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MOSFET conversions for upD858 and MB8719 boards

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

  1. guitar_199

    guitar_199 Well-Known Member

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    Could I be allowed to squeeze in a couple of related questions???

    Regarding replacing finals with a MOSFet.....

    1) Am I correct in saying that, when you install the MOSFET (assuming that it is done properly) you get higher output power that with the original transistor?

    2) If 1 is true, can you assume that the original radio can support the "current / power" requirements to support putting the MOSFET in?

    My thinking is that, generally the highest DC voltage available is on the order of 13.8 VDC. The only way you can get MORE power is to increase current draw through the final. So if putting a MOSFET in a typical radio and now you get 20 watts out.... that higher power almost HAD to come with more current through the MOSFET. How do you know if the radio can supply it??? Or is that just one of the things that you have to be aware of... and work to take care of it???



    Thanks,
    Bob
     

  2. kopcicle

    kopcicle Sr. Member

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    To oversimplify, Volts x Amps = Watts
    Then, whatever doesn't get out to the antenna is "dissipation".
    Volts x Amps x ?% (50% for a first go) = Watts
    Then, Volts x Amps x ?% (50% for a first go) x 4 = PEP
    ...because PEP is simply four times your carrier when 100% modulated.

    So you can increase the voltage , current, or efficiency, or all three.
    As an example NXP's bleeding edge is now 65V @70+% efficiency.

    https://www.nxp.com/docs/en/data-sheet/MRFX1K80H.pdf
    (note: see pg 9)

    ...but you just want to smoke the traces out of a stock radio and melt the main power switch.

    In all but a few even SSB radios all the power goes through the off/on switch and the mode selector.Then there is the HR2510 and those "like" it. Just put power to the driver and final all the time and only turn the "radio" off and on .

    It's never simple , it's never easy, it's always just numbers.
     
    LeapFrog likes this.
  3. guitar_199

    guitar_199 Well-Known Member

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    I understand the relationships between voltage, current and power.

    The part that is confusing me is this.... it sounds like, from what I have read.....

    Take a radio... currently puts out 4 watts or so....
    Take the covers off...
    Remove the final and a few components,
    Solder in a MOSFET and a few components....
    Power it up and....BOOM you are pushing 20 watts.

    Again.... that is what I think I am reading in these stories...and I may be wrong... but it is the impression I am getting.

    So what I am saying is... if you simply change out the final... and a small group of parts... and all of a sudden you are putting out 4 times the power.....

    Wait a minute.... that power has to come from somewhere simply because V x I = P and the voltage in the area of the final was not very likely to change... so where did the extra power come from?

    I am probably misunderstanding something and THAT is what I am seeking to remedy!

    :)
     
  4. kopcicle

    kopcicle Sr. Member

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    in the case of a change to mosfet, increased current and "some" increased efficiency while still @ 13.8V
     
  5. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    The 4 watts is just the carrier. Modulate it and you will be around 16w. The mosfet 20 watts is modulated so it is not much more than the 16 you started with.
     
    binrat likes this.
  6. guitar_199

    guitar_199 Well-Known Member

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    Ok....... the light is getting brighter..... :D

    Thanks!
     
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  7. nomadradio

    nomadradio Analog Retentive

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    That radio has a design feature that was mostly restricted to 23-channel SSB radios. It has a modulation transformer for AM mode.

    Every SSB CB built after about 1979 can deliver only the radio's DC-power supply voltage to the final and driver, no more. This voltage gets modulated with your mike audio to produce AM.

    The TRC457, 458 and the 449 mobile version have a modulation transformer that can boost the peak audio voltage to 20 Volts peak or more. This translates to a higher peak RF power in AM mode.

    The AM carrier-control circuit in this radio can be replaced by a TIP120 transistor to make the carrier variable. This mod also permits 25 Watt peaks from the stock final.

    We have never tried using MOSFETs in that type radio, but I'd expect to see overheating failures from the AM modulator, the carrier-control transistor, and maybe from the mode switch. All the power to the final and driver pass through that switch. Pretty sure it's rated for only about 3 Amps. When that switch fails, it's a bear to replace.

