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Marconi TF144H/4S

Discussion in 'Troubleshooting and Repair' started by Sinewave, Dec 8, 2019.

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  1. Sinewave

    Sinewave

    129
    2
    Feb 15, 2013
    Hello. I've been performing some restorative work to a Marconi TF144H/4S.

    So I'm going through the setting up process from the service manual and I'm stuck at the crystal calibrator section. I'm not so sure the crystal calibrator is working at all.

    If there's a way of sharing the service manual perhaps we can go over the circuit and I can try some suggestions?

    Thanks.
     
  2. 73's de Edd

    73's de Edd

    2,794
    1,171
    Aug 21, 2015
    Sir Sinewave . . . . .

    • Initial confirmation ;
    Is if you have powered up the unit and received a signal from the generator unit to a nearby or connected receiver in the
    G thru J bands of the signal generator.
    If so, then how about your having connected a low z set of headphones to the unit in order to connect to the internal RF mixer and af amp circuit within the unit ? This will then let you hear the two frequencies zero beat . . . .*** . . . . . the variable one of the signal generator and the fixed reference one of the 2 quartz crystal references being used.
    If so, then you would want to switch to the first band which would accomodate reception of the 400Khz internal xtal calibraton frequency . . .which would be the Sig gens G band which can be tuned to a 400 khz crystal frequencys 3 rd upper harmonic of 1.2 mhz. and its fourth at 1.6 Mhz and the final capable range of the 5th . . . . at 2 Mhz.
    Then you flip to band H where you note that H band range will let you tune in to 2 Mhz as a 5th harmonic again, then 2.4 Mhz, 2.8 Mhz, 3.2 Mhz,3.6 Mhz and a final 4 mhz

    Then, when you finally get up to I band range the reference crystal used, gets swapped upwards to a 2 Mhz unit and its 2nd harmonic
    will be used to get your first 4 Mhz reference beat note.
    The same referencing method to a 4 Mhz xtal holds for all of the 2 mhz freq check spreads of the I band and all of the J thru L bands.

    ***
    ZERO BEAT ? ? ? as you tune near the crystal reference frequency used with the frequency adjust knob of the generator and you approach that frequency, one typically hears a high pitch tone at ~ 5 Khz from the xtal frequency ( 22Khz for cats and dogs, 40khz for any bats in the attic- over hearers) and it then incrementally slides down the audio scale to progressively lower frequencies to a zero beat and then a backward transition to higher frequencies, as you tune past the xtal reference.
    So if you hear that tonal transition at all of the multiples of either 400 Khz or 2 Mhz meg multiples harmonics . . . . all is well with your calibrator and BOTH of its reference xtals.

    Finally . . . .
    Should you want UBER accuracy, you can use C192 and C144 to set your corresponding reference xtal to a zero beat against WWV , National Bureau of Standards . . . time and frequency references . ( Or the foreign equivalencies )

    Manual Sub Referencing Markups . . . .

    upload_2019-12-9_4-45-34.png




    Thaaaaaaassssssssit . . . . .

    73's de Edd . . . .

    Now, I am proofreading this, ever so carefully, to see if I have any words out.



    .
     
    bertus likes this.
  3. Sinewave

    Sinewave

    129
    2
    Feb 15, 2013
    Hi Edd, thanks for your response. I'll try later, I've got a set of headphones somewhere. I did recall making some strange readings around the oscillator valve, I'll post them up later. Otherwise the generator CW output is very good indeed.
     
  4. Sinewave

    Sinewave

    129
    2
    Feb 15, 2013
    Hi Edd, I tried the headphones check and I can hear a high pitch squealing noise which comes and goes as I move the tuning around the 1000 k mark.
     
  5. Sinewave

    Sinewave

    129
    2
    Feb 15, 2013
    Another check in the set up and calibration procedure was to couple a wire around V103 and measure the xtal frequencies with a scope, but I can only pick up a triangle wave of the CW output in one of the check ranges and just noise in the other, so I wasn't really sure if the valve circuit is doing what it should be. I did try another 12AU7 but it didn't change the results of that particular check. I'm just having another look over it now to see if I've missed something or am doing something wrong...
     
  6. Sinewave

    Sinewave

    129
    2
    Feb 15, 2013
    The page (page depending on which PDF you have) which has the drawing for the crystal calibrator circuit has some voltages to be checked. It gives voltages for V103, pin 1 and pin 6 between ranges A-F and G-L. Below are some measurements I made.

    Tuning scale set to mid-range. Set carrier control affecting readings on some ranges.

    Measuring pin 1 voltages of V103

    Ranges

    A 102.2V
    B 105.2
    C 111.2
    D 114.5
    E 118.4
    F 114.2

    G - L should be zero volts, but I see minus a few hundred milivolts to little over minus 1 volt on some ranges.

    Measuring pin 6 voltages of V103

    Ranges

    A 72.6V
    B 74.6V
    C 78.7V
    D 81V
    E 91.9V
    F 88.7V
    G 168.5V Set carrier control doesn’t affect readings
    H 168.5V Set carrier control doesn’t affect readings
    I 189.4V Set carrier control doesn’t affect readings
    J 189.4V Set carrier control doesn’t affect readings
    K 189.3V Set carrier control doesn’t affect readings
    L 189.3V Set carrier control doesn’t affect readings
     
  7. bertus

    bertus

    319
    104
    Nov 8, 2019
  8. Sinewave

    Sinewave

    129
    2
    Feb 15, 2013
    I don't have a cap leakage tester to hand, so I wonder if coupling/decoupling caps for the oscillator circuit might benefit from replacement?
     
  9. Sinewave

    Sinewave

    129
    2
    Feb 15, 2013
    I replaced the caps in the circuit but still no likey. I've tried the valve, everything else is simple passive components, so I wonder what it is that I'm missing?

    Any pointers?
     
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