Do the obvious things:
1. begin with easiest thing - switch to "mono", if the signal in the left channel appears - there is something in the input RCA connections (wiring or something), you should go to point 5;
2. if it doesn't help - check external connections (just swap left-right channels connections in the input and output of your Cornet, if the non-working channel is always the left channel - then something is wrong with the interconnects);
3. change tubes;
4. check the voltages in the non-working channel;
5. if everything is OK, check the input and output signal connectors wiring (disconnect power and check the resistance of the wires between the connectors and internal parts to which they should be connected - joint point of R201 and R200 - for input and C208 and R222 for output;
6. if you feel comfortable with working with high voltage (!!!) you can connect rather long (about a half a meter) insulated wire (with small cleared from the insulation part on one end) to the 2 and 7 pin of each tube (beginning from the last) - there should be 50Hz hum from your acoustic systems. This way you can find the stage which has the problem. Then you should check everything in the stage for broken contact or non-working part.
