Yamaha EM-300 mixer board troubleshooting

[Windhand]

Hi all! New to this lovely community and my first post. i'm also a beginner to the world of electronics repair.   

My band and I recently acquired a Yamaha EM-300 powered mixer board. At first it worked fine then one day there was no sound from the speakers.
Sending sound through the 'AUX IN' did produce some barely audible music if the output is turned way up.
The other outputs (MON OUT, G-EQ OUT,..), everything before the Power amp, worked fine.

Opening the board I first noticed some loose screws, could be they shorted something?
I removed them and looked for any signs of a problem. Only thing I could find is this bodged Diode that seemed out of place:


Checking the Service manual seemed to affirm my thoughts that this is a bodge.

My understanding sadly ends there. I shorted the clipped diode to the bodge on and that seemed to jump start something, as now one of the speakers works fine.
Could this be the problem? Should I reconnect things according the circuit diagram from the manual? 

I haven had time yet to check the MA transistors, I did manage to check that bodge diode and it had a FB of 0,3V. Any ideas why this was done?

And suggestions on what the next step should be or anything would be very appreciated. Have a great weekend!

[Andy Watson]

There is a relay that connects the signal to the input of both power amps - can you confirm that it is operating? Do you have facilities to measure voltages? If so, check the voltage around TR23/4.  Can you identify the diode(s) labels in the bodge - I would guess either D30, D31 or D32.?
Do the lamps behind the meters light ? This would confirm that F7 is intact.

[Windhand]

Thanks for the reply!
I can measure the voltages but it wont be until tomorrow when I can go back to the studio. The lamps behind the meters light up.
As far as I can tell, the added diode is an N4007 and judging by the diagram the lower D31 diode is an 1S1555 and the clipped D32 is a 1N34A.
I can do some measurements tomorrow and post them here.