FM-200-B Acquisition

Discussion in 'Fisher' started by audmod01, Sep 16, 2017.

  1. audmod01

    audmod01 AK Subscriber Subscriber

    Messages:
    901
    Location:
    Tioga, TX
    Dave;

    Thanks for noticing the knob issue. I had not looked that closely. I looked at it this afternoon and discovered that although the type of knob on the tuning shaft was correct (no pointer ridge on the side of the knob - just fine ridges all the way around) the knob cap was not correct. Tuning knobs do not use caps with the pointer dash on them. I retrieved the knob and brought it in and removed the cap. I installed a brass cap with no dash on it and will install it after the glue dries.

    I am glad you know the color sequence on the jewels. The service manuals I have do not specify the left to right color sequence. Now I will be able to reorder the sequence and get it right. On my 202-R I had to change them there too. It is surprising how often these get switched from original sequence. When I buy new Fisher brass knob caps I always order the set of 8 for the 800-B/800-C models as they come with two caps that do not have the dash on them. I have not restored an 800-B or 800-C yet, so I wound up with some extra plain brass caps. I used one on the 202-R and now one for this tuner.

    I probably will bring the tuner in tomorrow and test all the tubes just to see if any are way off on output level. Then I can order some new tubes if needed. I will give it a try on my stereo system to see if it actually works as claimed.

    Joe
     
  2. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    8,846
    Location:
    Ball Ground, GA
    Within the "Wide Band Multiplex" family, the one constant between all the tuners is that Green is always used to indicate Stereo MPX reception. The Red and Amber were reversed between the FM-200B and the R-200: On the 200B, Amber (on the left) indicated that the muting function was turned on, while Red (one the right) indicated that some level of AFC was active. On the R-200, Red (on the left) indicated that the tuner was in FM mode while the Amber (on the right) indicated AM operation. It is in fact amazing how often you see the jewels out of the correct sequence.

    Dave
     
  3. audmod01

    audmod01 AK Subscriber Subscriber

    Messages:
    901
    Location:
    Tioga, TX
    I had noticed that my FM-200-B had a missing AC accessory receptacle after examining it. Many of these older units had AC receptacles that require the use of AC plugs that are non-polarized. That is, each prong of any AC plug that is to be inserted needs to be of identical size or width. People seem to insist on plugging in newer equipment with polarized plugs and break the insulation surrounding the contacts inside the AC receptacle. This is a bad and unsafe practice! Never plug a polarized plug into a non-polarized receptacle!

    Late yesterday I removed a pristine non-polarized receptacle from a parts chassis to use on the 200-B. I may get a chance to install it today and restore that capability of my 200-B unit. I will be using nuts, screws and starwashers to install the AC receptacle as I do not have rivets and tools to install rivets. Choosing rivets and installing them is not easy. The thickness of the chassis metal plus the ID of the hole to be filled get into the process. It is one thing to install rivets in a factory environment and another in existing equipment that already has other switches, sockets etc. installed.

    Joe
     
  4. audmod01

    audmod01 AK Subscriber Subscriber

    Messages:
    901
    Location:
    Tioga, TX
    I managed to find time to do several things to the FM-200-B today:

    1) I installed the replacement AC receptacle on the back of the tuner chassis. This is a switched receptacle although I do not currently have a need for it. I just wanted the tuner to have all the capabilities it was manufactured with.

    2) I replaced both .01 AC input bypass capacitors with new disc ceramic .01uF @500VDC Y5P safety type capacitors. One B+ resistor above a ground lug of the electrolytic can capacitor one of the AC bypass capacitors was connected to was dressed dangerously close to the ground lug (probably bumped during a previous service event). I moved the lead of the B+ resistor so that it was well away from the ground lug.

    3) I replaced the 1uF electrolytic in the MPX circuit with a new 1uF polypropylene metalized film capacitor rated at 600VDC.

    4) I replaced the two 6uF electrolytics in the Ratio Detector circuit. All the electrolytics I replaced were OEM parts and definitely need to be replaced after this many years.

    I noticed that a number of original capacitors have already been replaced with modern orange drop mylar film capacitors, so that is a plus.

    Notice the green plastic part in the lower right corner of the chassis at the RF input circuitry area. That part is the balun (balanced to unbalanced transformer) which used to be referred to as an "elevator coil" by service technicians years ago. These are made using two colors of same size wire in a bi-filar winding arrangement. The fine wire used in these is susceptible to damage from lightning strikes. The same is true of more modern ferrite core type balun transformers. Mother nature can really rip either type of these. Usually the wires become vaporized in such an event.
    FM-200-B Under Chassis Repairs web 01.jpg
    Joe
     
  5. audmod01

    audmod01 AK Subscriber Subscriber

    Messages:
    901
    Location:
    Tioga, TX
    I decided to give the tuner a test. I turned it on after first connecting my outside FM only 9-element antenna and connecting the audio outputs to my stereo system. At first it came on with no indication of any stations - just white noise from the mixer circuit. Then I turned it off and swapped the two nuvistors. Then after it was turned on, it tuned stations very well other than some issues with the microtune circuit and AFC. The nuvistor sockets likely need some Deoxit treatment. I plan to do this to all tube sockets.

