As YIIi said, I think that the power supply is shot and so is the LM1875.
I looks like the power supply is putting out AC.

 

 

 

Couple of notes: Ive horizontally flipped the top view picture.
Pedal Input Ive indicated on the picture - is not just pedal input...I was slow to realise it also doubles as the Front Units Input from the Mixer board...(Inputs selectable by mechanical switch) so this is THE input connection to the Amplifier LM1875T on pin 1 VIN+
The Bridge Rectifier is a KBL406

Are these scorched diodes acting as a Voltage Regulator for the mixer board ?

ERROR: The 3 Pin power connection for the mixer board is depicted incorrectly ..from the top should be white -12v, 0v GND Blue, +12V Red

 

Im wondering if it might be worth finding and using a seperate +12vdc, -12vdc, power supply to use for the mixer board....?

What do people think?

There is plenty of room in the unit..(guitar amp enclosure).. If I did Would I need to join Grounds from the seperate Power supplies.?

 

Ok, I see the circuit now.

Full ±30V is supplied to the LM1875.
The 100Ω resistor and 12V zener is to create ±12V power for the mixer.
You want to replace the 100Ω with 100Ω 5W resistor.
You want to replace the 12V zener with at least 3W or 5W zener such as 1N5349B or NTE5127A.

 

Thanks Mr Chips ! Its making sense to me now... Im off to Jaycar (in Australia) tomorrow! ..the amp itself might actually be ok I guess.
Cheers

 

The parts burned out because the Chinese manufacturer of the amplifier skimped and saved a few pennies on the size of those parts.

 

For better performance, you can replace the two zeners with one LM7812CT and one LM7912CT.
You will have to do some drilling and board patching to get three leads into the board instead of two leads.

If you have never worked with these components before, be careful that you watch the pinouts of the two. They are not the same.

 

Right Audioguru ! ... the mixer board seems to be much better quality than the power / amp board..different manufacturer i guess....hoping the mixer board didnt get damaged in all of this....hoping to test tomorrow.

 

For better performance, you can replace the two zeners with one LM7812CT and one LM7912CT.
You will have to do some drilling and board patching to get three leads into the board instead of two leads.

If you have never worked with these components before, be careful that you watch the pinouts of the two. They are not the same.

Ive just been looking at dual output boards and circuits!..and noted the 7812 and 7912 are used a lot. Would I need to change / add any resistors etc....assume one of the three legs will goto ground on each.... I remember I wasnt sure of the max input voltage..but probably be ok behind the 100 ohm resistors ?

 

When you go to Jaycar, get one LM7812, one LM7912, and two 1uF/35V tantalum capacitors if you want to scrap the zener regulation.

I would eliminate the 100Ω resistors in this case. It really depends on how much current the mixer board is taking. If it is taking less than 150mA then it would be ok to leave the resistors in.

 

There are some tests you can perform. Do you have a DMM?

Unplug the mixer board.
Power up the amplifier and measure the positive and negative voltages feeding the amplifier chip.

 

You might want to upgrade all the components associated with scorching/darkening of the pcb.

 

So put the 1uf /35v tantalum caps across the output or input of the 7812/7912 to ground? ..also do you think ditch the Zener diode coupled 220uf Caps as well as the resistors you think ?

 

You might want to upgrade all the components associated with scorching/darkening of the pcb.

The scorching is under the two power resistors and two 12V zeners. We're working on it.

 

Ive got to go now...getting late.(01:45)..will check back in tomorrow morning...I will test voltages mr chips..and then report back.
many thanks to Mr Chips and to everyone who has replied. !

 

So put the 1uf /35v tantalum caps across the output or input of the 7812/7912 to ground? ..also do you think ditch the Zener diode coupled 220uf Caps as well as the resistors you think ?

Actually,

You can ditch the 100Ω resistor, zener and 220μF capacitor.
We are going to replace the zener and 220μF with an LM7812 and 1μF cap on the output of the LM7812 on the +12V rail to the mixer.
And LM7912 and 1μF cap on the -12V rail to the mixer.

You will need a small carbide bit .035 - .042" and a Dremel type power drill to drill holes into the PCB.

 

For now, leave the two 220μF caps.
Remove the two 100Ω resistors and the two zener diodes.

I will show you where to drill small holes in order to insert the LM7812 and LM7912 TO220 voltage regulators into the board.

 

Drill three small holes 0.035-0.045" where marked with red dots.