Blown RAM card?

Discuss construction, troubleshooting, and operation of the Altair 8800c computer

Re: Blown RAM card?

Postby AltairClone » January 26th, 2022, 8:21 pm

Try an 8.2K if you have one. If not, see if you have a 7.5K
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Re: Blown RAM card?

Postby Ttpilot » January 27th, 2022, 7:00 pm

AltairClone wrote:Try an 8.2K if you have one. If not, see if you have a 7.5K


Okay, I swapped an 8.2k in for R41. Here's the result:

phi1 + phi2.jpg
Phi1 + phi2


Also the measurements

Phi1:

phi1 stats.jpg
Phi1 measurements


Phi2:

phi2 stats.jpg
Phi2 measurements


Maybe phi2 is a bit long, too?
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Re: Blown RAM card?

Postby AltairClone » January 27th, 2022, 7:14 pm

Ok, that looks great on phi1 and got us up to the minimum phi2 to phi1 timing. Now let’s shorten phi2 a bit to get some headroom. I’d use a 5.6K for R42. That should get phi2 down to just below 250ns and gain us about 30ns of headroom.

Mike
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Re: Blown RAM card?

Postby Ttpilot » January 27th, 2022, 10:54 pm

AltairClone wrote:Now let’s shorten phi2 a bit to get some headroom


With 5.6K in R42

phi1 + phi2.jpg
Phi1 + phi2 (R41 = 8.2K ohms, R42 = 5.6k ohms)
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Re: Blown RAM card?

Postby AltairClone » January 28th, 2022, 12:00 am

That looks perfect. All pulse widths and the phi1 and phi2 clock edges relative to each other are well within spec. Any problems you have with the computer are now not related to processor clock issues.

Mike
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Re: Blown RAM card?

Postby Ttpilot » January 28th, 2022, 12:35 am

AltairClone wrote:That looks perfect. All pulse widths and the phi1 and phi2 clock edges relative to each other are well within spec. Any problems you have with the computer are now not related to processor clock issues.

Mike


Thanks very much for the help. It's been a fun learning experience.

When I power on the status lights and wait light are all correct. But I seem to have a stuck address line and data line, and a reset takes me back to address 02 instead of 00. I guess I need to take the front panel all apart and inspect everything on that. Sigh. At least I'm closer!
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Re: Blown RAM card?

Postby AltairClone » January 28th, 2022, 5:21 pm

Before you take apart the front panel, I’d remove and reseat the data connector from front panel to the CPU board a few times. Also look at the wires and female pins of that connector closely and make sure none of those wires are broken where they solder at the front panel.

Mike
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