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Fisher Radio Model 300 MPX Signal Generator

Discussion in 'Fisher' started by audmod01, Jun 19, 2018.

  1. audmod01

    audmod01 Super Member

    Messages:
    1,278
    Location:
    Tioga, TX
    I recently spotted one of the Fisher Radio Model 300 MPX Signal Generators on eBay and bought it. The seller was in Boulder, Colorado and so the shipment to my location in Texas was in record time. The unit is in pretty decent condition externally. The metal brackets that hold the leather strap handle on the top are corroded, but otherwise the case is in very good condition.

    Dave Gillespie has the service manual for these units and has offered to share scanned files of the manual. When I ordered the unit, it was claimed to have a service manual with it, which turned out to just be a copy of an article from the June issue of Audio magazine of 1962. The article was written by a Mr. H. Heinz, who was apparently an R&D engineer with Fisher Radio. The article explains how to use the Fisher 300 MPX Signal Generator to align MPX decoders of that era for proper decoding of the FM MPX signal format. It did include a schematic of the generator, but, I am not sure how accurate it is nor if it applies to all versions that Fisher may have produced. A test equipment item would necessarily be produced in far smaller numbers than the consumer receivers, tuners and MPX decoders sold to the general public. I wonder what these were sold for when new.

    The magazine article was a Xerox copy of an original magazine copy, so it is rather decent import from my scanner.

    Joe
    300 image web 02.jpg
    300 image web 03.jpg
    300 image web 04.jpg
    300 image web 05.jpg
    Audio June 62 page 20.jpg
     

     

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  2. audmod01

    audmod01 Super Member

    Messages:
    1,278
    Location:
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    Here are some views inside the unit before any clean-up. I had just finished cleaning some of the front controls and the Right Channel level control was frozen in place. I removed the knob by loosening its two set screws (Allen type) to be able to work some lubrication down into the shaft area. Then I applied lubrication and worked the control until it moved as free as the Left channel level control which was working fine. I then returned the Right channel knob only to discover that one of its set screws was missing. I spent an hour looking for it on the carpet in my study. I finally discovered that it had fallen underneath the knob when I put it on the shaft, so it was being held in place by the knob!:crazy: I removed the knob, retrieved the set screw and finished installing the knob again.:beerchug:

    Most all the tubes I looked at were OEM Fisher tubes. I have not gotten around to testing them yet. They may be OK as most test equipment does not see anywhere near the hours of operation as a consumer audio product. There is plenty of dust on some parts of the chassis. It needs a good cleaning. The chassis looks like it may be silver plated, or if not it may be stainless steel? The front panel is aluminum like many of the early 1960s amps/preamps, receivers/tuners. I had to use some Goof-Off to remove tape adhesive residue on one area of the front panel.

    Joe
    Inside uncleaned web 01.jpg Inside uncleaned web 03.jpg Inside uncleaned web 04.jpg Inside uncleaned web 05.jpg Inside uncleaned web 06.jpg
    Inside uncleaned web 07.jpg Inside uncleaned web 08.jpg
     

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    Last edited: Jun 19, 2018
  3. AudioKeith

    AudioKeith AK Subscriber Subscriber

    Messages:
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    Location:
    Cape Coral, FL and Stafford, VA
    Neat looking unit.
     
  4. AlTinkster92

    AlTinkster92 AK Subscriber Subscriber

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    NC
    Nice piece and I believe I see the good SS tube shields on this. they were also used on the FM200B and the FM1000. good luck with it!:thumbsup: Al
     
  5. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Joe; Theres a good possibility that the chassis is Cadmium plated, so due caution is advised. The reason I say that is Picture #1 which from the angle shows the spotty areas indicative of cadmium plating.
    Larry
     
  6. audmod01

    audmod01 Super Member

    Messages:
    1,278
    Location:
    Tioga, TX
    The tube shields are the type that have a spring and to install or remove you push down on the shield and rotate about 5 degrees and lift off or replace. One shield has a linear crack from bottom to top. I am not sure how such a crack could have been created. I should be able to find a replacement though.

