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Oscilloscope: screen seems dead

Discussion in 'Troubleshooting and Repair' started by muffinman, Jul 21, 2015.

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  1. muffinman

    muffinman

    17
    0
    Dec 10, 2012
    Hello All,

    I'm in mourning, my favourite oscilloscope seems to be dead (Kikusui COM7101A). At least, all seems to be normal (buttons, lights) all respond as expected. However, there is no output on the CRT screen at all, not even some variables in the screen. Turning up intensity all the way usually gives some glow, nothing either. Though being an electronics noob, this leads me to suspect that either the CRT or the power supply of the CRT is faulty. Last time I used it (x months ago) all worked without a glitch.

    My question is: with the help of the attached pictures below, can someone give an indication on what to measure to find out whether the CRT is dead or the power supply (or tell me I got it all wrong and this quest is hopeless)?

    Hope someone can give me advice on this.

    Thanks in advance and kind regards, Maarten

    I suspect this connector is hooked up to the power supply..
    [​IMG]

    This is the back of the CRT, in the previous picture we saw the top side this pcb.
    [​IMG]
     
  2. shrtrnd

    shrtrnd

    3,732
    477
    Jan 15, 2010
    If you've got a 'glow', I'd be looking at your horizontal and vertical drive boards, not the CRT itself.
    Any heat/physical damage to components in your horiz/vert driver circuits?
     
  3. muffinman

    muffinman

    17
    0
    Dec 10, 2012
    Hello shrtrnd,

    I should have phrased it more clearly but there is no glow at all when turning up intensity... that's exactly why I suspect either of these two components.

    Kind regards, Maarten
     
  4. shrtrnd

    shrtrnd

    3,732
    477
    Jan 15, 2010
    Don't give up hope. Take a good careful look at the component area (and the contacts on your connectors) for burn marks, indicating obvious damage to isolate the fault.
    I always check for obvious damage before getting down to the nitty gritty. A lot of times you can isolate the problem quickly that way.
    Look carefully for excessive heat/burn damage.
    (and if you've got compressed air, blow out all that dust. Dust settles on the circuits which causes overheating and early compoent failure. In this case it might also help to locate a problem)
     
  5. davenn

    davenn Moderator

    13,577
    1,866
    Sep 5, 2009
    On that board on the back of the tube, the first 2 wire solder joints (top left) labelled H and H ... these are the heater (filament) connections
    you should be seeing something less than 12VAC ... usually around 6V

    confirm its presence or absence


    Dave
     
  6. muffinman

    muffinman

    17
    0
    Dec 10, 2012
    Hello all,

    To Dave: I tried to measure AC on these points, assuming one is phase and one is null. Got no significant measurement there (~0.2V).

    To shrtrnd: I took halve the thing apart including the power supply board. I did not see anything suspicious. Below are some pictures of the same board as in the first picture in my initial post (front and back). From the back of the metal box a black there exits a thick black wire which goes to the front of the crt. All in all, the board seems to be a bit dirty but otherwise ok.

    Kind regards, Maarten

    [​IMG]

    [​IMG]
     
  7. davenn

    davenn Moderator

    13,577
    1,866
    Sep 5, 2009
    just put the meter probs across those 2 "H" points ... phase or whatever doesn't enter into it

    now, assuming you tested it the way I suggested ? ....

    OK, then the first thing to do is figure out why you have no heater voltage .... trace it back from
    that board, through the wires and connector and then back across the main board....
    If done the usual way, it should just go back to its own winding on the transformer secondary side


    Dave
     
  8. dorke

    dorke

    2,342
    665
    Jun 20, 2015
    to Dave,
    It easier to disconnect the board from the back of the CRT and measure with scope disconnected from mains the 6.3vac winding resistance...

    In general, as a first step in that kind of fault:
    I would look for the filament light from the back/neck of the CRT
    and if not visitable touch(carefully!) the CRT's back/neck to feel if it is worming at all.

    and while at it,
    check the CRT filament resistance with the board disconnected.
     
  9. davenn

    davenn Moderator

    13,577
    1,866
    Sep 5, 2009
    not really

    its even simpler just to see if filament voltage is there ;)
     
  10. mrmagoolew

    mrmagoolew

    4
    0
    Aug 1, 2015
    This may sound silly, but try unplugging the CRT connector and the plug in the first picture. It just might be a dirty connection. Had it happen on a computer power supply, and when I unplugged it and plugged it back in several times, the computer worked again. Just a thought. Lew
     
  11. nepow

    nepow

    99
    1
    Jul 18, 2011
    Failing this advice and checking for the heaters glow you need to take some voltage measurements. However there are hazardous voltages around the CRT socket connector including the final anode voltage connector which is plugged onto the CRT glass envelope on it's own towards the front of the CRT. Be very careful here as this is EHT_ extremely high tension!!!
    I had a similar problem with a Hamex scope, the EHT block had failed. I sold it on as spares/repairs but if the same component has failed you may get a replacement part off Ebay.
    Last point the final anode connector point can still hold a charge long after the scope has been switched off!!
     
  12. mr fixit

    mr fixit

    10
    2
    Jun 23, 2013
    From reading back, the CRT heater has not been eliminated.
    Check looking at the back (PCB end) of the CRT, with the CRO switched ON, for the heater glow - an orange incandescent lamp glow.
    You should be able to see it through that hole in the CRT PCB.
    If there's no glow (and your measurements seem to indicate there wont be), measure the heater voltage - on the PCB tracks - not the CRT pins. If there is voltage there (6.3volts AC usually) THEN measure on the CRT connector pins.
    Why? because allowing for the lo-res picture above, those solder joints look a bit "dry" to me.
    If you do find a dry joint there, I would resolder all the CRT PCB joints, both at the CRT end and where that power supply connector plugs in.
    If there's still no heater voltage, work your way back - carefully, to where it connects to the main power transformer.
     
  13. davenn

    davenn Moderator

    13,577
    1,866
    Sep 5, 2009

    Yeah I have tried to get him to do this, but he hasn't responded as to what he discovered

    until we all hear back from him, it would be better to await a response rather than adding more advice without knowing earlier test results


    Dave
     
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