.285kHz frequency while looking with the scope connected to the base of Q5 of the receiver. Now the RK-20 remote receiver responds to both channel and volume frequencies pretty much equally!


I also had to repair another broken trace, followed by changing the capacitor C28 which is a .05uF disc ceramic to the base of the oscillator transistor. This improved the symmetry of the oscillations of the circuit. I changed C29 from a .5uF electrolytic to a 0.47uF 100V metalized film polypropylene capacitor and the 250Hz modulation frequency began to work!
So now I have proved that the transducer will work for both channel and volume frequencies (with and without the 250Hz frequency which modulates the continuous wave frequencies for motor reverse) the 0.5mil metalized film that I bought to begin with. What a relief!!!

Now we know that a defective hand transmitter transducer can be successfully rebuilt and perform all the functions it was designed to do when new.

Joe

 

[gadget73]

So the issue was that the new film requires a bit different loading then? Thats not so bad. I don't have one, but thank you for your efforts on this.

 

[audmod01]

Gadget73;

Yes, I suspect that if I had film that was about 1/2 as thick, say 0.25mils that it would work better and might not have to be tweaked by adding extra parallel capacitance. The output level is not as high as Matt's original unit I worked on. I am going to try one of the 2N4403 transistors that I also bought to see if it will produce more output from the transducer. I may see if I can find a metalized film capacitor of large enough physical size and tear into it to get to the film and hopefully get some that is thinner than what I currently have, however if the 2N4403 does the job it will be time to stop and call it finished. The frequencies are all good, just needs more output level. The perforated aluminum grille on the front of the hand unit knocks down the output level considerably. I might also try a fine mesh screen wire grille to see if that helps.

Joe

 

[rufleruf]

Joe,

That's great news!! The hand unit and receiver arrived safe and sound yesterday and now I am thinking it's time to dig into my tuner.

I have a 500B to work on for another member here first, a 500-C to finish, and about 50 pounds of vintage electronic parts to sort first though.

 

[audmod01]

After I had finished work on Matt's RK-20 and remote hand control, I became dissatisfied with the results I had managed with my own unit. I had discovered that if I added about a 100pFd capacitor across the transducer in circuit that I could get it to tune to the right frequencies, but it had very poor amplitude and only worked a few inches from the RK-20 receiver. I had set the units aside until I felt like tackling it again. I began trying new pieces of metallized Mylar film of the same thickness I had on hand. It still refused to improve performance. I knew that if I managed to get more capacitance in circuit without using a capacitor in parallel, that it should improve the performance. I took the transducer out of circuit and measured its capacitance which was only about 13pFd! I knew from measuring Matt's transducer that it needed to be close to 100pFd. I had placed the Mylar film on the back side of the thin aluminum plate and decided to move it to in front of the thin plate. I did that and discovered that the capacitance had risen to about 98pFd as measured on my capacitance meter. This was a vast improvement compared to previous efforts.

I reinstalled the transducer and began to measure amplitude of the signal at the base of Q4 in the RK-20 receiver. The signal level is significantly improved. I have not finished the alignment of transmitter to receiver circuits yet to see just how far away I can get the transmitter to work, but I believe it may work out to a reasonable distance. More to come later on the progress. In the meantime, I modified a drawing I had made previously to reflect today's discoveries.

Joe

 

[matt e.]

I have a MF-300 with 34 foot RK10 wired remote. Thanks for your great posts and maybe someday I will get lucky and find an RK20. BTW I did open up the RK10 and discovered the internal pcb is the same as used in the RK20 although of course none of the parts are there on the wired version.

 

[audmod01]

Matt;

This is interesting. I wondered what the RK-10 looked like. Could you take some external and internal pictures and post them here? Your wired remote control may well be even rarer than the RK-20 remotes.

Somewhere I still have the extremely thin metallized Mylar film that I bought from a specialized Yahoo group dealing with radiation detectors. That will be my last resort if I am unable to get the 1/2 mil Mylar to work. One other possible way to improve distance would be to improve the sensitivity of the RK-20 receiver, either by the use of a better ultrasonic microphone or using a modern Op Amp chip instead of one of the transistor gain stages in the receiver. Op Amps are easy to use and gain can be set quite easily.

There is still the possibility of creating a hand held transmitter using micro-touch switches and 555 timer ICs plus a way to couple the energy to the transducer using a step-up transformer. It might be possible to use one of the step-up transformers made for dynamic microphone work. At some point I may pursue this route anyway, because not everyone who has an RK-20 receiver has the hand held unit.

In the meantime the Hayseed 800-C electrolytic kit arrived yesterday, so I need to stop and get those installed and finish up the 2nd 800-C I started restoration on.

Joe

 

[matt e.]

