Icom IC-730 heats up after first transmit

Discussion in 'Radio Circuits, Repair & Performance' started by KN4HSD, Nov 15, 2019.

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  1. KN4HSD

    KN4HSD Ham Member QRZ Page

    I have a well loved Icom IC-730 which has been working well since I found it on eBay. Lately, the PA will continuously warm up after the first time it transmits. The only way to "reset" it is to remove DC power entirely. As it warms up, the fan will eventually kick on as if the PA has overheated (high VSWR). A supply power meter shows normal current increase at transmit, then down to a higher receive current than first noted. The current remains high even after the power button is pressed off. DC power removal starts it all over again.

    Anyone ever seen anything like this? I've ruled out a sticking relay because of the DC power cycling vs. just turning it off to reset it.

    Thanks for the help,

  2. KA9JLM

    KA9JLM Ham Member QRZ Page

    Does it do that when testing with a dummy load ?

    Is it the same on all bands ?

    Good Luck.
  3. K7TRF

    K7TRF Premium Subscriber QRZ Page

    That suggests something that isn't part of the switched Vcc system which pretty much means either the reverse polarity protection diode or the finals devices both of which are directly connected to the input DC power.

    It sounds to me like you could have a final amplifier device that's on the verge of failing and it draws excessive leakage current after you transmit and it continues to draw leakage current even after the rig is powered down. There should only be a few devices that are always connected across the DC input and not switched via the main power switch. I'd take a good look at the schematic and focus on those devices.
  4. KN4HSD

    KN4HSD Ham Member QRZ Page

    Yes, with a dummy load.

    Yes, on all bands.

  5. KN4HSD

    KN4HSD Ham Member QRZ Page

    Good advice, I'm digging in now. I'm guessing those amplifing transistors are long gone off the market. Do alternatives exist for a radio from this era?

  6. KN4HSD

    KN4HSD Ham Member QRZ Page

    Found a burned resistor near the finals, on the base leg of one of the transistors. Looking into it further.
    W1TRY likes this.
  7. G4COE

    G4COE Ham Member QRZ Page

    Not necessarily the finals, I had a couple of Icom's where indeed the finals passing a heavy current as though in TX mode, the set heated up in minutes even on receive, the problem was a leaky driver transistor, with a base collector leak, I can't recall the models.

    As you probably know the +13V supply is on all the time except for the TX biasing which is switched by the PTT line.... the short was turning the TX biassing line hard ON.

    Should it be ANY transistor, be very, very careful where you get them from, lots of 'fakes' knocking about on Ebay - ALWAY'S buy from as reputable supplier, these fakes may last two seconds or two or three day's, they usually have high gain and low output and their die, the silicon slab is uaually third the size....!

    I'm guessing right now they are 2SC1971 or 2SC1972's... you needn't take them out to check, just lifting the base lead will do.... NOTE: The middle pin is the emitter and some may have a insulating mica and washer because they *may* be a emitter resistor used, if this is shorted out by not insulating the transistor from ground the new transistor may be damaged through increased gain and thermal run-a-way.

    Sorry didn't see the last post about finding a burnt resistor.

  8. KA9JLM

    KA9JLM Ham Member QRZ Page

    That could do it. Why it burnt is another problem.

    Good Luck.
    K7TRF likes this.
  9. KN4HSD

    KN4HSD Ham Member QRZ Page

    This week I slowly added power (slowly increased current) while watching the board with a thermal camera. Made it to about 0.50 amps. One of the final transistors clearly heats up when it shouldn't. The culprit is a 2SC2097. Any advise on who a reputable supplier is? Should both finals be replaced as a pair?


  10. KA9JLM

    KA9JLM Ham Member QRZ Page

    Does the bias look OK ?

    A matched pair would be best. RF Parts has them. $125 for a matched set.

    K0UO likes this.

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