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ZX Spectrum Plus

This Spectrum provided my second attempt at a repair.  I knew that this one was going to be more challenging than the 48k Rubber Key model because of my own stupidity.  At some point, the wire had broken off in the power supply plug, and I made the mistake of cutting it off, and resoldering it the wrong way around!

At that time, my electronics knowledge was limited, and didn't realise that the Spectrum has the positve on the outer ring of the plug, and the negative on the centre pin.  Having realised my mistake, I changed the voltage regulator - but the Spectrum still didn't work - so it was confined to a cupboard for years.

Upon opening the machine up, the first thing that I noticed was that the ULA socket was soldered in upside down.  This was corrected before I continued with the diagnosis.

Having measured the voltages that the internal power supply was generating, it was pretty obvious that the main fault lie here.  In fact, TR4 had failed - this is a common failure mode, as the supply volage at the wrong polarity means that TR4 gets the supply voltage straight up its emitter.

Having changed the Transistor, I got a little more life out of the machine - at least the voltages were correct, but I only had a herring bone pattern on the screen.  Suspecting the lower memory, I changed the 4116s for socketed 4164s

Once again, it is common that the memory chips fail when the power supply is not working as it should.  The old 4116 chips need +5V. -5V and 12V - hence the additional wires which make the changed for the 4164s which only need +5V.

Next, the machine was tested and found to be working.  The only remaining issue is that the speaker is a bit on the quiet side, which may be as a result of a failed TR7 - but that is another problem for another day.

I also have another eight 4164s to replace the upper memory, but didn't bother as the upper memory was still working.  After all, if it ain't broke, don't fix it!