Blown RAM card?

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

Blown RAM card?

Postby Ttpilot » January 1st, 2022, 10:44 pm

I finished building my 8800c. My RAM card is an old Compupro RAM 17 from the 80s. It used to be installed in my old North Star Horizon.

When I fire up the 8800c the data bus lights are all on, which doesn't change no matter what I do, and the address and status lights are flaky. Same thing if I take the RAM card out. I suspect when the old Horizon died it took out my RAM card as well. I've ordered a new chip set for it and will hope for the best.

I'd also hoped to use my Morrow Designs DJDMA disk controller from the Horizon. I'll be that's fried too.

Sometimes getting these things working is like death by a thousand paper cuts

But it's fun ;-)
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Re: Blown RAM card?

Postby AltairClone » January 4th, 2022, 7:39 pm

Missing RAM, or RAM that does not respond, would result in all data LEDs being on like you’re describing. I really like the RAM 17 for Altair and other early computers. Do you have any other RAM card you can try to make sure the CPU and front panel are working properly? The address and status lights should not be “flakey.”

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Re: Blown RAM card?

Postby Ttpilot » January 5th, 2022, 11:20 pm

AltairClone wrote:Missing RAM, or RAM that does not respond, would result in all data LEDs being on like you’re describing. I really like the RAM 17 for Altair and other early computers. Do you have any other RAM card you can try to make sure the CPU and front panel are working properly? The address and status lights should not be “flakey.”

Mike


Thanks. I have 3 old North Star 16k DRAM cards, but I think they all met the same fate. I should be getting a set of replacement chips for the RAM 17 later this week. I'm hoping that works.
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Re: Blown RAM card?

Postby Ttpilot » January 15th, 2022, 5:56 pm

Update. Finally got the chips to reload the Compupro RAM 17 board. I also went back through and resoldered any suspicious looking spots on the 8800 front panel, front panel controller, and 8080 cards. Not much changed as a result. When I turn it on and do a stop/reset I get this:

after stop-reset.jpeg
After 1st stop/reset


After waiting 5 minutes or so and repeating the stop/reset, I get this:

after 5 min.jpeg
Stop/reset after 5 minutes on


D4 seems to be stuck on, also A1 intermittently. I don't know of those bits are actually stuck or not. I can load code into memory, such as as simple add routine. The examine switch works, but clicking it continuously seems to jump around in memory at random. Examine Next seems to work okay. Clicking Single Step seems to light up a lot of address lines, and I can't tell if it's actually going through the program. Run lights up everything but INTE, PROT, WAIT, and HLDA, plus A15 and A14 are flickering.

I saw someone mentioned in an earlier thread that they had timing issues with the 8080 board. Could this be the problem? I don't have any diagnostic equipment, so I'm at a loss
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Re: Blown RAM card?

Postby AltairClone » January 15th, 2022, 7:13 pm

Yes, it’s quite common that the CPU board needs its clock adjusted. You don’t have an oscilloscope?

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Re: Blown RAM card?

Postby Ttpilot » January 15th, 2022, 11:04 pm

AltairClone wrote:Yes, it’s quite common that the CPU board needs its clock adjusted. You don’t have an oscilloscope?

Mike


Nope
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Re: Blown RAM card?

Postby Ttpilot » January 16th, 2022, 12:15 am

About a year ago BillO started a thread about the CPU card where he mentioned needing to use a 74ACT04 (instead of a 7404?) and change two resistors to correct the timing. Is that the sort of change that would work?
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Re: Blown RAM card?

Postby AltairClone » January 16th, 2022, 8:59 am

The existing 7404 is fine. It’s typically just a matter of tweaking one resistor to get the clock phases correct relative to each other.

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Re: Blown RAM card?

Postby TronDD » January 16th, 2022, 2:06 pm

I had accidentally swapped a 7404 and a 74LS04 on the CPU card resulting in no clock signal.

Double check you have exactly the right chips in the right places.

I also had a bad solder joint on a ground pin on the front panel S100 card which caused the front panel to act erratically. The data bus would float high and the CPU would read a 377Q or a RST 7 jumping it to address 000 070Q. Temperature was a factor in the connection which makes me suspect a similar situation because of your front panel acting differently after 5 minutes.

And I also had to change resistors a little to get my clock within spec. You will need an oscilloscope for that.
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Re: Blown RAM card?

Postby Ttpilot » January 16th, 2022, 8:15 pm

TronDD wrote:You will need an oscilloscope for that.


Ordered one. I’m about here on the learning curve

9E765A7D-5CFD-4E6E-B5A2-987707EF8541.jpeg
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