    All the control shafts have knobs except one at about the middle of the right side. That one has a knurled shaft and turns easily. I need to inspect and see what its purpose is.

    The unit has an audio sine wave oscillator circuit that can produce 50Hz, 1kHz and 8kHz tones. These then can be selected and sent through a Left and Right channel amplifier circuit with pre-emphasis, then applied to diode bridge modulation circuits and mixed with 38kHz developed from a reference 19kHz oscillator circuit that is crystal controlled. Fortunately the crystal is still in its socket! There is an output jack that provides the composite signal that includes the L+R and L-R signal plus the 19kHz pilot tone and there is also an RF output around 100mHz that can have the Composite signal modulated onto it to feed an RF signal into a tuner or receiver front end. That RF frequency might be variable to some degree so that the user does not have to worry about dealing with a local FM station on the same frequency. I will have to learn more about this unit.

    I have done some preliminary cleaning of the chassis now and will take some more pictures of the results.

    Joe
     

     

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  7. audmod01

    audmod01 Super Member

    Messages:
    1,278
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    Larry;

    You could be right. The spotty areas are on the one surface that had most of the dust. There were a few spots where corrosion has come through the top plating? It looks like some liquid must have dropped down on that surface several places. I have seen similar spots on some tuner or receiver chassis, but usually with the typical rust of steel. This one does not seem to have normal steel rust for some reason. I wonder sometimes if people used to stand over their receivers and be carrying drinks that they spilled down inside the set causing significant corrosion due to not cleaning the residue off. Too much beer?:)

    Joe
     
  8. gadget73

    gadget73 junk junkie Subscriber

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    36,973
    Location:
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    Make sure that the rock hasn't drifted. I've had a number of things with crystal controlled oscillators over the years that the crystal has gone off. Sometimes they have trimmer caps, and once in a while I've even gone so far as to alter the value of the padder caps to get them in range but it doesn't always work.
     
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  9. audmod01

    audmod01 Super Member

    Messages:
    1,278
    Location:
    Tioga, TX
    In most cases I have seen over the years, crystal controlled oscillators are usually right on and may only be off a few Herz. The Fisher documents I have in hand now state the 19kHz oscillator is accurate to within 2Hz. Fortunately I have a decent HP frequency counter to check it with.

    Dave is getting his service manual for the 300 scanned in so he can supply me with a copy. I think we can transfer the files using my DropBox account without resorting to US Mail. In the meantime I acquired a German language document for the unit and have spent time translating it, then proof-reading and editing. Next step is to capture the illustrations and block diagrams as image files and incorporate into an overall *.pdf document. The original document had three columns of text with images and schematics. I am not that good at inserting image files within columnar text files, so those will go into separate pages. The document explains the various circuits of the Fisher 300 MPX Signal Generator and gives the technical specifications of the unit. This has taken two days of work and I am not done yet.

    I ordered a Feeltech FY6600 60mHz Signal Generator through eBay. It will do lots of signal types and also includes a frequency meter to measure external frequencies. At under $100 it fit the bill. It will do sweeps and allow me to check amplifier frequency response etc. It comes with software so my computer will help it do various things and a set of cables. My existing signal generator just would not provide the frequency resolution I needed for working on Matt's Fisher RK-20 ultrasonic remote control system (or my own either). This one will provide me with lots of capabilities I never had at home.

    Joe
     
  10. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    36,973
    Location:
    Southern NJ
    Seems to depend on the crystal and storage conditions. Most of my rock oscillator experience is CB gear, and its possible they did not use the best quality parts. Some of the ones in my Browning are off a few kilohertz, beyond the point of being usable. A few were completely dead.
     
  11. audmod01

    audmod01 Super Member

    Messages:
    1,278
    Location:
    Tioga, TX
    Some of RCA's early Color TV sets had their 3.58mHz crystal enclosed in a miniature tube glass envelope normally used for a 9-pin tube and sealed in a vacuum. Later on they went to the more common metal cases that appear to have been hermetically sealed. I only occasionally ran into a defective crystal of either type. I would say that the failures were probably less than 1% of total units I serviced.

