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View Full Version : Osborne Executive Blank Screen on Boot



jj_darling
July 24th, 2012, 10:04 PM
A few years ago a friend of mine gave me an Osborne Executive as a gift knowing that I was interested in vintage computers. It had belonged to his sister's in-laws, and he wasn't sure whether it ran or not. Upon powering it up I found that it ran (fans whirred, caps charged up, power light came on), but the screen never lit up. I was disappointed (always loved that amber glow), so I tried troubleshooting and trying to figure out how to make the thing go-- the external display jumper has been removed, cleaned, and replaced; the internals have been inspected for damage (none visible, at least not in terms of blown up components, missing/corroded connectors, etc); and I connected the composite video out to a TV and verified that the internals function properly (got the boot screen and was able to launch Zork from a disk, although the screen overscanned so much that I couldn't read the command prompt). As far as I can tell, there's nothing physically wrong with it, and I couldn't figure out how to test the display independently from the rest of the internals so I can't verify if it works.

Any tips on how I might proceed here? I can't wait to see the "East of house: There is a mailbox here." prompt on this thing!

Chuck(G)
July 24th, 2012, 10:25 PM
Take a peek at the base of the CRT near the socket. When you have power applied, can you see the CRT heater glow?

jj_darling
July 25th, 2012, 10:08 AM
Yes indeed. So that means the tube is good, right? So then it must be an issue in communicating data to the display. Or a calibration issue? Since the Executive can't do graphics does that mean this display is just a terminal display (like a teletype) that's being fed serial character data in stead of an analogue image signal that would need calibration?

Chuck(G)
July 25th, 2012, 02:40 PM
It does mean the CRT is most likely okay, however there's a pile of circuitry on the display itself. Let's see what we can find out. The Executive uses a Zenith DT4 7" display (almost no manufacturers back then made their own displays, but rather purchased OEM kits). The schematic is here (http://z80cpu.eu/mirrors/oldcomputers.dyndns.org/rechner/osborne/executiv/manual/sheets/osbe_video.png).

As you can see, there's not much to it. The usual culprits should be looked after; bad solder joints (the DT4 seems to have been afflicted with that ailment in particular). The schematic shows where to check voltages. Make sure that all connectors are fully inserted (they could get bounced loose). If you turn the brightness full up, can you see a faint raster on the screen? Since we know that the CPU section works fine, this breaks down to a basic B&W TV repair task.

barythrin
July 25th, 2012, 02:51 PM
I know this is a bit basic but have you checked the brightness/contrast controls on the front?

jj_darling
July 25th, 2012, 03:39 PM
Okay cool, I'll have to find a multimeter and start looking around inside the CRT housing. And no, playing with the knobs has no visible effect on the screen. I've never used a schematic as reference for diagnosing circuitry problems before-- where should I start? Should I just be testing at the points in the circuit where the schematic shows a hexagon or circle with a voltage in it?

Thanks for the help!

Chuck(G)
July 25th, 2012, 04:16 PM
Start by doing a visual inspection of the video board. Do you see any obviously damaged components? In other words, are there any charred resistors, bulging capacitors, etc? A sniff test can turn up some amazing things...

For now, don't bother to disconnect the anode (high voltage) lead from the CRT--although I doubt that there's still a charge on the tube, it can zap you if you're not careful.

Now flip the board to the solder side. Look carefully (use a magnifier if you have to) at all the solder joints--are there any that look questionable? A joint that was marginal when this was still a young box can go bad with age. Resolder any that look iffy.

Now, put everything back, reconnect the connector to the CPU board and power the thing up. If you still don't get video, carefully take some measurements where marked on the schematic. Stay away from the horizontal output section for now--that's anything that's connected to the flyback transformer, including the large horizontal output transistor.

Note that all voltages are measured with respect to chassis ground and that they're all DC, so make sure your meter is set to read DC volts. How do the voltages look?

While we're at it, is the multimeter the only piece of test equipment that you have right now?

jj_darling
July 25th, 2012, 04:40 PM
The only pieces of test equipment I have right now are my five senses-- though I'm planning on picking up a multimeter sometime soon. Perhaps I can hope for a bad solder joint to fix everything, but I imagine this project is going to be disassembled on the table for a while. I'll perform the sniff test and visual inspection and see if that turns anything up.

Hah, I wish I could just do what the service manual says:

1. Screen remains blank.
REPLACE video monitor (sections 8.1.3 and 8.1.4).

If I did want/need a replacement at some point, do you have any suggestions on where to look for one? I imagine a quick googling isn't going to get me far in finding something like this.

Chuck(G)
July 25th, 2012, 06:57 PM
ah, I wish I could just do what the service manual says:

If I did want/need a replacement at some point, do you have any suggestions on where to look for one? I imagine a quick googling isn't going to get me far in finding something like this.

An awful lot of OEM monochrome monitors had standardized connections. I suppose that you could look for a similar one--Ball Bros. (the Mason jar people) manufactured an awful lot of them, for example. But basically, it's the same with all of this old gear--try to repair what you can and hope that a replacement will come along.

Do you have any electronics-handy friends? A monitor isn't really the place to start because of the high voltages that can be involved.

Regardless, you can get some of the simple tools, such a multimeter locally. Radio Shack still sells soldering equipment and Harbor Freight offers a very inexpensive line of multimeters that will do just fine for your needs.

smp
July 26th, 2012, 03:06 AM
Hah, I wish I could just do what the service manual says:

If I did want/need a replacement at some point, do you have any suggestions on where to look for one? I imagine a quick googling isn't going to get me far in finding something like this.



I have a box of pieces and parts from a disassembled Osborne 1. This computer was "working" but had problems with the disk drives, so I sacrificed it to become my Osborne parts bin. I have a complete CRT assembly that may be useful. Also, with some guidance from others more knowledgeable than I am, perhaps some of the other components may also be useful. If you are interested (and if the parts are actually interchangeable), I would be glad to sell some pieces at reasonable cost plus shipping.

smp

nige the hippy
July 26th, 2012, 08:42 AM
Just another daft thought, I haven't got an executive but is the little jumper on the right hand side of the front panel in place? as far as I know it jumpers the video from the processor to the CRT.
http://www.old-computers.com/museum/photos/Osborne_Executive_FrontSide_s2.jpg
& if you haven't got one, as far as I know it's 1:1 top to bottom (but please check!)

Chuck(G)
July 26th, 2012, 08:58 AM
...the external display jumper has been removed, cleaned, and replaced...

As stated in the first post.

patscc
August 22nd, 2012, 08:47 PM
Well, five senses are better than nothing. On the tube, there's the anode cap with a wire sticking out of it. It leads to the flyback. If you listen closely, do you hear a high-pitched whine at all ? (kinda like in an old tv)
Since the CRT heater is directly from 12V, it's perfectly possible to have a glow with a hosed horizontal section.
When you get your meter, check the +50V (+B) supply voltage.
If you're new to electronics, instead of getting a cheap DMM, see if you can't find an old-style analog multimeter (RatShack may even still sell them). The circuit-loading behaviour between DMM's and an analog (not a FET or VT meter) is different, and the readings on the analog should be easier to interpret.
Oh, and get test leads that you can clip to stuff. It's easier to not zap yourself if you're not trying to hold two probes and read the meter, all at the same time.
patscc