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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
    So on the subject of biasing with the new transistors I made some measurements which were puzzling.

    Background, I have used these Si transistors in another CT471c, which reads low, but otherwise the set up/calibration checks tests out fine. Perhaps sorting out that low reading unit can be for another time.

    However in this unit tonight, measuring at the collector of VT13 I got 13.6v which I should. At the collector of VT18 minus 3.6, fluctuating to 3.8v. It should be 5v +/- 0.1v.

    So I've got some thinking to do here.
     
  2. Sinewave

    Sinewave

    119
    1
    Feb 15, 2013
    I've been away from this for a while. So tonight I though I'd revisit.

    The meter was still exhibiting violent fluctuating of the needle. I chose C29 to replace and replaced it and it appears to be behaving for now.

    Just as I were setting up to make some more tests and measurements, the movement began to oscillate again. Replaced C15, movement stable, will observe further.
     
    Last edited: Feb 12, 2019
  3. Sinewave

    Sinewave

    119
    1
    Feb 15, 2013
    Collector of VT13 13.6V

    Collector of VT18 adjusted to 5.2v via RV12 (CT471C set in VAC range) and set in the VDC range measuring 3.96, it should be 5v.

    So it's improving.

    Chopper cct on the scope shows a nice square wave, 1.96v pk-pk at 507hz.
     
  4. Sinewave

    Sinewave

    119
    1
    Feb 15, 2013
    I'm quite puzzled. I've switched on again this evening to discover the violent oscillation is back. But it doesn't do this on AC functions.

    A few on and offs and a few clicks and it stops, but may well return later.

    So far every electrolytic has been replaced on the amplifier board.

    Here's what it looks like

    [​IMG]

    Puzzled
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Sounds like there may be some issues with some switch contacts.
     
  6. Sinewave

    Sinewave

    119
    1
    Feb 15, 2013
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Watch out. Vibration on a meter movement can snap the needle off. Watching that video I felt I was going to see it happen. Of course, you may be nowhere near the resonant frequency, but it can't be doing the needle any good.
     
    Sinewave likes this.
  8. Sinewave

    Sinewave

    119
    1
    Feb 15, 2013
    I'm wondering if the chopper circuit is playing up on the DC ranges. There's not much to that circuit really, so maybe I could try some new transistors. I'll need to see if I can find a good Si alternative.
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    I've just looked at the schematic again, and also had another read through the thread.

    Have you checked the two BAY31 diodes? A failure of either of these could damage the silicon transistors far more easily than to the original germanium devices (ge devices allowed reverse base emitter voltages up to 30V, whereas si is limited to about 7V).

    I would be concerned that replacing them with silicon diodes would affect the accuracy of the meter, but a temporary replacement would either identify or eliminate them as the cause of the problem.

    If they are a problem, it might be possible to replace them with shottky diodes, but leakage may again be a problem, so I don't recommend trying them as a diagnostic.

    If you have an oscilloscope, it would be very interesting to look at the waveforms on the bases of those transistors. The waveforms should be very similar (but operating alternately to each other).

    It is an interesting circuit, I'll try to find time for a closer examination.
     
  10. Sinewave

    Sinewave

    119
    1
    Feb 15, 2013
    Do you mean the BAY31 diodes could damage the Si transistors I placed in VT16-18 positions?

    If I get a chance tonight I'll scope up the bases of VT16-18 if they're the transistors you mean. Or did you mean the two transistors in the chopper? I haven't replaced them as of yet, I'll get them out and test them with a meter. I also have a Peak Atlas semiconductor analyser, I could test them with that and post the results up.

    Thanks.
     
  11. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Sorry, I thought you had replaced the transistors in the chopper.

    My first step would be to take the meter out of circuit and replace it with an equivalent resistor. That way you can leave the unit powered up for extended periods without writing about damaging the totally irreplaceable meet movement.

    The next thing I would confirm is whether the frequency seen at the meter is the chopper frequency (or maybe double the chopper frequency) just to confirm that it is this breaking through.

    If it isn't the chopper frequency, then probe back to where it starts and see what you find.

    If it is the chopper frequency I would start with the output side of the transformer and check for symmetry in the circuit signals.
     
  12. 73's de Edd

    73's de Edd

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    Aug 21, 2015
    Sir Sinewave . . . . .

    I say there . . . . let's break your unit down by sections . . . and then do an analysis of the cause of your instruments recurring / onset case into VNS Syndrome .
    (Violent Needle Swing ).

    Starting with the AC/DC amp section and detector / meter amp circuitry.

    Consulting supplied referencing markup, input is at VT11 base and makes a direct DC coupled flow thru as far as the collector of VT18 . . . just follow the GREEN ARROW path.
    Then there is AC coupling into the final V20-21 complementary driver stage

    I am quite surprised by seeing your units needle action, as it is responding at much less than 5 cps and must have a quite significant recurring voltage change on it.on it.

    Initially can you vouch for the "health" of capacitors in the unit ?
    Specifically C22 . . . . . C26 . . . . . C16 . . . . C29 . . . . . C15 . . . . . C13 . . . . . C14 . . . . . C12 . . . and . . . C34
    By that . . . I mean , not having had capacitance decline by virtue of time onset electrolyte dry out.

