Built a ZX Spectrum but having Upper RAM Issue?

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DistantStar001
Posts: 27
Joined: Sat Feb 29, 2020 2:07 am

Built a ZX Spectrum but having Upper RAM Issue?

Post by DistantStar001 »

Like the title says, I've finished building my 3B board. It boots! The keyboard works (not surprising since it's a new membrain). It even loads 16K programs and executes BASIC. So for all intents and purposes, it's a fully functional 16K Spectrum. Just one issue... I installed the full 48K on it. So everything seems to work except the upper RAM.

Attempting to load larger programs causes it to either crash/reboot or gives me an error message: "M RAMTOP NO GOOD 20:4". I did test the programs (mostly .tzx files) on my Spectrum + (a 6A under the hood) and they load fine.

I've also tried two RAM test programs. The first (MEM Test) reports that RAM 1 is bad. I've replaced it several times, but it still insists that RAM 1 is bad. The second is in German, but if I'm interpreting it properly, the lower is fine, but the upper is scrambled?
Screen Shot 2022-05-16 at 7.48.05 PM.png
Screen Shot 2022-05-16 at 7.48.53 PM.png
So far, I tried replacing the 4164s several times, all tested good chips, but I still get the same errors.
DistantStar001
Posts: 27
Joined: Sat Feb 29, 2020 2:07 am

Re: Built a ZX Spectrum but having Upper RAM Issue?

Post by DistantStar001 »

Figured out the problem! All the of the chips I have require a 256.0-cycle refresh. I need chips with a 128-cycle. I guess I'll have to stick these things in a C64 or something.
DistantStar001
Posts: 27
Joined: Sat Feb 29, 2020 2:07 am

Re: Built a ZX Spectrum but having Upper RAM Issue?

Post by DistantStar001 »

OK! Not the problem... Just replaced the RAM with known good chips and the correct refresh and it's still not seeing it.
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