MF-300 & Remote Control Acquired

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

  1. audmod01

    audmod01 AK Subscriber Subscriber

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    Tioga, TX
    Larry and Gadget;

    Yes, sometimes I feel like my hands and tools are just like using a crowbar! Originally the unit did not have K3 engaging as soon as power was applied. This happened after I measured the DC voltage on C8. I don't know what I did to cause it but have to find the problem and repair that before going on to other things within. Someone suggested using a 2N1307 to replace the 2N632 transistor in the remote hand unit. Another person suggested an NTE160 if I remember correctly. In any case I am studying the available parts to see what might work best. I have to get the hand unit working before tackling the receiver to any great effort.

    Joe
     
  2. audmod01

    audmod01 AK Subscriber Subscriber

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    I just found and made a correction for connection of C17 on the phenolic board. The current version posted is the most recent.

    Joe
     
  3. dcgillespie

    dcgillespie Fisher SA-100 Clone

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    Location:
    Ball Ground, GA
    How was it wrong? Is this a correction you made to the original schematic?

    Dave
     
  4. audmod01

    audmod01 AK Subscriber Subscriber

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    Location:
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    Dave;

    No, I had C17 connected to the wrong solder lug 11, top row on the illustration instead of lug 10. I also had forgotten several jumpers from lug to lug. On the bottom row, lugs 14 & 15 are connected; lugs 17, 18 & 19 are connected; lugs 20 & 21 are connected; and lugs 23 & 24 are connected. I worked from a hand sketch of the board and was duplicating it in Adobe Photoshop and stopped my work at one point, thus loosing my place in the process - sorry! I probably should go back to the schematic and add dashed lines around the Q6 & Q7 related parts on the phenolic board, then post an updated version of the schematic with the lug numbers designating either top or bottom row lugs. I also need to make an illustration of the bottom side of the phenolic board since there are some wires there that I am not sure were duplicated on the top side.

    Joe
     
  5. audmod01

    audmod01 AK Subscriber Subscriber

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    Location:
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    To clarify issues related to the Phenolic Board further, I examined the underside of the board.
    There are numerous connections on the underside of this board as follows:
    Solder lug 1 connects to solder lug 17
    Solder lug 15 connects to solder lug 20
    Solder lug 19 connects to solder lug 9
    Solder lug 9 connects to solder lug11
    Solder lug 12 connects to solder lug 13
    Solder lug 26 has a light blue wire that connects to a lug on Black Relay K2
    Solder lug 8 has a red wire that connects to a terminal on K3 and a terminal on K1.
    Some of those jumper wires have black Cambric sleeving on them while some are bare wires. Obviously it is important to watch lead dress very carefully when reinstalling and securing the Phenolic Board to the chassis. It would be very easy to short out or pinch either the red wire or the light blue wire to the board or the chassis. Also, C8 being underneath the board, has to be carefully positioned so that it does not interfere with other parts such as the relays under it. The new parts used to replace existing electrolytics should be chosen smaller physically if at all possible to free up space and make assembly less critical.

    Cambric sleeving is pretty tough, but if replacing items such as C8 the 250uF@35VDC electrolytic It may be wise to use Teflon sleeving on its leads.

    I have made additional notes on my hand sketch of the chassis related to the physical components such as the volume control & motor assembly, the relays, power transformer, bridge rectifier and some related terminal strips and connectors. I will be working on an illustration of these parts showing some of the interconnecting wiring for a future post. Everything is so compact and packed that tracing wires and parts is quite a task. Anyone who tackles one of these needs to be really on guard to make sure they are tracing the correct wires to various parts. Some wires are the same color and only differ by the wire gauge used to connect certain parts.

    I have mentioned that there are two flat washers that go between the Phenolic Board and the chassis mounting tabs under the board. Today while handling the unit a third flat washer fell out of the chassis and so far I have been unable to locate a spot where it may have come from. It has no marks on it to indicate that it was ever used in fastening any other parts such as a screw or terminal strip ground lug. I am beginning to suspect that it may have fallen loose into the chassis at the factory and has been lying inside ever since.

    K3 and K1 are small white relays very similar to what Fisher used in their 500-C and 800-C receivers. K2, the reverse relay, is larger and has a thin black plastic cover over the top of it. The relays are arranged in this order: K3 (Volume) is close to the volume control motor, K1 (Tuning) is just behind K3 and K2 (Reverse) is behind K3.

    I first thought that the center relay was K3, but instead it is K1 used for activating the tune feature. I need to check to see if K1 is still activating as soon as power is turned on. If it is not, perhaps the extra flat washer that fell out was causing that relay to turn on.

    Joe
     
  6. audmod01

    audmod01 AK Subscriber Subscriber

    Messages:
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    Location:
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    Here is a picture that shows some of the component locations of the RK-20.
    Component Locations.jpg
    Joe
     
  7. audmod01

    audmod01 AK Subscriber Subscriber

    Messages:
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    Location:
    Tioga, TX
    Here is an illustration of the underside of the Phenolic Board so that you can see the jumpers that were added there.
    RK-20 Phenolic Board Underside.jpg
    Joe
     
  8. audmod01

    audmod01 AK Subscriber Subscriber

    Messages:
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    Location:
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    Here is a drawing that shows some of the power wiring of the RK-20. Most of the wire colors are consistent with others carrying the same voltage or function, but there was one exception in the orange wire to the solder peg connecting to the - terminal of C13 on the Etched Circuit Board. On that board the peg was labeled "RED". This orange wire connects to the same terminal of K1 that has a RED wire connected to it from K3. I suspect that in other units that a RED wire was used here instead of an Orange one. I think this drawing will be helpful to people who may be servicing their own units. The chassis wiring is quite congested and not easy to trace out.
    Components Illustration.jpg
    Joe
     

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