Two more Electron repairs


At the recent ABUG Scotland meeting I picked up a couple of Electrons that were in an unknown condition. My initial idea was to use them as donor machines for my clone but when I got them home I thought it would be worth checking them out first. So here goes..

Electron 1

First look and it's a bit dirty..I'll give it a quick clean before I open it up.

On the inside we can see it's an issue 4 with a removable ULA, on the left hand side I can see a corrosion on the modulator but other than that it is quite clean.

Time to turn it on..and right away it can be seen that the screen text is blue and there is no CAPS LOCK LED. Testing the keys and about four didn't work.

Pressing each faulty key a few hundred time and they slowly came to life.

I thought the blue text would likely be caused by a bad connection between the ULA and its socket so I removed it and inspected and cleaned both the socket pins and the ULA contacts. Some corrosion can be seen on the ULA top & bottom left.

With that cleaned I popped it back in and tested it again. Blue text sorted.

Now to replace the CAPS LOCK LED. The new one is a lot brighter than the original so its current limiting resistor will need to be replaced with a higher value to dim it down a bit.

That's one working..

Electron 2

This was a bit cleaner on the outside but not so on the inside. The corrosion is a bit worse on the modulator and the big concern is the corrosion on the keyboard ribbon cable. This Electron is a version 6 with the soldered in ULA so if there is a problem with that then games over..

Time to switch on..white bars..this normally points to the ULA or memory being faulty or bad solder joints round these IC's.

Due to the poor condition of the keyboard ribbon cable I tested the continuity of each wire and surprisingly they were all ok. I plugged this keyboard into Electron 1 to test the keyboard. It had just a couple keys not responding, again just a few hundered key presses and they started working.

Looking round the underside of the circuit board there were two areas of concern, the first was around the 6502 processor with about half a dozen bad solder joints and the other was one pin on the ULA. Once resoldered it didn't make any difference, it still powered with the white bars. Time to get the logic probe out.

Checking the 6502 Address and Data pins all show activity, the reset pin goes high on power up but when checking the NMI, IRQ and R/W there was no activity and that tells me the ULA is dud. So at the moment this machine will be set aside for spares.