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G & E Bradley CT471C

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

  1. Sinewave

    Sinewave

    119
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    Feb 15, 2013
    Hello, does anyone here have any experience in the workings of the G & E Bradley CT471C?

    Thanks.
     
  2. Bluejets

    Bluejets

    3,432
    697
    Oct 5, 2014
    As in using the meter or repair to some problem?
     
  3. Sinewave

    Sinewave

    119
    1
    Feb 15, 2013
    I have two here which are behaving oddly. I can find little information on how they work exactly. I have schematics for them.
     
  4. Bluejets

    Bluejets

    3,432
    697
    Oct 5, 2014
    You will have to be more specific as " behaving oddly" is not really an electrical term.
     
  5. Sinewave

    Sinewave

    119
    1
    Feb 15, 2013
    Are you familiar with these units then?
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,173
    2,689
    Jan 21, 2010
    If you havea schematic and a good description of what is going on, then we may be able to help.

    With neither of these (especially without any description of the fault) then there is no way to say if we can help.
     
  7. 73's de Edd

    73's de Edd

    2,616
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    Aug 21, 2015
    Sir Sinewave . . . . .

    Hmmmmmmmmm . . . with that being circa 1964-66'ish.

    Now, if I gave you this "bone" would you wag your tail wildly, and then fully be able to handle the task at hand ?

    http://www.vmarsmanuals.co.uk/archive/1248_CT-471_Multimeter.pdf

    73's de Edd
    . . . . . . . . . . .


    You DO realize that the PRIMARY difficulty with working with a college research grant, is that if you solve the problem, you're out of a job.
     
    Last edited: Dec 10, 2018
    davenn and Richard9025 like this.
  8. Sinewave

    Sinewave

    119
    1
    Feb 15, 2013
    No, firstly I'm not a dog. Secondly as said previously I have schematics. I'm specifically looking for someone with experience on these meters which I understand is unlikely.
     
  9. Bluejets

    Bluejets

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    697
    Oct 5, 2014
    People in here are experienced in many areas of electronics and before you poo poo them it might be advisable to know that because of the former, it is highly likely for many to diagnose whatever you think may be a "specialised" piece of gear.
     
    davenn and (*steve*) like this.
  10. Sinewave

    Sinewave

    119
    1
    Feb 15, 2013
    Hello and happy new year.

    I hadn't intended to poo poo anyone at all, I was genuinely interested in anyone who may have touched these before, sorry if that had came across in a different way.

    So, since you ask, I have a meter here which works to a degree, gives reasonably accurate readings. But after a short while the needle begins to rapidly and I mean very rapidly oscillate, even when measuring DC. It's got me stumped.
     
  11. Bluejets

    Bluejets

    3,432
    697
    Oct 5, 2014
    Show what you are measuring, how and what meter.
     
  12. 73's de Edd

    73's de Edd

    2,616
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    Aug 21, 2015
    Sir Sinewave . . . . .

    Dad was an EARLY day geophysicist . . . . which involved cross association with physics-mechanical engineering-hydrodynamics-chemistry-mathematics-drafting-electricity and photography.
    So in my very early years . . his words . . . . of throwing ME a bone . . . . was giving assistance on explaining something I was having problems understanding.

    I have a meter here which works to a degree, gives reasonably accurate readings.

    Need to know what functions and ranges that you are having abnormalities on.

    But after a short while the needle begins to rapidly and I mean very rapidly oscillate
    Even when measuring DC.


    I don't know if you were . . . . around, active and hands on in 1964, but I certainly was well established.
    That unit of yours was using germanium semiconductor design . . . .WOE . . . be the headaches . . .experienced with that families shortcomings of
    leakage and beta/gain shift with temperature .
    The real players in the game had personnel right at the semi manufacturers site so that they could pull just off the line transistors and diodes
    and test / hand pick the . .sometimes . . .select few units that could meet their required design specs.
    Making a constantly reliable, instrumentation quality DC amplifier was an uber technical challenge at that time frame.
    What they opted to do on this unit was to take the front end and its attenuation capabilities to reduce any over amplitude voltage down to a common voltage that would fall within the specs of not overloading an AC gain block of discrete transistors designed to operate in their most stable linear gain function .
    Those were being your units VT-11-12-13 transistor string.. They produced a bit more than 20db gain so that it could be calibrated / pulled back to get 20, in order to get your typical .012mv/40mv/.12v/.4mv/1.2v/4v/12v/40v/120v/400v/1200v full scale readings of that amplified and detected frontal sampled voltage.
    I said amplified . . . but that was being as an amplified square wave . . . akin to the same treatment that an amplified AC signal would be receiving.
    That transition was created by the use of a two transistor multivibrator operating at approx 550 cycles that would chop the incoming DC voltage into a stream of square waves.
    Hypothetically . . . . . .in setting your meters range at 12mv DC scale and taking a DC sample with your meter probe and reading out as a 10mv level. That voltage would then be routed to the chopper circuitry where that 10mv DC level then gets dropped down to 0 voltage for 90.5 us of every 181 us ..
    Then that 10 mv in amplitude square wave stream , thereby created , was amplified by the VT11-12-13 amplifier section and detected to then transform it back to a scaled up DC which is then being final meter drive .


    If you want to potentially skip / shorten the troubleshooting chase, jump directly to an early stage C34 electrolytic in the unit and see if replacing that doesn't stop the 2-3 cps meter needle flutter.
    Since that filtering of the detected voltage is the most likely to be causing your described problem.
    C22 is also being associated with the end circuitry meter drive filtering.

