Comparing Transistor Output to Gain More Power Output ?

Discussion in 'General Technical Questions and Answers' started by K2WH, Dec 1, 2019.

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  1. K2WH

    K2WH Premium Subscriber QRZ Page

    I am restoring an old ICOM 202 SSB 2 meter portable. I have determined, the final transistor a 2SC-1947 is bad since I have zero output but monitoring the driver signal it is clear and intelligible.

    I would like to replace it with a transistor with similar ratings but possibly more power output than the 3 watts specified for the ICOM-202.

    I have repaired these little rigs before, and this "New" one, I would like to try a different transistor than the 2SC-1947 if available.

    I have already order 2 of the 1947's, but would like to know how to search or compare to a transistor with more power gain but yet operate within the same circuit parameters with minor adjustments.

    2sc1947.jpg
     
  2. KD8DEY

    KD8DEY Ham Member QRZ Page

    NTE-488
    3.5W
     
  3. KA9JLM

    KA9JLM Ham Member QRZ Page

    That most likely will not happen with minor adjustments.

    But could be done with some work. A bigger heatsink would be required.

    Have Fun.
     
  4. K2WH

    K2WH Premium Subscriber QRZ Page

    Thanks for the NTE info.

    I should have known that and just to be sure that transistor is bad, I'm going to check the meter diode detector to see if that is bad, just in case I don't have a blown transistor but just a bad meter rectifier.
     
    Last edited: Dec 1, 2019
  5. KB0MNM

    KB0MNM Ham Member QRZ Page

    That 2SC1947 reminds me of a transistor which was frequently replaced by a 2N3055 in television flyback circuits. Be certain that you consider: 1. Gain is usually a function of a capacitor in certain circuits. 2. Electrolytic capacitors can develop a high Equivalent Series Resistance ( ESR ), sometimes also described as a 'lumped-sum' impedance issue. Lumped-sum describes how by either manufacture or age, all components actually have attributes of others- sometimes more pronounced at high frequencies. Capacitors and inductors both have inherent resistance, yet capacitors tend to exhibit more resistance with age. 3. There is almost always a penalty if a semiconductor device gets hot, which is why temperature-compensation / shutdown diodes are frequently used in output circuits.
     
  6. KM3F

    KM3F Ham Member QRZ Page

    Check the specs for max frequency and power first, then if the radio power supply is capable of the power input needed for the power out specified.
     
  7. KL7AJ

    KL7AJ Ham Member QRZ Page

    And, don't confuse power gain with total output power capacity. Some of your more powerful transistors don't have as much gain as "weaker" ones, but need a lot more driving power, which could require a bit of redesign work.
     
  8. W1VT

    W1VT Ham Member QRZ Page

    Yes, Eric makes a good point. A bigger device usually has more input capacitance and a lower input resistance, which requires more driving power. Sometimes you can go to much hotter technology, but this usually requires a lot of work to avoid creating an oscillator.

    If your electrical engineering skills are weak, perhaps the most straightforward redesign is to substitute a 28 volt device, and run just the final off of 24 to 28 volts.

    Zak W1VT
     
  9. KP4SX

    KP4SX XML Subscriber QRZ Page

    Easy to do. All you need is an extra battery bag hanging off of the Icom 202 :rolleyes:
     
  10. W1VT

    W1VT Ham Member QRZ Page

    https://www.rfparts.com/mrf237-mot.html

    RF Parts has the MRF 237 NOS, which is rated at 4 watts at 175MHz, 12 volt supply. Unusual pinout, as the emitter is connected to the case instead of the collector.
     

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