Another ZX Spectrum's near-death experience
Approximately one year ago I got two Sinclair ZX Spectrums (48kB versions) from Cyberpipe's computer museum to repair. I was quickly able to put together an undamaged circuit board and a working keyboard into one working machine which was then on display in the museum for some time. After that I was left with enough parts for another Spectrum which I declared unrepairable because it seemed that every imaginable component was burned out.
However after tinkering with it every once in a while when I found some free time I got it back to a perfectly working condition. Looking back it was quite an amazing experience. Here's a list of all things that needed repair in the order I repaired them:
There was a big hole in the 5V regulator. Replacing that was the easy part.
The ferrite core of the little transformer on the circuit board was also shattered, probably because of over heating. The wiring also appeared to be damaged. When I couldn't find a similar new core I ended up gluing back together the original core piece by piece with epoxy resin and then making new windings.
Continuing with the obvious stuff, there was also a resistor with a crack along it's body. Perhaps the transformer core shattered because of a mechanical not thermal shock after all?
Since the transformer of the DC-to-DC converter was obviously shorted at one time it was easy to guess that the switching transistor TR4 (ZTX 650) was also burned out. Getting a replacement however wasn't that easy because this NPN transistor has to withstand 2A of pulse current but also has to be in small a TO-92 or TO-18 case in order to comfortably fit on the circuit board and under the keyboard. I tried a bunch of transistors from a basic 2N2222 to a fancy 400V ST13001 I got from an old fluorescent lamp. They all eventually burned out (with ST13001 holding a record at several hours of operation). I only found a satisfactory replacement two weeks ago when I noticed that one of the local electronic component shops had an old stock of ZTX 651 transistors.
Now it was finally time to fully power up the computer and see how extensive was the damage to the digital part. As expected the first time only four characteristic black and white vertical bars appeared on the screen. I suspected that the low 16 kB of RAM was to blame and it took me a couple of months to find a supply of chips that could be used to replace the original 4116 ICs (4164 ICs by the way and I only got four for quite an outrageous price). After experimenting a bit with RAM chips however I couldn't find specific chips that were bad.
Suspecting the CPU, I took the Spectrum to my Faculty's lab and removed all the big chips from the circuit board and put them into sockets. Replacing the CPU with a known-good chip however didn't help. After having some wacky ideas how to diagnose this problem I got some good advice from Winston on the World of Spectrum forum to suspect the ROM.
After confirming that the ROM chip was certainly acting strangely (I definitely wouldn't suspect it myself, thinking that a mask ROM was the most robust IC in the Spectrum) I've found an old 27C128 EPROM, programmed the original ROM contents in it and modified the circuit to accommodate an EPROM instead of ROM.
Success! Spectrum now boots to the "Copyright by Sinclair" screen, however the keys don't work. As usual the keyboard membrane broke in several places. I first tried to repair it, but soon gave up (there's a funny story here about how the membrane worked properly each time I tested it on the table but was dead when I connected to the machine).
Luckily these guys offer cheap new (manufactured in 2007!) replacement membranes. Now it was just a matter of undoing all my previous keyboard-related modifications.
Keys now work, but there's no familiar clicking sound from the speaker. Interestingly one last component needed replacement - the protection diode on the audio output pin of the ULA chip. This also made me wonder if the damage to this board was made by connecting the power supply with a wrong polarity. This diode would form a direct short circuit in that case and I can't imagine any other fault which would damage it.
So, now I have two Spectrums in a working condition. Maybe there's a game that supports multiplayer on two connected machines?
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This is great! I`m trying to fix a zx spectrum myself.
Same problem, computer dead, tested voltage regulator giving out 6.5 volts, changed that, as you say the easy part.
My suspect is now the coil I have resistance readings of 0.001 ohms across one coil and 0.007ohms across the other, is this faulty?
TR4 I think has definately blown.
My question is how did you make new windings on the coil?
Thank you for an informative post.
All the best.
Ade.