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[Osborne 1]: Osborne 1 CRT functional, but very bright dot appears upon machine power-off

zeroandone211

zeroandone211

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Aug 6, 2023
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Hello, world! I recently acquired an Osborne 1A, which came with the double density card installed. Unfortunately, I lost my B drive (unsure why, I attempted to repair it, but just ended up altering the speed of the spindle, and cannot get it back to the required RPM, but have ordered a GoTek to use with the Osborne. Anyway, when I first powered the machine on, then off, a very bright dot appeared in the middle of the CRT, and stayed there for about half a second before slowly losing its glow, and then going away completely. Does anybody perhaps know how to fix this problem? I just watched Adrian Blacks' video on fixing a CRT spot on a Commodore PET so I'm curious to know whether or not this can be performed on an Osborne CRT. Also, when I first received the Osborne 1A, the picture on the CRT was very obviously tilted upwards (pointing upwards towards the right upper side of the CRT). I made some adjustments to the raster rings, but unfortunately the CRT yoke is actually glued into place (fixed yoke), and this prevents me from making any adjustments to fix this. Is there any other way to fix this problem? Adjusting the raster adjust rings did make a significant difference, but I can still tell that it is slightly tilting upwards. Thank you all very much, this is my very first post on the VCF Forums!!! I've been a big fan for a long time now!

Thanks :)
 
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Sorry I can't help with your questions on the Osborne. But, Welcome to the VCF Forum. I will be interested to follow along and if you are able to get these issues solved.

Seaken
 
The Field Service Manual (2F00040-00_Service2ndEdition_1983.pdf) does contain the crt alignment proceedure, which looks like it will involve removing the glue.
 
Its a 'feature' of some old CRT circuits. Hugo has done a fantastic piece on why it does it on a PET CRT and what to change to stop it happening.

I will see if I can find it.
 
And the gotek

have you got the interface board to connect from the Osborne cable to the gotek and pick out the Power ? (its not just plug and play)

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zeroandone211 said:
Anyway, when I first powered the machine on, then off, a very bright dot appeared in the middle of the CRT, and stayed there for about half a second before slowly losing its glow, and then going away completely. Does anybody perhaps know how to fix this problem? I just watched Adrian Blacks' video on fixing a CRT spot on a Commodore PET so I'm curious to know whether or not this can be performed on an Osborne CRT.
Click to expand...

I mean you can do anything, LOL, but is it necessary? I have had / repaired a number of O1's over the decades, your description sounds "normal" to me. A video of the issue ( use fixed exposure in normal room lighting ) may help those with O1 experience tell if this is "normal" or not. I don't think the VDU subsystem in the O1 was originally designed for spot elimination. If you wanted to add this feature, the place to look would be that capacitor. ( Edit: on the original O1, this is C103 on the VDU schematic. )

zeroandone211 said:
Also, when I first received the Osborne 1A, the picture on the CRT was very obviously tilted upwards (pointing upwards towards the right upper side of the CRT). I made some adjustments to the raster rings, but unfortunately the CRT yoke is actually glued into place (fixed yoke), and this prevents me from making any adjustments to fix this. Is there any other way to fix this problem? Adjusting the raster adjust rings did make a significant difference, but I can still tell that it is slightly tilting upwards.
Click to expand...

Similarly, a photo of the issue ( and what you were adjusting ) would speak volumes - I'm not sure what you mean by tilted upwards. I would avoid adjusting the focusing magnets / yoke ( on any CRT ) until you have eliminate any electronic sources for the issue - because there are a lot of them. I guess I'm counseling against getting sucked into "perfection spiral" for something that was not really all that perfect to begin with.
 
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The problem with a bright spot on the CRT when you switch the machine off (not having seen how bright however) is that the CRT phosphor can become damaged with the result that the screen has a dark spot on it under normal operation.

This really should be avoided - unless it is a static museum exhibit!

Dave
 
daver2 said:
The problem with a bright spot on the CRT when you switch the machine off (not having seen how bright however) is that the CRT phosphor can become damaged with the result that the screen has a dark spot on it under normal operation.
Click to expand...

Everyone has a different idea of what "bright spot" means, and mucking about "fixing" things which are not actually broken is also to be avoided - that is all I was suggesting. The other option is see if lowering the brightness knob before power off eliminates the spot. If it does, boom, done, no soldering required - only discipline. Unlike the PET, the O1 VDU does not seems to have been designed to prevent the spot, the TC on the grid circuits is like ~10ms. The other thing, the OP also mentions what sounds like keystoning. The spot could be secondary to some issue causing the distortion. I'd try to resolve that first and the other issue may resolve itself.
 
Which is why I put the text in parenthesis.

I agree, if it ain't broke don't fix it.

Sometimes the spot is designed to de-focus at the same time, so it is not a problem.

It is only a problem if it is a bright, pin-point spot. But we do need a video to confirm.

Dave
 
I can certainly get a video! I'm doing some work on the machine at the moment, so it is outside its case, however I can still operate it. When I get the keyboard put back together, I can take a video of normal operation! Unfortunately, my CP/M disk (both master AND copy) went bad, and neither boot. It's definitely not the floppy drive however, because I can boot Wordstar, and other programs that are bootable just fine. I can even back out to the Wordstar "system", and from the A prompt I can switch disks to the CP/M disk and can get directory contents, and run the CP/M utilities from the disk. It just won't boot. However, I will post a video soon! Thank you all for your help!
 
There's supposed to be a circuit that turns the brightness all the way down when power is lost - this effectively "eats" the dot. There's nothing wrong with the picture tube. The dot is caused because the CRT is still emitting electrons for a short while as it cools, and there's no longer any horizontal and vertical pulses to form a picture.

If you're really concerned, either turn the brightness down before switching off, or get the schematic and find/repair the circuit concerned. If you're not sure what you're doing, a TV technician could fix it, since the same circuit is in all CRT-based TVs.
 
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