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28volt RF driver transistor MRF485

Former Member

I'm rebuilding this old two way radio. I'm looking for a cost effective replacement.  This discontinued/limited availability transitor was a very common Ham radio part.  It was used to Drive the MRF422 finals on several transmitter amplifiers as well as The Kenwood TS930/TS940.  They were prone to reliability problems and their heat sink mounting was poorly done from the manufacturer. For example:  The frequncy controll memory was supplied by backup batterys, once they failed the radio would automatically change transmitting frequency, this would casue sudden severe heating of the final output board, the insulation varnish on the ferrite chokes was too thin and failed, shorting the chokes, the resulting surge blew the barrior diode and the result ++ heat melted the cheap plastic heat sink mountings. (Kenwood factory reps attributed the faiures to the power supply instead of the usual cause and effect.) It would take a few trys before the operator realized that the radio was transmitting on one frequency and recieving on another but by then the rest of the chain of events of the failure had begun.

 

Looking at the efficiancy of the MRF485 and the MRF 422, I'd rather not pay premium prices for a less than excellent results.  Are there higher quality replacement parts? What is a Bipolar transistor used for anyhow, why are the Motorola/Kenwood specs so confusing?  I have a difficult time believing that this radio has to run as hot as it's vacuuum tube predecessor, or that the final PA circuit is much more than a trasisorized version of the design that preceeded it.  With any Transmitter, any gains in efficiany always seems to have a double performance payback.  (Also true to radio transmtters, any mistake in operating it tends to result in needing to rebuild the final output amplifier as I'm doing now.)

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  • 0

    Joe,

     

    You should check out the Amateur Radio Group on element14.

     

    I do not think the FETs get any more premium than Motorola in the HAM area. However, if you can find the obsolete Texas Instruments TIXM101, you would have a fairly good noise ratio compared to modern alternatives.

    The bipolars are often used as low noise pre-amplifiers.

    BFP620

     

    In HAM, some people use the bipolar transistors as switches, replacing mechanical relays. If this is your application, most FETs will be fine.

     

    Cabe

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  • 0

    Joe,

     

    You should check out the Amateur Radio Group on element14.

     

    I do not think the FETs get any more premium than Motorola in the HAM area. However, if you can find the obsolete Texas Instruments TIXM101, you would have a fairly good noise ratio compared to modern alternatives.

    The bipolars are often used as low noise pre-amplifiers.

    BFP620

     

    In HAM, some people use the bipolar transistors as switches, replacing mechanical relays. If this is your application, most FETs will be fine.

     

    Cabe

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  • 0 in reply to Catwell

    Thanks for the response Cabe,  I didn't realise that I had posted in the wrong group until your mention of better horizons.

     

    FYI, and for those trying to amplify in the GHz band.   I'm mentioning this subject because I noticed my IBM laptop's WiFI does have noise problems and not ot speak of that fact it's much more polite than letting Computer tec readers think that transistor would be of any help to them.. 

     

    In HF radio, that is below 30mhz, we commonly have used circuits similer to CB radios.  In fact, that is what most of the older CB's are actually designed around. Those old standard and inexpensive ham designs are being eteranlly reused.  In this case, the MRF485 transistor would make a rather low power replacment for the commonly known,"CB final."  Where one might thing we would want a maximim amout of gain between stages, we also have full intentions of continuing to increase power until we run out of fuse boxes, therfor  we also would rather filter any interna harmonic noises at acceptable points.  The points are allways between stage of the amplification.  In our usual CB radio, we only have intentions of a pre-driver and audio amplifer providing current through one driver and one final amplifier stage, a classic design, which in some cases does include more elaborate methods of post filtering.

     

    On the subject of transistors, MY transistor isused in a design where it is asked to take a known and controlled amount of AC current, at a known frequency and amplify it by approximately eight times.  More specifically, it inputs to it's base, a filtered 1.5-30mhz at 100-750mw and a amplifies it into the next filter and consquent stage.  That whole previously mentioned assembly is and has a been universally known as a "broadband Linear amplifer" since we were using vacuum tubes and slide rules.

     

    Although I have huge range, I also have no need for further inmprove my imfomation density, WiFs do and should improve through better interstage filtering.  Some hams do use the HF band for digitial transmissions and they have assured me there is plently of ground to cover in that area of software and engineering.

     

    As you may have noticed, the MRF485 can carry 30 watts of resistive heat through its body to a heat sink, where the transistors you have mentioned can only carry a fraction of that.  I only have intentions of continuing postponing the accumulation of that heat at least until my MRF422's contribute and possibly the 13ga coaxial cable can't take it anymore, or I run out of money, it's the nature of the addiction.  The use of germanuim in such an area is usually of little value, but it may some day be an interesting method of passive filtering. (I assume you noticed it in a relay control circuit.  In such "dow relays", the leakage of a germanum is intentional but all the better to be designed a part of a Schmitt trigger circuit.  Remember that lightning is a problem, and a find a nice place to add a ridiculously gigantic schottky as a reverse voltage saftey; it's also the nature of the Linear circuit to look at what any such an animal is intended to do with it's entire life.)

     

    I should inform your page, and therefore I will, that your computer information tecs could also spend unfathomable amounts of money on such an endeavor, that is what endeavors are for. If you would like to look up the laws, remember that High powers of microwave enegy are best filtered, directed at the highest possible concentration, and not applied to anyone's bone marrow.

     

     

    Thus far, I have found some vague efforts to the effect that the IGBT is a somewhat more efficient voltage regulator that our usual.  If you find any better designs in your 100w+ computers, I'd find it interesting subject.  I have an entire Gulf of Mexico to provide liquid cooling, but I've worked on water cooled welders before and I'd rather not try it again.

     

    Thanks again, Joe