MF-300 & Remote Control Acquired

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

  1. audmod01

    audmod01 AK Subscriber Subscriber

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    Early today I scanned the pages of the MF-300 Owner's Manual. I have cropped and straightened the pages, enhanced contrast and removed dark edges of the pages and specks etc. When I finished I concatenated the pages and found the file is too large, so I need to massage each page to about 200dpi in order to get the document below 20MB for the database. I should get that done tomorrow.

    Joe
     
  2. audmod01

    audmod01 AK Subscriber Subscriber

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    RK-20 Hand Unit Battery: I went to a battery specialty shop that has batteries for computers plus phones and power tools etc. today to find a battery solution for the RK-20 hand remote control. The original is a type H-163 rated at 4.2VDC which is no longer available. The solution the shop came up with is to use 3 button type batteries, DANV640ALK, which are 1.5VDC alkaline batteries. The shop can electrically spot weld them together to make a single 4.5VDC battery the same package size as the original battery. That way the battery will be a perfect physical fit and no adapters will be needed. The transistor in the hand unit is not going to know the difference from the 4.2VDC original battery and will work OK. These battery specialty shops can spot weld batteries together successfully without damaging the battery. It is not good to try to solder batteries together due to the heat buildup inside the battery.

    I had to have them order the needed batteries as they only had one of that type on hand, so I will be getting all new fresh ones to fill my order. They will contact me by phone and/or e-mail to let me know when they arrive and are ready. It will probably be next week some time. So, for anyone who owns a MF-300 with the RK-20 remote and hand unit there is hope to get it working again! The same would be true for the MF-320 which includes the RK-20 elements with tuner in one package.

    Joe
     
  3. audmod01

    audmod01 AK Subscriber Subscriber

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    Today I picked up the custom battery for the RK-20 remote control hand transmitter. The total cost was $14.60 with tax. While that may seem expensive for a battery, it is actually three button 1.5V cells spot welded together, so there is some labor involved and I was just glad to get a decent substitute in order to be able to check out the remote control system. I will take a picture tomorrow of the custom battery and also of it installed in the hand unit.

    Joe
     
  4. larryderouin

    larryderouin You can be sure if it's Westinghouse??????? Subscriber

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    That's actually a decent price considering most places charge $5-$7 or a button cell.
     
  5. audmod01

    audmod01 AK Subscriber Subscriber

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    I had the remote hand unit with me and I explained to the young man what it was for and that I was restoring a classic high-fidelity stereo FM tuner that used vacuum tubes and he was intrigued and very helpful. He said he would personally see to it that the job was done right and would have a heat-shrink tubing outer cover and he was as good as his word. I will eventually go back by and thank the young man and show him some pictures of the units in operation. Most young people have never seen equipment with vacuum tubes so they are very curious as to how they work. We get a chance to educate the younger generations about old technology.

    Joe
     
  6. gadget73

    gadget73 junk junkie Subscriber

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    Cool that you have a place that will do that kind of thing. Finding specialty places like that can be pretty difficult.
     
  7. audmod01

    audmod01 AK Subscriber Subscriber

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    I used Batteries + in Sherman, TX which is a city of about 38K population. There should be similar places within reasonable distances for most people I would hope.

    I have spent time today cleaning the battery terminals inside the remote hand transmitter. One thing I discovered is that the circuit board does not have an identification for where the + and - terminals of the battery are to be positioned. I took time to trace the circuitry and found which terminal is the + terminal. I had some white enamel paint and painted a plus symbol on the board just above the plus battery terminal tang. I don't want to ruin the single transistor oscillator circuit by plugging in the battery backwards!

    This remote hand unit appears to have had some repairs made in the past. One circuit trace was replaced by a piece of wire and an orange wire jumper was added on the top side of the circuit board. I also cleaned the switch contacts where the push-buttons engage them. These are similar to relay contacts.

    Joe
     
  8. audmod01

    audmod01 AK Subscriber Subscriber

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    I installed the new battery and plugged the RK-20 into AC power and turned it on. There is no reaction to hand unit volume or tune buttons. I do not believe that the hand unit is putting out a signal. The next step will be to troubleshoot the hand unit. I will also make some power supply checks of the RK-20 remote receiver unit. There are a number of possibilities that could cause the remote hand unit not to work. Ohm meter checks and the oscilloscope may have to be brought into play.

