Wonder if this is a no load issue....somehow ? given the voltage output is steady...the 7912 I was given at Jaycar is a L7912CV

Can you post photos of both sides of the board?

 

Found this ..might be relevant ?! https://forum.allaboutcircuits.com/threads/baffled-by-simple-linear-regulator.118289/

 

 

I don't see any mention of a minimum load in either TI or ST datasheets.

A 2k2Ω load resistor would draw 5mA.

btw, the 1μF capacitor on the input side should go directly on pins-1 and 2 on the LM7912.

 

Will goto Jaycar tomorrow and grab a 2k2ohm 5 watt and test.

 

If the 7912 is oscillating it will give spurious readings on a DMM.

 

Will goto Jaycar tomorrow and grab a 2k2ohm 5 watt and test.

You don't need 5W.
¼W will do.

I tried to replicate your situation and cannot get the LM7912 to malfunction under any conditions.
Double check your GND connection on pin-1.

Else try another LM7912CT.
Get some small heatsinks while you're there and mounting nuts and bolts.

https://www.jaycar.com.au/to-220-mini-6073b-type-heatsink/p/HH8502

 

The next thing is, once you get the power supply working, do you know that the mixer was not damaged?
How much of a travel to get to Jaycar for more replacement parts?

 

The next thing is, once you get the power supply working, do you know that the mixer was not damaged?
How much of a travel to get to Jaycar for more replacement parts?

its about an hour to jaycar....yeah no idea of the mixer status!... just back from Jaycar..bought heatsinks and resistor and also as a back up plan zener diodes and 100ohm resistors...will test in a few hours

 

OK ! 2k2 resistor across negative output of 7912 and gnd... = -12Vdc !

 

So far so good! its alive! ...need to leave it on and see how hot those regs get...Mixer circuit OK!..guitar sound amplified again!

 

So far so good! its alive! ...need to leave it on and see how hot those regs get...Mixer circuit OK!..guitar sound amplified again!

Glad to hear your amp is up and blasting away again.
What guitar do you play? I have a Fender Telecaster.

 

Thanks for all your help Mr Chips ! Cant say Im a Player really..but my daughter is not bad... its just a 'Phoenix' strat copy -made in Japan a few years ago.. but it sounds pretty good...even better with the Amp! I left the Amp going for five minutes....and regs were not hot....then have had to go out....will try tomorrow for a longer test. Cheers

 

Did a long test today..all was well...heatsinks and regs ok to touch (temperature warm but not hot)....now Im done I thought!
But no!

Put the board and unit back together..turned it on...made a lot of hiss and crackle....then settled....then goto plug in guitar almighty crack and pop out of speakers... it settled again..and then I realised tapping the metal case for the amp and mixer circuit made it happen....anyone any ideas what this is likely to be. The case is earthed. I cant see anything obvious TIA

 

Try to isolate where the noise originates. Is it coming from the mixer or from the amplifier?
Unplug the mixer and see if the noise goes away.

 

Thanks Mr Chips .. Im going to isolate/test tomorrow... It seems to have started after I pushed the metal case back into the wooden amp enclosure... it maybe Ive inadvertently got a track fracture....my dremel is worn and I used a drill to clean out some of the existing component holes...then also the heatsink may have got caught a bit when pushing the case in... tomorrow will see if its the mixer board or the amp/power board...take it from there.... Im thinking I may have to build a new power reg circuit on a new board if I get trouble with tracks. Cheers.

 

Fixed ! Back in case ..nice and quiet again... Isolated the mixer board..found its all on the amp/power board..... Gave the amp/power board a close inspection with magnifying glass ..found a few dry joints and one track fracture...resoldered dry joints and soldered a jumper in and now all good. The board is quite poor quality. Thanks all and again Mr Chips!

 

Now you owe me two Fosters.
Cheers, mate.