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Old HC 5502 analog scope ( some display problems )

Discussion in 'Repairing Electronics' started by BGAmodz, Jan 21, 2014.

  1. rumpfy

    rumpfy Active Member

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    Just to add a comment; I have an old Tektronix 564 CRO which is about 50 years old. Over the years, the tube socket has been subject to small corona discharges at the high voltage pins(2400 Volt). The corona has tracked across the socket and has etched the moulding material. The corona has also etched some of the plated pins of the CRO tube itself. It one is not careful in pulling off the CRO tube socket, one of the pins will pull out of the base of the CRO tube.
    It is conceivable that your problem may have been something like this or maybe just a slight corrosion problem due to the CRO not being used very often. On another occasion, another TEK cro had a unsteady display, and in desperation we cleaned around the socket and also the deflection plates connections which are formed into the glass of the tube itself. This cleared the problem.
    I try to keep my TEK clean on the inside as well as the outside by keeping it's cover on and in a cupboard when it's not in use.
    Good luck with your CRO; you'll fall in love with it when you uncover its full capabilities.
     
  2. BGAmodz

    BGAmodz Member

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    Of course i will take care of it .

    As previously mentioned , Cowboybob said that the flickering i got in screen when displaying sine waves was due to CRT persistence duration degradation ( see post 41).

    Can this be a bad caps issue?
     
  3. dr pepper

    dr pepper Well-Known Member Most Helpful Member

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    I think bob meant that your screen has less persistance than it had, so low frequencies will appear to flicker more, unless your screen has been sat in direct sunlight this shouldnt be all that bad.
     
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  4. dave

    Dave New Member

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  5. BGAmodz

    BGAmodz Member

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    Hi again everybody .

    I just want to recall that the scope i was trying to repair is still working but not completely, i can measure dc signals as well as sine waves within 60 Hz but not high frequencies.

    The problem is that i can't measure high frequencies on switch mode psus . when i put test probes across gnd and gate of the igbt ( the igbt is removed) the negative test lead blows up immediately.
     
  6. Les Jones

    Les Jones Well-Known Member

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    If you don't understand why this is happening then you should not be attempting to work on switch mode power supplies. The negative (ground) lead on the scope will be connected to mains earth which will be connected to the mains neutral at some point. (In the substation or where the supply enters your house.) I assume that you are attempting to connect the scope ground to the negative of the bridge rectifier. On every half cycle this point will be connected to mains live via the diodes in the bridge. To work on switch mode power supplies you need to power the switch mode power supply from a floating supply. (By using an isolating transformer with no path to earth on the secondary.)

    Les.
     
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  7. BGAmodz

    BGAmodz Member

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    Thanks for your replay .

    At first i was not working this way , i have been aware of the consequences of this method . But after working with a welders repair personnel , he was checking the PWM signals this way and was displaying them without any damage at all .

    I should point out that the negative side of the IGBT is tied the PWM ic's GND pin .
     
  8. Les Jones

    Les Jones Well-Known Member

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    But the PWM ic's ground pin will almost certainly be connected to the negative of the bridge rectifier which is fed directly from the mains. Have a look at the schematic. It's a good job you did not touch the PWM ground pin and something that is at ground potential. If you had you would probably not be around to write this post. I think you should stop working on it until you FULLY UNDERSTAND what you are doing.

    Les.
     
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  9. BGAmodz

    BGAmodz Member

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    Ok how do i check the PWM signal safely ?
     
  10. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    In the way you want to check them A-B which is usually invert and add. You take the cha A probe for one and the channel B probe for the other.

    Basically NEVER use the ground lead of the scope UNLESS you KNOW the probed circuit is isolated.

    Other choices are a differential amplifier or isolation amplifiers. Both pricey.

    FWIW, in most of the line operated supplies, the mains are either half or full wave rectified depending on 120 or 240. So one side is something like 155 volts away from ground.
     
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  11. BGAmodz

    BGAmodz Member

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    I see KISS .

    But just like the repair technician did on his hameg , i was using the crocodile clip attached to the probe for the gnd .

    I don't know what could be wrong with my scope witch checks sinewaves perfectly using the same clip.
     
  12. cowboybob

    cowboybob Well-Known Member Most Helpful Member

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    BGA, what everyone is pointing out is that the ground lead on your scope probe is chassis ground (or neutral): it is NOT (normally) a floating ground.

