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This page last updated on 14 August 2001

[ULA] [AY Sound Chip] [Keyboard] [Keyboard Repair] [128K Composite Video] [128K RGB] [Power Supplies]

He multiplexed address-lines.

/WR   2    39 Q          One of the +5V is decoupled through a RC-low-pass.
/RD   3    38 /MREQ      U,V are the color-difference signals.
/WE   4    37 A15        /Y is the inverted video including sync.
A0    5    36 A14        D are the data-lines, decoupled from the CPU by
A1    6    35 /RAS       resistors.
A2    7    34 /ROM CS    T are the data-lines to the keyboard (address-lines
A3    8    33 /IO-ULA    through diodes).
A4    9    32 CLOCK      SOUND is the analog-I/O-line for beep, save and load.
A5   10    31 D7         CLK is the clock-source to the CPU including the
A6   11    30 D6         inhibited T-states.
/INT 12    29 D5         IO-ULA is (A0(CPU) OR /IORQ) for the I/O-port FEh
+5V  13    28 SOUND      Q is the 14MHz-crystal, other side grounded through
+5V  14    27 D4         a capacitor
U    15    26 T4
V    16    25 D3
/Y   17    24 T3
D0   18    23 T2
T0   19    22 D2
T1   20    21 D1

SOUND C   1    28  D0       Vcc is +5V.
PORT      2    27  D1       SOUND A, B and C can be tied together.
Vcc       3    26  D2       CLOCK can be some MHz.
SOUND B   4    25  D3
SOUND A   5    24  D4
GND       6    23  D5
PORT      7    22  D6
PORT      8    21  D7
PORT      9    20  BC1
PORT     10    19  BC2
PORT     11    18  BDIR
PORT     12    17  A8
PORT     13    16  RESET
CLOCK    14    15  CLOCK

  OUTER SIDE   A 15 14 8  13 12 9  10 11  INNER SIDE
             D
             0   BR EN CS P  0  A  Q  1
             1   SS L  Z  O  9  S  W  2
             2   M  K  X  I  8  D  E  3
             3   N  J  C  U  7  F  R  4
             4   B  H  V  Y  6  G  T  5
  INNER SIDE

  [BR] BREAK   [EN] ENTER   [CS] CAPS SHIFT   [SS] SYMBOL SHIFT

In real the matrix connections are in one row on the top side of the membrane.

Replacing keyboard membranes on the Spectrum, Spectrum + and Spectrum 128:

[Thanks to Brian Gaff for most of this information]

First find your good membrane: this seems to be the impossible part! If you have a black 128K machine, this uses the same membrane as the Spectrum +.

It may be the case that the connectors on the keyboard PCB have scratched off the silver from the ribbon, in which case it is possible to buy some so-called "conductive paint" which is a suspension of fine silver particles. If you let it dry, it conducts. Use a fine brush to replace what is scratched off from the ribbons. This paint is available from most electronics hobbyist stores (for example, from RadioShack in US).

If not, you'll need a new membrane: see the Spare Parts section for some possible sources.

Firstly, remove all the screws in the bottom of the case. On the rubber keyed model, two of these may be under the front feet. Also note that the screw at the rear in the centre is different to the others in some cases. Turn the computer the right way up and gently lift the top of the case. You should see two flat cables linking the keyboard to the PCB.

Carefully pull these upward without twisting them. They usually pull out easily. Grasp them as near the PCB sockets end as you can. They also tear easily, so try not to leave any bits behind in the sockets. On the Spectrum + and 128, remove the keyboard and take note of how the rear sprung feet are assembled. Murphy's law dictates that these are bound to fall out as you re-assemble the case later!

Back to the rubber keyboard now, and there were two versions of these. Look beneath the keyboard. If there are any small metal lugs at the outside edges, gently prize them upwards. This may allow the top metal plate to be removed. However, the majority of Spectrums relied on good old glue to secure the top plate, and getting these apart can be a slow job if you wish to avoid bending the plate.

The best way is to slowly pull it off. Try not to kink it though. Once this is free, there is a rubber mat and then the membrane. This part is easy. Just replace the membrane, making sure it is the correct way round with the flat cables poking through the slots as before. If there is not enough sticky left to hold the metal plate, then contact adhesive or double sided tape work well. Try not to get any of the adhesive on the membrane or rubber mat though.