    But only after turning over enough rocks to score a compatible replacement part.

    A MOSFET final will definitely draw more current through that switch than the stock final.

    73
     
    NZ8N likes this.
  8. ExitThirteen

    ExitThirteen Grumpy and Cranky

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    I had a Cobra 142GTL a while back that I did some experimenting on, I decided to put in an IRF520N mosfet in place of the 2SC1969 final. When I got done putting in everything into place I was producing 30W PEP output on SSB. However....


    ... it took out the mode selector switch, more specifically, it burned out the contacts on the LSB side of the mode switch, which was the mode I was using at the time of testing. Too much current draw. That is the caveat, as Nomad and Kopcicle have suggested. Something's gotta give.

    I have found that 25W PEP on SSB keeps everything pretty happy after the conversion has been done.

    Again, I state that this is an alternate method to keep/get your radio going, not to make a big power upgrade. There's very little to be gained by putting in a mosfet for the sake of trying to squeeze every last watt out of a radio. Unless a person is willing to make several upgrades (power supply, mode switch, AM pass regulator, etc), it's going to cause more problems than what it's worth.


    ~Cheers~
     
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  9. Robb

    Robb Yup

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    Never realized that the mfrs of the 10m export radios w/dual MOSFETs are using upgraded mode switches and beefier circuitry to keep their radio from expiring in the most predictable ways. One would think they would get the spurs out of them - too . . .

    I had also rebuilt a 458 and used the MOSFET mod. Radio output runs nearly as cool as a cucumber; because I didn't try to squeeze the last watt out of it. Put in a Galaxy 55 meter in it. Looks and runs great. One of the better radios that I've gotten, repaired, and rebuilt.
     
    #144 Robb, Oct 30, 2019
    Last edited: Oct 30, 2019
    NZ8N and binrat like this.
  10. 711

    711 Well-Known Member

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    Modulation transformer radios like the 29, pump out up to 35-40 volts peak to peak to the final. My take on the mosfet mod is to “volt” the final and prop up a voltage divider to the gate to drop 3.5 volts from a 8v TX source. Treat the final as a PA stage rather then a modulated stage, maybe even modulate the predriver. Just another way of doing things
     
  11. Harley3315

    Harley3315 Member

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    How would one go about modulating the predriver in a 29? I used to upgrade the predriver to a 1973 and then 1969 final and see 30-35 watts out of those. Also my initial question still remains about the ge am only radio which has no mode switch problem to over come its am only I would think beefing up some traces and such would help on this. I did see a video on YouTube by a guy out side the us doing the mod to an ssb pll02a radio but noone has said they have done an am only yet...
     
    Shadetree Mechanic likes this.
  12. guitar_199

    guitar_199 Well-Known Member

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    That's just it! I would not be doing it looking for converting my mobile into a commercial radio station..... I am more about "I have some old radios I may need to fix and, if they don't sell the finals any more, what can I do".

    I wouldn't care if the power didn't change at all..... I would just be looking for a replacement for what is there.
     
  13. Robb

    Robb Yup

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    People will push up the watt output; but that is the VERY thing to be careful not to do.
    I turned down the dead key way down after the TX coils were peaked.
    So Xit13 came up with the design!
    Thanks again!
     
  14. Harley3315

    Harley3315 Member

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    Well the other option would be a 1969 and some kinda npc or the other one Aystemic or something like that handy Andy was doing with pc122 or similar. Which I've also not seen on any pll02a radios the goal is to get out fairly well in town on the wire antenna it's the only option at work. If the gain was good I'd be willing to do the 1969 I usually save them for bigger better radios this radio has extra channels which is what I want to use it for mostly below 1.
     
    Shadetree Mechanic likes this.
  15. Harley3315

    Harley3315 Member

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    My am only radio has no mode switch is a mobile on a variable power supply I have a lot of options!
     

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