    Yesterday I made a couple of negative discoveries on this unit. 1) the 19kHz Xtal Phase Adj shaft and knob are both gone. There seems to be the remnants of a white plastic adjustment hex-end tool in the adjustable coil. I may be able to find a new hex-tool that I can cut up to create a suitable hex working end and have the shaft pass through the front panel for a knob to be placed on. 2) The 19kHz Amplitude shaft to the right of the 19kHz Osc. Phase Adj. position is missing its knob too.

    In looking at pictures in the copy of the service manual that Dave Gillespie kindly shared with me, I see that the knobs for those two controls were hex-shaped knobs that were smaller than the other knobs on the front of the unit. I should be able to find knobs of similar size to fit the shaft lengths and diameters involved. Antique Electronic Supply has a large selection of knobs that will likely have something that will work.
    Fisher MPX 300-04.jpg
    Front Panel Illustration from Service Manual

    Joe
     

     

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  12. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Go Joe, GO! Besides Dave, You'd be the one to get this unit working. So far you've been batting 1000.
     
  13. audmod01

    audmod01 Super Member

    Messages:
    1,278
    Location:
    Tioga, TX
    Larry;

    Thanks for the words of encouragement!:)

    This morning I took the knobs off and removed the trim nuts from the two top right hand switches, then the nuts on control shafts that hold the front trim panel in place. The front trim panel then just lifts off. I found that there is no remnant of an adjustment shaft for the Xtal Phase Adj. slug in L1 a 5-25mH coil. I tried several alignment Hex tools and found that the most common size normally used for IF alignment in Tube type radios works well. I plan to search for a set of radio/TV alignment tools and get a new set. I will likely find at least one that I can sacrifice to provide a hex-end mate for the slug of the coil and also fit the shaft ID of the control bushing.

    For now the front panel and knobs are back in place to prevent any loss of parts.

    Dave had made several corrections and added comments in his service manual, so I am taking time to modify the appropriate sections in the service manual as I go. I am sending files for those pages back to him for his verification before I consider the changes permanent.

    Joe
     
  14. audmod01

    audmod01 Super Member

    Messages:
    1,278
    Location:
    Tioga, TX
    I finished printing the service manual for the model 300 MPX Signal Generator. I used Adobe Reader to view the 11 X 17 inch file I had made of the composite schematic. It was pieced together in Adobe Photoshop from two page scans to create the single page. I increased the resolution to 600dpi before saving it as 11 X 17 inches. Then I changed it to 300dpi in Photoshop before saving. In Adobe reader there is an option for page sizing and handling and I selected Poster which prints the drawing to 4 pages of 8.5 X 11 inch paper. I then carefully combined them using some 3M Magic tape into a C Size drawing which I was able to fold and place into the folder I created.

    Joe
    C Paper Cut Fold.jpg
    Make fold #1 first after inserting lower left edge into folder/binder. Then fold the right end toward the left edge, stop short of the center crease of the folder and line up the fold 2 line at the edge of other 8.5" wide
    pages, crease the fold, the fold the outer edge back toward the outer edge of the other pages along fold
    line 3. It is an accordion type fold which will pull out to the right when access is needed, then fold up from the bottom until the top is exposed. This can be reversed to put back in the folder without having to remove any pages and keeps the drawing neat, plus it is far easier on the eyes to view a schematic in a C size drawing.
     
    Last edited: Jun 25, 2018
  15. gadget73

    gadget73 junk junkie Subscriber

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    Just a mention, if you want this on a single large sheet I've had that done at Staples before and it wasn't a whole lot of cash. I actually did it online, I just sent the PDF and they mailed me two poster sized prints of a schematic. I expect it can be done in the store too. I store them rolled up in a shipping tube.
     
  16. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Ball Ground, GA
    Joe -- As built, the 19 kHz Phase and Amplitude Adjustments do not have knobs, and only the Amplitude control has a shaft, but again, no knob.

