Clean the solder from the pads where the zener was removed using solder wick or a solder sucker.
Avoid using too much heat otherwise you could lift the copper trace off the PCB.

The negative voltage LM7912 will be inserted oriented as shown with pin-1 (GND) closest to the mixer power connector. Pin-2 (INPUT) goes into the new hole that you drilled.

 

Positive regulator LM7812 will be oriented in the same way, with pin-1 (INPUT) closest to the mixer power connector.
The centre pin (pin-2) GND goes into the hole vacated by the zener diode.

 

On the underside of the PCB you will run three jumpers with 24AWG insulated jumper wire.

 

You might have to install 1μF caps on the underside of the PCB across the existing 220μF caps to prevent the regulators from oscillating.
If you have to do so, make sure you match the polarity of the new caps with the existing caps, i.e.,

-VE on cap goes to -12V, +VE goes to GND at the LM7912

-VE on cap goes to GND, +VE goes to +12V at the LM7812.

 

Might want to put those 1 uF caps on the regulator *inputs*. The 7905 in particular can become unstable without an input C.

 

Many thanks Mr Chips...really do appreciate the time you have spent helping us out!...this is my daughters and she also appreciates your help thankyou... Just an update..Ive removed all scorched components..also the two 220uf Caps....tested board and measured output from Bridge rectifier... we get +28Vdc and -28Vdc...so quite high. Thats as far as Ive got so far...Ive been to Jaycar got a 7812 and a 7912..also bought four 1uF tantalum bead caps...just in case we need to put them on the input of the 78/7912 's ?! ...Ive got to go out for a bit...then later will start drilling and assembling......do we need to bring that Voltage down (28V+- dc ) supplying the 78/7912's ? Im not sure.....will be back in around a couple of hours....many thanks to everyones thoughts and comments. Cheers.

 

If you have removed the 220μF caps then put the 1μF caps in its place.
Watch the polarity!!!
If you don't the caps will explode.

 

Just for grins..... Assuming source voltage of 28 volts is normal, with 12 volts output - and those resistors were actually 100 ohm, then....

The voltage across the resistors would have been 16 volts. That's 160 mA, and a dissipation of 2.56 watts in the resistors. They look like 2W resistors - really should have been 5 watt parts. With no load on the +/- 12 volt output connector, the zeners would dissipate 1.92W. These look like 1N47xx parts which are rated at 1W.

Not surprised things look toasty.

 

Just for grins..... Assuming source voltage of 28 volts is normal, with 12 volts output - and those resistors were actually 100 ohm, then....

The voltage across the resistors would have been 16 volts. That's 160 mA, and a dissipation of 2.56 watts in the resistors. They look like 2W resistors - really should have been 5 watt parts. With no load on the +/- 12 volt output connector, the zeners would dissipate 1.92W. These look like 1N47xx parts which are rated at 1W.

Not surprised things look toasty.

Yes. I did the analysis based on 30V from the supply, supplying 180mA to the mixer board.

oops! Forgot to tell @jikonn he will need to put some heatsinks on the LM7812 and LM7912 regulators.

 

Just for grins..... Assuming source voltage of 28 volts is normal, with 12 volts output - and those resistors were actually 100 ohm, then....

The voltage across the resistors would have been 16 volts. That's 160 mA, and a dissipation of 2.56 watts in the resistors. They look like 2W resistors - really should have been 5 watt parts. With no load on the +/- 12 volt output connector, the zeners would dissipate 1.92W. These look like 1N47xx parts which are rated at 1W.

Not surprised things look toasty.

Thinking the unit is about 15 years old..been on the boil for a while !

 

Yes. I did the analysis based on 30V from the supply, supplying 180mA to the mixer board.

oops! Forgot to tell @jikonn he will need to put some heatsinks on the LM7812 and LM7912 regulators.

Thanks Mr Chips
I meant to ask you about heatsinks...I can get some later..guess we will be ok for a quick test without them? ....apologies I missed your post about leaving the 220uF caps in place....although room is tight and removing them gives more room for heatsinks...still I might put new caps on underside to leave more room for heatsinks....or might put one of the regulators on the underside..I could do this as this board is mounted upside down..fixed to the metal box with screws into the amp heatsink........just got to be careful with polarity..note to self!...will be doing in a few hours..Cheers

 

Yes, you can do a quick test without the heat sink.

Is there space for the LM7812 and LM7912 standing up?
Be careful where and how you mount the heatsinks.
The LM7812 tab is GND.
The LM7912 tab will be at -30V. You want to keep this and its heatsink from touching anything.

I think you will have space on the topside to put the 1μF caps.

 

Wait. We don't know how much current the mixer requires.
Since both zeners were getting hot maybe the mixer doesn't need a lot of current.
Do the test and see how hot the LM7812 and LM7912 get after 30 minutes. Then you can decide if they need heatsinks.

 

Wait. We don't know how much current the mixer requires.
Since both zeners were getting hot maybe the mixer doesn't need a lot of current.
Do the test and see how hot the LM7812 and LM7912 get after 30 minutes. Then you can decide if they need heatsinks.

Im thinking then If I can touch the Regulators after running for 30 minutes (after turning the unit off) we should be ok ?!

 

Assembled and tested.... +12Vdc Check! -12Vdc Fail....instead I get steady -19Vdc? ...cant see a reason for it...+-28Vdc Volts feeding regulators still Ok. Dodgy 7912 ?

 

Assembled and tested.... -12Vdc Check! +12Vdc Fail....instead I get +19Vdc? ...cant see a reason for it...+-28Vdc Volts feeding regulators still Ok. Dodgy 7812 ?

Try temporarily tacking on the 1μF cap across the input side and GND and see if that helps (assuming the rating of the cap exceeds 30V).

 

I made an error in measuring.... +12vdc ok.....problem is getting -19Vdc ..so 7912

 

assume tack positive of cap to 0volts gnd ..and Negative to input (-27vdc)

 

tested with tacked 1uF tantalum on input to 7912 ..same result steady -19vdc weird ? ! the 7812 is still fine +12vDc

 

tested with tacked 1uF tantalum on input to 7912 ..same result steady -19vdc weird ? ! the 7812 is still fine +12vDc

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