While fixing previous spectrums I bought a cheap motherboard in poor condition to use for spares. I knew it was rough from the sellers pictures, and when it arrived in the post it was obvious that it had suffered from several previous attempt to ‘fix’ it – there were multiple visible broken tracks and many of the chips had obviously been replaced. Rather strangely there were also a number of bodge wires on the top side of the board running under chips – WHAT ??
Over Christmas 2020, with Covid lockdown, we weren’t going out much and I had run out of retro computers to fix; so I decided to see what I could do for this abused Spectrum.
The first stage was to remove all of the main chips to see what state the motherboard was in. If it was beyond repair I could at least test the chips and reuse them later. I also removed the TV modulator – this was not going to be used again.
With all of the chips removed I checked the board over and replaced the previous bodge wires (on the back of the board this time). I also checked the continuity of all the data and address lines to check for other broken tracks – there were a couple around the lower memory, which I also fixed. Happy that I had done enough I added nice new sockets for all of the chips.
Installing chips back into the board I would love to say that it worked first time – it didn’t. Following many more hours of debugging with my oscilloscope and tracing tracks against the schematics I finally began to see life as the diagnostics began to run and finally a BASIC prompt was displayed.
With it finally working I decided it was going to be my ‘test bed’ Spectrum, everything was in sockets making it easy to test chips from other Spectrums. What I needed was built-in diagnostics. I decided to replace the standard EPROM with a custom multi-rom EEPROM. There are lots of guides online so I gathered a few components (mainly diodes, a switch and a couple of bodge wires). I also replaced the TV modulator with a bespoke PCB composite mod.
The result was a custom ROM containing two images, selectable by a switch. I programmed two custom ROMs, one with diagnostics images and another with ‘enhanced’ BASIC and diagnostics – and it works great.
What do I do with it now ? I wanted to have it displayed, accessible and functional. The idea of the ‘Naked Spectrum’ was born. The bare Spectrum motherboard mounted on a display board, combined with a modern power supply and small monitor. I could then use it either as a rolling display or soak test for suspect chips.
The visible power supply is just the outer case from an original supply, the system is actually powered by a modern switching power supply mounted on the back of the board. I have included volt and amp meters to monitor the current draw of the Spectrum (helping to identify problems/shorts quickly). The final piece is an 8″ TFT display running off the component signal. I added a Dandanator mini (see my previous post) to fast load games and other diagnostics – the joystick port also allows me to drive the menus using a standard Atari joystick.
All done 🙂