The RAM

RAM types 
The serial chip is a NEC uPD42832C. This is a 32K pseudo-static RAM chip. These chips are like dynamic RAM but have the ability to retain data under a standby voltage (around 2V) with a self refresh circuitry.Dynamic RAM chip are incompatible. Static RAM chip can be use without any problem. The replacement is recommended because their power drain is the lowest (1/10 ratio).

RAM socket 
The motherboard layout has 32 pins. It is tracked for a 128K chip. On issue 4 machine a 32K chip is soldier using the 28 low pins. Here is the socket pinout from the component side view.

    +--------------+
POE |1    +--+   32| VCC
A16 |2           31| A15
A14 |3           30| VCC
A12 |4           29| WE
A7  |5           28| A13
A6  |6           27| A8
A5  |7    Z88    26| A9
A4  |8    RAM    25| A11
A3  |9    PCB    24| POE
A2  |10          23| A10
A1  |11          22| CE
A0  |12          21| D7
D0  |13          20| D6
D1  |14          19| D5
D2  |15          18| D4
VSS |16          17| D3
    +--------------+


This table describes the 128K chip pinout and Blink signals.

Pin     Chip    Blink   Pin     Chip    Blink
1       POE     POE     32      -       VCC
2       A16     MA16    31      A15     MA15
3       A14     MA14    30      [CS]    VCC
4       A12     MA12    29      [WE]    WRB
5       A7      MA7     28      A13     MA13
6       A6      MA6     27      A8      MA8
7       A5      MA5     26      A9      MA9
8       A4      MA4     25      A11     MA11
9       A3      MA3     24      [OE]    POE
10      A2      MA2     23      A10     MA10
11      A1      MA1     22      [CE]    IRCE (internal RAM chip enable)
12      A0      MA0     21      D7      MDH
13      D0      MDA     20      D6      MDG
14      D1      MDB     19      D5      MDF
15      D2      MDC     18      D4      MDE
16      VSS     GND     17      D3      MDD

Internal RAM upgrade 
It is easily possible to upgrade the internal RAM to 128K. For a 512K upgrade some hardware modification are needed. The OZ version 4 is the only one able to recognize a 512K internal upgrade. For a 128K upgrade you can put a Toshiba TC551001 static RAM chip. Obviously, a backup of your Z88 is required before upgrading. 
 

Disassembly the box without batteries. Deconnect the two keyboard ribbons, the screen ribbon. Put your motherboard on a dry table without metal. Unsolder the old chip. Solder a 32 pins flat socket, and insert the new chip. You will have to cut some plastic structures (like an X) in the keyboard plastic support.

For a 512K upgrade, you will have to wire the A17 and A18 pins with a link directly to address lines on to corresponding slot connector pins.

Replacing the old 42832 Ram will spare your battery life time. The slowest rams consumes the least (120 or 150 ns). For example :

Size  Chip    Type            Speed           Power (mW)      Manufacturer
(K)                            (ns)            Act/Stdby
32K   (PS)    uPD42832C        -15L (150)      220 / 2.75      Nec
128K  (S)     TC551001BPL      -10L (100)      27.5/ 0.02      Toshiba
512K  (S)     TC554001BPL      -70 (70)        50.0/ 0.30      Toshiba
128K  (PS)    TC518128PL       -12 (120)       275 / 0.55      Toshiba
512K  (PS)    TC518512PL       -10 (100)       275 / 1.00      Toshiba
128K  (PS)    HM658128ALP                                      Hitachi
512K  (PS)    HM658512LP                                       Hitachi
512K  (S)     BS62LV4006PCP55  -55 (55)        290 / 0.15      Brilliance Semiconductor, Inc.

(PS=Pseudo-static RAM chip, S=Static RAM chip)