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Heathkit Digital Clock

Discussion in 'General Electronics Discussion' started by ajh, Nov 28, 2010.

  1. ajh

    ajh

    2
    0
    Nov 28, 2010
    Can anybody help.
    I have a Heathkit Digital Clock GC1195/GC1197 that was built by Heathkit staff in Gloucester England in the 1970's. The clock has be perfect, apart from the lamp issue, until now.
    I now have a problem with the clock gaining time.
    I am a mechanical not electronic engineer but am willing to have a go at repairing it myself.
    Could anyone advise.
    Regards,
    AJH
     
  2. Resqueline

    Resqueline

    2,848
    1
    Jul 31, 2009
    That clock gets its timebase from the mains so you're probably just having issues with noise on the mains circuit that the clock is on.
    Failing to locate the noise-maker you could try to improve on the noise filter on the pcb in the clock. You can download the diagram here.
    The filter consists of R111 (100k) and C103 (0.01uF). These have a combined time constant of 1ms = a cutoff frequency of 159Hz.
    You can try to increase the capacitor value up to a maximum of 0.027uF (=27nF) to get a 60Hz cutoff frequency and hence improved noise rejection.
    Increasing the time constant however will lower the amplitude going into the clock chip, and taking it too far the clock will not run at all.
     
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,178
    2,690
    Jan 21, 2010
    The schematic is available here.

    It suggests some waveforms and voltages you should expect.

    You haven't moved from somewhere with 50Hz power to somewhere else with 60Hz power have you?
     
  4. ajh

    ajh

    2
    0
    Nov 28, 2010
    No. Its been in the same place for about 15 years.
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,178
    2,690
    Jan 21, 2010
    Then the best bet is Resqueline's suggestion about noise.
     
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