Friday, 23 August 2019

ZX Spectrum 16K, 48K and Plus models - DC-DC CONVERTER MODIFICATION

ZX Spectrum 16K, 48K and Plus models


Updated and corrected by 1024MAK. Version 7E3823_02.

This modification and upgrade is only needed on ZX Spectrum boards that are issue 4a or earlier where it has not already been performed.

The modification results in improved converter operation, prevents loss of the -5V supply if the input voltage of the DC supply to the computer rises above 11.9V and improves the voltage on the -5V power rail.

The following new materials are required:

D15 - BA157 (it may be possible to reuse the existing D15)
D17 - BA157
D18 - 1N4148
D19 - BZY88C 5V1 500mW 5.1V Zener

C46 - 1uF 50V WKG 105°C Electrolytic or ceramic axial (only required on issue 1 and issue 2 boards)
C47 - 1uF 16V WKG Electrolytic axial or ceramic axial
C49 - 560pF ceramic axial (if not already this value)
C74 - 4.7uF 16V WKG Electrolytic radial (if not already fitted)
C78/C80 - 22uF 25V WKG Electrolytic axial
C79 - 1uF 16V WKG Electrolytic axial or ceramic axial

R60 - 220Ω 5%, 0.25W
R79 - 2.2kΩ (2k2) 5%, 0.25W

Remove the following components from the printed circuit board:
D12- 1N4148 Diode
D16 - 5.1V Zener
C47 - 22uF Electrolytic
C49 - If not already a 560pF ceramic axial
C77 - various values used
R54 - 100kΩ
R55 - 56Ω
R60 - 270Ω, various values used

Embody the modification as follows:

Remove the anode of D15 from the board.
Connect C78/C80 between the former anode PCB pad for D15 (the PCB track that connects to C49) and the now free end of D15 anode, positive end of C78/C80 to the PCB pad.
Connect D17 between ground (anode end of D17) and the new connection of C78/C80 to D15 anode.

Substitute components as follows:
Remove Substitute New Value
D12 R79 2.2kΩ 5%, 0.25W
D16 C79 (positive to ground) 1uF 16V
C46 (only issue 1 & 2 PCBs) C46 1uF 50V 105°C
C47 C47 1uF 16V
C49 C49 560pF
C77 None Open circuit
R54 D19 (cathode to ground) 5.1V Zener
R55 D18 (cathode to C46) 1N4148
R60 R60 220Ω

Install C74 if not already fitted between R58 lead (junction with R58/R59/TR5 base) and C34 positive lead (+5V connection) with C74 positive to C34 positive lead.

Circuit Operation
The modification is confined to adding/changing various components in the DC/DC converter circuit to improve reliability. Circuit function does not change.

Please take the notes above into account when reading the above diagram.

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