MOST COMMON PARTS FOR FISHER, Sticky needed?

Discussion in 'Fisher' started by larryderouin, Oct 15, 2010.

  1. larryderouin

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

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    Yep. unscrew the "jade" screw and screw in the Pomona. the thread on the pomona is 6-32. If you aren't sure of your screw size, take it down to the local Home Depot and size it in the hardware section.
     
  2. audiodon

    audiodon AK Subscriber Subscriber

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    The first time I bought them, I bought a dozen of each color of the 8/32 by mistake. The 6/32 is a perfect replacement with one caveat.

    You may need to use one or two 6/32 washers as spacers because the thread doesn't go all the way down the stalk of the pomona so the washers act as a spacer.

    I'm going to include the link from AK'er dadbar because it shows the stalk. You can see that the last 1/16 inch isn't threaded and you'll need the threaded grip to get it snug. The other good news is that the ends are shaped like a nut so you can use a nut driver to tighten them, but be careful because the receptacle on the terminal strip is quite thin. Snug is done. Very snug is lucky. Extra snug will strip the lug.
    http://www.pomonaelectronics.com/pdf/d5699-5700-5701_100.pdf
     
  3. birdgirl

    birdgirl New Member

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    great thread!
     
  4. larryderouin

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

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    Thermistor part #'s from Mouser
    CL-80 527-CL80A $1.78 Each
    CL-70 527-CL70A $2.30 Each

    Instock as of 7-27-14
     
  5. larryderouin

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

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    RUBBER GROMMET's for the PHONO STAGE Tube isolation. This will minimize any chassis vibration from getting into the tubes and being amplified. Also good if you have a somewhat microphonic tube and don't want it picking up chassis noise. The originals are for the most part, dried out, crusty, dusty, and generally no good.

    It's a Keystone part. Rubber Grommet .187 P/N 534-730. This is @ Mouser. The fit is exact and no fussing with over or undersized grommets.

    Mouser pricing as of 03AUG2014
    1: $0.18 ea
    10: $0.175 ea
    25: $0.149 ea
    100: $0.108 ea
    250: $0.093 ea
    500: $0.089 ea
    1,000: $0.078 ea
    2,500: $0.071 ea
    5,000: $0.067 ea
     
  6. larryderouin

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

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    And for the HYBRID AMPS......Thermal grease for the transistors. RATSHACK part 276-1372
     
  7. vsat88

    vsat88 Like my Bluetooth ?

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    How about multicap replacement values (under amp separates) and parts number/ BOM from someplace like Mouser ? For 500C Anyone ??
     
  8. larryderouin

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

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    There are so MANY choices for Electrolytics, that it's not something that everyone can agree on. Take the values and voltages off the cans themselves and match up with separates.
     
  9. vsat88

    vsat88 Like my Bluetooth ?

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    Thanks for the reply

    Thanks Larry, that's what I will do!
     
  10. larryderouin

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

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    Some Changes to POST #1.

    With the Demise of Ratshack, parts are no longer available from that former source. Use Mouser/Digikey/Newark or your favorite LOCAL Mom and Pop Electronics shop.

    As I use Mouser pretty much exclusively, any parts I list will have MOUSER PART NUMBERS. I've already changed the BRIDGE RECTUMFRIER to a FAIRCHILD PART.

    The Silicone based Heat sink Compound can be had @ Mouser.Digikey/nNewark, Parts-Express, etc. PLEASE DO NOT USE ARCTIC SILVER on TRANSISTORS. The stuff is conductive and WILL KILL your OUTPUT's and other components as it's Silver Bearing. The SAFE White Heatsink Compound is basically Zinc Oxide in a Silicone carrier.
     
    Last edited: Aug 6, 2015
  11. SoundOfSound

    SoundOfSound Super Member

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  12. larryderouin

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

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    Gonna stick a couple of links here for items/mods for the 400-500c-800-c. This one will be a work in progress.

