FAQ X108G not transmitting

Discussion in 'X-108' started by Bob N7SVI, May 9, 2020.

  1. Bob N7SVI
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    Bob N7SVI Beginner X-108 Member

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    My radio is still receiving very well but the transmitter is not transmitting. I had issues with an antenna switch in my shack and tried to transmit several times without an antenna, only a few feet of coax. After fixing the switch problem, the radio transmitted but would get "stuck" in transmit mode until I hit the ptt several times. Today I replaced the battery successfully (CR1220 in a holder) but the transmitter is now completely dead. Any ideas on what may be messed up and how to fix it. Or any ideas about anyone that repairs these radios. Mine is an early version.
     
  2. Chris Wilson
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    Chris Wilson Chef-Boarder X-108 Member

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    Hey Bob
    I was given one which is no longer transmitting. I have a serial beginning with an 11. slightly diferent scematic. There is no transmit to speak of. Maybe 10mW.
    I have reseated the ribbon cables, replaced the battery (socketed same as yours) and flashed the 7 May 2017 firmware.

    I have noticed that the drive is a little low to the output stage. There is a long RF fly lead that connects the top logic / signal board to the RF Amp / PSU board. That is the feed to the power amp. Make sure that is plugged in.

    Just to let you know, that did not fix my problem.

    That said, I jerry rigged a Pixie QRP rig and cranked a half decent signal into the amp and it does amplify, I got around 12 Watts, though the power setting makes no difference to the output. So the question is: How much drive does it take at the input to the amp, and should I see a change in the output when I change the power even though the signal to the amp is constant
    I'll let you know how I go
     
  3. Bob N7SVI
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    Bob N7SVI Beginner X-108 Member

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    Thanks for that Chris. I look forward to you solving the problem with a simple fix I can do. Hi Hi !!! I have two radios that really want to work and get back on the air. These are really fun and simple radios.
    Bob Rodman N7SVI
     
  4. Chris Wilson
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    Chris Wilson Chef-Boarder X-108 Member

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    OK
    I am u and running now

    The schematic on line is different to the version I have for the input stage on the RF power amp.
    The long IPX jumper feeds the modulated RF signal to the board. Its pretty low level. Maybe 50mW. You can see a test point next to the jack you can connect a CRO up to.

    I disconnected that and fed a 7.023MHz signal ftom a Pixie QRP transeiver into the power amp and discovered the first transistor Q5B on the input was not working. If I fed the signal to the collector of the transistor, I got more output.

    Measured the device with a meter in circuit and didn't get the usual diode readings from B to C or B to E. I carefully removed it with a tweezer iron I have for this work, and tested it out of circuit. Base was open.. So I cleaned up the pads and soldered a 2SC8050 in its place. That got it working with full fignal.

    After attatching the jumper, the device seemed to work OK, however, I carelessly hit the top of the antenna switch relay which made a bit of a mess so it wouldn't switch. I clipped off the melted plastic top and its OK now. I will repalce it one day

    Here are some photos.

    I put fiber washers on the transistors as the screws into the heatsink protruded past the base of the heatsink plate underneath the board. Now th eteatsink is plat and makes a solid contact with the case itself

    To troubleshoot yours, disconnect the screen and top board.
    Remove the tinned covers from the amp baord as I have done
    Check all of your transistors / fets in circuit for obvious short circuits

    If you dont find any...you'll need to test on a live board.
    Hook up the ribbon cable between the boards. Don't worry about the coax jumpers
    Hook up the front pannel / screen baord and carefully rest it so you can operate it, without shorting anything out
    Connect up a power meter and dummy load.
    Connect up a CW key or a switch to the Key jack
    Connect up power
    Set the radio to CW
    Set the keyer to manual
    Tune your radio to the frequency of your signal generator which you set to the frequency you want to test
    Now
    Feed a signal into P4B - You can hook up a piece of Cat5E solid cable and use that as a pin into one of the coax jumpers, and plug the other end into P4B. Ground anywhere convenient.
    Close the CW Key.
    Check the bass and collector signals on Q5B. You should see amplification. If not. That's what I had.
    Follow the signal through the stages and make sure your'e seeing a bigger signal on the output of each stage.

    With the blown transistor, I was getting 7W with around 2V of drive. Once I changed the transistor out, I pegged the 20W meter for the second I gave it 100mW. Everything worked.

    I could not have fixed it without a CRO or an RF signal source. Having a really good soldering setup made life much easier, as well as a jewelers magnifying visor.

    The original chip transistor is available from Mouser though I am not what gain rating it has

    Hope this helps
     

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