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MF-300 & Remote Control Acquired

Matt;

Is your MF-300 fairly clean overall? How about the remote unit? Fortunately the tuner can be operated without the remote so it is OK to check out the tuner before tackling the remote. I noticed when looking at the remote receiver of my unit today that I can hear the motor for the volume control energize and stop like it is at the end of its travel, but it is not actually moving - another condition to investigate.

Have fun!

Joe
 
Joe, I have yet to clean or attempt to test any of it. I expect I will need a new battery for the remote at a minimum. Cosmetically it's not too bad. Need to make a cabinet for it.
 
Matt;
I understand. There are only so many hours in a day and if you also are working full time, there are even less you can apply to restoration of any item.

One thing I thought of is that since I have a signal generator, I could apply an ultrasonic signal at the microphone input of the correct frequencies for at least a couple of functions in order to see the rest of the RK-20 receiver circuit operate. You are right though in that a working hand unit is going to be the most helpful in the long run. The functions activated by rotating the hand unit sideways adds modulation to the output and the receiver then shifts to the reverse direction for a given command, either station change or volume/on/off.

Have fun!

Joe
 
Today I ordered a set of electrolytics for the RK-20 receiver and hand unit. Now to order some transistors to use. Q5 (Raytheon 57D1-70) was definitely shorted e to c. If C12 was partially or completely open, it could have allowed kick-back from K1 to cause Q5 to short. I need to check Q4 and make sure it is OK (it seems to be since it does have collector voltage). Unless there is ultrasonic signal energy present at the base there is no bias voltage to turn Q4 or Q5 on.

I had a couple of 100uF@16V radial lead electrolytics that I used to replace two on the etched circuit board. One still needs to be replaced plus C12 rated 100uF@70V. The 0.5uF electrolytic in the remote hand unit will get replaced by a 0.47uF and the 25uF will get replaced by a 30uF. Those are all part of the order I sent to Mouser today.

Joe
 
Matt;

Based on the pictures of the internal parts of your remote hand unit, It appears that the parts visible in your unit are exactly like mine complete with the small dial-cord string restraining the position of the one capacitor right behind the transmit transducer! Even the orange wire winding around on its way to the oscillator coil is arranged in the same manner. So I conclude that both units are as they were manufactured at the factory.

I do not see any indication that your RK-20 receiver had a foam rubber pad against the large .68uF capacitor at the rear of the chassis. I have found that with the cover removed and the original foam pad removed the .68uF capacitor will move about when handling the chassis since it is physically so large and can vibrate. I am pretty certain that is why the foam pad was placed inside the chassis at the factory. If a unit is shipped a similar material needs to be placed between the cover and somewhat around the capacitor to keep it from moving while being shipped. Otherwise the capacitor leads could develop metal fatigue and fracture.

In my private conversation with you about these units I made these comments:
There is a mica compression trimmer capacitor in the hand transmitter. The oscillator coil appears to be adjustable too as it has a hex-shaped core that a plastic alignment tool can fit into. I don't see any evidence of glue down inside the coil to keep it from being adjusted. The factory alignment information talks about adjusting first L2 of the receiver while looking at the base of Q5 with an oscilloscope with the "Tune" button depressed while tilting the hand unit to the right. L2 is adjusted for maximum signal of the transmitted signal. Then the same procedure is done while depressing the "Vol." button looking at the base of Q4 and adjusting L1 for maximum signal. It says if the two signal levels differ by a factor of 2 or more, the hand unit is to be returned to the factory for alignment. Since the Fisher factory no longer exists, that cannot be done. It seems to indicate that the hand unit might need to be adjusted some in order to assure it is putting out the correct frequency or close to those shown in the schematic. L2 is supposed to tune to 38.285kHz and L1 to 41.805kHz. Fortunately I have a digital display signal generator that I can rely upon to check for correct frequency.

