I have a model 28 ASR wired in half duplex. I had been successfully using it with an interface I am building until it stopped operating. I measured the resistance of the loop and it's a dead short. After unpluging evreything from the Electrical Service Unit there is still a short on the loop. I checked the coils and they are not shorted. I am not very knowledgeable on the workings oif these machines apart from the basics. Is there anything that come to mind that might be the source of my problem?