You will have a problem replacing the 1N3754, so check it but you will need to try and fix the bias with the other two didoes (see above warnings). If your meter has a diode test function test all of the diodes and see what voltage readings you get when you test them. The meter will read the voltage at which the diodes conduct. You can use these voltages to select the right mix of diodes to get the bias right.
I know that the board had been messed with previously, did you get it all back to normal? Have you checked the other voltages on the board? Any signs of burned resistors or wrong value components?