What we don't know is how far off a hand transmitter might be from the ideal 38.285kHz and 41.805kHz frequencies. A receiver can be adjusted within some range, but what the range of adjustment is we do not currently know. Another unknown is what any pull in frequency is, or may be involved when shifting from the hand transmitter being held level in the hand (pure sine wave output) versus rotated to the right (clockwise) when it is transmitting a sine wave modulated with an audio tone (60Hz or other). Some oscillators shift in frequency some when modulation is added.

Sure wish some old Fisher factory service people were still around who could provide more details of the interaction between remote transmitter and receiver and their frequency adjustment ranges. We will probably never know. We may have to see what these do and get a feel for how they interact in order to get close to all the original design goals. The transmitter transducer and the receiver microphone may also create some loading of the circuits that could affect frequency as well. There are lots of unknowns.

Joe
 
Yesterday I was provided with some additional information on transistor suggested substitutes for the RK-20. Here is a list:
2N632 (Hand Unit Osc.) use 2N1307
57D1-70 use 2N4403 or ECG102A
57D1-88 use 2N4403
57D1-89 use ECG160
One source said the 2N4403 would probably replace all of them. I plan to look at specifications on each substitute and see if I find any clues there.

Joe
 
I found a set of 5 RCA SK3004 which are equivalents to ECG/NTE102A transistors and ordered them. I also ordered some 2N4403 transistors to try. The 2N4403 transistors may well be more reliable than the germanium types. They have a higher e to c breakdown voltage.

Joe
 
Today I installed new electrolytics throughout the RK-20 Remote Receiver. Larry had mentioned that the OEM electrolytics were all Frako parts which are known to go bad. I made a complete sweep through the receiver and replaced them all. I also replaced Q5, the shorted 57D1-70 transistor, which operates the K1 relay for Tuning function. I used an RCA SK3004 transistor to replace it. When I applied power to the unit the power supply voltage rose to about 15VDC with the 122VAC input. One relay chatters while the power supply voltage stabilizes at turn-on. I am not sure which relay it is yet. However the relays are all stable after a second or so. The relay was chattering at turn-on before I changed the electrolytics or the Q5 transistor.

I notice that the I1 lamp works for both standby indication and for volume control relative position. The lamp receives about 1.8VAC after the 2.8VAC voltage from the power transformer passes through a 10 ohm 1/2 watt resistor, R28. Fisher did not indicate the voltage of the lamp and only assigned a Fisher Part Number, 150461-1, to it. It could be replaced by an LED if need be. Caution would need to be exercised about the leads to the lamp or LED as they do flex considerably during rotation of the volume control. If replacing the lamp with an LED I would suggest that the original lamp wires be used. The ones in this unit are blue stranded wire leads probably about 26AWG. They plug onto a terminal strip near the front where the on-off manual push-push switch is located. R28 is soldered across two of the terminals of the terminal strip. The wire supplying the 2.8VAC to R28 is green.

I discovered some time ago that K2, the reverse relay, was loose in the chassis. That did not strike me as being wise. Today when replacing capacitors and Q5 on the etched circuit board I looked closer at K2. The foot of the relay had threaded screw holes in two places. I turned the chassis over and noticed that there was an indication that two screws had originally secured K2 to the chassis. Fisher used flat-head slotted screws and they were missing from those holes. I found that 4-40 X 1/4" screws would work to secure K2 and added them. K2 is now well secured to the chassis. I suspect that someone in the past had removed those screws and forgot to replace them when reassembling. This might also explain the extra flat washer that fell out of the chassis. I suspect the flat washer would have gone between the chassis metal and the foot of the relay. There would have been two two flat washers used in installing K2.
Here are a couple of pictures:
Rcvr Parts Replaced web.jpg
K2 Fastening Screws web.jpg
Notice that there is a phenolic insulator that K3 is secured to using two 4-40 X 1/4" flat head slotted screws. K1 is just to the right of K3 and secures directly to the chassis metal like K2 does. I used Phillips head 4-40 X 1/4" screws which have a slightly higher head surface. I hope they do not interfere with the back cover of the receiver. If they do I may need to file them down some or find some slotted flat head screws at a fastener specialty store. There is one fastener specialty store I know of which has hard to find hardware at decent prices. However it means driving about 45 miles to get to the store.

