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Thread: Osborne 1 & Executive dead CRTs ?

  1. #1

    Default Osborne 1 & Executive dead CRTs ?

    Sorry, I had posted this in the wrong forum (Vintage Hardware Review) and wasn't getting any responses so I moved it to this forum:

    Greetings all and thank you for your help in advance. This is my first post here !!

    I just picked up 2 Osborne computers. 1 is the original Osborne 1 and the other is the Osborne Executive. Both coincidentally have the same exact issue. I can not for the life of me get the CRT's on either of them to work. There is no high voltage or neck glow on either. I confirmed that both units have working main boards as I can use an external monitor. I only have 1 shunt. It is the shunt for the Osborne 1 that has the composite out connector on it. This shunt when installed connects the main board the the CRT. Here is what I know so far:

    I get 12VDC at the CRT board.
    I get about 12Khz on my oscilloscope on the Horizontal circuit.
    1 can see adjustments being made on an oscilloscope when I move the brightness control.
    I have done a complete electrolytic cap replacement on the CRT board (OCC2) except for 2 caps as I didn't have them. I pulled and tested and they tested good.
    I checked the flyback on the OCC2 monitor. I removed it from circuit and used 18VDC and a VOM and got 12VDC across the primary. I would think if this was bad, it would be an open circuit ?

    Both CRTs have the same issue. I am not sure why they just don't power on when being supplied with 12VDC. I think I read somewhere that these TTL monitors require a horizontal sync to power on which is present.

    Is this a common issue with these ? Is there a component I can check right away that might be the culprit ? Find it strange that both have same issue.

    Thanks !!

  2. #2

    Default

    Quote Originally Posted by MaCJaX View Post
    Sorry, I had posted this in the wrong forum (Vintage Hardware Review) and wasn't getting any responses so I moved it to this forum:

    Greetings all and thank you for your help in advance. This is my first post here !!

    I just picked up 2 Osborne computers. 1 is the original Osborne 1 and the other is the Osborne Executive. Both coincidentally have the same exact issue. I can not for the life of me get the CRT's on either of them to work. There is no high voltage or neck glow on either. I confirmed that both units have working main boards as I can use an external monitor. I only have 1 shunt. It is the shunt for the Osborne 1 that has the composite out connector on it. This shunt when installed connects the main board the the CRT. Here is what I know so far:

    I get 12VDC at the CRT board.
    I get about 12Khz on my oscilloscope on the Horizontal circuit.
    1 can see adjustments being made on an oscilloscope when I move the brightness control.
    I have done a complete electrolytic cap replacement on the CRT board (OCC2) except for 2 caps as I didn't have them. I pulled and tested and they tested good.
    I checked the flyback on the OCC2 monitor. I removed it from circuit and used 18VDC and a VOM and got 12VDC across the primary. I would think if this was bad, it would be an open circuit ?

    Both CRTs have the same issue. I am not sure why they just don't power on when being supplied with 12VDC. I think I read somewhere that these TTL monitors require a horizontal sync to power on which is present.

    Is this a common issue with these ? Is there a component I can check right away that might be the culprit ? Find it strange that both have same issue.

    Thanks !!
    I guess you already have found this:
    http://www.bitsavers.org/pdf/osborne...ition_1983.pdf
    Didn't you?
    Starting on page 236, there's the manual for the Motorola MD1000-190 that I assume was used in the Osborne 1.
    There're theory of operation and schematic for the monitor PCB.
    Yes, the high voltage is generated by the HSYNC signal anyway as it seems with a very quick glance.
    If you have some test equipment, we can help checking what's going on with those monitors.

    Frank

  3. #3

    Default

    I just looked at the monitor schematic. The monitor has its own H scan osc. It will run with no H sync and no V sync input either. The V oscillator and H oscillator only synchronize to the sync signals, the syncs do not have to be there for it to free run.

    With no video though the screen would be blank to black, but if the monitor is working, you should likely see the scanning raster appear with the brightness control turned right up, even with no video signal input, if the monitor is working.

  4. #4

    Default

    Guys, thank you so much for taking the time to help me. It's greatly appreciated !!

    So I have an update. The smaller 5" CRT from the Osborne 1 is now working. I did a complete cap kit first before troubleshooting and to my surprise, it works perfectly now. As for the larger Osborne Executive CRT. That one I initially did a complete cap kit on the board but it still has no video. I do have neck glow now but definitely no high voltage. I have tested it's flyback and CRT with my now working board from the Osborne 1 and it works. I get the Osborne screen so I can rule out the flyback and CRT as the issue. I started checking transistors and resistors which I though might be the problem but they all test fine. I'm wondering if the issue might be the NE555N timer IC. I will check to make sure its getting voltage and its outputting a clock. Besides this IC, I really don't know what else to check. I will test it tonight and report back.

    Thanks, Again !!

  5. #5

    Default

    It is much easier to help if you can post the schematic of the particular monitor you are dealing with then also indicate where on the schematic you have tested it and how you have tested it.

    For example when you say "no high voltage" its far too vague. There are a number of high voltage points derived from the secondary of the H output transformer to test, as well as the EHT (Extra high tension), but you might not want to go under the CRT's anode cap without the right kind of EHT probe.

    So if you could provide more detail of the tests and the schematic on one post, it should be possible to get to the bottom of the fault.

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