UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Computers

Notices

Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

Reply
 
Thread Tools
Old 10th Nov 2019, 2:04 pm   #1
ct92404
Pentode
 
Join Date: Sep 2015
Location: San Bernardino, California, USA.
Posts: 101
Default Osborne Executive monitor problem

Hi all,
I just recently bought a vintage Osborne Executive computer. It's in fairly good cosmetic condition, and it is mostly working...but it definitely has some issues. I don't know how well Osborne computers sold in the UK (from what I've read, the company itself didn't last very long). But I'm hoping maybe someone here can help!

Luckily the computer came with some disks, including the master disk with CP/M and BASIC. At first, it was working ok and was able to read disks, load programs, and even format disks fine. But I noticed the front cover was a little loose and sometimes the computer would turn off and restart if it was moved. I decided to open the case to see if I could find any obvious problems like loose connections, etc. (Maybe that was a dumb idea?) I realized that someone had definitely messed with it at some point. I saw a couple of jumper wires on the top side of the motherboard...one wire was just loosely clipped between two IC chips? What the heck?! It was just a clip lead, like what you would use for testing something with a meter! That wire just came off. The other wire was soldered in place, and I decided to leave it alone for now.

I put the computer back together, and now unfortunately the built-in monitor is having problems. The image is slowly rolling, as if the vertical hold is off. But I don't know if there is a vertical hold adjustment, I don't see a knob or anything anywhere. I connected an external tv to the composite video jack on the computer, and the image was ok. So apparently, it's just a problem with the built-in monitor.

Does anyone have any idea what the problem might be? Is there any way to adjust the vertical hold? I don't know what the heck someone was trying to do with that jumper wire clipped on the motherboard, but if they were trying to "fix" an earlier problem, that seems like a pretty dumb way to do it!

- Chris
ct92404 is offline   Reply With Quote
Old 10th Nov 2019, 4:13 pm   #2
Refugee
Dekatron
 
Refugee's Avatar
 
Join Date: Apr 2012
Location: Worksop, Nottinghamshire, UK.
Posts: 4,265
Default Re: Osborne Executive monitor problem

You must have found the vertical sync wire and detached it from the position it was bodged to when the manufacturer changed there monitor supplier.
It is common to see things like this in products that are not made in vast numbers.
Refugee is offline   Reply With Quote
Old 10th Nov 2019, 7:19 pm   #3
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 3,178
Default Re: Osborne Executive monitor problem

Hello Chris
The case history so far seems to be -

-You got it and it was working.
-You took it apart to rectify a physical / case problem.
-You accidentally changed something (removed that link)
-Now you have no vertical hold

If you were Sherlock Holmes you would probably see this as a likely case of cause and effect, so it seems you need to try putting that link back where it was before you do anything more complicated. I'm guessing you didn't take any photos of the insides before this happened so you may not know exactly which two pins the link should be on.

Have you tried looking around for internet images of Osborne main boards to see if there are any showing this link in position?

Should also ask if you have tried asking about this on the Vintage Computer Forums:-

http://www.vcfed.org/forum/forum.php

Unlike this forum, VCFED is entirely dedicated to retro computers so I would also try posting your problem over there as well (apologies if you already have).
SiriusHardware is offline   Reply With Quote
Old 11th Nov 2019, 3:14 am   #4
ct92404
Pentode
 
Join Date: Sep 2015
Location: San Bernardino, California, USA.
Posts: 101
Default Re: Osborne Executive monitor problem

I just can't imagine the company putting in that jumper wire. I mean, it was just a clip lead stuck on between two chips! It seems so lazy and sloppy. I'm pretty sure of one chip that it was connected to (the other jumper wire is soldered to it). But I don't know which pin or the other chip that it was connected to.

I guess that wire was obviously for vertical sync, but I just don't understand why anyone would "fix" something in such a haphazard way.
ct92404 is offline   Reply With Quote
Old 11th Nov 2019, 6:14 am   #5
TonyDuell
Dekatron
 
Join Date: Jun 2015
Location: Biggin Hill, London, UK.
Posts: 3,393
Default Re: Osborne Executive monitor problem

I wonder if that kludgewire was part of a 50Hz/60Hz conversion. Perhaps it grounded a pin on an input port to get the CPU to set up the video side for 50Hz. A user who couldn't solder added it, it was not the official way to do it. And then the vertical hold preset in the monitor was tweaked, so it locked to the 50Hz vertical frequency.

I'd be inclined to mark the current possition of the vertical hold preset and then give it a cautious twiddle. You may find the video monitor jumps into lock.
TonyDuell is offline   Reply With Quote
Old 11th Nov 2019, 9:58 am   #6
ct92404
Pentode
 
Join Date: Sep 2015
Location: San Bernardino, California, USA.
Posts: 101
Default Re: Osborne Executive monitor problem

Mystery solved! I figured it out!

I posted a topic in Vintage Computer Forums, and a member gave me a link to diagrams. One of the pins with a jumper wire was indeed part of the vertical sync circuit. I also got a clue from seeing a listing on the diagram for "Spare Gates," which I assumed were unused pins on IC chips. It turns out that the jumper wires were connected to another chip which apparently had unused pins and one of the "spare gates." So I took a guess that what happened is that at some point a gate failed in the chip and someone decided to make a quick and dirty fix by connecting it to a gate in the other chip instead of just replacing it.

I did a quick test and connected pin 5 and pin 6 of the "bad" chip to pin 5 and pin 6 of the other chip with thr "spare gate", and that fixed it! The image on the internal monitor looks normal!

What an annoying mess! But at least it wasn't something more serious!

Probably the best thing to do would be to just replace that chip.

If anyone is familiar with the Osborne Executive computer, UF 7 is the chip that apparently partially failed. It's a 7406 chip. I don't know much about logic circuits, so this is all still kind of new to me! I was able to read the schematics though and pretty much figure it out.
ct92404 is offline   Reply With Quote
Old 11th Nov 2019, 10:45 am   #7
JohnBHanson
Hexode
 
Join Date: Aug 2009
Location: Worthing, Sussex, UK.
Posts: 451
Default Re: Osborne Executive monitor problem

I have had a similar fault with 7406 chip on a different machine. Easiest fix was to solder a good 7406 chip over the top of the first one. Worked a treat as the outputs are open-collector and can be wired together wtihout problem.
JohnBHanson is offline   Reply With Quote
Reply

Thread Tools



All times are GMT +1. The time now is 3:02 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2019, vBulletin Solutions, Inc.
Copyright ©2002 - 2019, Paul Stenning.