Next I will work on the remote hand unit after lunch. I will begin by replacing the Frako electrolytic capacitors in it, then re-install the battery and see if the oscillator circuit is working with just those parts replaced.

Joe
 
Damn Joe. Your perseverence is paying off. I wish I had 1/2 of the knowledge you do with Electronics. But we all have our own fields of endevour, and mine was EMS. If you have to file those 2 screws do so. Beats a 90 mile trip. Unless you bring the Dog and roll all the windows down (providing it's warm enough).
 
Larry, I enjoyed your story about your grandson and your gift to him of some earphones. Your greatest gift is your affection and teaching him to appreciate good music. He is a fine looking youngster!

Enjoy!

Joe
 
After lunch I tackled the remote control hand unit. I changed both C31, a 25uF@16V (since I could not get a 6V part) and C29, originally a .5uF@6V to a .47uF@16V (since a 6V part was not available). I checked the transistor using the ohms scale of my Fluke 8600A and the junctions read normal front to back and there is no short from e to c. I cannot get any oscillation from the oscillator. I checked the resistances of the oscillator coil. Here are the measurements from the lowest leg shown in the hand unit schematic to the next lug is 0.66 ohms, to the next higher lug is 1.8 ohms and to the highest lug is 115 ohms. I suspect that the ultrasonic transducer is bad. The next step on this is to get an electret condenser microphone and try that as the load for the oscillator circuit. These metalized mylar film transducers have a tendency to go bad with time and use. I will have to search for one on-line. I might have one somewhere in my pack-rat collection, but the problem is finding it!

I decided to check the RK-20 receiver by using my digital signal generator. It can put out frequencies from 0.1 to 10Hz, etc. in powers of ten jumps all the way to 1mHz. I tuned it to the Channel tune frequency of 38.285kHz and it triggered the K1 Tune relay just by placing the output lead of the signal generator near the microphone input. Next I tuned the signal generator to 41.805kHz and held the lead near the microphone input and the volume control relay K3 was activated. So I believe the RK-20 remote receiver is pretty well shown to be operational. I don't yet know about the modulated tone case for volume up and channel up, but that depends on having the hand unit fully functional.

I used my Metric dial caliper and the opening for the ultrasonic transmit transducer is about 1.35cm or .406 inches in diameter. The original unit has two leads that solder to the circuit board and it can be removed. The parts that it is made from can be disassembled to some extent as it uses some screws akin to those that might be seen in a watch or clock. The problem would be knowing how critical the thickness of the metalized mylar film is and how to cut a new piece to the correct size and round shape. I think it would be much easier to just find an electret condenser microphone element and adapt it to this application. The typical ones seen in portable digital recorders or analog cassette recorders used as microphones will also work as transmit elements if appropriate signals are applied to them. I will look for some sources.

Joe
 
I should add that in changing the electrolytic capacitors in the RK-20 remote receiver the ones that mounted with radial leads on the etched circuit board were replaced with new radial lead types. On the bakelite phenolic board where axial lead parts were used, I cut the existing component leads flush with the part leaving the old lead in place. Then I wound a close coil in the leads of the new capacitor around a small paper clip lead so the coil would slip over the old component wire and then crimped them with small needle nose pliers, then soldered them. Larry has mentioned this method and it works well. It makes for fewer errors and less possible damage to the lugs you may be soldering to. It is also less work!