    The whys and wherefore's of the screen, cathode, grid resistor changes/additions and output coupling caps.
    This is a reply DAVE Gillespie made in response to questions about them. Thansk Dave.
    "QUOTE"
    If I may, here is some detail behind the recommendations for these modifications:

    1. "Reducing the grid resistors to 220K": The published maximum DC resistance for grid #1 of 7591 class tubes is 300K (by RCA, Sylvania, and others). This is the maximum value allowable when the tube is operated under conditions of maximum dissipation -- which many designs nearly did -- so yes, using 330K is using a value that is right on the ragged edge.

    The reason for the maximum resistance specification is to limit the value of reverse grid current voltage that can build up under conditions of maximum dissipation (read that, maximum heat). But there are other concerns that can aggravate this issue as well. Higher line voltages push the heater voltages higher, which means that today, even more heat is generated. The basic concern is that excessive heat (however it's generated) can cause cathode material to boil off and deposit itself onto the grid. This means the grid has now become a partial emitter, with any current flow it provides developing a voltage across the grid resistor that is opposite in polarity to the negative voltage applied to the bottom of the grid resistor. This negative grid current voltage then works against the negative bias voltage, causing the tube to conduct more current, generating even more heat, boiling off more cathode material, and ultimately causing thermal runaway. The production of gas comes into play with these events as well to further aggravate the issue. Therefore, to help control this condition, a maximum DC resistance is specified for the control grid to limit the amount of ill effects that can be caused by reverse grid current.

    As a result, with the high temps that these units typically run at inside their cases, and importantly, because of today's higher line voltages, and because the tubes are biased "on" pretty hard to begin with, it makes very good sense to pull the value of the grid resistors back a little to help minimize the chances of developing any significant reverse grid current voltage, and the possible thermal runaway that can produce -- and this is even before the quality of modern production tubes is considered!

    2. "Double the value of the coupling caps": This has nothing to do with loading on the driver stage, in fact, lowering the value of the grid resistors actually increases the load on the driver stage. The reason the cap values are increased is to keep nearly the same time constant in the RC coupling circuit between the driver stage and the output stage with the new lower value grid resistors installed.

    3. "Install 10 Ohm cathode resistors": This makes for the easiest way to monitor output tube current draw, and therefore allows for the easiest way to set the bias and balance of the output stage (if such controls exist), or check to see that matched output tubes are in fact matched. These resistors are typically used for balancing currents under quiescent conditions, but can also be used to check for balance under dynamic conditions if the proper equipment is available. Balancing output tube quiescent current maximizes OPT low frequency power handling ability (by minimizing core saturation), while balancing dynamic currents minimizes overall distortion.

    4. "Installing a 10K bias pot": Modern tubes are manufactured with far less precision than the best tubes from yesteryear were, and so, fixed bias amplifiers operating with tubes of modern manufacture on higher line voltages will usually need some adjustment from the original fixed setting a manufacturer provided. Installing some type of adjustable control is therefore highly desirable, and adding a DC balancing feature to that is even better. Installing these controls so that they cannot be inadvertently altered from their setting goes hand in hand with the installation of any such controls.

    5. "Installing 100 ohm Screen Stability Resistors": So many folks have posted on various forums -- from all degrees of experience -- how everything is as it should be, but the output tubes will arc for no reason that they can determine. How expensive does that get when output tubes can be ruined in an instant?

    Long ago, I determined that these arcs were in fact occurring from the screen grid. Many pieces of old hifi gear were designed with very little resistance in the screen circuit, which usually didn't present too many problems as the power supply caps of the day had much higher ESR values, which tended to help keep things stable. Even at that however, some designs (Eico HF-89 for example) were simply notorious for arcing output tubes because of the larger overall power supply capacitance values they employed.

    Today it is all too common, with the installation of new power supply caps that have much lower ESR values, and which are so often increased in size as well (all installed on new drop in power supply boards), to have all manner of output tube arcing events occur, which leaves folks puzzled because it just never used to do that before. New manufacture tubes got the lion's share of the blame -- until even NOS tubes were arcing in these refurbished pieces of equipment as well.

