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Thread: Osborne 1 dead video / schematics

  1. #1

    Default Osborne 1 dead video / schematics

    Hello, I can't find the schematics of this board, coming from an osborne 1:

    photo_2020-07-27_12-40-25.jpg

    photo_2020-07-27_12-40-29.jpg

    photo_2020-07-27_12-40-33.jpg

    photo_2020-07-27_12-40-20.jpg

    photo_2020-07-27_12-40-14.jpg

    I checked the field service manual here:
    http://bitsavers.informatik.uni-stut...ition_1983.pdf

    on page 10 of the appendix (246 of the pdf) there is a schematic of the motorola MD-1000
    which looks different from my board:

    6691-motorola-md1000-190-5-inch-crt-display-ttl-input_6691b.jpg.jpg

    it sports two 555 timers, while my board has only one.

    I need to troubleshoot this unit which was occasionally working, but often the video collapsed horizontally.
    see the video here:
    https://www.youtube.com/watch?v=kQHoRrJifRs

    Now it's completely dead.
    Thanks

  2. #2
    Join Date
    Oct 2016
    Location
    Dutchess County, New York, USA
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    Check for cold solder joints or cracked solder joints and reflow any that look suspect. You might get lucky and need to do no more than that.
    Crazy old guy with a basement full of Pentium 1 laptops and parts

  3. #3

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    Just by looking at the photo of the underside of the board I already see multiple candidates for resoldering.

    BTW if your Osborne power supply still has the original RiFA capacitors in the power supply, flicking the power switch this often could easily trigger them to going into 'smoke escape' mode..

  4. #4

    Default

    I already replaced the RiFA caps, i found them already exploded!
    I tried to fix some of the soldering, but it's still not working. Also, I noticed that 12V and GND on the connector are shorted (they beep with the multimeter) is this by design?!

  5. #5

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    Quote Originally Posted by Elder0010 View Post
    I already replaced the RiFA caps, i found them already exploded!
    I tried to fix some of the soldering, but it's still not working. Also, I noticed that 12V and GND on the connector are shorted (they beep with the multimeter) is this by design?!
    Well a 'dead short' would not be good but then the whole powersupply would go into protection. Did you measure this with the connector on or off ?
    I do not know at what resistance level your multimeter beeps but I suspect it could be in the tens of ohms range which would be normal for a device which draws maybe 1 Amp at 12 Volt. Measure it again on the (low) ohms range.

    These are the solder joints I noticed in your photo, I marked the suspect ones with a red circle.
    osborne_bad_solderjoints.jpg

    In general: all solder joints which show a black circle near the protruding pin are suspect. Most of the time this is caused by thermal (or mechanical) stress on the pin. Big resistors, transistors, transformers and connectors often have these solder problems. Also areas on the PCB which are discolored by high temperatures are prone to having bad solder joints.

  6. #6

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    You don't often see a horizontal scan collapse with the CRT beam visible, in other words a vertical line, because the EHT for the CRT is generated by the H scan output stage. On the other hand when the vertical scan collapses you see a horizontal line because the H scan and CRT's EHT is still running as are the sub-circuit that support the CRT's gun voltages.

    You need to get the schematic for the VDU.

    It looks like it failed, so it could be that the horizontal output transistor failed or perhaps the drive to it went missing and the EHT charge storage just stayed up just long enough to see the CRT beam and to witness the event of the H scan dying, and you saw a vertical line, briefly.

    If you can get the monitor schematic, I could suggest some things to check. The circuitry in these small VDU's is very straightforward and fairly easy to fault find.

    I have seen these particular small VDU's on ebay for sale and wondered what they might have been used in. There are some very similar ones made by Panasonic.

    It would be very important to find out of this VDU has its own independent H scan oscillator (probably it would ) unlike say the IBM5151 that does not.

    Does this monitor have an H.Hold control preset potentiometer ? If it does, then you can be fairly certain that the fault is in the monitor itself and not in the video/sync signal originating from computer.

  7. #7

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    ...I looked at the schematic with the two 555 timers, one delays the sync so as to make an H position control, the other is the H osc. They probably got rid of the first one in some models, and just synchronized the other 555 H osc directly with the sync and had an H Hold control with that, which would also act as the H position control to a more limited extent.

    If it is still not working after the bad solder joints fixed up, the first thing to do with the monitor powered, check for an approximate 15,750 Hz waveform at the output (pin 3) of the 555 IC and that it has normal 12V power on its power pin/s.

  8. #8

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    @gertk: thanks! I didn't spot all of them!

    Unfortunately, resoldering the solder joints didn't fix the issue!

    @Hugo:
    I was looking for the correct schematics because I hadn't luck finding them!

    The only 2 potentiometers that I see are for the focus and the vertical size!
    The 555 is getting the 12V supply, and here is the output of pin3:

    photo_2020-07-28_20-15-37.jpg

  9. #9

    Default

    Quote Originally Posted by Elder0010 View Post
    @gertk: thanks! I didn't spot all of them!

    Unfortunately, resoldering the solder joints didn't fix the issue!

    @Hugo:
    I was looking for the correct schematics because I hadn't luck finding them!

    The only 2 potentiometers that I see are for the focus and the vertical size!
    The 555 is getting the 12V supply, and here is the output of pin3:

    photo_2020-07-28_20-15-37.jpg
    That is probably ok.

    With a x10 scope probe, and the scope set on 50V/cm, put the probe on the collector of the horizontal output transistor (this is ok in a small VDU like this, the expected peak voltage is only about 100v, but it is not ok in some sets like color monitors where it can be 1kV and needs a special probe). That waveform will tell us if the H output stage is working properly or not. It should look roughly like the positive half of a sine wave with a base around 10us long and a period of around 63uS).

  10. #10

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    While doing some oscilloscope tests with iz8dwf (which is a friend of mine IRL), we discovered that there was a short preventing the horizontal output transistor to work properly.
    I removed the short, and measured the signal on the collector pin:
    Screenshot-(510).jpg

    (it was a random waveform before).
    Still no output on video, we will continue the tests tomorrow!

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