Memotech 32k RAM pack failure

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ujko
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Joined: Wed Aug 18, 2021 5:07 pm
Location: Croatia

Memotech 32k RAM pack failure

Post by ujko »

Hi,

I have one of this in mint condition, boxed, with manual, no scratches - real beauty :mrgreen:

Of course, something must be wrong - it does not work, produces blank screen when attached to ZX 81. I tried it on three of them so it really does not work.
20211125_222341.jpg
So, I have replaced both 74157's and instead of OKI M3732L-20 I put KM4164-15. It still does not work, likely due to a different matrix but can somebody confirm this ? 3732 is crippled 3764 and it seems it is organized as 256 rows x 128 columns matrix (according to datasheet). But what about 4164 ? It does work as a replacement for TMS4532-NL3 (at least the type which I have), so maybe it is organized as 128 rows x 256 columns ?

In the mean time I'll replace the rest of IC's (except PROM), just to be sure.

:?:
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1024MAK
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Location: Looking forward to summer in Somerset, UK...

Re: Memotech 32k RAM pack failure

Post by 1024MAK »

I've been inside a 16k Memotech 16k RAM pack and reverse engineered it, but have not done so for their 32k RAM pack.

The KM4164 should work, as they are also 128 cycle refresh DRAM chips. The logic level on pin 9 (A7) should not matter (as long as it is a valid logic level).

This being the only address input that is not required for refresh. But note that there may be a difference between the A and B versions of the DRAM chips. What is the full part number of the chips you are using?

The ZX81 display system means that the Z80 refresh register (and hence the refresh address) does not cycle through all 128 counts as often as it should. So if the DRAM chips are sensitive to not being refreshed as often as they should, this may cause problems.

Have you tested the removed chips?

What test gear do you have?

By the way, Texas Instruments (TMS) 64k bit DRAM chips (such as TMS4164) have a 256x256 matrix and so need a 8 bit, 256 cycle refresh. The 32k bit versions are connected up such that the multiplexers supply the correct logic level to the relevant pin via a fixed track connection or via a link wire.

Mark
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