Been doing some more testing with the scope this lunchtime, concentrating on the RAM. One of the 4116 chips in the bank IC6-13 has a much lower voltage (and different shaped trace) on the DOUT pin and one of the RAM chips in the bank IC15-22 has no voltage at all. If the bottom 16K of RAM is faulty then that would explain the unsuccessful boot. I’ve got no reference trace for a working 4116 …
Finally got around to powering up the motherboard this morning and have done a preliminary check of the board. First problem was to find a good ground connection for connecting the crocodile clip of my probes too. Thankfully the Spectrum has a common ground and it turns out the heatsink is grounded, so that problem was solved fairly quickly. I thought I’d start off with a simple test. Is the Z80 receiving a 3.5Mhz clock …
I’ve had a stab already using my limited tool kit by following this guide (http://www.worldofspectrum.org/SpectrumRepairGuide). I’ve got as far as checking the board visually (well, it looks okay) and metering out the power (that checked out fine). The rest of the guide is all cutting tracks and bending pins and to be honest some of the legends are difficult to read on the hand-drawn diagrams. As luck would have it I will be getting a …
