Mixer inputs

[argonrepublic]

On page 2.9 The Mic, inst and line are shown. The input board is dead on channel 10 only. There is no output from the TL072. Could you please assist me with some strategic steps to diagnose the problem? I swapped in aother opamp and it is still the same.

 

[abraxalito]

First step is to verify all DC voltages starting with the power supplies. So pins 4 and 8 in the case of the opamp. Then check the DC voltages at each of the inputs (2,3,5,6) and the two outputs (1,7).

 

[argonrepublic]

The good circuits have 0v on1, 10v on 2, 10.3v on 3 -15v om 4, 0v on 5, 0v on 6. 0v on 7 and + 15v on 8.


The bad channel has 13v on 1, -15v on 2, -13v on 3, -15v on 4, 0v on 5, 0v on 6, ov on 7 and +15v on 8

 

[abraxalito]

Could be RC11 open circuit - measure the voltage on both sides of it.

 

[indianajo]

Could also be CC8, the electrolytic cap from pin 3 to analog ground, leaking electrically. This looks old with 4 transistor input, rubber sealed wet caps don't age well. Hint. The freight from a distributor will be $9, all those 10 or 22 uf caps will be $.09 each in 10000 hour service life grade. Buy more than one in the box.