The Spectrum + calls for slightly more labour to do the job. Turn the keyboard upside down and remove all the screws in the backing plate. Also remove the two clamps at the rear where the flat cables run. Once this is done, the backing plate can be removed, leaving the membrane exposed. This is a more complex beast than the rubber keyed one.

Now, replace the old membrane with your new one, making sure it is located on the spigots correctly, and replace the backing plate and screws. These do not need to be too tight.

Care must be taken to get the flat cables laying straight through the clamps. Try to do the screws up a little at a time evenly. Do not over tighten them though. As a matter of interest, these clamps are used to make connections between various layers of the membrane, so any mis alignment could cause strange keyboard faults! [I'd go along with that alright - I threw away a perfectly good membrane when all that was wrong was that one of the clamps hadn't been tightened enough. - Damien]

All that remains is to carefully replace the ends of the flat cables into their sockets. Again, try not to buckle them, as they are fragile. Once this is done, lay the keyboard on the machine and test it. Test ALL the keys and combinations. If all is well, then the screws can be replaced. If it isn't, check that the cables are in the connectors properly, and that there is no debris in there from the old cables. On the + and 128, suspect the clamps first! Simply loosen them and realign the cables. Remember when replacing any screws, that they are only screwing into plastic, and you only have to over tighten them once, and the pillar they screw into could either crack or just end up with a large hole in it!

[If the ends of the membrane cables where they attach to the PCB have been cracked, you can get away with cutting them off past the cracked point and inserting the cable back in the slots. There is not much slack in the cables though, so you may find this tricky if the split is any more than around a centimeter away from the socket - Damien].

[Thanks to Henning Bog for this information]

Pin 1 is composite PAL, 2 is GND. There's no audio on any pin of the RGB connector.

The picture is a bit dull unless your TV's video input is High Z (high impedance) However, that's quite unlikely. Normally the impedance of the video input is around 75 Ohms. To alleviate this problem, short the 68 Ohms resistor inside the Speccy that's in series with pin 1 (follow the track on the PCB). Or you can hook it directly to the input of the RF modulator.

The audio can be taken from the Mic socket but a better balance between 48K and 128K sound is obtained directly from pin 5 of IC38.

For the 128K machine:

        RGB              Pin  Signal              Level
                         -------------------------------------------------
   --7--   --6--         1    Composite PAL       75 Ohms, 1.2 Volts pk-pk
         |               2    0 Volts DC
    3--  8  --1          3    Bright output       TTL
  --     |     --        4    Composite sync      TTL
      /     \            5    Vertical sync       TTL
     5   |   4           6    Green               TTL
    /    2    \          7    Red                 TTL
         |               8    Blue                TTL

And for the +2A/+3 (thanks to Mike Wynne):

        RGB              Pin  Signal              Level
                         -------------------------------------------------
   --7--   --6--         1                        +12V
         |               2    GND                 0V
    3--  8  --1          3    Audio Out
  --     |     --        4    /Composite Sync     TTL
      /     \            5                        +12V 
     5   |   4           6    Green               Analogue 1.67V p-p
    /    2    \          7    Red                 Analogue 1.67V p-p
         |               8    Blue                Analogue 1.67V p-p

The power supplies for the various machines are rated as follows:

Machine             Voltage   Amperage  Centre Polarity
-------------------------------------------------------
ZX-81                 9V DC     500 mA        +ve
Spectrum 16K/48K/+    9V DC     1.4  A        -ve
Spectrum 128          9V DC     1.85 A	      -ve
Spectrum +2           9V DC     2.1  A        -ve

Timex 1000/1500       9V DC     500 mA        +ve
Timex 2040           24V AC     1.2  A        n/a
Timex 2068           15V DC     1.0  A        -ve

TC 2048		      9V DC     800 mA        -ve
TC 2068               9V DC     1.0  A        -ve

Z88                 6.5V DC     500 mA        +ve

Alphacom 32 Printer  24V AC     1.2  A        n/a
Timex Modem           9V DC     500 mA        +ve

Flat-screen TV      5.9V DC     100 mA        -ve

From Ian Collier:
This is what the end of the +2A/+3 PSU lead looks like (which is obviously a mirror image to what the power socket looks like):

    ---u---
  /         \
 / +5    -12 \
|      0      |
 \ +5    +12 /
  \   +12   /
    -------

The 5V line is rated at 2A, the +12V at 700mA (200mA for the +2A) and the -12V at 50mA.

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