    Furthermore, if you want to do the right thing on that unit, for further proper functionality and reliability, you would also want to replace the units now 55 year old electrolytics group of C1-2-7-8 and C3-12-16-and 29 ( using 10 ufd units ) and C15 and C22.

    ASIDE . . . .
    Upon initially opening one of those pristine and stored units . . . place your nose right down into the midst of the unit and let your nose experience the cacophony of different smells emanating forth from that unit.
    ( It reminds me of when I have to open the case of my 50'ish vintage TV-7 tube tester. )


    73's de Edd . . . . .
    . . . . . . . . . . .


    Now . . . . . . mind you . . . . I'm not a gynecologist . . . . . but I'll certainly be happy to take a look.
     
    Last edited: Jan 2, 2019
  13. davenn

    davenn Moderator

    13,225
    1,734
    Sep 5, 2009

    it did pretty much to everyone here

    we are now 13 posts into the thread and you still haven't answered the basic question from way back in post #4 by @Bluejets

    you have written us all off before even giving us any info to go on.

    Electronics is electronics, it's predictable between a multitude of devices




    Dave
     
  14. davenn

    davenn Moderator

    13,225
    1,734
    Sep 5, 2009

    finally :)

    OK so, this meter appears to be capable of running off main or battery supply
    Does the osc. fault occur when running of either of the supplies or just one of them ?
    if on mains supply only, it may be a smoothing capacitor that has failed and is letting too much ripple through

    there's a whole bunch of power rails detailed in the service manual ...... have you checked any of them yet to see if they are correct when the fault is occurring ?
    if so, what did you find ? .... If not .... DO IT
    That would be about the first thing to check

    With a piece of electronics this old, I would suspect pretty much every electrolytic capacitor in the unit
    and you saying that the fault comes on after a short while almost guarantees hat it could be one or more of the electros as they warm up


    You now have lots of work to do .... time to get started


    Dave
     
  15. Sinewave

    Sinewave

    119
    1
    Feb 15, 2013
    Like I say, I didn't mean it to come across that way, but we don't always come across in words on a screen as it would appear in person.
     
  16. Sinewave

    Sinewave

    119
    1
    Feb 15, 2013
    There are issues with Ge semiconducors and the meter was rarely responding to anything at all until I replaced the three transistors which are prone to 'tin whisker'. I used Si small signal PNP transistors which appeared to wake the meter up into reasonable responce.

    I shall check the capacitors you suggest. Thank you.

    And yes, I do love the smell of these vintage units. I especially love the smell from the inside of an old Avo and the smell of their cases.
     
  17. Sinewave

    Sinewave

    119
    1
    Feb 15, 2013
    I don't have the mains power supply, I believe this may have been an optional extra. So I'm using the batteries and a bench power supply when I've got the meter open.

    I am quite slow at this at the moment but I'll try the things suggested and let you know the outcome. Thanks.
     
  18. Sinewave

    Sinewave

    119
    1
    Feb 15, 2013
    I'm going to take a look at this tonight. One thing I've observed is that the fast ocillations of the movement is occuring only on the DC ranges.
     
  19. 73's de Edd

    73's de Edd

    2,616
    1,067
    Aug 21, 2015
    Sir Sinewave. . . . .

    See the need for initial info . . .relevent to the instruments pristine ORIGINAL condition . . . .versus the install of silicon transistors versus the OEM germanium transistors.
    Without your telling, I wouldn't have known that changed situation . . . . in the half lifetime of the decay of yttrium.
    To see a fast flicker of a meter pointer is more relevant to a square wave than a sine. And 5-7cps is about the very-very upper limit of even being able to detect that.
    If you did not accordingly, reestablish the proper biasing for the new SILICON transistors versus the initially installed GERMANIUM, that dead state time alone could cause a signal drop out problem.
    One would need to take a low level . . .10-50mv . . . . 1 Khz input into the VT 11-12-13 amplifier block and check gain to see how much more gain is now present than with the old germanium clunkers
    and get gain back down to ~25db . .to potentially be decreased downward .
    Adjust the biasing of the UNK silicon PNP transistors to get a linear amp performance and use scope observance for quality of waveform.
    Retest.
    The info on the E-caps still holds true . . . if they are being of original stock.

    ASIDE . . .
    Odor . . .
    Probably its being punctuated by the phenol , bakelite and formaldehyde .


    73's de Edd . . . .
    . . . . . . . . . . .


    I started jogging for my healths sake, but had to give it up for my lifes sake,
    my thighs kept rubbing together and setting my underwear on fire !
     
  20. Sinewave

    Sinewave

    119
    1
    Feb 15, 2013
    Hello.

    Yes the biasing was of initial concern seeing as Ge were the orginals. The replacements are BC557B.

    So some progress tonight based on your previous advice gave the following results...

    C34 was replaced which stopped the oscillation for a short while. Oscillation still present though after a time, so C22 was replaced after which the oscillation would happen only in short bursts rather than persist once started. Most of the time the movment has settled down, only for some oscillation to re appear if the meter is making measurements.

    I'm testing the DC current and voltage ranges using a small current and voltage calibrator which can supply up to 20v and 20mA, but at the moment I've the meter on the 12V/I range and testing it with 10V and 10mA to observe its behaviour.

    There continue to be some flucutations when measuring, but the overall rapid osciallation has settled down, but I do need to do more as you've advised to completely eliminate any fluctuation. Currently I have 10v going to the meter and the movement is slowly moving between 9v to 9.5 volts. When the meter has been off for a while and then switched back on, it will read very close to 10v before any fluctuations begin.
     
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