OK, so I got a multimeter. I had a look at one that had continuity mode but they didn't have it in stock, so I got one without for now under the impression I can still test the continuity using resistance mode (without the beep albeit).

So I guess that means I'll be testing resistance and continuity at the same time. I've done a quick little test and the positive (inside pins) on the end of each cable does indeed have a current, whereas the negative (outer barrels) don't. On the switch, you get a current through it as long as the switch is set to the right position (as it should, I guess)

I'm not quite sure about the female 3.3/5.5mm jack as the female hole for the 1mm pin is so small that the MM contact won't fit in and touch any of the metal that I'm assuming is inside. However, I unscrewed the barrel on the connector and just touched the red contact to where the red wire is actually soldered into the connector instead, which got a current. Putting the red contact onto the outside contacts of the plug momentarily gives me a -1, which I guess is right, but then it changes to bigger numbers then slowly goes down 0.04, not sure if this is meant to happen?

A couple of questions though:

1) How many ohms should the positive pins on each end of the cable be reading - if they're working properly that is?
2) Is there a way to check for shorts or reversals?

Thanks

 

It should be as close to 0Ω as possible, single digits at most.

Shorts will show up as continuity between the pin and the barrel on the same end when not plugged into anything, and reversals will be the same but on far ends.

You can also check for the presence of the expected voltage at the end of the line when turned on.

 

Hey-I return with strange, but good and unexpected news!

So, I just plugged the PSU in and tested the voltage at the end of the OEM DC plug, got a 12.17v current. Then added the 1st cable and tested the plug on the end of that and got almost the same, then the switch, the same. Turned the switch to off position to test that and it cuts the power as it should. Added the last homemade cable and got 12v once again.

So I thought what the hell. Got the mixer and plugged it in-viola-it works! This time it was plugged into the actual plug it will be plugged into.

The weird thing is, I plugged it into 2 completely different plugs the first time to test it worked (when it powered on momentarily) and there was obviously a problem there. But then on the same plugs, removing the made cables combo and just going with the OEM power supply got it turned on just fine. But now on a different plug with the cable combo, it's fine.

So a happy ending - Thanks for the help!