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Old 10th Nov 2019, 2:04 pm   #1
ct92404
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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
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Old 10th Nov 2019, 4:13 pm   #2
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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 their monitor supplier.
It is common to see things like this in products that are not made in vast numbers.
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Old 10th Nov 2019, 7:19 pm   #3
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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).
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Old 11th Nov 2019, 3:14 am   #4
ct92404
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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.
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Old 11th Nov 2019, 6:14 am   #5
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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.
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Old 11th Nov 2019, 9:58 am   #6
ct92404
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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.
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Old 11th Nov 2019, 10:45 am   #7
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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.
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Old 15th Feb 2023, 12:42 am   #8
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Default Osborne Executive monitor problem (again)

Hello all.

I was going to send the following message to Ct92404 but was prevented from doing so until I have posted one message so posting the same message.

I have an Osborne Executive from the US recently but it was dead when I got it. After spending a few days on it, it is almost fully working except that the V-Sync is constantly scrolling down the screen.

I've read your post of a similar problem but don't know which IC and pins that I need to joint to resolve the problem. I would be grateful if you would advise me please. I have the Zenith 7" circuit diagram. I even put a socket on UF7 but the 74LS06 seemed to be tested fine and replacing it with a new one didn't resolve the problem of the rolling v-sync problem. An external monitor works fine with a rock solid display.

Any help would be grateful appreciated. Thanks!

Regards,

Kwong Li
London, UK (230V 50Hz)
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Old 15th Feb 2023, 2:45 am   #9
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Default Re: Osborne Executive monitor problem (again)

Looks like ct92404 hasn’t been active on the forum since June, maybe sending him a PM will generate an email.

Last edited by Station X; 15th Feb 2023 at 9:17 am. Reason: Threads merged.
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Old 15th Feb 2023, 2:52 am   #10
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Default Re: Osborne Executive monitor problem (again)

Did you try the vertical hold preset?
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Old 15th Feb 2023, 5:46 am   #11
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Default Re: Osborne Executive monitor problem (again)

Have you tried an external composite video monitor connected to P10? If the vertical sync locks with that, it means most of the logic on the mainboard is working.

Do you get the vertical sync signal at pin 6 of UF7 on the mainboard?

Unfortunately my copy of the Osborne field service manual is missing the 7" monitor schematics. Did you find them on-line somewhere, if so where?
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Old 15th Feb 2023, 9:19 am   #12
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Default Re: Osborne Executive monitor problem (again)

Quote:
Originally Posted by Mark1960 View Post
Looks like ct92404 hasn’t been active on the forum since June, maybe sending him a PM will generate an email.
ct92404 accepts emails via the forum. Left click on his username to the left of his posts.
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Old 19th Feb 2023, 8:22 pm   #13
Happy Oldie
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Default Re: Osborne Executive monitor problem

Sorry about the delay and thank you all for the responses.

I believe that ct92404 has corrected the UF8 V-sync not gate problem but is probably not the problem that I am experiencing.

Oddly, there is no V-sync/hold on the monitor. I need to check UF7 pin 6 for the enternal V-sync signal. I know the signal paths are slightly different but the external V-syn is fine and I get a solid picture on an Apple IIc monitor. It is looking likely to be problem inside the internal monitor.

The internal 7" Zenith monitor would be a pain to probe due to the way the PCB is recessed in the cage and the stupidly short cable to the motherboard. The CRT circuit is missing from the serrvce manual as TonyDuell said but I did find a Sams manual with an almost identical Zenith CRT V-sync circuit with very useful scope images.

https://ia903001.us.archive.org/21/i...%20Monitor.pdf

I will be having a good check on the Zenith CRT if I can somehow manage to run it outside the Osborne Executive case.

Thanks to all for replying!

Regards,

Kwong Li
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Old 21st Feb 2023, 12:29 pm   #14
TonyDuell
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Default Re: Osborne Executive monitor problem

That manual I do have as it's the 5" (or so) monitor used in my Osborne 1A.

It may be worth taking the time to make up an extension cable so you can run the monitor on the bench. I've made many for such purposes over the years and they've always saved time and hassle in the end.
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Old 21st Feb 2023, 11:28 pm   #15
Happy Oldie
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Default Re: Osborne Executive monitor problem

I have three Osborne 1As too but two of the keyboards are suspect. The problem with these membrane keyboards is that I couldn't simply replace a key or two like I could with an Apple II or Commodore PET. One keyboard has had something heavy smashed into it in the past with a few keycaps and stems missing or broken. The keyboard frame was bent. Even with the frame bent back (quite hard to do due to the thickness of the metal), I am getting a stream of characters on start-up (the infamous flashing on the start up screen). I might have to disassemble the entire keyboard which would be a long and tedious job.

Regarding the Executive, I only have one and it is in very good condition. I believe due to a manufacturing defect on the PSU, the computer was very intermittent and mostly wouldn't power up in it's early life. I eventually spotted the hairline crack around the 12V pin header to the motherboard. A healthy dose of solder with the others reflowed just in case and restored the PSU and the computer operational state. If only I have sorted the internal monitor, the machine would be fully working. I am alway wary on working inside CRTs.

I would make up the extension cable as you have suggested, thanks.

Regards,

Kwong Li
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Old 11th Mar 2023, 9:35 pm   #16
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Default Re: Osborne Executive monitor problem

Hello.

After many hours of troubleshooting, I have managed to fix all the problems on the Osborne Executive. The following are what I have done, in case it is useful to others with similar problems.

1. Replaced the RIFA capacitors with X2 ones as normal.

2. Corrected the no 12V power problem - power light is on but no sign of life. It turns out to be a all round hairline crack on the solder side of the PSU DIL +12V pin. Measuring the pin would get zero volt but the track side would show the normal 12V. Reflowed the solder corrected it. It pays to remove the PSU first and not just measure in place on the component side.

3. The CRT was continuously scrolling down. The V-Sync pulse is there on pin 9 of the CRT connector and an external monitor is working fine. Checked through all the resistors, capacitors and transistors from pin 9 to the CRT tube and all were fine except the CR301 zener. This was shown to be bad with forward and reverse of around the same voltage. The marking on the zener is Motorola 279-23218 but couldn't find the part anywhere. Tried a couple of guesses (1N4736 and 1N4737) but not only they didn't stop the scrolling, the screen display collapsed into a single vertical line after around 30 seconds. Eventually asked a friend (IZ8DWF, the Youtube fame) who in turn ask someone with an Exec to check. It turns out that the zener is 13.8V 0.5W. As 14V is easier to get, replaced the faulty one with a Multicomp 14V 0.5W zener and the CRT is now working fine without scrolling.

The Osborne Executive is now fully working. Onto the next retro fix.

Regards,

Kwong Li
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Old 12th Mar 2023, 1:05 pm   #17
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Default Re: Osborne Executive monitor problem

Kwong Li, thanks for coming back to us to let us know the eventual cause and outcome, and also the actual value of that diode.

The way you were able to have someone measure up the mystery diode for you on their working machine is a great example of the worldwide spirit of co-operation which exists between owner-drivers of old systems like this.
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