Joe
 
I ordered an ultrasonic transmit element made by Murata from DigiKey which is Murata P/N MA40S4S and has an operating center frequency of 40kHz, capacitance of 2550pF and can produce up to 120dB output with a 10V rms sine wave input. The diameter is 9.9mm which is very close to the size currently in the hand unit. It comes with attached leads for soldering to a circuit. Care has to be exercised to avoid damaging the device during solder operations. I can use siezers to carry away heat from the lead and avoid problems that way. These typically have a 4kHz bandwidth which will easily cover the 38 to 41kHz frequencies used by the Fisher remote control system. There are some other sources for similar parts, such as Jameco. I had no luck with Mouser.
 

Attachments

I am jealous seeing these hand held remotes. I have the tuner and the receiving unit, but unless a spare remote shows up, I'll end up with some sort of modern remote that can do the job. Still great reading this thread in case I ever do get a remote.
 
hammr7;

The Fisher RK-20 is an ultrasonic sound system, receiver included. Modern remotes all operate at infrared light frequencies, many of which incorporate digital pulses encoded into them in order to support a whole host of functions. Those remote hand units are totally incompatible with ultrasonic systems. To use a modern remote would require obtaining a receiver for the infrared hand unit and accessing the infrared receiver's control output leads in order to control the MF-300 and volume change and on-off functions would have to be handled by an appropriate solid state variable attenuator arrangement. There are some ICs that are made to use digital control for analog audio levels that could be utilized. I have seen a few independent systems from some sources that can do at least some of the functions. You would have to search for them.

It would be possible to make an ultrasonic substitute hand unit using some modern IC op-amps and 555 IC timers to generate the required frequencies. The volume up and frequency tune up operations of the RK-20 use the basic 38.285kHz and 41.805kHz modulated by an audio frequency and is detected by the Q6, CR2, CR3 circuitry to turn on Q7 which changes the state of the K2 Reverse relay. If I can determine the modulation frequency after getting the hand unit for this receiver working using my O-scope, I will post the information in this thread. Then if someone wants to design a remote hand unit using an ultrasonic output and modern transistors/ICs etc., I am sure it could be done. There are plenty of small plastic project boxes that could be used to house such a system. I don't want to undertake such a task myself. My only focus in this thread is to restore the existing system.

Joe
 
This morning I carefully unsoldered and removed the Transmit Transducer from the remote hand unit circuit board. I looked at the back side of the metallized Mylar film and I spotted a puncture in it. Looking from the front using a jeweler's loupe I could see the same hole through the front metal screen of the Transducer housing (not the anodized aluminum screen at the front of the hand unit). As I examined several ultrasonic transducer specifications, I found that the impedance of these transducers ranges from about 2K ohms to about 9K ohms varying with frequency. The capacitance seems to range from about 1500pF to about 3000pF. So with the transducer removed from the hand unit I am going to put a 3.3K resistor in series with a1500pF capacitor to simulate the load the transducer would normally present to the oscillator and see if I can get any oscillation from the unit.

Joe
 
For those of you who own a Fisher RK-20 or the Remote Control Hand unit, I need some information from you. There is an R30 shown on the schematic with a value of 27K ohms. In my remote hand unit, R30 is an 18K ohm resistor. I suspect that this resistor was reduced from 27K ohms to 18K ohms in order to assure higher bias to the base of the oscillator transistor in an attempt to make sure the transistor oscillator circuit would oscillate easily. I would like to know if your hand unit has the R30 27K ohm resistor or perhaps a value lower than 18K ohms.
Remote Transmitter web.jpg
So far removing or disconnecting the transducer does not help the transistor to oscillate. C29 has been replaced as has C31, a 25uF@25V electrolytic. The transistor does not oscillate even with an approximate 2000pF capacitor and 3.3K ohm resistor in series substituted for the transducer either. I tried a new RCA SK3004 transistor with no change in the condition. My next step is going to be replacing C28 a .05uF disc ceramic capacitor and if that makes no difference, I will also try a new C26 a 5000pF capacitor.

Joe
 
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