    Knowing for sure that you have a good 100 ufd for C22 across the meter, find another of that capacitance and WV rating and then you will use it for test damping of the amplifier at different stages.
    Since the majority of stages use PNP xstrs lets place one clip of a double clip leaded test lead on the E-caps POSITIVE lead and have the other clip of the test lead connected to the common ground buss.
    That then leaves you with a floating NEGATIVE cap wire to use for probing.
    Then you power up the unit and confirm that it is still being in its "shaaka shaaka shaaka your money maka mode"
    If so, then put that capacitors probe lead at the base of VT11 and see if the fast meter action either totally stops or has some noticeable degree of abatement ?
    Short the capacitor leads to fully discharge it between test steps.
    Then move the cap probe lead on down to VT18 base and evaluate, then move to the shared emitter junctions of VT20-21

    Tell us your findings.
    If no effect was made to the meter . . . . . then we move on to check out all the blades and rotor of ye olde Huey Cobra and warm up the "chopper".

    Relevant Marked Up Techno Referencing . . . . .

    upload_2019-2-18_3-14-28.png

    73's de Edd . . . . .

    If nothing ever sticks to teflon, how do they make teflon stick to the pan ?

     
    Last edited: Feb 18, 2019
    (*steve*) likes this.
  13. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Thanks Edd, that makes it so much easier to read and understand.
     
  14. Sinewave

    Sinewave

    119
    1
    Feb 15, 2013
    Thank you to both of you. I'll do some testing tonight and report back. In the meanwhile what I might try and find is a 1mA general meter from Farnell or somewhere to connect in place of this meters movement, as has been pointed out this is irrelplaceable.
     
  15. Sinewave

    Sinewave

    119
    1
    Feb 15, 2013
    Ok, so far this afternoon, when the needle is oscillating, touching down with a 100uF cap to the base of VT11 and ground does stop the oscillation.

    Will try more tests in a moment.
     
  16. Sinewave

    Sinewave

    119
    1
    Feb 15, 2013
    Ok, so I made the same test to the base of VT 18, oscillation stops.

    To the emitter junctions of VT20-21 the oscillilation is still there, though localised at the bottom end of the scale rather than higher in the scale.
     
  17. 73's de Edd

    73's de Edd

    2,615
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    Aug 21, 2015
    SIr Sinewave . . . . .

    I acknowledge your tests and their results, now we need to further delve into it, to find that if any circuitry tied into that amp string, is not additionally influencing it .
    That specifically would be the additional use of that same E-cap to additionally shunt . . . to ground
    1 . . . . . The shared cathodes of MR14 and MR15 in the meter bridge rectifier.
    That line is being a ground return and gets routed to ground thru a different series of resistances, in accordance to which range SC3 switch is being set to.

    2 . . . . . The junction of VT19 and the left lead of shared R109 resistor.
    That is being two incoming phase detector reference samplings.

    3 . . . . . The shared and common junctions of R-145 / R107 / R83 / R135.
    Which are associated with meter zero centering.

    Additionally, I saw no feed back from you on the given E-capacitors ( unless you shotgun / replaced them . . . . and had not mentioned it ).

    No way would I give creedence to the E-caps of 50+ Years of ageing in this unit, for their bypassing, decoupling and Hi freq suppression capabilities * ( * since you see no .01 or .1 ufd ceramic caps shunting them.) , unless I had tested all of them.

    My EXACT manner would be to initially confirm the the meter fluttering and then
    go to C34 (22 ufd unit) and either hold or solder tack another 22 ufd across / in parallel with the original unit to see if there is a decrease or elimination of meter flutter.

    Then pull the sub capacitor and move on to use the same manner of testing for the following units.

    C12 (10 ufd unit)
    C14 (1 ufd unit)
    C13 (1 ufd unit) . . .unless it is a paper unit, then check for DC leakage with a 50 vc test voltage.
    C15 (47 ufd unit)
    C29 (10 ufd unit)
    C16 (10 ufd unit)
    C26 (1 ufd unit)
    and C22 (100 ufd) , which I hope that you have changed out already.

    Only then . . . . would my mind be at ease . . . . in the aspect that this massive gain string of transistors, is not breaking into sporadic, self induced oscillation, due to inadequate capacitive decoupling .

    Thaaaaaaassssit . . . . .

    73's de Edd . . . . .

    The probability of someone watching you is directly proportional to the stupidity of your action.


     
  18. Sinewave

    Sinewave

    119
    1
    Feb 15, 2013
    Thank you Ed,

    Do you think that any poor capacitors may have damaged Ge transistors? I know Si is more prone to this, but just a thought.
     
  19. 73's de Edd

    73's de Edd

    2,615
    1,067
    Aug 21, 2015
    Nope . . . .
    ASIDE . . . . I just now, read back, and saw that you did replace C34 and C22 . . . that's good . . .

    I did do some ESR testing with a scope . . .
    Was that being in the manner . . . . that you feed a 100KHZ sq wave signal into the scope . . . from a function generator and then shunt the signal with the E-cap, being tested, to see how much it attenuates that square waveform ?
    That works . . . . . if you are additionally comparing against a known low ESR E-cap, of the same cap value and voltage rating . . . for reference .
     
    Last edited: Feb 22, 2019
  20. Sinewave

    Sinewave

    119
    1
    Feb 15, 2013
    Thanks Ed,

    With regards to the capacitors, earlier you mentioned some should be replaced. In the absense of time to test every characteristic of the capacitors (I did do some ESR testing with a scope but I don't know how accurate my method is) I replaced the following

    (copy and pasting from notes I'm making as I work on this unit)

    C22, 34
    C16, C26, C13, C12.
    C29, C15, C11
    C14.

    I think that covers all capacitors in the amplifier board. When replacing some in groups, it appeard to have an effect, only for the oscillation to return.
     
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