Which power supply? Original sinclair zx or modern? Which one do you prefer? Which one to choose?

[Someone]

The Speccy’s power supply circuit is quite interesting. If I’ve correctly interpreted it comprises of input voltage 9-11V DC (unregulated) to generate +5VDC (regulated) from a linear voltage regulator and its other voltages from a oscillator with some voltage multipliers to get +12VDC, 12VAC and -5VDC (with zener diode). The heat generated from a linear voltage regulator is the excess unused power which is approximately the difference between the input voltage and output voltage multiplied by the current drawn (i.e. power - the higher the input voltage the more heat is dissipated). Usually I’d suggest using an LDO (Low Dropout Out) voltage regulator and a lower input voltage to lower the heat dissipation but in the case of the Speccy other output voltages are reliant upon a minimum input voltage of 9VDC. There are many SMPS replacements for the 7805 coming from brands such as Murata, Traco, Recom, OKI, XP power and others. Just ensure that they meet the specs you need.

 

[cj7hawk]

The new switching modules are incredible. And in the past, more than once, I considered desoldering the spectrum PSU and installing a SMPS from a video game...

The problem with the Speccy design - is if a lower RAM chip fails, it cooks the transistor in the PSU oscillator, and then the rails die, and more ram chips die. It's a bad situation.

If it wasn't for the aesthetics of the old PCB, I would replace the entire circuit with a 12v input, and a +5-5 micro converter and do away with the oscillator based system.

OTOH, I have gotten pretty good at fixing spectrums, and have better equipment now, so fixing them a lot isn't so much of a problem.

 

[Someone]

…if a lower RAM chip fails, it cooks the transistor in the PSU oscillator, and then the rails die, and more ram chips die. …

I saw that as an potential problem and might be mitigated if you fused the -5V (and other oscillator derived voltage) lines.
Just a thought.

 

[cj7hawk]

I saw that as an potential problem and might be mitigated if you fused the -5V (and other oscillator derived voltage) lines.
Just a thought.

That's the problem. Once the -5v rail fails, it causes the other 4116 ram chips to fail. They will be damaged if run without the -5v rail. You would have to fuse each of the chips individually which isn't practical at the low currents involved... Though I suppose you could limit them via resistors though I don't know how well that would work in practice. It would probably require a slightly lower voltage to offset the load.