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G & E Bradley CT471C

Discussion in 'Troubleshooting and Repair' started by Sinewave, Dec 10, 2018.

  1. Sinewave

    Sinewave

    119
    1
    Feb 15, 2013
    Yes I used my function generator as you suggest and the calculatons suggested replacement.
     
  2. Sinewave

    Sinewave

    119
    1
    Feb 15, 2013
    Right, lets get to it, I'll now do some of the new tests you've suggested.
     
  3. Sinewave

    Sinewave

    119
    1
    Feb 15, 2013
    So far...

    1 Not performed yet as I still can't locate MR14 and 15

    2 Touching down the VT19 and R109 junction (connection to emitter?) stops the meter fluctuation.

    3 Touching on the shared -4v commons (I went onto R107) stops the massive oscillation, but takes the needle to the left of the zero scale and there is an ever so slight fluctuation, really very minute movement.

    Still looking for MR14 and 15, where have they got to? Maybe they're out to lunch....

    Oscillation is out at the moment, so waiting for it to return before doing the rest of the tests.
     
    Last edited: Feb 22, 2019
  4. 73's de Edd

    73's de Edd

    2,615
    1,067
    Aug 21, 2015
    Referring to the schematic . . . . MR 14 & 15 are being two of the diodes in their configuration as a Full Wave Bridge rectifier that feeds their developed DC to the instruments 1 ma meter movement.
    They are on the right half of it and I erroneously referred to a pair of cathodes, while it is actually being an anode and a cathode from each. They then route upwards to pass to the left to then go over to SC3 switch, terminal 3.
     
  5. Sinewave

    Sinewave

    119
    1
    Feb 15, 2013
    Hi Edd. Yes I see them on the schematic, but struggling to locate them in the meter. I will take another look later.
     
  6. Sinewave

    Sinewave

    119
    1
    Feb 15, 2013
    So, for test 1, I have some belief that I'm on the correct side of D14 and 15, touching down doesn't stop the oscillation.

    They were very hard to find, buried deep down in the switch wafers.
     
  7. Sinewave

    Sinewave

    119
    1
    Feb 15, 2013
    Suspect diodes?
     
  8. Sinewave

    Sinewave

    119
    1
    Feb 15, 2013
    From the PDF page 34...

    When measuring direct voltage up to 400 volts the unknown voltage is applied to TL5. It is fed via contact 3 or 5 (don't they know then? -sinewave) and the rotor of SB3 (according to polarity), the rotor and contact 10 of SC1 and the rotor of SA8 to the appropriate point along the series resistor chain R19-R23, R37-R40, R117-R121, R114, R115 and R131. The output from the resistor chain is applied directly to the input of the chopper circuit. When making measurments with the 40v, 120v and 400v ranges, voltage divider resistors R16-18 are bought into circuit by SA14. These resistors in conjunction with capacitor C20, also for the low-pass filter which minimizes any superimposed alternating voltage. On the 1200v range, the unknown voltage is applied to terminal TL3 which is connected to the chopper circuit via R154, R155, R156 and R15.

    So, SC being the measuring finction range, SA being the range value and SB being the main on/off/+/- switch.

    From what I can gather the numbers suffixing 'SC' on the drawings related to the wafers, with 1 being close to the front of the meter, with the numbers climbing the further to the rear of the meter the wafers go.

    I'm going to see what I can spot, considering this oscillation happens only on DC, current or voltage.
     
  9. Sinewave

    Sinewave

    119
    1
    Feb 15, 2013
    I've given the swtich contacts a good going over. I don't think it's them. Along the amplifer all connections coupled to ground have been checked.

    I'm wondering if the issue is in the chopper...
     
  10. Sinewave

    Sinewave

    119
    1
    Feb 15, 2013
    I have the chopper out.

    I have discovered two things, which may not make much difference, however in my other unit and according to the drawings, the collectors of VT23 and VT 15 are connected to resistors R73 and R89 which then are connected to diodes MR16 and 18.

    However, in this unit the diodes are connected to the collectors with the resistors before the diodes.

    I wonder if this is enough to cause this issue?
     
  11. Sinewave

    Sinewave

    119
    1
    Feb 15, 2013
    That's a no, as the oscillation still occurs with the chopper from another unit fitted.
     
  12. Sinewave

    Sinewave

    119
    1
    Feb 15, 2013
    This has taken me a few hours, including make notes and taking photos, but I've removed the amplifer board to make it easier to make some checks. Also I might remove the amplifier board from the other unit and try it in this one.
     
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