    To be safe, before you hook up the ground lead of the 'scope to anything, use your multimeter (or whatever) to check the voltage between your 'scope ground lead and whatever you intend to hook it to - if there is voltage present, of whatever level, DO NOT connect the ground lead to it!!!

    I say whatever level voltage because if it's mains, you've got a HUGE safety issue and if it's less, you're grounding something that isn't supposed to be grounded and the release the magic smoke will be your reward :banghead:.
     
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  13. BGAmodz

    BGAmodz Member

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    Ok i must admit i don't quite understand well.

    The scope i got has a Ground connector , and the test probe has one alligator clip attached to it and using the DMM i see continuity between them ( ground connector and clip) . Are they supposed to be like that ?
    Please i need a tiny explanation .
    [​IMG]
     
    Last edited: Nov 14, 2015
  14. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Yes. Ground of the scope is Earth. The clip is EARTH.

    For the NEXT trick, you need TWO probes.

    Note, the Vertical Mode switch that labeled CHA1 ADD CHA2. You want ADD only depressed.

    The Position Channel 2 has a "Pull to Invert"; Pull the knob out.

    Now, there HAS to be some sort of a connection to ground, for this to work. In general, you don't use the clip.

    You can now use two probes (no clip) and measure across something.

    As what was said before, measure with a battery powered DVM, both AC and DC the voltage between the clip and what you THINK ground is on the device your trying to test.

    When you test a battery powered devices which is isolated will need to use the ground clip. Otherwise, it stays in your pocket. You know, the "One hand in the pocket when your measuring HV"

    For now, that's your rule.

    Don't put the clip anywhere you would be uncomfortable connecting to earth.
    ==
    Have you ever heard of Probe compensation? Do you know how to use the Cal 1 V point on the scope?
     
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  15. BGAmodz

    BGAmodz Member

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    Ok . as far as i remember , the technician was using two different ac inputs for for powering both the scope and the unit to test .

    I guess this where my problem is right ? i need to isolate the unit's ground from the scope's ? and of course the grounds are tied to the neutral in my case right ?
     
  16. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Likely. The device under test usually gets the isolation transformer.

    Isolate, probably. The scope should also have a rating as to how far above ground you can bring the inputs.
    I have some Keithley instruments which allow up to 30 V for the "ground". The scopes I've used, it's been a "hard" connection to earth. When you try to "float" a metal cased scope the whole scope case is floated at the high voltage, so you really want your test equipment grounded and the device your working on floated.

    My bench as as isolation transformer followed by an instrumented Variac (amps/volts). It was used to test the TV's with the non-isolated line voltage filament string. 2-prong polarized cords.

    I've since acquired an Isolation amplifier.
     
    Last edited: Nov 14, 2015
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  17. cowboybob

    cowboybob Well-Known Member Most Helpful Member

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    YES.
    NO!!!

    Reread KISS's post. You must check that the ground of the device you wish to connect to the 'scope is, in fact, earth ground!

    Otherwise, use his two channel 'scope probe inputs method.
     
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  18. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    This http://www.smps.us/power-supply.html little blurb should give you an idea why you can't ground stuff willy nilly.

    Since we don't know where you are in the world, we can't say much about AC distribution, In the US, neutral and ground are connected at one point - true, but the bridge rectifier across the line poses the problem. The last transformer provides the possibility of isolation.
     
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  19. BGAmodz

    BGAmodz Member

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    Ok for now i got an idea , is it safe to check the signal across the pulse transformer's primary since this is isolated from the IGBTs ?
     
  20. cowboybob

    cowboybob Well-Known Member Most Helpful Member

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    OK. I give...
     
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  21. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    That's why I waited for CBB's reply. I couldn;t figure out a good answer. "I give" is as good as any.

    BTW: when you use oscilloscope probes the input Z of the scope becomes 10 Meg, so you don't need much of a path to measure something.

    You have to use the differential method and there has to be some signal or reference to the two probes.

    The scope only measures CHA #1 to Earth and CHA #2 to Earth. It CAN display the difference, but only if some path to EARTH exists.

    I picked one of these http://www.ko4bb.com/manuals/index.php?dir=Philips/PM8940 up from ebay for good price. Keep watching.
     
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