The type number of the chips on your board is KM4164B-15
The three digit number below the type number is either a batch number or a date code, so where three digit numbers are used, possibly 624 could be week 24 of 1986. Where four digits are used, the first two are the year and the second two are the week number. Apart from some Japanese manufacturers, week one is the first week in January.
Different batch numbers or different date codes normally make no difference at all, they all normally work the same.
The type number KM4164B-15 is broken down as
- ‘KM’ a code used to indicate the manufacturer, in this case Samsung Electronic,
- ‘4164’ chip type, in this case a one bit wide 64k bit DRAM chip,
- ‘B’ you would have to check the datasheet to see what this means, often a ‘A’, ‘B’ or ‘C’ means an improved revision, a ‘N’ or ‘P’ often means a plastic DIL package,
- ‘15’ access time (speed), here it means 150ns.
Please note that there are TWO different types of 4164 DRAM chips. The Z80 microprocessor uses a 7 bit / 128 cycle DRAM refresh system. So you have to use a 4164 DRAM chip that is compatible and which uses a 7 bit / 128 cycle / 2ms DRAM refresh system.
A Texas Instruments TMS4164 is one example of the other type of 4164 DRAM chip, it uses a 8 bit / 256 cycle / 4ms DRAM refresh system and will not work correctly.
Any replacement chip must be rated at 150ns or be quicker (faster).
In this model of the ZX Spectrum, eight DRAM chips are used together to form one area of memory (each of the eight chips provides one bit of data per byte at each address, each chip only being connected to one data line). There are sixteen chips because there are two areas of RAM, each is 64k bytes, so two lots of 64k bytes gives the total of 128k bytes.
As long as all the DRAM chips have a quick enough access time (speed) and are 7 bit / 128 cycle / 2ms DRAM refresh types, in most cases you do not have to use chips from the same manufacturer.
Before buying some DRAM chips, I would recommend carrying out some tests to try to narrow down the cause of the fault. Do you have a multimeter? Do you have any other test gear such as logic probe, logic analyser or oscilloscope? Or better yet, do you have an expansion that can run a diagnostic ROM so that a memory test can be performed?
If you have a multimeter, start by checking the +5V supply to check that it is between +4.75V and +5.25V.
If you have a logic probe, logic analyser or oscilloscope, check pin 6 of the Z80 to see if it is getting the 3.5MHz clock. Then using a meter, logic probe, logic analyser or oscilloscope, check pin 26 to check that this is logic high. If you press the reset button, this should change to logic low, then go high again once you release the button.
There is a pin-out of the Z80
here.
Mark
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