    Here is a picture of the hand unit and the new battery. I marked the plus and minus ends of the new battery and placed a white plus indicator on the circuit board just above the positive terminal tang or lug. If you look closely you can see the added orange wire above the board. I may need to replace the two electrolytic capacitors in the hand unit. Both appear to be quite old. The receiver also has had no electrolytics replaced in it either.
    Hand Unit w New Battery web.jpg
    On similar remote control units I serviced in the past, an O-scope probe placed near the transducer of the hand unit will normally pick up the output. A direct connection would also work to see if the oscillator is even working.

    It has been a long day with some work being done on the Electra for my granddaughter too. It is time to rest!

    Joe
     
  9. larryderouin

    larryderouin You can be sure if it's Westinghouse??????? Subscriber

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    Both of those 'lytics are FRAKO's. Junk. I'd have a hard time deciding which is worse. FRAKO or NTE. Plus new caps should be markedly smaller.
     
  10. audmod01

    audmod01 AK Subscriber Subscriber

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    Larry;

    Your comment confirms my suspicions. The receiver unit is full of them too, so I am strongly thinking about making a clean sweep and replace them all before going any further. And after all, look how old these units are by now - prime candidates for replacement. Capacitors are strange little beasties anyway. I have seen so many curious and odd failure modes in capacitors over the years I could almost write a book on them. One of the worst I ever had to deal with was a Goodall capacitor used to couple audio in a multiplex circuit. It would open intermittently at odd times, in some cases hours after the unit was turned on. Anything the user did to the tuner would make it start working again. It might go hours again without failing only to repeat the sequence with different timing the next day. Changing stations, flipping a slide switch, turning off and on, bumping the set - all would make the condition clear and it might or might not do it again all day. I finally found it using an O-scope. I had to leave the scope connected in circuit and on for multiple days before I found the culprit. Once it found the problem capacitor I changed the one in the other channel. Sprague Orange Drops were used to replace the tubular Goodall caps. I never liked Goodall capacitors after that.

    Joe
     
  11. gadget73

    gadget73 junk junkie Subscriber

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    yeah those Frakos aren't awesome, but the original ones in my Metz console amp from that same time period were functional until I pulled them. You could see the crust at the ends though. The Frako caps in my Bozak speakers were done for, they sounded much better with fresh parts.
     
  12. audmod01

    audmod01 AK Subscriber Subscriber

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    I think the quality problems with many of the parts years ago were due to poor process control and lack of understanding about the chemical properties of materials being used. In the case of capacitors the sealing of internal parts had a lot to do with how long and how well they performed. Stabilizing lead entry points and sealing against moisture and air contaminants penetration is a very important aspect of capacitor construction. With the advent of ceramic capacitors there were both improvements and failures. Dendrite growth affected many of them as it did in poorly constructed mica capacitors. Over time we learned of this due to the evidence presented by the "silver mica disease". Fortunately manufacturing and component design has improved and many new parts are much better than ones from years ago. Now we can enjoy the benefits of modern improved components with better confidence that they will perform well for many years.

    I remember hearing a fabulous Bozak speaker system years ago while working in Austin, TX. A customer had a McIntosh system and Bozak speakers that were very large and featured multiple drivers. The entire system was very impressive and sounded great. The Bozak speakers would have done a great job with a nice Fisher system too.

    I must put together a list of electrolytics and get them on order. I have some on hand, so I will check the list against what I have and order those that I don't have.

    Joe
     
  13. gadget73

    gadget73 junk junkie Subscriber

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    improvements in materials and construction certainly do play into reliability. Rubber deteriorates, more modern rubbers hold up better though. The old glass end seal caps are pretty durable but they were a lot more expensive to make too. I have some glass end seal tantalum caps from the mid 1960s that are still in perfect working condition. Thats top grade stuff in test gear though, not what your average consumer electronics would have seen. Mine are in an old Singer spectrum analyzer which I really need to get back on the bench and try to fix. The sawtooth oscillator is snoozing instead of sawing so it has no horizontal sweep.
     