    With some form of transient trigger (music, an open ground, whatever), the lower ESR value caps can set up incredibly strong oscillations in what is effectively a tank circuit that's formed with the screen grid, and can cause an arc at any time. Installation of 100 ohm Screen Stability resistors (for 7591 class tubes) stops this problem virtually every time. I've never had anyone tell me that installation of these resistors did not stop the problem. Personally, I've never had an output tube arc event in nearly twenty years now, where before I had boxes of blown tubes until I could determine what the cause was, and develop an appropriate cure. In equipment such as this Fisher represents, it is very cheap insurance against such events, and produces no audible or measurable difference in performance.

    Hopefully, this will help with an understanding of why these modifications can make such worthwhile improvements in so many of these wonderful instruments. With some components being virtually unobtainable now, minimizing any damaging events that might occur helps all of us to enjoy this equipment even more.

    Good luck with your receiver!

    Dave
    "END QUOTE"


    The Phase Inverter adjustment thread.............. http://audiokarma.org/forums/index.php?threads/quick-and-easy-400-phase-inverter-adjust.559415/

    Then there is Dave's EXCELLENT "Improving the 400" thread..http://audiokarma.org/forums/index.php?threads/improving-the-fisher-400.511867/
     
    NorthPoleGuy and bolero like this.
  13. NorthPoleGuy

    NorthPoleGuy New Member

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    image.png
    So could you please explain a little further how you wire up the CL? Do you wire up two separate Current limiters in series with each leg of the primaries of your power transformer? So these act as high resistance devices at first and then slowly go to a low resistance to act as a soft start?
     
    Last edited: Dec 1, 2015
  14. bhamham

    bhamham AK Subscriber Subscriber

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    I don't use two anymore, one does the trick for me now. I remove one of the black transformer primaries from the terminal strip and put the CL-80 there with a flying connection to the PT primary you removed. Not the best practice but works for me. Yes, high resistance until it reaches a high temp. Keep it away from other components. Stick your DMM on it and the other AC side and watch at start-up.
     
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  15. NorthPoleGuy

    NorthPoleGuy New Member

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    I will order up a few and give it a try, guessing one might not hurt on some of my other old tube radios to help preserve the switch contacts. Thanks for the explanation Bruce. O
     
  16. larryderouin

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

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    TUBE SUBSTITUTION;
    6AQ8. Use the 6DT8. Is preferred sub and 1/5th the cost.
    6HR6/6HS6. Use 6AH6. The specs are very close to both tubes and is a sub for the 6hs6. In a pinch you can use in this order, 6AU6, 6BA6, or 6CG6. I've used all of these tubes in 400's and Custom Electra tuners without noticible degradation. I prefer the 6AH6 as it's spec's are closest to the 6HS6. AND it's ALSO CHEAP. About $4. to $5. each
     
  17. RUmad

    RUmad Well-Known Member

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    I scanned through all four pages here and didn't see any info for the festoon lamps. It's possible it's been covered elsewhere, but I've seen LED festoons on that auction site, not my thing, but they're there. Also found a variety of sizes on P-E, which Fisher people had verified in the reviews. Mine ended up working after a good cleaning but figured I'd add the info here.

    http://www.parts-express.com/6-volt...ntz-sansui-kenwood-yamaha-sony-and-o--070-118
     
  18. arts

    arts Super Member

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    Your best bet for the original festoon bulbs is an electronic surplus store.I recently picked up 5 at one of my local haunts.

    DSCF2235.JPG
     
  19. vsat88

    vsat88 Like my Bluetooth ?

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    I buy all of my bulbs from Dwojo. DGWOJO.COM Cant say for sure he has them but I can say for sure he will not rip you off or steer you wrong. 100% honest fellow in my opinion .
     
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  20. RUmad

    RUmad Well-Known Member

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    Electronic surplus store. I should be so lucky! No such animal 'round here.
     

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