ZX
Spectrum 16K, 48K and Plus models
DC-DC CONVERTER MODIFICATION
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:
Diodes:
• D15 - BA157 (it may be possible to reuse the existing D15)
• D17 - BA157
• D18 - 1N4148
• D19 - BZY88C 5V1 500mW 5.1V Zener
Capacitors:
• 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
Resistors:
• 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:
• 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.
