Sinclair ZX80 / ZX81 / Z88 Forums

Hi.

I am trying to fix my Toastrack and ZX-Diag while testing lower ram shows "RAM FAIL 4" that maps to the IC10 on the board. So I will have to replace it.

i can see that all of the 16 RAM chips on the board are of the same type: OKI M3764A-15, therefore I have the following questions:

1. is it so that the same chip is used for both Lower and Upper RAM in toastrack?
2. Is it OK to mix different types of chips, for example if I change only IC10 to a different model, would that be ok?
3. if so then what kind of options do I have to replace this 3764 chip? Can I mix 4164 in? Any other options?

Thanks!

Contended video RAM is IC 6 - 13. Uncontended 'upper' RAM IC 15 - 22.
Note from the service-manual: All RAM chips should have 150ns access time and 128 row refresh.
Because the 128K uses "real" 4164-chips (the 48K used "broken" 4164-chips which where declared as 4132-types) there is no hint to use only chips of the same type/manufactory ...
in short:
1 - yes
2 - yes
3 - yes you can take any 4164 RAM chip.

Thanks for the reply!

A side question, Is it possible to test 3764 chip on a tester like this that is for 4164?


Thanks!

tridy wrote: ↑Tue Nov 09, 2021 11:32 am Thanks for the reply!

A side question, Is it possible to test 3764 chip on a tester like this that is for 4164?


tester4164.png

4164.jpg3764.jpg


Thanks!

Looks like being 4164 pin compatible
https://pdf1.alldatasheet.com/datasheet ... 3764A.html

In case you're having trouble finding 4164 ram chips, you can get 10 for about GBP2.5 from Aliexpress. Delivered.

I started mixing chips, but found that some of the chips in my 128K toastrack were a little unreliable, and while they tested OK, I had to replace them. Had I simply replaced the lot to begin with, it would have saved time.

Also, the Fruitcake ram test rom is pretty useful. You don't have to plug it in externally - just replace the onboard ROM with it.

David.

Tiger wrote: ↑Sun Nov 07, 2021 1:16 pm Note from the service-manual: All RAM chips should have 150ns access time and 128 row refresh.

Not all 4164 or equivalent DRAM chips are the same (and the various manufacturers used various numbers/codes). Two very important points:

• The chosen chip has to have an access time that is fast enough, otherwise strange and weird problems will occur.
• The refresh has to be the correct type. See below.

There are two memory layouts in this DRAM chip size. Both have exactly the same pin-out. All are 64k bit (65536 bit) in size.

One has a memory cell matrix of 8 x 8 and requires a 256 cycle refresh address. These are not compatible with a Z80, as it only has a 7 bit (128 cycle) refresh system.

You need the other type. These chips that have a 7 x 9 matrix and hence use a 128 cycle refresh address. These were deliberately designed to be compatible with Z80 and other systems that only have a 7 bit (128 cycle) refresh system.

OKI M3764A-15 DRAM chips fit the above requirements (128 cycle, 7 bit refresh and an access time of 150ns).

TI TMS4164-15NL DRAM chips do not fit the above requirements. The access time is okay, also 150ns. But they require a 256 cycle, 8 bit refresh.

The only way to know is if you happen to find a copy of a chart on the internet that lists these details. Or by very carefully reading the manufacturer’s datasheet or data book.

Or if you buy from a reputable seller, but make sure you tell them you need the type with the 128 cycle refresh.

On trading sites, be careful. As it’s not uncommon for descriptions to be copy and pasted, and hence be misleading…

Here is one trusted seller

Mark

Your tester is useful for 4164 and 41256 RAMs - for 4164 you have to set the jumper as printed on the pcb!
As I told, you can use 4164 types with 150ns access time and 128 row refresh. This includes parts from the
following manufacturers: Hitachi, Intel, Mitsubishi, Mostek, Motorola, NEC, OKI, Panasonic, Toshiba and National.
I also had no problems with "higher" RAMs as NEC D41256C-12, OKI M41256A-10 (wich also works as "Upper-RAMs" in 48K speccys)

I mean the TMS4164 is pin-compatible with the OKi-type , but the 128 row refresh should be a problem. Also Siemens HBY4164 and Samsung KM41464 (not KM4164) have 256 refresh ...

KM41464's are not good for Toastrack, because they are 4bit DRAMs and were used in +2A/B and +3.

For Toastracks and the upper RAM of any Spectrum 48K, suitable Samsung brand chips are KM4164B

McKlaud wrote: ↑Wed Nov 10, 2021 6:27 am For Toastracks and the upper RAM of any Spectrum 48K, suitable Samsung brand chips are KM4146B

Do you mean KM4164B ?

Yes, just corrected.

The data sheet attached.