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spaghetticode
April 26th, 2017, 12:00 PM
Hi, I recently bought an Osborne portable computer, which seems to be working fine except for the monitor: at powerup the boot screen image scrolls vertically fast, so I am assuming there is something wrong in the vsync circuit.

I have very basic electronics skills, but I have access to a DMM and oscilloscope, so if properly guided I think I may be able to eventually fix the computer and enjoy it. Does anybody know where to start? I've dowloaded a PDF titled "Osborne 1 technical manual", which has schematics that may be helpful for the task.

Thanks,
Andrea

JonB
April 26th, 2017, 10:48 PM
Hi

This might be a better source of information: http://bitsavers.informatik.uni-stuttgart.de/pdf/osborne/2F00040-00_Service2ndEdition_1983.pdf

Check first for adjustment. You're looking for a trimmer potentiometer on the monitor PCB called "V-Hold" or something similar. You should be able to see it on the schematic. It will probably look like this http://p.globalsources.com/IMAGES/PDT/BIG/728/B1053031728.jpg although there are lots of different types. Use a small flat bladed screwdriver to adjust until the screen rolling has stopped.

Usual disclaimers when dealing with CRTs apply:
- Don't do it unless you're confident.
- Don't touch any metal parts inside the CRT chassis.
- Stay well away from the HT lead (the fat wire plugged onto the side of the CRT tube) and transformer, there are very dangerous voltages.
- Use an insulated screwdriver and do not touch anything else other than the trimmer.
- If you have a long screwdriver that means you don't need to put your hand inside the chassis, use it.
- Consider wearing rubber gloves, just in case (these may not protect you from the HT voltages, but will help if you accidentally touch something else that you shouldn't).

Regards
JonB

gertk
April 26th, 2017, 11:40 PM
I think you need to adjust V-Hold if the screen is scrolling vertically

JonB
April 27th, 2017, 06:33 AM
Thanks, gertk - post amended. Couldn't find H-HOLD trimmer on the schematic but it has V-HOLD. Duh... (slaps head).

Also.. there's a picture of the CRT chassis in the service manual. There isn't much clearance if the V-HOLD is somewhere in the middle of the PCB (but good designs tend to locate these trimmers at the edges - I couldn't make out where it was from the poor quality picture). Be careful in there.

gertk
April 27th, 2017, 10:58 AM
My Osborne is in parts at the moment, already got a bunch of sockets for the RAM chips of which some are bad.
Now I just have to get myself to desolder those 32 chips, put in sockets and go test each of the 4116 chips.
Already made an adapter with the extra supply voltages for my Atmel 328 based 4164/41256 DRAM tester so I can test 4116 DRAMs

spaghetticode
December 9th, 2018, 12:36 PM
Unfortunately there is no VSYNC trimpot for this monitor.

After a long time I was able to confirm the issue with my monitor is in the monitor PCB electronics by swapping the board. Unfortunately I could not find any significant issue with diodes, transistors and electrolytic caps. The only voltage issue I found is 120V instead of 140V on the red/orange cable that goes to the CRT.

Here is the monitor schematics, if somebody more experienced would like to check it: http://www.vintage-computer.com/SAMSOCC1/Sams%20-%20Osborne%20OCC1%20-%20Monitor.pdf

Next week when I find some time I'll try to check also scope waveforms if it's not too complicated given the small spaces and high voltages involved.

spaghetticode
December 23rd, 2018, 02:12 PM
I finally had enough time to troubleshoot the monitor properly. It's been hard, it took me quite a few hours, but I did it. The SAMS manual helped me a lot, without it I think I would have never succeeded.

I found the issue by looking at the waveforms on the oscilloscope, I was able to diagnose CR301 was not working properly, as it was degrading the signal.

I replaced it with a 9V zener and now the monitor works fine. I'm not sure about the replacement part, the manual says it's a 279-23218 diode, but there's no reference online with that number. On one side of the diode there's 4.5V, on the other there's 11V.

spaghetticode
December 27th, 2018, 01:59 PM
Dead moderator,

a few days ago I elaborated a reply to this thread, posted it and it's not yet visible. I'm wondering if you ever actually got it. I'm not implying this forum does not work properly, maybe it was my fault, but I was not informed about the issue.

I can't find a "help" section, I can't find the list of my messages yet to be moderated, that list may be helpful to see what's still pending and if the message was actually delivered to the server or what. And I wonder why, after writing so many messages (I guess I'm at about 10 on this forum) you still need to moderate me. But hey, that's your forum, so you can have your rules, of course :)

mbbrutman
December 29th, 2018, 11:40 AM
Dear spaghetticode,

Thank you for your note. The moderators are not dead; at least as far as we know. They do get busy with work and real life at times.


Your message has been approved. After a few more messages you will no longer find your message being held in the moderation queue.


Regards,
One of the undead Moderators ...


PS: Do not take this too seriously. As bad as moderation is, the spam problem is worse ...