    The Phase Adjustment is designed to be adjusted with an ordinary hex tool, which is then covered with a black cap once set, so there is no permanent shaft or knob per say.

    The Amplitude Adjustment is a conventional control with a short stub shaft that is to be adjusted with your fingers, and then also covered with a black cap, so there is no knob associated with this adjustment, either.

    The caps are meant to prevent inadvertent adjustment once these controls are set, as once properly done so, will rarely if ever require adjustment.

    Sorry for the delay in proofing your work. I should be able to do that later on today.

    Dave
     

     

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  17. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    I do the same at Office Depot. But I have them print the schematics out on 24x36 poster size so I can read the damned things.:biggrin: Cost per is less than $4.00 each. As long as the original schematic is fairly clear the repro will turn out clear.

    Now Dave and Joe are colaborating on stuff. The Technical expertise on the Forum is going to leap by a couple of orders of magnitude. Hang on guys, it's gonna be a bumpy ride!!! (I'm kidding of course.)
     
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  18. audmod01

    audmod01 Super Member

    Messages:
    1,278
    Location:
    Tioga, TX
    Dave;

    I wondered about the lack of knobs. The picture of the front panel in the service manual appeared to show them with knobs, so I thought that they came that way. Apparently the caps screwed onto the threaded shank protruding through to the front panel. It is easy enough to adjust the coil with a hex alignment tool as you suggested. The level adjustment control shaft on mine is knurled, and easy enough to set with just fingers. This will be just that much less to have to do on the unit. Thanks for the input!

    I need to check all the tubes to see if they are OK. I suspect they may be since test equipment is not likely to have lots of hours on it compared to consumer electronics.

    Larry;

    I have done the same thing, only I go to Fed-Ex office and get the same size print. This was just a quick way to get a full size schematic. I am like you and need something large enough to read without glasses. This fills the bill. I will download the schematic file to a thumb drive and stop by Fed-Ex to get a nice print on a single sheet of C-size paper in the near future. I took time to straighten connecting lines and corrected any off-set lines so that the scanned image-disjoint at their meeting edges do not cause confusion.

    Joe
     
    Last edited: Jun 26, 2018
  19. audmod01

    audmod01 Super Member

    Messages:
    1,278
    Location:
    Tioga, TX
    I got around to testing the tubes in the signal generator using my B&K667 tube tester. Here are the results
    V1 12AT7 both sections = 60; questionable
    V2 6AU6 = 78 good
    V3 6AB4 = 68 low end of good
    V4 6AB4 = 66 barely good but grid leakage
    V5 7247 Plate 2 = 42 bad: Plate 7 = 81 good
    V6 6AU6 = 83 good
    V7 6AB4 = 66 questionable
    V8 12AT7 both sections 68-70 low end of good
    V9 6BL8 Plate 2 = 63 bad: Plate 9 = 53 bad (Sylvania replacement)
    V10 6BL8 = Plate 2 = 58 bad: Plate 9 = 51 bad
    V11 6AW8 = Plate 2 = 68/69 wiggles meter needle low end of good: Plate 7 = 69 questionable
    V12 OA2 = will test regulation in circuit later
    It appears that my first thoughts on the tubes was wrong. Most of these show considerable use, measuring low or questionable emission. All the tubes except V9 are Fisher OEM tubes. Looks like I will be ordering a set of tubes.
    I have one other tube tester that I might check these on again just to see if I get similar results.

    Joe
     
  20. audmod01

    audmod01 Super Member

    Messages:
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    Location:
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    I tested the tubes again this time using an Allied Radio model 600. During the process I had to stop and tighten contacts in both the 9-pin and 7-pin sockets - not an unusual situation for such an old tester. After doing that, I continued with testing and found that all of the tubes tested good except the V4 6AB4, so I am inclined to get that one tube and check for proper operation with the existing tubes using the service manual going through calibration procedures and suggested checks. If the unit meets its required output levels the tubes may truly be OK. Some tube testers were designed to show faults in tubes tested to sell more tubes. I am not sure about the B&K tester's reputation in that regard.

    Joe
     

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