Image Map Image Map
Page 1 of 2 12 LastLast
Results 1 to 10 of 13

Thread: CRT width adjustment...

  1. #1

    Default CRT width adjustment...

    I picked up a NEC JC-1215MA off of eBay in really nice condition to match my PC-6001A. One thing I'd like to tweak is the horizontal size if possible. There is a V.SIZE adjustment and I found some horizontal adjustments like H.CENTER and H.HOLD, but I don't see a H.SIZE. I carefully pulled the case (I know there is HV inside and have worked around monitors before), but on the chassis/pcb, I don't see any sort of adjustment for this. Any hints on where it might be? There is a large white block that I think is the flyback and it has a couple of pots mounted near it, one for focus and one for screen which I think is the master brightness...

  2. #2
    Join Date
    Jan 2007
    Location
    Pacific Northwest, USA
    Posts
    32,686
    Blog Entries
    18

    Default

    I don't have the prints for your monitor, but width is very often controlled by a coil with a slug that can be moved (using a nonmetallic adjustment tool).

    Here's a video that explains it

    This is assuming, of course, that you've checked the power supply voltages.

  3. #3

    Default

    Quote Originally Posted by alank2 View Post
    I picked up a NEC JC-1215MA off of eBay in really nice condition to match my PC-6001A. One thing I'd like to tweak is the horizontal size if possible. There is a V.SIZE adjustment and I found some horizontal adjustments like H.CENTER and H.HOLD, but I don't see a H.SIZE. I carefully pulled the case (I know there is HV inside and have worked around monitors before), but on the chassis/pcb, I don't see any sort of adjustment for this. Any hints on where it might be? There is a large white block that I think is the flyback and it has a couple of pots mounted near it, one for focus and one for screen which I think is the master brightness...
    Mostly, the width control (coil) that ChuckG describes is either on the main PCB or occasionally it is attached to the Yoke. In both cases it is a coil in series with the H yoke deflection coils.

    Chuck G's remark about the power supply voltage is very important. Aside from a series inductor, the width (all other things equal) is directly linearly proportional to the power supply voltage that runs the H deflection output stage. So if that voltage is reduced it is possible to have insufficient width, but still enough range on a height control to get normal height. So it is worth checking the power supply voltage is normal before adjusting the width inductor.

    (Of interest if you look at the rise of current in an RL circuit with time after the voltage is applied, it is the usual inverted exponential form. However, if you look at the first part of the waveform, which is substantially linear, the part used for H scan in a video monitor and you differentiate the usual equation for the rate of current rise with time in an RL circuit with a step voltage applied, you will find that it is equal to V/L where V is the supply voltage and L the inductance)

    A few extra details: In practice of course if the power supply voltage feeding the H output stage is reduced, it doesn't result in an exactly linear reduction in width of the screen raster. The reason being the the EHT drops too and as it does it encourages a larger scanning raster. The size of the raster is inversely proportional to the square root of EHT voltage in magnetic deflection systems, so this partially compensates the width reduction if the power supply to the H output stage is dropped to a lower value.

  4. #4

    Default

    Thanks guys. I was looking for the coil, but didn't see it. I did not check the yoke though so maybe it is there. I took some pictures last night when I had it apart, but they turned out blurrier that I wanted. I'll post them in a moment.

    It is entirely possible that the monitor width is adjusted properly - the PC-6001 has always had wide borders on every TV I've tried it on. I'd like to make the picture a little bigger if possible.

  5. #5

  6. #6

    Default

    Quote Originally Posted by alank2 View Post
    It is entirely possible that the monitor width is adjusted properly...
    Yes, it is possible. FWIW, that display also looks quite similar to that of the NEC Multisync in TTL mode.
    PM me if you're looking for 3" or 5" floppy disks. EMail For everything else, Take Another Step

  7. #7

    Default

    Is that white block on the yoke the coil that could be adjusted?

  8. #8

    Default

    One thing that is not entirely clear, is it the actual scanning raster that has reduced width (& height) or is it the picture content in that raster, the photo does not allow this to be checked. To see the edges of the scanning raster one can turn up the brightness control.

    If it is the actual scanning raster that is smaller than it should be and there is no actual width inductor (some sets do not have one) then the HT supply to the H deflection stages needs to be checked.

    If you had a schematic it would be easy to see if there was a width inductor in the circuit or not and what the supply voltages should be.

  9. #9

    Default

    I don't have the schematic for it unfortunately. In the picture above, the height is about where I'd want it - if I could get the width to around the same level of border, I'd prefer that.

  10. #10

    Default

    I can't see clearly how things are mounted but if one can more to yoke back without moving the centering magnets, that will increase the horizontal and vertical spread. You can readjust the vertical after. This works until the beam hits the sides.
    Dwight

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •