The Lil Old ZX Spectrum 48k Service Manual

SUPPLEMENT No 2

LIST OF CONTENTS

Introduction

The ZX Spectrum+ is basically the same as the 48k ZX Spectrum with a different keyboard and case, and a reset button. The board circuitry is similar apart from the reset button and the fitment of an extra resistor.

The layout of the keyboard is different, which because of its complexity now needs a five-layer membrane. The spring action of the keys is provided by a bubble mat.

CONVERSION PROCEDURE - SPECTRUM TO SPECTRUM+

  1. Remove board from the early version case.
  2. Change heatsink (on Issue 1 and Issue 2 boards only), retaining the triangular insulator, if originally fitted.
  3. Connect the reset switch across C27 on the top side of the board.
  4. Test keyboard, especially STOP.
  5. If STOP does not work, connect a 22k resistor across R68. (On the Issue 1 board, connect a 22k resistor between pins 1 and 5 of RP1, on the reverse side of the board).
  6. Fit four rubber feet to the underside of the case.
  7. Fit board to case bottom.
  8. Press the reset switch into the recess in the side wall (ensure that the leads are not trapped).
  9. Fit foam pads: one to the top of the modulator; and on Issue 3 (and higher issue boards), two on the top of the heatsink. These are to protect the membrane tails from chafing.
  10. Drop folding legs into slots. Sit the springs on top of the legs.
  11. Connect the keyboard flextails.
  12. Fit the top cover, taking care not to crease the tails.
  13. Test.
  14. Fit warranty seal, if applicable.

For details, see the ZX Spectrum+ upgrade instructions, which accompany the upgrade kits.

ASSEMBLY OF THE KEYBOARD FOR THE SPECTRUM+

The flextails running from the keyboard to the clamps should lie in an S shape, to prevent tension in the membrane. This is achieved by ensuring that the end of the tracks on the middle layer protrude by 1mm past the clamps. This is easily seen on the wider tail, but not so clear on the narrow one. Visible markers are provided on future membranes.

When clamping the flextails, ensure that an extra packing piece is included under the tails at the clamp position. (These are fitted to current production models). These should be approximately 0.75mm thick, and as wide as the tail. Paxolin is a suitable material. This section does not apply to any Spectrum+ which has S as the first digit in the serial number.

When assembling the keyboard, it is advisable to tighten the screws in the following order:

The screws should be tightened to the minimum to give a firm fit, (eg a quarter-turn back from full tightness).

Ensure that on each membrane tail the 2 tail surfaces are stuck together, to avoid short circuits on the board. This is best achieved by using a piece of double-sided tape, or a pair of tabs around the edges, near the connector end. This is current practice in production.

Both metal and plastic reaction plates are possible. The metal plate has to be accompanied by a plastic insulator. The plastic reaction plate is similar and cheaper. The metal plate needs 6.5mm fixing screws and the plastic plate needs 8mm.

It is advisable to check the operation of all the keys on a new assembled keyboards.

KEYBOARD OPERATION

The Spectrum+ keyboard is in effect two switching membranes in parallel, a 'top' membrane and a 'lower' membrane.

The top membrane is similar to that for the Spectrum 48k, except that pressing any one of the following keys also makes contact between row 1 and column 6 on the top membrane in addition to their dedicated row/column positions:

TRUE VIDEO EXTEND MODE CURSORright
INV VIDEO EDIT CURSORleft
BREAK CAPS LOCK CURSORup
DELETE   CURSORdown
GRAPH    

For example, pressing TRUE VIDEO connects row 1/column 6 on the top membrane as well as row 3/column 1 on the lower membrane.

Also pressing any one of the following keys also makes contact between row 2 and column 8 on the top membrane in addition to their dedicated row/column positions.

. (full stop)

; (semi-colon)

" (inverted commas)

, (comma)

For example, pressing the full stop key connects row 2/column 8 on the top membrane as well as row 3/column 8 on the lower membrane.

SPECTRUM+ KEYBOARD : TOP MEMBRANE

Spectrum+ Keyboard Top Membrane Diagram

Click on the above image for a larger version (44.8k GIF file)

SPECTRUM+ KEYBOARD : LOWER MEMBRANE

Spectrum+ Keyboard Lower Membrane Diagram


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