    The stations tune in properly manually with full stereo and quieting. The signal level indicator shows 4 or above on most stations. A few weak ones show less, but still produce stereo reception in automatic mode. For a first try, I would say this is quite good with no tube replacements. Some of the tubes are original Fisher tubes. My favorite station tunes in with no interference from the local relay station 4.5 miles from my house. This one is definitely a keeper! It appears to work very close to the 202-R unit I have already worked on.

    It will be interesting to see how the tubes check on my tube testers.

    Joe
     
  6. audmod01

    audmod01 AK Subscriber Subscriber

    Messages:
    901
    Location:
    Tioga, TX
    Here is a picture of the tuner operating after I corrected the positions of the jewels. It looks nice with my favorite station coming in at about level 4 on the signal level meter.
    FM-200-B Jewel Positions Corrected web.jpg
    Only one dial lamp is lighting and it is the original incandescent type. The one on the right is burned out. I will look for some LED types for replacement.

    Joe
     
  7. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    8,846
    Location:
    Ball Ground, GA
    Very nice Joe! Back in the day, a properly operating Fisher tuner could take on all comers regardless of brand and always come out on top. The 200B is such a perfect example of that. Glad you found a one that was in such great overall shape!

    Dave
     
  8. audmod01

    audmod01 AK Subscriber Subscriber

    Messages:
    901
    Location:
    Tioga, TX
    Dave;

    Yes and thank you! I felt like I had earned at least one good one after all the ugly ducklings I have had to deal with. I will take time to rebuild the electrolytic in the power supply after I get some more individual ones to add to the two I have left on hand. The one in the tuner is still working, but gets rather hot to the touch, so I don't want to press my luck on the power supply. Other than that it works surprisingly well even after all these years.

    Joe
     
    Last edited: Sep 28, 2017
  9. audmod01

    audmod01 AK Subscriber Subscriber

    Messages:
    901
    Location:
    Tioga, TX
    Notdigital;

    Today I managed to shape the piece of walnut wood that you sent me to fit the right front trim of the cabinet that the FM-200-B came in. It is currently being glued to the remainder of the original piece which has a groove that was either made by a router or a dado blade set-up on a table saw. The glue should be fully cured by tomorrow about noon. That piece in turn mates with the right side of the cabinet. The left side trim also has some damage at the front edge, but can probably be repaired using some wood filler and then sanding lightly and applying some walnut stain.

    I was able to use my stationary belt sander to get the part shaped precisely to fit the remainder of the right front trim piece. I could not have done the job without this tool. I do not have a table saw or a router table to precisely control cuts and bevel shapes. It took hours of work to get the pieces to fit but I am happy with the result.

    The lower edge of the right side of the cabinet came completely apart from the cabinet bottom plywood piece. A bit of sanding to clean off some glue residue will allow reassembly with some new glue. I have some corner clamps I can use to keep the pieces at an accurate 90 degrees to each other while the glue cures. This tuner will look really nice in its wood cabinet when finished.

    Joe
     
  10. audmod01

    audmod01 AK Subscriber Subscriber

    Messages:
    901
    Location:
    Tioga, TX
    Dave;

    I noticed that in the Z5 IF circuit there are two back-to-back diodes inside. There is a resistor capacitor network externally then going to ground. This appears to be an additional means of providing limiting if signals get high enough to forward bias both diodes. Limiter V8, a 6AU6, provides the final limiting ahead of the ratio detector. Do you know what type of diodes are used in this back-to-back circuit? Are they germanium or silicone?

    Joe
     
  11. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    8,846
    Location:
    Ball Ground, GA
    Joe -- I've seen that Limiter transformer used in a couple of Fisher's more elaborate designs, but have concluded that the diodes are not there to enhance the limiting action -- particularly in this case because they follow two stages of limiting before them. The diodes can't provide any effective limiting because the diodes are driven via capacitive coupling from the plate of V7 -- with the cap being required or else the diode at terminal #4 would shunt the plate voltage of V7 to ground.

    Examination of the circuit shows that the two diodes, together with the coupling cap that drives them and C71, forms a full wave voltage doubler, with some designs using the output of the doubler to drive an eye tube or trigger the muting circuit. The 202-T tuner/preamp for example uses this transformer for these functions. But in the FM-200B, the output of the doubler circuit drives -- nothing. Therefore, at best, all it can represent is a load on the output of V7 due to the charging of the coupling cap and C71. R86 would help define what that load is, but its value is 15K whether the doubler drives any other circuits, or drives nothing. In any event, since the diodes can't clip the signal, all they can do is load it down some.

    I've often thought about those diodes -- but it may be nothing more than just using up inventory, I don't know. Fisher changed to silicon for their small signal diodes, but I would bet these are still germanium -- but I am basing that on the time period only, not actual testing.

