No sound, high impedance on input XLR
Posted: Sun Dec 18, 2011 9:04 pm
I built my fourth A12 and it had no sound, only noise and no output. I measured the resistance between pins 1 and 4 of the input transformer at 38 ohms, but initially my measured resistance between pin 2 and 3 of the input XLR was 13.9 K ohms ( the same as the series resistance for R 36 and 37.) Pins 2 and 3 of the input XLR should have the same impedance as the input transformer pins 1 and 4, 38 ohms (give or take a few ohms). Carefully tracing back, the measured resistance on the top of the jumper pads at J1 and J2 it was 13.9 K, on the bottom of the pads it was 38 ohms, the same as the transformer pins 1 and 4. Therefore there was no connection between the top and the bottom of one of the pads. In the instructions the standard is to leave these jumper pads uninstalled. I installed a short piece of wire through each hole and soldered on both sides and everything worked perfectly. I went back and did the same to my other three modules just in case. I guess there was slight defect in the board on the last module, but I want to let everyone know, this is the fix to this problem.