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MOSFET conversions for upD858 and MB8719 boards

Discussion in 'General CB Services Discussion' started by ExitThirteen, Mar 10, 2011.

  1. ExitThirteen

    ExitThirteen Grumpy and Cranky

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    Dave239 nailed it. The reason I made this post is to provide information for a viable alternative to replacing a final other than a C1307 or a C1969. Yes, I know they are still available. But they are no longer being made and will eventually dry up. Some people may not want to pay 16 bucks for a final, when with a little time and about 4 bucks worth of parts you can resurrect your vintage radio.



    I've had many people hack on me about this (and other mods, even though I provide pictures of results with the proper test equipment), not understanding the purpose of posting it in the first place.... and people wonder why I'm not real active on the forums anymore... there's your answer.


    ~Cheers~
     
    LeapFrog and 222DBFL like this.

  2. Robb

    Robb Yup

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    I think there are a great many of us on this forum that appreciate all of your contributions, Xit13. I know I do. I was able to get an old 148GTL to work well with this upgrade. Also, a TRC-458 that is still being gone through.
    Thanks!
     
    Chipper and 222DBFL like this.
  3. Crossbow

    Crossbow Active Member

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    I to enjoy your post ExitThirteen. I'm not a big poster but I do a lot of reading and learned quite a bit from you. Keep'em coming Sir!
     
    Chipper and 222DBFL like this.
  4. Odball

    Odball Member

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    does anybody know how to convert a TRC 459 with the LC7133 or 32 chip
     
  5. Xitami

    Xitami Member

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    So what mod do i use for a President Madison that runs the 8719 PLL, 4 led and 3 push switch model?, is it dual or single conversion?
     
  6. ExitThirteen

    ExitThirteen Grumpy and Cranky

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    For the Uniden/President Madison with the MB8719 PLL, it's the dual conversion chassis, like the Cobra 148GTL/2000GTL.


    ~Cheers~
     
    Chipper likes this.
  7. Xitami

    Xitami Member

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    Many thanks Exit13, thought it might be but was not sure, thx
     
  8. Robb

    Robb Yup

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    Yep, did it to a 858 chassis/TRC-458. Seeing about 20w out AM/SSB.

    Should the bias on the driver be set at more than the factory spec 15mA? Using a 2sc2166 as unit_399 suggested. Or, should I set it higher?

    Used the 39pf cap on C167 with no change.
     
    #113 Robb, Apr 5, 2017
    Last edited: Apr 5, 2017
  9. ExitThirteen

    ExitThirteen Grumpy and Cranky

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    Robb,

    I set the driver bias at minimum of 20mA on all my radios, I firmly believe 15mA bias current on the driver is too low for an AM/SSB chassis, since you want things as clean as possible in all stages of amplification for SSB.


    ~Cheers~
     
    Robb likes this.
  10. Ridgerunner0788

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    If ya can post pictures step by step on how to do the mod it be easer for people like my slef to do please and thank you
     
    Chipper likes this.
  11. loosecannon

    loosecannon Sr. Member

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    ridgerunner,

    these mods are not posted to be "cb school", and basically, if you need pics to do it, you probably shouldn't do it. I don't say that to be elitist or rude, it's just that there is more going on than just replacing a few parts here.

    you need to have some knowledge of what's going on in the circuit so you'll know how to interpret the instructions. every radio is different, and yours may require a small variance in a part value, or your board is a variant of the one used in the mod and some parts may be different.

    However, if you decide to attempt the mod, there are people here who can help you if you get stuck along the way.

    good luck!
    LC
     
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  12. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    IT might help you too, if you can get a PDF of the IRF520 - it's a bit dry in reading - but there are several key pointers that stand out.

    One, the Bi-polar Paradox, the Smith Chart was used back then to plot out the input admittance of a particular transistor. Now, if you can't find the part, or that part is made by a company that would rather sell you an automotive MOSFET for hi-voltage gating for a spark plug - you're gonna' be hard pressed to use that supplier - even though the part MIGHT fit the profile it's labeled as. That admittance was a means to match the part to the signal path and amplify or mix that signal and pass it along. The Base region - in the biasing of it, let you tailor the output and depending upon selection - a knee that you can use to obtain nearly uniform and linear amplification. You don't get that readily with FET-based designs.

    Second, MOSFET - the Spongy Trampoline - at least if you get the wrong part, that's what it will act like in the replacement part - Bipolar was analog direct current munchers. MOSFET are more like Tubes, use a Gate - like a Grid, only the Cathode and Anode was changed to Drain and Source (don't ask) but handle current quite well and act more like a switch than an analog device - the Gate being insulated - again makes the part act more like a tube than a silicon-based semiconductor. Even though the Drain To Source connection is direct and doped - formed - shaved to a thickness - to perform like a switch - a VERY DARN FAST switch.

    Thirdly - our biggest benefit was the advent of Class C and Class D (and above) types of amplification. At least we can apply an analog signal mix it in with the bias - apply it across the Drain to Source connection and only make the Gate switch it on and off 27 million times a second, once cycle on to off is about 25~35 PICO seconds in duration - so it's a very fast spike. Another advantage is the "Built in capacitance" these MOSFET have. Granted Capacitance is a bane and can be an unwarranted aftereffect, but the capacitive effects of how the gate works against the Drain to Source junction - makes PROPER SELECTION paramount - but knowing the On and Off Times let alone the rise and fall rates (skew and Slew) can provide a knee similar to the Bi-polar - but takes a trick to do it and no smith chart can help - it's still a best guess game to get the best results by trial and error in most cases.

    Lastly - bias itself - the early Bi-polar brothers use current rushing thru the line - and their voltage is set quite low - just enough to keep the base region off, but enough there to spill the RF wave over into the base region and away she goes. The current rushing thru, helps to keep the Bi-polar forward conducting until the cycle swings into the positive side and the base doesn't need the bias reserve pool - just the power of the incoming wave. This current also provides a means to wash away noises and cutoff events that can ruin the waveform and even damage the Bi-polar. In MOSFET the "bias" has to be set to a level of VOLTAGE (with some current to cleanse off noisy switching spikes and other artifacts) and that voltage level is pretty tricky. Too low, you are chopping the RF wave and cutting off your signal - too high and she's latched on and in a meltdown - not unlike a Slo-Blo fuse - can take some hits but will pop and then you're done. And on top of that, not too much power as voltage or current - for she will puncture the Gate and no, sorry, it's not the way you make a transistor. The insulation used for the Gate to Substrate (the area of Source and Drain) is an oxide - and that means a temperature sensitivity and if run too hard or hot - breakdown occurs.

    So, twist a fresh LED "easy on the eyes" light into your reading lamp and crack open some PDF's about MOSFET's - because as Bi-polars that once were the norm are drained off - lots of the dies, artwork and the art of making them work - will be gone forever - and MOSFET is the replacement - a high speed switch that transistors once held.

    Good luck
    :+> Andy <+:
     
    Shadetree Mechanic likes this.
  13. Robalo

    Robalo Active Member

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    Many thanks Exit13
     
  14. Low_Boy

    Low_Boy Well-Known Member

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    Exit 13. Nice post.
    Thank you for all the time spent on this. I am sure this will be helpful for many people.
     
  15. Two Pack

    Two Pack Active Member

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    Just finished doing this mod to an old school 148. 18watts am and ssb.
     

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