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How To Measure CRT Voltages? DC or AC?

Discussion in 'Electronic Repair' started by Rock Braud, Apr 2, 2004.

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  1. Rock Braud

    Rock Braud Guest

    I am chasing down a problem with my Emerson 27" model MS2700RDA.

    The picture has about 20 "retrace" lines across it, the image is
    barely visible, and red dominates the color.

    I have done some research and am trying to determine if there is a CRT
    short and if so where it is.

    When measuring voltages on the CRT pins should I have my DMM set on DC
    or AC.
    Also, I am placing the negative lead on the CRT Header Element ground
    pin; is this correct?????

    When I measure the cathodes, Red, Green, and Blue, wrt heater filament
    ground, I get the proper voltages; 155-165 VDCon all three and they
    are usually within 1 volt of each other.

    However, when I measure VDC "across" the heater filament I get 0 VDC.
    But, when I measure it for AC voltage, I get around 2 or 3 VAC. What
    is the correct way to measure heater filament voltage???

    Finally, when I measure Pin 5, a.k.a. screen grid, I get 0 volts, both
    AC and DC settings. Shouldn't I be getting 21-26 Volts? How should I
    measure screen grid voltage?

    I have measured resistence across all combinations of pins. All
    combinations show infinite resistence EXCEPT across the header
    filament AND Pin 4 ("blanK") and the filament which is 150 ohms if I
    recall correctly. Or is it 150 kohms? Anyway what resistence should I
    get across the heater filament and what is Pin 4???


    Thanks.
     
  2. Patch

    Patch Guest

    All voltages except the filaments should be DC, they will be AC.
     
  3. RonKZ650

    RonKZ650 Guest

    All voltages except the filaments should be DC, they will be AC

    Correct, you should have 6vac across the filiments, but you need a decent meter
    to measure it accurately because its at 15.7khz. A cheap meter isn't capable of
    accurately measuring AC at higher frequencys.
     
  4. You need true RMS capability also.

    Tom
     
  5. Is the FBT screen too high? Does it need a colour setup as well?
    I think you can verify a CRT roughly by something like this (I read
    this here years ago, and I don't know how well I remember it, but)...

    After removing the CRT board, apply 6.3 volts to the heater pins on
    the CRT. Measure the voltage with respect to ground on the 3 cathodes.
    They should be roughly the same, and about 1.5 volts (I think) for a
    healthy CRT.
    That's what I'd expect to see with a garden variety (not true RMS)
    meter. The best way is with a scope and/or true RMS meter. If I
    remember correctly, the scope will yield readings of about 27 V P-P.
    If my memory can be trusted at all, 0 is correct.

    Tom
     
  6. Also note that the screen/G2 voltage is from a very high
    impedance source (several hundred Megohms). So, a normal DMM will
    load it excessively. You need a high impedance meter or HV probe.
    The focus is also from a high impedance source and is several kV
    so you need a HV probe. If the focus looks OK, no need to measure it.

    --- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
    Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
    +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
    | Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html

    Important: The email address in this message header may no longer work. To
    contact me, please use the feedback form on the S.E.R FAQ Web sites.
     
  7. Guest

    Thanks, that is exactly what I was looking for.

    Later.
     
  8. Guest

    Not sure I follow. What should I measure for the FBT and what are the
    expected values.

    What is "colour setup??"
    I have measured the cathode voltages and ther were all within 1 VDC of
    each other. However, when I measured again last night the red was 20+
    VDC lower. I plan to re-measure tonight both with the PCB connected
    and not.

    I am not sure what you mean by apply 6.3 volts to the heater pins.
    Isn't this 6.3 VAC or 6.3 VRMS? Did you mean to jumper the PCB heater
    connections to the CRT Pins??
    This is actually very good info for me. I hadn't realized the heater
    filament was being fed by a varying source. I just received my
    Photofact schematics, and sure enought the show a waveform across the
    heater filament which shows 23 volts p-to-p!

    I plan to obtain an RMS meter this weekend to better measure these
    values.

    I also now have the correct pinout on the CRT pins. I plan to
    re-measure tonight to see if things are correct.
     
  9. Arthur W.

    Arthur W. Guest

    The best way to measure CRT voltages is: very carefully!!

    You are dealing with DC potentials of 15 to 20KV/
     
  10. Guest

    Well, I went to Fry's and couldn't find a cheapo Vrms meter. I was
    looking for one of the "old" meter types. I know, I know.

    Anway, I really enjoy building my own tools and was wondering if
    anyone knew of a good schematic for a Vrms meter. I have seen various
    schematics for in-circuit Cap testors and so fourth.

    Anyone can point me in the right direction.

    Thanks.
     
  11. Well, only one of them should be that high - or higher. The ones on the
    neck PCB are 200 V or so for the cathodes, a few hundred for the screen/G2
    and a few kV for the focus. But the last two are extremely low current.

    Still, I agree, very carefully is most definitely important!

    --- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
    Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
    +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
    | Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html

    Important: The email address in this message header may no longer work. To
    contact me, please use the feedback form on the S.E.R FAQ Web sites.
     
  12. If it's to measure the filament voltage, really not necessary. What color
    is the glow?

    --- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
    Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
    +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
    | Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html

    Important: The email address in this message header may no longer work. To
    contact me, please use the feedback form on the S.E.R FAQ Web sites.
     
  13. Guest

    Yes, it is to measure the filament voltage. I get around 2-3 Vdc when
    measured. I am pretty confident it is correct but just wanted to be
    completely sure.

    And, yes there are three "glowing" sets of tiny wires in the neck of
    the CRT.

    Thanks for your help.
     
  14. If they are visibly glowing, they are working well enough to get a picture.

    I don't recall what your original problem was but if it's no picture or
    a color missing, the filaments aren't the cause.

    --- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
    Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
    +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
    | Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html

    Important: The email address in this message header may no longer work. To
    contact me, please use the feedback form on the S.E.R FAQ Web sites.
     
  15. Rock Braud

    Rock Braud Guest

    Hello Sam,

    I agree with you. I am now planning to troubleshoot a possible Screen
    Grid (G1) to Red Cathode short.

    Thanks for your input.
     
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