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ROBYN T-123B help needed

Discussion in 'General CB Services Discussion' started by Crambone, Nov 2, 2019.

  1. Crambone

    Crambone Active Member

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    i have a T-123B that would quit on me after a few minutes, RX, TX. The TX only showed a dead key of 2 watts and no modulation. I installed new Filter Caps and all the Electrolytic's this solved the quilting after a few minutes and shows me a dead key of 2 watts still with very small modulation. I tried to align the Radio per the owners manual but need a SAMS Photofact or something with voltage check points. I’m not sure how to check the modulation transformer? I followed the 4 leads and they all seem to have voltage where they terminate. I didn’t disconnect the wires just tested in place but I’m not sure what voltages I should be seeing.
    Am I looking in the right direction? Oh I also tested all the tubes and they are all about each 95%.

    Thanks, Andrew


     

  2. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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  3. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    s9_mag_nov_1972_pg28_pg29_sm.jpg
     
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  4. Crambone

    Crambone Active Member

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    Ya I used that for alignment but still only 2 watts and faint modulation.
     
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  5. Dmans

    Dmans Sr. Member

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    Are you using a High or Low impedance microphone?

    73
    David
     
  6. Crambone

    Crambone Active Member

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    I’m using the stock hand mike.
     
  7. Dmans

    Dmans Sr. Member

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    Crambone,
    I can think of a couple of possibilities here:

    1. Is the element in the microphone any good? Have you tried another microphone?
    2. Does the modulation light flicker with voice input?
    3. Have you thoroughly cleaned the contacts in the CB/PA switch and the Range Expand Switch?
    4. Is output audio from the speaker in RX normal? If somewhat low, measure voltage at tube pins per the Owners Manual (page 14) or try another 12AX7 in the V5 position. (Half that tube is used as the mic pre-amp)
    5. Measure voltages at pins of tube V6. Swap another tube in the V6 position.

    V10 (RF output) and V6 (Audio Modulator) can be swapped (if both are still 6BQ5's). 2 watts output power is lower than this unit is capable of with good tubes and voltages to tubes. Perhaps both of your 6BQ5's are bad??.

    I'm not sure what testing was done to pronounce the tubes "all at about each 95%" but there is no better test than an in circuit operational test with voltage measurements.

    Good Luck. Please post your findings.

    73
    David
     
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  8. Grinder74

    Grinder74 KE8EOJ

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    Did you get this straightened out? I had no modulation on my Teaberry Model T and swap 6bq5s around and got modulation back but almost dead recieve. Gonna replace the one bad tube with a 7189 my brother gave me.
     
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  9. Crambone

    Crambone Active Member

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    No not yet it was packed for a move and I haven’t had the opportunity to get back to it yet. I will keep this in mind, I do remember checking the tubes.
    Thanks for the reply
     
  10. nomadradio

    nomadradio Analog Retentive

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    Do NOT use a 7189A for the audio tube in those radios until you have a look at pin 6 on the socket. This pin has no connection inside a 6BQ5 and the factory used it as handy tie point for some other part of the audio circuit.

    7189A tube has a jumper connecting pin 6 to pin 9 inside the tube. This will cause smoke in a radio built to 'hijack' the pin-6 lug on the socket.

    As if that weren't confusing enough, the old Sylvania data for the 7189 (without the "A" suffix" is wired just the same as a 6BQ5, no internal jumper from pin 6 to pin 9.

    Don't remember the final socket having this bad habit in those radios, just the audio tube.

    73
     
  11. GTC

    GTC Member

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    Any progress with this?
     
  12. Crambone

    Crambone Active Member

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    I have not had time to get back to it yet, new house and lots of work before boss “wife” will cut me loose toplay with my radios!
     
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  13. GTC

    GTC Member

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    Ah, 'She Who Must Be Obeyed'.

    I have a 123B in my project pile with a suspected dud modulation transformer. Will be interested to follow your progress.
     
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  14. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    I also have a 123B with no transmit, I haven't even looked at it yet. I bought it for parts, but it would be neat to get it going.
     
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  15. GTC

    GTC Member

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    It's not a complicated radio and being tube-based it has a certain charm for me, so I do intend to get mine going.
     
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