718 Final Repair

Discussion in 'Radio Circuits, Repair & Performance' started by N5BOY, Dec 30, 2019.

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  1. N5BOY

    N5BOY Ham Member QRZ Page

    I just completed a successful Icom 718 PA board repair and thought that some may benefit from what I learned.

    I will put the most important information first and you can read the extended story if you have time and find it interesting.

    FYI: My 718 is the older model with bipolar transistors on the PA board.

    I replaced my leaking driver transistors (2SC3133) with a matched pair of Eleflow 2SC1945 transistors sourced from weazle66 on ebay. They fit the board without modification and are working perfectly after I reset the driver and final bias and reset the max power out adjustment.

    Here is the extended story:
    My radio was running hot to the touch, very hot, when I was not transmitting. I checked the current draw and found that it was drawing 1.5A when off and almost 3A on receive. There should be no current when the radio is off and less than 1.3 A on receive. When I checked the bias voltages I found 800 mv on receive on the driver transistors when there should have been 0 volts. The same 800 mv was present when the radio was turned off. On the 718 (and most other radios) the power remains connected to the finals when the radio is turned off. I lifted the collectors on each of the drivers and found that one of the transistors was leaking voltage from the collector to the base causing the drivers to conduct. Though I have not figured out how; the unwanted bias voltage was also present on the basses of the final transistors. After replacing the drivers I had 0 volts bias on receive (and when the power was off) on all of the transistors on the PA board and 600 mv on the drivers and finals (as there should be) in transmit mode.

    My Icom 718 is now cold to the touch at all times on receive (or when off) and only slightly warm as you would expect, when talking on SSB.

    When researching my problem on the internet I found several people asking about their 718s running hot but found no explanations or solutions.

    I expect that the same problem, running hot and drawing too much current when turned off or on receive, can and does occur in other radios.
     
    Last edited: Dec 30, 2019
    G4COE and K6RBK like this.
  2. G4COE

    G4COE Ham Member QRZ Page

    I had a Icom for repair, can't remember model this one was drawing many amps, it wasn't the PA transistors as the guy said but one of the drivers, not sure if it was a 2SC1971 or 2SC1972 pair, just one of them had a base to collector leak, this turned the biasing supply hard on in received mode, causing the radio to become very warm in a matter of a minute or so.

    Getting these 'slabs' are like rocking horse manure, like one or two others I've got a couple of fakes, Ebay is a big No No... these devils won't even fit in a TS2000 (they use the same drivers) and have a gain in their 90's there are other tell tale signs of being fakes.

    Dave
     
  3. N5BOY

    N5BOY Ham Member QRZ Page

    This particular Ebay seller was recommended on one of the forums. It seems that he gets the parts from Eleflow, tests the parts and supplies Ham operators and CB radio enthusiast. Getting out of production parts straight from China is risky. I have seen videos where people have tested Chinese transistors and found that their PNP transistors were actually NPN!
     
    Last edited: Dec 31, 2019
  4. G4COE

    G4COE Ham Member QRZ Page

    That's where I got mine from and Eleflow didn't want to know... see post here to avoid repeating. Further to that post I'm still waiting, batch numbers, jpeg they got all the details, still no answer!

    Dave
     
  5. N5BOY

    N5BOY Ham Member QRZ Page

    The Ebay seller I used was up front about the transistors being slightly bigger and cautioned that you may have to take steps to make them fit in some applications. I have seen or heard (don't remember which) of someone using a spacer, a piece of copper would do, to raise the transistor just above the circuit board and still conduct the heat to the radio heat sink. This solves the problem of the part not fitting down into the hole in the board.
     

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