  14. audmod01

    audmod01 AK Subscriber Subscriber

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    A nice spectrum analyzer tells a lot about amplifier intermodulation and harmonic distortion. With dual tone signal input look at the harmonics and watch as signal input level increases. You can also see the point at which an amplifier goes into clipping with sudden increase in harmonics as the third-order intercept point nears. It happens more gradually with vacuum tubes and suddenly with solid state devices. Of course the spectrum analyzer has to be able to cover audio frequencies in order to do that. Some only cover RF frequencies. There are some that used plug-in modules that allowed them to cover multiple bands of frequencies.

    Joe
     
  15. gadget73

    gadget73 junk junkie Subscriber

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    The module in this one is an RF sweeper, so it would be most useful for doing FM IF strips since the output can be tuned into that range. Thats actually what I intended to do with it when I got it. Never spent enough time to make it go, and I have since purchased an ST 1000a that I haven't spent enough time using to be really competent with it.

    The Singer is kind of an oddball, almost all solid state except it has one 12AX7 and one 12AU7 in there as vertical and horizontal amp stages.
     
  16. audmod01

    audmod01 AK Subscriber Subscriber

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    Today while waiting for parts for another receiver, I decided to take another look at the Fisher RK-20 that came with the MF-300 auto-tune FM tuner. I began by measuring the AC input voltage to the RK-20 from the wall outlet, it was 122VAC. The output of the unit's power transformer produced -14.4VDC at the 250uF@35VDC electrolytic that I found underneath the phenolic circuit board that contains Q6 and Q7. The filter capacitor used is not shown on the list of materials or on the schematic, but it exists!

    Many of the transistors used have a beginning type designation of 57D1-nn where the nn is two extra digits. It turns out that these are mostly Germanium PNP transistors that were made by Raytheon (if I am interpreting the "RAY" manufacturing label information correctly).

    There are two circuit boards in the RK-20 receiver. One is a conventional etched circuit board which is populated with discrete transistors and radial electrolytic capacitors plus a few discrete resistors. Much of the circuitry is inside an encapsulated stacked 3-D module on that board, so it would prove to be difficult to service if it ever had a failed internal part. The other circuit board is simply a phenolic or bakelite board with many eyelet terminal lugs mounted along either long side of the board (which is 3.75 X about 2 inches). It contains transistors Q6 and Q7 of the schematic. There are two white relays and one black relay underneath this board along with the 250uF@35VDC filter capacitor for the power supply. The space underneath this phenolic board is very close and two flat washers are fit between the back side of the phenolic board and the chassis metal tabs that support the board when two 1/4" hex screws secure it to the chassis. The placement of the 250uF capacitor is quite critical due to the original part size. It is likely that modern replacements will be significantly smaller physically.

    I drew a sketch of the phenolic circuit board in Adobe Photoshop to show the parts l
    I uploaded a new version of this board illustration after discovering I had left off a few parts and some jumpers.
    RK-20 Phenolic Board.jpg
    I also took a couple of pictures showing some of the internal boards and parts. I will post those separately after I finish processing them.

    Joe
     
    Last edited: Feb 8, 2018
  17. dcgillespie

    dcgillespie Fisher SA-100 Clone

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    Any chance that cap is C8? On the schematic, it is shown as a 250 uF 25 volt cap that is connected across the output of the bridge rectifier, but not part of any board. Located near K1 on the schematic.

    Dave
     
  18. audmod01

    audmod01 AK Subscriber Subscriber

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    Here are the two extra photos I took showing some of the details inside with the cover removed from the chassis:
    RK-20 Inside View web 01.jpg
    RK-20 Inside View web 02.jpg
    Dave;
    Yes it could be C8, I just did not spot it being so far away from the bridge rectifier and being a different voltage. I think the center white relay is K3 for the volume control. It is currently engaging as soon as power is applied to the unit. I have some troubeshooting to do.

    Also C11 (100uF@15VDC); C12 (100uF@70VDC) and C13 (100uF@15VDC) are not shown in the parts list nor are the transistors in the parts list. I am researching substitutes for the transistors.

    Joe
     
    Last edited: Feb 7, 2018
  19. gadget73

    gadget73 junk junkie Subscriber

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    I think you can fit at least 3 more parts in there. Plenty of room.
     
  20. larryderouin

    larryderouin You can be sure if it's Westinghouse??????? Subscriber

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    Need a bigger crowbar, Joe????
     

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