You may find
this web page helpful
As it says, be VERY careful when testing not to short anything out!
Myself, I use a spare PCB mounting DIN socket when testing, but I very much doubt that most people will have one of these lying around.
Alternatively, as long as you are careful with the keyboard ribbons (and do be careful, as membranes for these machines are not readily available as spares), you can test the voltages on the main PCB of the computer.
All voltages are DC and all are referenced to the common 0V/GND (ground). Note, this is not the same as mains earth/ground.
An easy place to put the negative (black) multimeter lead/probe to connect to a 0V point, is the outer of the TV/UHF output on the modulator. Use this as your 0V/GND reference point.
A pin-out of a Z80A is linked to in my sig, repeated here:
ZX81 Chip Pin-outs. All DIL/DIP chips have their pins ordered in a similar way. As you will see for the other chips in that topic.
The Z80A is IC2. Set your multimeter to the 20V DC voltage range (or equivalent). Measure the +5V supply on pin 11. It should be between 4.75V and 5.25V. If it is under, or over this range of values, then the PSU is faulty, or there is a problem with the cable or one of the connectors.
The colour encoder chip is IC9 (the chip will be labelled TEA2000). Measure the +12V supply on pin 11. It should be between 10.8V and 13.2V. If it is under, or over this range of values, then the PSU is faulty, or there is a problem with the cable or one of the connectors.
Be very careful when testing voltages on the pins of the chips, it’s very easy to slip the probe between two pins, and this could cause damage to that chip or others that are connected to it.
Please report your findings back here if you want more help.
Incidentally, if you intend to use this machine a lot, we recommend using a RGB SCART lead like
this one . This is a good quality type. This seller also has their own website:
https://www.retrocomputershack.com/. This is my preferred option. But as you can see, they are having a break at the moment.
An alternative, that is just as good, is
this one. Don’t let the name put you off.
I’ve bought from both these sellers in the past, and am happy with the products.
However, ZX Spectrum computers were never designed to work with modern TVs, and the leads can’t overcome the lack of a proper +12V supply from the ZX Spectrum +2A/+2B computer. So TVs that require working automatic function switching inputs on their SCART input sockets may not work. Unfortunately, with so many different models of TV that are in use, it’s impossible to known in advance if a particular model will work okay.
Mark