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Thread: IBM 5153 - part of image missing

  1. #1

    Default IBM 5153 - part of image missing

    Hi,

    this is my first post here but I have been reading this forum for quite a while. So far I've always found a solution to most problems, but, as you can imagine, not this time!
    As this is my first post, I'm not able to post any pictures, without which this thread is somewhat useless as it's very difficult to describe the problem. I didn't find any rules that say anything about the post count required to post images, but I suppose I have to start somewhere.so here it goes...

    I have finally managed to get a 5153. The seller said that it had a strange problem where after a while the top right part of the image would disappear. I really had no idea of what he meant, so I got it.

    Well now that I have it I still have no idea of what's going on, but the problem seems to be there right after being switched on. It appears that there is no raster in the top part of the screen. But it's not a straight line either, more of a wavy line where no colour is displayed. Only about an inch on the left hand side, but a good 2-3 inches on the right hand side. Strangely, text is somewhat visible but rather faint. Adjusting the vertical size seems to move the problem too, and stretching the image vertically seems to bring colour into affected area too.

    Strange! Well maybe not.. if someone wants to explain, I'm all ears.

  2. #2

    Default

    It is not possible to answer this question with any degree of certainty without a picture of the problem. If you can find out how to post an image of it (usually just click on "go advanced" and "manage attachments") most likely a number of the forum members would know what was going on.

  3. #3

    Default

    Just post the pic(s) elsewhere and give us the link.
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  4. #4

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    For some reason I wasn't looking at the full site and therefore didn't see any options for attachments... D'OH...

    Here are some pictures of the fault.

    1.jpg2.jpg3.jpg

    Picture 3 show the monitor without any input.

    I have taken some videos too so I'll try and put up a link to those later.

  5. #5

    Default

    Ok, You will need the schematic.

    The middle image shows what is going on. The raster scan is still present and the CRT's beam is being over blanked after the vertical flyback with a non rectangular vertical blanking pulse whereby its trailing edge is lengthening a little over the horizontal scan time frame, giving the tilt. All three beams are being blanked about equally. So what you are seeing appears to be a defective vertical blanking pulse.

    (The application of both the vertical and horizontal blanking is to blank or extinguish the picture tube's beam current and brightness during vertical retrace and horizontal retrace. In the 5153, the transistor that does this is Q202, and its output is coupled into the Red, Green and Blue CRT gun amplifiers by three diodes).

    Q202's input comes from both the horizontal scan circuitry and the vertical scan circuitry. Since the three beams are being blanked about the same, and the H blanking looks ok, I think most likely Q202 & the diodes are ok and there are also some clamp diodes in the base circuit of Q202 that need checking. There may be a defect is the H blanking too as shown by the wedge of brightening on the lower left, but likely there is only one fault.

    The first component I would check is C318 (could be 316 hard to read on my schematic), a 3.3uF 160V electrolytic cap that couples the vertical blanking signal via diode D309, it could be leaky, possibly. After that I would check the diodes D309, D201,D202,transistor Q202 to find out where the fault is. It would be straightforward with a scope.

    I am reminded of something here:

    On one of my 5153 monitors, it developed a blanking issue (not exactly the same) but it turned out that the blanking transistor Q202 had a disconnected emitter. The reason was this transistor and the blanking components are close to the front of the pcb (closest to the CRT face roughly near the center area) and there is a vulnerable area of pcb trackwork there that had cracked, near a screw, along with a small part of the pcb, because the monitor had been dropped. So, it would be worth a good physical inspection of the area and tracks and components around Q202 and the pcb near the screw.
    Last edited by Hugo Holden; October 13th, 2018 at 05:23 AM.

  6. #6

    Default

    Thanks for the detailed reply. That's good stuff. My component level knowledge of CRTs is fairly limited but what you are saying makes sense to me. Some of the stuff I need to get my head around first.

    I now have the circuit board sitting on my bench and I'm going to give it a good visual inspection first and touch up any dodgy solder joints.

    More troubleshooting to follow.

    I've also uploaded a video that shows the fault in action. I hope this will confirm your diagnosis.

    https://youtu.be/0ux52Rxr9FU

  7. #7

    Default

    As my posts are still being moderated, there is a bit of a delay in seeing my posts.

    Just an update: C318 checked out OK, but after a visual inspection D309 showed badly corroded legs. Testing it in circuit didn't give any proper results so after desoldering it just crumbled to pieces. Any further testing showed it was definitely gone now!

    I'm now looking for a replacement diode but so far can only find Zeners and rectifier diodes in my component box. I assume a rectifier diode is just not fast enough so I'll keep looking; if I can't find anything I'll have to order some.

    Will keep you posted about the results.

  8. #8

    Default

    I just checked D309, it is rated at 200V reverse voltage so I'll definitely have to wait until I get some delivered. I don't want to chance a 100V 1N4148, which is very borderline looking at the 5153 schematic.

  9. #9

    Default

    Hi,

    Sounds like you are onto it. The 1N4148 is not suitable also it needs to be a low capacitance fast switch diode or the leading and trailing edges of the blanking pulses could get slowed down, so the usual junk box 1N4007 etc is not suitable either.

    The diode I would recommend as a replacement is the BAV20 or BAV21, 200 & 250 V rated fast switching:

    https://www.vishay.com/docs/85543/bav17.pdf

    Probably these are easy to get from the usual suppliers or Ebay.

    Also, thinking about what you said, it raises the question why the diode was so corroded. Check nearby for any leaked electrolyte from electrolytic caps. They can still test ok with some leakage. To see if they have leaked, remove any suspect ones and put the meter probe tips (on Ohms range) onto the capacitor's rubber base, if there is any conductivity, the cap has leaked in the past.
    Last edited by Hugo Holden; October 14th, 2018 at 04:00 PM.

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