Dragon 32 - REPLACEMENT OF POWER SUPPLY AND CENTRAL PROCESSING UNITS ON THE DRAGON 32

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Since the introduction of the Dragon 32 there have been several versions produced of both the Central Processing Unit (CPU_ and the Power Supply Unit (PSU).

Most versions of the CPU and PSU will work together and this service sheet identified which combinations will, and the modifications necesarry to implement each of them.


Identification of boards

The first setp when replacing either unit with one of a different version is to identify the version of each of the boards you wish to use.


PSU's can be identified by the type of modulator used. The model number being clearly marked on the top as follows:

MK1 - 1286 Modulator

MK2 - 1233 Modulator

MK3 - 1287 Modulator


CPU's can be identified by comparing the RAM area with the descriptions below:

16x16 - Two rows of RAM chips, 1x9 and 1x7

Piggy Back - 6 chips on main board, piggy back board in sockets 1 and 8, 11 chips on piggy back board

Siemens - 1 row of 16 RAM chips, 2 chips per socket (one chip on top of another)

OKI - 8 RAM chips (usually labelled OKI)


Next from the table below, check that the combination you have chosen is a usable one.

PSUPIGGY BACK CPUOKI CPUSIEMENS CPU16x16 CPU
MK1yesyesyesyes
MK2nononono
MK3yesyesyesyes
  • NOTE: MK2 PSU's are obsolete and when encountered should always be replaced with MK1 or MK3 boards.



MODIFICATIONS

Having selected a useable combination carry out the appropriate modifications from the following:-


Contents

MK1 PSU and Piggy Back or OKI CPU

CPU:

    270R resistor in series with cassette input, i.e. the connection to Pin 4 of PL4
    *Sony Mod.

PSU:

    10R resistor in series with +5v feed to modulator.


MK3 PSU and Piggy Back or OKI CPU

CPU:

    150R resistor short circuit over R9.
    Cut resistor R6 (C49 side) and solder single cone wire to cut end, then solder other end of wire to Pin 14 IC11
    Drill back of board to open circuit track, i.e. between IC11 Pin 13 and cut end of R6.
    *Sony Mod.

PSU:

    Remove R1. This will either be a 1.2K or 68R resistor. For the 68R resistor, this was a modification and may be 
    found on the back of the board and linked to R7; Replace 330R resistor in original R1 position.


MK1 PSU and 16x16 CPU

CPU:

    1K resistor across PL6 Pin 9 and 0v. If background lines are prominent, fit 0.22uf capacitor in C58 position.
    *Sony Mod.

PSU:

    10R resistor in series with +5v supply to modulator.


MK3 PSU and 16x16 CPU

NOTE: With the 16x16 CPU, all mods (except possible add on capacitor to C58 position) are completed at time of manufacture, howvever some boards may still be present without mods. Mods as listed below:-

CPU:

    6.8K resistor from sholder pad near C6 to bottom of R6 (C49 side).
    220R resistor short circuited across R9 (R9 being 270R). Alternatively replace R9 with 120R.
    If background lines are prominent, fit 220N capacitor in C58 position.
    *Sony Mod.

PSU:

    Remove existing 1.2K resistor in position R1 and replace with 330R resistor.
    Remove existing 120K resistor in position R6 and replace with 22K resistor. Retune modulator sound core.


MK1 PSU and Siemens CPU

CPU:

    Change C7 to a 2.2-22pf trimmer. Place a 1K resistor between 0v and PL6 Pin 9.

PSU:

    10R resistor in series with +5v supply to modulator.
    *Sony Mod.


MK3 PSU and Siemens CPU

CPU:

    6.8K resistor from bottom of R6 to solder plate near positive side of C28, 200R resistor short circuited over R9.
    C7 change to 2.2-22pf trimmer.
    1K resistor between 0v and PL6 Pin 9.
    *Sony Mod.

PSU:

    Remove existing 1.2K resistor in position R1 and replace with 330R resistor.
    Remove existing 120K resistor in position R6 and replace with 22K resistor. Retune modular sound core.


*Sony Mod

This modification was originally introduced to overcome loss of colour band problems encountered when the computer was used with Sony 22" televisions. It has been found however, that it also cures other colour problems and for that reason it is now considered standard on all CPUs.

The modification is carried out by fitting 1 15K resistor across IC10 Pin 4 and IC12 Pin 10, or for neatness on the 16x16 and Siemens board the resistor may be fitted from C28 negative side to a solder plate on IC12 pin 10 line.

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