I consulted the Rider service info and they mention this specific issue. It seems that he original 17AP4 required a stronger magnet than the 17BP4. They suggest sliding the original all the way back over the base.
That did the trick
I'll try to find an RCA to F connector at Radio Shack tonight so I can feed a cleaner signal into it.
While working on this issue, I would occasionally hear a loud squeal from the HV area. Eventually, I tracked it down the the HOT plate lead. Moving that wire just slightly on the end going into the cage can eliminate or bring back that squeal. I suspect that's why there's tape wrapped around it - to keep it from moving. I guess I'll pull the chassis again and take a closer look. Probably a bad connection at the flyback or a break inside the wire.