    Dave
     
  12. audmod01

    audmod01 AK Subscriber Subscriber

    Messages:
    901
    Location:
    Tioga, TX
    Dave;

    I should have looked closer and spotted the voltage doubler topology. In the final analysis its presence in the 200-B only promotes using similar circuit values for V7 and V8 as compared to similar Fisher products by maintaining equivalent loading as you mentioned.

    Joe
     
  13. notdigital

    notdigital AK Subscriber Subscriber

    Messages:
    2,214
    Location:
    NYNY
    I never throw out pieces like the one I sent you for the reason you are using it: artful restoration. I actually love doing the type of job you're doing. On top of being weirdly relaxing, it is very satisfying when the end result is worth all the effort. Please do post a pic when you're done. I'd love to see it!
     
  14. audmod01

    audmod01 AK Subscriber Subscriber

    Messages:
    901
    Location:
    Tioga, TX
    notdigital;

    Yes I tend to do the same thing. I have one piece of walnut wood that came from a record changer drawer that I took apart last year. It supplied some parts for another project I am working on. Walnut is actually a very tough wood. I need to make a run to my local hardware store and get some more plywood for the cabinet I am making for the Electra IV/VI combination I am putting together for my granddaughter. I will pick up some walnut stain while there to use on this cabinet repair.

    I will post some pictures as soon as I get the cabinet done.

    Yesterday I listened to my favorite station on the 200-B and found that it works best when set to AFC 2. If allowed to be in standard Microtune or AFC 1 it lets the local relay station cause background interference and occasionally blasts in. Adequate warm-up time is also required. This condition varies with weather conditions. The station I like is over 60 miles away to the south and the interfering station (alternate channel frequency) is north of my house. I need to also modify my old Winegard 9 element antenna by adding a reflector shield similar to the Kroner FM antenna design detailed on Brian Beezeley's website. I have some aluminum rods I can use to modify the antenna on hand and just need to get busy doing it. The relay station comes in at -9dBm and the desired station at -59dBm, so you can understand the challenge the situation presents. Not many tube tuners can deal with this condition. My best results are obtained here using a Kenwood KT-7500 which has multiple ceramic IF filters and needs to be set to Narrow bandwidth to keep the interfering station from bleeding into the audio of the desired station.

    Joe
     
  15. audmod01

    audmod01 AK Subscriber Subscriber

    Messages:
    901
    Location:
    Tioga, TX
    I did manage to get some walnut wood filler to use in filling a few small cracks and to build out some of the left front side trim. I looked at walnut stains at the store but did not find anything that truly matched the color of this cabinet. It appears to be a mix of standard walnut stain with a small amount of red mahogany in it. There is a somewhat red tint to the stain that was used. I have some red mahogany stain so I am going to try mixing the two and try it on some scrap pieces of wood until I get a good match.

    Unfortunately the weather is not co-operating today. There has been a light rain all day so far. I was lucky that I managed to get a 2' X 4' piece of plywood into the car without it getting rained on after check-out at the store. I just happened to catch a time when the rain had stopped. As I started home I ran into more rain.

    I bought a small iron for steaming out small dents and scratches for less than $10, so that is on hand now. I hope this works well. This cabinet has some small scratches and the one with my 202-R has an even larger and deeper scratch on its top. I will take before and after pictures to provide evaluation information to our members. Those who have tried it in the past say it is beneficial, so it is at least worth a try.

    Joe
     
  16. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

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    19,963
    Location:
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    Get SWMBO to give you a couple of "OLD USED" towels to be used exclusively for this. If you use any other beat to hell towel, the claws from hell come out and she'll let you have it 6 ways to sunday. :)
     
  17. audmod01

    audmod01 AK Subscriber Subscriber

    Messages:
    901
    Location:
    Tioga, TX
    Larry;

    Forewarned is forearmed!:rolleyes: Advice well taken.

    Dial Lamps:
    Since my FM-200-B is a low serial number sequence, 20233E, and one of its incandescent lamps is burned out, I decided to measure them. They measure about 3.262cm long and their contacts at the end are about 0.56cm OD and the glass is about 0.615cm OD, so I need to look for replacements that will be close to that size. I am lucky that the sockets in this tuner are undamaged. They are the dark umber brown type sockets.

    The later serial number 200-B tuners may have used different lamps than this version. I do not have a correct serial number sequence service manual for this tuner. Instead I have the 3K-4K and 6K serial number sequence manuals from the AK database. Those manuals both show P/N 150082-6 lamps for those serial number sequences.

    Joe
     
  18. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

    Messages:
    19,963
    Location:
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  19. AlTinkster92

    AlTinkster92 AK Subscriber Subscriber

    Messages:
    3,026
    Location:
    NC
    Joe, let me check my stash, might have a couple spares when Dave G redid mine a couple of years ago....I'm in the process of changing everything to LED Al
     
  20. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    32,777
    Location:
    Southern NJ
    I've run into this a few times. Honestly I'd love to find polarized sockets that will drop into the old spots without any modification just to make this a non-issue.

    On the other hand I own files, and its really easy to make things not have polarized plugs anymore :)
     

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