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View Full Version : Repro SCELBI interfaced with Popular Electronics Scopewriter and ASCII keyboard



mwillegal
December 31st, 2018, 06:52 PM
This is something that was done back in the day and I felt that I had to recreate this combination of devices. I also have a little 256 byte monitor hacked together. The monitor supports reading and writing memory and executing programs through normal keyboard input. These add ons turn the SCELBI 8H into a machine that approaches practical usefulness. More details can be found on my blog.

http://www.willegal.net/blog/

Regards,
Mike Willegal

8008guy
December 31st, 2018, 07:19 PM
Thanks for this Mike. it inspires me to build my scelbi soon.

Len

Slob
January 9th, 2019, 08:10 PM
Very nice, nothing like an analog X-Y display. Too bad analog scopes are getting older and AFAIK aren't being made anymore. That output on a digital scope looks bad (I've tried).

I've build a Spacewar display for my PiDP8 and used a (fairly rare cheap) Tek 604 display in great shape for it. It's far better in resolution and linearity than my in rather good shape 464 (which is a relative of your 465). Being a few inches larger helps. I have a few Tek 620's with dubious aging tubes and/or bad switching transistors, but the working ones were noticeably better than the 464.

You always read about projects involving converting a CRT TV to an XY display, but few have actually done it (and lived, apparently). And then, I wonder about the frequency response and linearity.

mwillegal
January 10th, 2019, 03:47 AM
The Scopewriter is not, strictly speaking, an X-Y display, as it only draws in the X axis. This makes it a vastly simpler solution than some others, at the cost of supporting only one line of output.

The SCELBI CRT interface does draw characters in an X-Y fashion. It is interesting in that it does this with two OP-AMP integrator circuits, one for X, one for Y. The main problem I have with my Tek465 in this application is that the Z input is capacitor coupled and reacts a bit slower than would be optimal for the SCELBI CRT interface. The thing I like about the Tek 465, is that when something goes wrong, it seems to be quite repairable, and as it is 40 years old, I do have an occasional issue with it. I agree that many other scopes have better display resolution.

regards,
Mike Willegal


Very nice, nothing like an analog X-Y display. Too bad analog scopes are getting older and AFAIK aren't being made anymore. That output on a digital scope looks bad (I've tried).

I've build a Spacewar display for my PiDP8 and used a (fairly rare cheap) Tek 604 display in great shape for it. It's far better in resolution and linearity than my in rather good shape 464 (which is a relative of your 465). Being a few inches larger helps. I have a few Tek 620's with dubious aging tubes and/or bad switching transistors, but the working ones were noticeably better than the 464.

You always read about projects involving converting a CRT TV to an XY display, but few have actually done it (and lived, apparently). And then, I wonder about the frequency response and linearity.

Slob
January 10th, 2019, 03:57 AM
I bought a broken 464 as a parts source for my good one. Then I repaired it. Diode bridges, of all things. These were among the last TEK oscilloscopes without (for the most part) unobtanium semiconductors.