More Video Hum Bar Problems

Around 1988 I had taken over a project of a very large audio/video installation involving a GE Talaria video projector that was specified into the project by my predecessor.

It turns out that the projector was built to run on 220V AC, and the building was 110/208 3-phase, which, according to GE, was not enough to properly drive the projector. I contacted the building's electrician to see if he could provide a 220V AC feed for me, and he said he could do that. Shortly after that, he said that he had provided the line and had tested it.

We installed the projector and when we fired it up, it had outrageous hum bars in the 24-ft diagonal projection. Clearly, this would not please the customer! Since the feed for the projector was more than 100 feet away (RGBS over RG-8 coax through conduit), my first thought was that we had a loose ground somewhere. Nope, that wasn't it. No problem with the signal being provided, either. So we called GE, and they sent out a tech to align the projector. He hadn't seen the hum bars before, so he had no idea what to do about them. Of course, I didn't suspect the power line because the electrician assured me that he had tested it. But I was running out of ideas, so I hooked up a volt meter to the 220V AC line, and it was sitting at 196V AC -- clearly the source of the problem.

I contacted the electrician and told him that the power he had installed was not only not 220, it was lower than the building's power line. It turned out that he had installed a 220V AC to 12V AC transformer in the line with the intention of wiring it as a voltage boost, but had hooked up the secondary's phase backwards and had it in back instead. So how did he "test" the voltage he gave me? With a neon light tester -- that told him he had at least 80V AC, but certainly didn't tell him he didn't have 220V AC as he had promised.

Tell us your experience in solving a knotty engineering problem. Send stories to Jennifer Campbell for Sherlock Ohms.

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