Using the 8800b CPU board in the 8800C

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

Re: Using the 8800b CPU board in the 8800C

Postby AltairClone » September 10th, 2022, 7:19 pm

I’m wondering if it was a trim pot as opposed to a capacitor? Based on the trace cut, the trim pot may have been in series with the crystal drive circuit to not over-drive the xtal.

Have you looked at phi1 and phi2 to make sure the CPU is getting a good clock?

Mike
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Re: Using the 8800b CPU board in the 8800C

Postby TheoAU » September 10th, 2022, 8:08 pm

AltairClone wrote:I’m wondering if it was a trim pot as opposed to a capacitor? Based on the trace cut, the trim pot may have been in series with the crystal drive circuit to not over-drive the xtal.

Have you looked at phi1 and phi2 to make sure the CPU is getting a good clock?

Mike


I can't check those yet, I'm waiting for my oscilloscope to arrive. Good news is, it should be here any day.
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Re: Using the 8800b CPU board in the 8800C

Postby TheoAU » September 12th, 2022, 8:40 am

Now that I've got the scope I tried to look at phi1 and phi2 of the 8800b clock. I think I checked the correct IC and pins. The 8800b manual states IC-F which is an 8224. Looking at the spec sheet, pins 10 and 11 are phi2 and phi1.
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8800b_CPU_Clock_8224_10_11_.png
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Re: Using the 8800b CPU board in the 8800C

Postby AltairClone » September 13th, 2022, 6:06 pm

The clock is obviously running and the timing looks fine - which is expected with an 8224 clock chip.

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