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Cappellanus 07-16-2007 06:57 PM

"tuning" the altair CPU card?
I hate to post a "newby" question, but I've seen talk of "tuning" any Altair CPU card, and I'm confused...

I think Grant promised to tune any card for anyone, as long as postage was paid both ways.

I'm not sure what this means. Is that tuning the clock frequency, so that some timing software is more accurate? I know of no adjustable parts on the CPU board, there are a few analog components (resistors and capacitors) but again, nothing that could be adjusted.

I went so far as reading the altair assembly manuals, but didn't see any reference to a CPU tuning procedure.

So, what's this all about (again, excuse the newby uninformed question, but I've been unable to figure this out through extensive searching.)

- Steven.

Grant Stockly 07-16-2007 10:01 PM

There is a 2 phase clock on the CPU card generated from two one shot timers. The parts listed in the manual for this part were starting points. The CPU card even has two sets of pads for each resistor and capacitor to allow for paralleling parts.

When I was debugging my first CPU card I looked at the clock on two other vintage cards. According to Intel's spec sheet those two vintage cards should not be working at all. If an older Altair is running badly then its possible that the 2 phase clock is responsible.

The closer the clock is to spec the less temperature or electricity fluctuations might impact the stability of the Altair. The ultimate fix is to use an Intel 8224 like this guy:


Cappellanus 07-17-2007 05:04 AM

Oh.... that makes perfect sense. Sometime I'll look at my clock signals and see what they look like. Funny, I never saw another Altair site or document mention this potentially inaccurate clock phase generator.

Looked at some board photos, and there does seem to be some variation on how many resistor/capacitors are in that top left row. I'll look at my own board sometime. Very interesting.

Another long-standing mystery solved.

- Steven.

marty 08-12-2007 01:19 AM

CPU Clock Mod
There is a clock MOD in Altair (Mits) Computer Notes Volume 2 - 3, page 10 When they have a modification to make the CPU card more like Intels spec's.
There is also another Clock article in Computer Notes Volume 3 - 7 , where they show how to comvert it to a 8224 clock Ic and a 18 MHZ Crystal.
Thanks Marty:p :p

Cappellanus 08-12-2007 04:34 AM

the Definitive Altair Clock documents...
Thanks a bunch Marty! This is just what I needed. I was surprised how little I could find about the "faulty" or "buggy clock" online and through Altair sites, or in other Altair doucuments, but the Computer Notes articles explain it all.

I read the articles here...
and while it's hard to follow exactly what they're doing (why couldn't they just show a schematic diagram) it is very interesting how they explain the problem, and the symptoms of a failing clock. Now I can see why different Altair CPU boards seem to be a bit different. I suspect most builders implemented the revision in Vol 3 num 7.

Rarely do posts hit such a "bullseye" with such a definitive and complete answer. Thanks for the pointer to the historical documents.

- Steven.

marty 08-12-2007 01:01 PM

Computer Notes
If you haven't looked at ALL the Computer Notes, There are alot of interesting articles, there to look at and read :o :o Which may take some time to read through, and use.
I can't say how many people implimented either mod, but the only one I have seen is the one Grant referred to earlier. I plan to write up a schematic for the mods, as I have time, and see what they are doing :) :)
Thanks Marty

jessi_rubin 11-19-2007 12:34 PM

when can we have that schematic?

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