Matt;

This is interesting. I wondered what the RK-10 looked like. Could you take some external and internal pictures and post them here? Your wired remote control may well be even rarer than the RK-20 remotes.
Joe

Pictures attached of the RK-10. The pcb is warped but does not interfere with the simple switch action. Actually it's in pretty nice shape. I bought the MF-300 recently from an estate presale. I don't know how rare these are, but I don't recall ever seeing one so I was lucky to have found it. When I asked if the tuner had the remote they told me it was too old for that, but after being persistent and texting a picture the nice lady miraculously was able to find it in a box in the guy's basement. No manuals were found, but the MF-300 came with the Allied made walnut case so I would assume they sold it originally. I uploaded additional pictures of the unit to rm.org

 

[audmod01]

Matt;

Thank you for sharing the pictures! The inside looks very clean. The pictures show very clearly how the traces are laid out. Part of some traces in my ultrasonic remote transmitter are missing. I had to use small wires to extend to leads of the transistor in order to get mine working at all.

I need to make another effort at replacing the .5 mil metallized Mylar film to see if I can get one installed without causing any wrinkles in the film as the four screws are tightened. I believe that any wrinkles in the film can cause the transducer not to operate correctly or with best efficiency.

It would be interesting to learn what the nominal inductance of the adjustable coil is. It has a fairly broad tuning effect as the core is adjusted.

Joe

 

[matt e.]

A Zenith ultrasonic remote.
http://digitalcollection.chicagohistory.org/cdm/singleitem/collection/p16029coll3/id/1832/rec/43

 

[audmod01]

Matt;

I am not sure the particular remote control by Zenith mentioned in the thread was the one introduced in 1956. The 1956 unit I remember was a type that used tuned steel rods to emit the ultrasonic sound being used for volume up/down or channel change. It was a mechanical device with spring loaded "hammer pieces" that struck the rods when a button was pushed. My dad bought such a Zenith B&W TV in 1957 that had that type of remote control. Zenith did not produce their own color TV sets for a few years after RCA introduced them in 1954. RCA had a 7 button transistorized remote control by about 1958-59 that used ultrasonic sound and had a transistorized oscillator built into the remote hand unit. It allowed the customer to control color level, tint, brightness, volume, channel up-down, fine tuning, and on-off. That was available on their CTC-9 color TV chassis. I serviced some of those when I worked for RCA Service Co. in Dallas years ago. Later (1960s) RCA changed to a more simple remote control, still ultrasonic, that only changed volume/on-off, color level, tint and channel change. They continued to use ultrasonic remote controls up until the XL-100 color TV sets in the early 1970s. Somewhat later the development of infrared LEDs brought about the introduction of remote controls using light energy instead of sound. The biggest issue with ultrasonic remote controls was that random noise from items such as keys or coins could activate different functions on the televisions which had remote controls.

Admiral produced the RK-20 for Fisher and had their own line of ultrasonic remote controls for their line of televisions. Magnavox used "whistles" that were activated by bellows to produce ultrasonic sound in their early remote controls for television. We Americans were a lazy bunch and wanted to do everything by push-buttons from our easy chairs!

Repairing ultrasonic remote control hand units is quite a challenge in units that use transistor oscillators to produce the output. Various types of transducers were made, but most all of them used metalized Mylar film inside to produce the vibrations emitted. Some of the mechanical type remotes were paired with receiver circuits that utilized stepper-motors to do certain functions.

Joe

 

[matt e.]

Wonder if your dog could hear those ultrasonic remotes? That must have driven people crazy when coins in your pocket could change the channel. Some early fuel injected cars suffered a similar fate during the CB craze when rf would enter the efi system and cause all kinds of weird unpleasant results. Museums do great work, but sometimes get the facts a little bit blurred. Not to get way out of this thread, but last night I was wasting time looking at the Henry Ford Museum's radio collection and noticed they listed a Grebe RORN as an audio freq amp while it was actually an rf amp. BTW the Henry Ford does not have a single Fisher piece in their collection. Anybody want to make a donation?

 

[larryderouin]

FISHER was too "highbrow". Maybe Soundesign, or LLoyds might be more up FORD's alley.

 

[rufleruf]

Joe,

You are reminding me it's time to restring my MF300 and see how it does. I purchased a second remote receiver that I need to test too.

matt e. - thanks for posting a picture of an RK10 - been wondering what it looked like.

 

[audmod01]

Larry;
A Fisher donation would need to go to a "Lincoln" department or a "Lincoln" museum. Fisher specialized in home entertainment equipment, but I bet they could have done a great job creating some AM/FM radios for autos - dream on!

Matt e;

Again thank you for sharing the RK-10 pictures and description. I believe there were some ultrasonic remote controls that dogs and cats could hear rather well, especially those that operated in the 20-30kHz region. I also remember women often could hear the 15,750kHz frequency of television flyback/horizontal output stages of tube type televisions. Most technicians (men) struggled to even hear it, especially if they had been in the US Military and were around artillery, fire-arms or rockets in firing mode. Tanks were not friendly to ears either. Loud repetitive noise damages hearing, especially in the higher frequency region.

