Installing EPROM in ZX Spectrum
When I was repairing an old Spectrum I saw that every byte coming from ROM has a one bit "hole" blown in it. ROM was damaged probably by accidentially shorting one of data lines and unstabilized 9V DC a mistake when plugging cassette recorder into Speccy. How to replace a ROM with another one?
First, you should burn an EPROM using ROM image coming with emulators. The image fits to 27128 (or 27C128) but you can burn 27256 with some alternate ROM. Now let's look at differences between Spectrum's ROM and an rodinary EPROM:
- Pin 1 in ROM is not connected, but in EPROM it's its Vpp, which must be powered to 5V for many EPROMs.
- Pin 20 is Chip Select in ROM, Negated Chip Enable in EPROM.
- Pin 27 OEI in ROM, it's PGM in EPROM (or additional address line in 27256).
What we need to do?
- Connect EPROM's pin 1 with pin 28, which is not dificult, in Speccy Issue 2 pin 1 of ROM is not connected.
- EPROM's Pin 27 must be kept high (in 128) or switched high-low (in 256). So in 128 we eill connect it to pin 28, in 256 we'll connect it to switch.
- EPROM's pin 20 must be supplied with pin 20 OR 21. We need to build a simple OR gate.
Havina g EPROM we must:
1. Connect pin 1 with 28 with wire. If 128 we must bend pin 27 upwards and connect it with 1 and 28. As in a photo:
2. Bend pin 20 up and solder a 10-20K resistor to it
3. Solder a 6-10K resistor to pin 14 (without bending it) and connect it with free wire from the first resistor.
4. Solder a wire to junction of these resistors.
5. Find a good access to pins 27 and 20 of ROM on mainboard. In Issue 2 it's shown below. Solder two 1N4148 signal diodes to them. Although some recommend using Schottky's diodes I had no problems with signal ones. Solder these diodes to holes with their anodes (the part without black ring). Now connect their cathodes (designated by ring on diode housing) together. Solder a wire from EPROM to this junction.
In Issue 2 the points are right below power socket and near D1 and D2.