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kendecom PA-100 tuning (micro control specialties)

Discussion in 'General Technical Questions and Answers' started by K1LBG, Oct 22, 2021.

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  1. K1LBG

    K1LBG Ham Member QRZ Page

    Hi all, my first post/query. I have two Kendecom/Micro Control Specialties PA-100 repeater amps that I am trying to tune. The only manual I could find online is for Issue 1, May 1985. The tuning procedure is similar to what I have but the schematic has a bias resistor and only 4 variable caps, while my units have no variable bias resistor and 5 variable caps. My board is a MCS PA-100 Rev A. I'm hoping to find out how to adjust the fifth variable cap, as the other four seem to be in the same location as the schematic in the manual. The power supply is totally different as are the filters after the RF transistor but that won't affect tuning. Thanks in advance. K1LBG
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    MCS pretty much copied the original Motorola application note design and added an output filter. It should have the bias control. The tunable capacitors are for input and output matching. No clue about the fifth one, that's not in the app note.:)

    Super simple one-transistor amplifier with input and output tuning. Why not draw a small schematic showing where the 5th capacitor is located in the circuit and see if we can determine what it's supposed to do?

    Original app note with their schematic:
  3. K1LBG

    K1LBG Ham Member QRZ Page

    I drew one, but it is not exact. I'll go back and touch it up and upload it in a bit. thanks
  4. K1LBG

    K1LBG Ham Member QRZ Page

    Here is the schematic of the PA-100 amp I have. The manual schematic has a variable resistor for bias fed by about 5 volts. it also shows L2 and L3 but I don't have discrete components on the board for them. PA-100 amp.jpg
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Well, the mystery capacitor is obviously part of the output tuning, along with C7 and C8.

    You sure L2 and L3 aren't even there? They may just be narrow printed circuit strips; but without L2 I don't see how C2 could tune anything to resonance.

    Without the gate bias, zero-signal drain current would be about zero so the amp is relegated to Class C only and wouldn't be linear; but if you're using this only on FM, that shouldn't matter.
  6. KA5IPF

    KA5IPF Ham Member QRZ Page

    What band?
  7. K1LBG

    K1LBG Ham Member QRZ Page

    Thanks for the feedback. 144Mhz. Yes, only FM. I was reading another post about not having enough bias current and going to class C instead of AB? I suspect L2 and L3 are carefully designed into the foil because there are no soldered in, above board components. The path on the printed circuit board is clearly not shortest path, so probably somebody calculated how to do the inductance in foil. I tried adjusting the mystery cap but since I did not see any changes in the output power or consumed current, I put it back to where it was. I guess the next step is to put in on the spectrum analyzer and look for harmonics or distortions.
  8. K7JEM

    K7JEM Ham Member QRZ Page

    What is the part number of the transistor? It's possible that it's a bipolar and not a FET.
  9. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Harmonics should be taken care of by the output filter.

    "Distortion" doesn't really mean anything on FM.
  10. KA5IPF

    KA5IPF Ham Member QRZ Page

    The inductance is built in for Class C operation. It should have fans on the heatsink, if not limit tx time. What input does it want? If 10-15w give it 5or so watts. Tune the output side caps first to get them in the ballpark so you don't dump 100w into an untuned circuit. Then go back to the input side and tune all of the caps, in order. Probably have to do it more than once. Last pass with whatever power you are going to give it. Watch the current draw at full smoke and make sure you have an adequate PS, probably need 20a minimum and that's the continuous rating not ICAS. It goes without saying have a good dummyload. Fans running now check it out, let it run a minimum of 30 min.

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