Joe

 

[fred soop]

.....
I also remember women often could hear the 15,750kHz frequency of television flyback/horizontal output stages of tube type televisions.
.....

Back in the day, I could align the horizontal oscillator without looking at the screen. The 15,750 became 15,734 after color was introduced. Much easier to hear.

 

[hammr7]

Joe,

Too much great work. And one of the best labors-of-love I have seen on AK.

Since I already have the receiving unit, I think I'll have to be like the "I want it now" millennials and ask when you are going to start mass producing these remotes so I can get a couple!

If I do ever find a sending unit in the wild, I'll be living this thread.

Hank

 

[gadget73]

Some early fuel injected cars suffered a similar fate during the CB craze when rf would enter the efi system and cause all kinds of weird unpleasant results.

I had an 88 Dodge Dakota that would shut off any time I keyed the mic, even with a stock, legal radio. I lined the ECM case with foil tape which fixed it until I added an amplifier to the mix.

 

[audmod01]

Hank;

I might be able to come up with a replacement for the original Fisher hand unit, but it would not have the mercury internal switch which is activated by rotating the hand unit on edge while pushing the same buttons to get volume down or channel down functions. I have thought about replacing one of the RK-20 receiver circuits with an OP amp and adjustable gain control to allow for tweaking the sensitivity as parts age. I keep acquiring more Fisher project units to revive - as I told Dave, it is a disease!

gadget73;

Yes, early efforts at computer control of various engine functions just did not consider RF interference. I remember a case in Dallas years ago where a person driving a Cadillac was driving near a city truck whose driver keyed his mike to talk to a dispatcher and killed the Cadillac's computer bringing the car to a slow stop in a lane of a major freeway. It caused a major multi-car pileup and many people were hurt. The car manufacturers learned they had to shield computer equipment and the leads going into and out of the computer to protect them from strong RF energy.

Applying new technology in any field that is radically different than the original technology often leads to new problems to have to solve. Some troubles do not show up in factory field testing but in the real world more factors exist to really test a product.

I need to get back to the hand unit alignment and see how good I can get it. One possibility that I have been considering would be to replace the metallized Mylar film one more time. The idea I had is to keep the film tight while mating it to the metal ring plate. The thin backup plate that directly touches the metallized film might be adhered to the film while the film is under tension by applying some heat from a flat iron. This would basically somewhat melt the film enough to make it stick to the backup plate and create a more unified part while keeping the part of the film that vibrates smoother and less likely to touch the convex screen piece and short out. First I want to try replacing the film one more time to see if another method of keeping the film taut will work without applying heat to the film.

I did some research and I believe the first Mylar film that had vapor deposited metal adhered to it was in 1955. Capacitor manufacturers began to use this method to make capacitors that were physically smaller for a given capacitance and better methods of sealing them from air and moisture began to be applied in manufacturing materials. Research chemists were great to have on staff during those times.

Joe

 

[audmod01]

Today I finished aligning the frequencies of the hand unit. The distance achieved is not very great - only a few feet. I am going to try using some Op Amp ICs I have on hand and some variable trimmer resistors to make a different receiver unit amplifier chain so that I can vary the gain in the receiver. Then at the least I can have control over how much gain to use on the receive end. I did find that when I have an oscilloscope test probe attached to the base of either the station change or the volume change transistor that I get the best results by switching the probe to X10 so that it does not load down the circuit much. Each function works correctly when the hand unit is held close enough to the RK-20 receiver.

I need to order some variable trimmer resistors to use in the Op Amp feedback path. The input resistors will be 10K ohm to match the base bias resistors in the transistor circuits. There is not an actual termination resistor as such in the circuit with the receive microphone element, so I am going to duplicate the lowest resistance seen at the base of Q1 in the receiver. The ICs I have are NE4560 types which are a pair of ICs in one package. It may be that one IC stage may have enough gain at the 38.285KHz and 41.805KHz frequencies and the second stage may only be configured as a unity gain follower, but I will get enough parts to be able to do it either way.

I can see both the CW and the modulated tones on the scope screen, so both modes in the hand unit are working.

I think it would be easy enough to create stable oscillators at those frequencies using 555 timers and have one Op Amp stage to use as a driver for a transducer from one of the modern automobile ultrasonic proximity detector designs to emit the desired CW and modulated tones the RK-10 likes to see. It should be easy enough to build a hand unit that would fit into a small plastic project box no larger than many other modern remote controls, say 1 inch X 2 inch X 3 inch or so. There are small DPST switches that are momentary contact spring loaded and hold connection until pressure from a finger is released. Some have contacts rated for a very long life. This will be one option if I am unable to get the RK-20 receiver to work with the extra gain and the hand unit as it currently is. I may still try once more on assembling the hand unit transducer with a different technique than I have used so far.

Joe