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Thread: Yaesu FT-900 with no Power output

  1. #1

    Default Yaesu FT-900 with no Power output

    Yaesu FT-900AT was bought in 1994 (Serial number 4104-0812) and tested for a few hours before doing a separation install inside the trunk of a car. Installation was never finished and the car was stored for 25 years. In 2019 removed FT-900 from trunk and powered up. Reception is good, and front panel controls work on all bands with the exception of MOX, CW, or Antenna Tuning, that is, there is NO POWER OUTPUT. Transmit Busy indicator goes RED but there is no PO or SWR indication on display panel. Pushing “Tune” does not show PO or SWR. Will not transmit on CW with a CW key.
    Already tried both a partial and full reset. Replaced CMOS battery behind front panel. Carefully reseated all connectors and RF cables within easy reach. No physical damage, rust, overheating, arcing, or smoke traces observed.
    Put unit with an antenna next to another transceiver (with an antenna, of course) at the same frequency, but keying the Yaesu FT-900 in any mode does not produce the smallest detectable signal. All this done inside the amateur bands where the FT-900 can transmit. Have been away from the ham shack for a very long time and no longer have oscilloscope, counter, signal generator, or other troubleshooting/service gear other than a DMM, SWR meter and load. Please advice. Tnx and 73 de AB6WP.

  2. #2

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    Does the TX/RX relay click when you hit PTT? Have you tested the finals?

  3. #3

    Default Tx/Rx relay clicks ok

    Quote Originally Posted by brandon lind View Post
    Does the TX/RX relay click when you hit PTT? Have you tested the finals?
    Tnx fer reply. Yes, the Tx/Rx relay clicks when pushing MOX, TUNE, and when Keying on CW. Also tested the functioning of the ground switch near that relay. After such a long time, the original microphone is lost so cannot test the PTT on it, but FT-900 goes into transmit if using the built-in keyer while on CW and VOX is on. Still NO PO.
    At present time, trying to find out how much drive to put into final ampl. Have a Norcal 40A capable of putting out 2 to 4 Watts (QRP rig). Any comments?

  4. #4

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    Not sure what you mean. Are you trying to drive the final inside the ft900 with an external source? If everything else on the radio still works, just no PO, I would start by testing the voltages around the final transistors (Q4004 and Q4005, both 2SC2879's), or at least test the voltage drop across the junctions when powered off to see if they are still working. Don't try to drive the ft900 final with an external rf source, the bias networks for the amplification stages only has power during TX, which is controlled by the radio. Also, I think the input for the final unit is in the mW range, not sure. I dont know how the rest of the radio will like a foreign signal being back-fed into the radio.

  5. #5

    Default Yaesu FT-900 with no Power output

    Quote Originally Posted by brandon lind View Post
    Not sure what you mean. Are you trying to drive the final inside the ft900 with an external source? If everything else on the radio still works, just no PO, I would start by testing the voltages around the final transistors (Q4004 and Q4005, both 2SC2879's), or at least test the voltage drop across the junctions when powered off to see if they are still working. Don't try to drive the ft900 final with an external rf source, the bias networks for the amplification stages only has power during TX, which is controlled by the radio. Also, I think the input for the final unit is in the mW range, not sure. I dont know how the rest of the radio will like a foreign signal being back-fed into the radio.
    Since I keyed the FT-900 next to a known working receiver, at the same frequency and with both antennas a couple of feet from each other, and did not get any faint signal on the receiver, I would estimate there is no drive to the final amp. My logic, however non-academic and without any means of signal detection such as oscilloscope, RF probe, etc., was to remove the input to the final ampl., feed it with a low power signal, then key the FT-900 to see if there was any actual power out of the final ampl. If there is any power out, then the problem is with the drive or pre-drive. Have not done the above because I am traveling and away from the radio, but will heed your warning and start with very low power first, as you said, in the milliwatts range. Clearly, the above is nothing short of a "hack" to fool the radio, but I am also weighting the economics of repair: a 12 lb radio will cost nearly $100 to ship round trip for repairs and the repair itself may be $200-400. So the cost could easily add to $500. One can actually find a used FT-900 for about that much and if I got it, I would have spare parts left over. For a bit more one could actually buy a new radio. This one is at least 25 years old. In any event, I do value your comments and as you suggested, will take a few measurements both with power off then with power on.

  6. #6

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    it is unlikely you would hear the signal trying to drive the final. I have had a few instances where I know for sure it didnt make it out of the radio..

    Just start out seeing if the diode junctions in the final transistors are functioning with a DMM on a diode test setting. Unplug radio, get to final section. Positive lead to base and negative lead to collector and emitter. there should be a reading of around .65 to .75 volt if they are good

  7. #7

    Default Yaesu FT-900 with no Power output

    Quote Originally Posted by brandon lind View Post
    it is unlikely you would hear the signal trying to drive the final. I have had a few instances where I know for sure it didnt make it out of the radio..

    Just start out seeing if the diode junctions in the final transistors are functioning with a DMM on a diode test setting. Unplug radio, get to final section. Positive lead to base and negative lead to collector and emitter. there should be a reading of around .65 to .75 volt if they are good
    Dear Brandon,
    I value your comments greatly. Once I get to the FT-900, I will make the few static measurements with the DMM as you suggested in your last posting. For now have been able to construct an RF probe as per VA7OJ/AB4OJ article on the net. I can also make a simple oscillator to put out a few milliwatts.
    73 de AB6WP

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