Spotty picture on VHF only

[kbmuri]

I have this unrestored original Zenith 24-inch B/W I picked up off of eBay a couple of years ago. It gets turned on once a month to check it, otherwise it's just on display (it's not a daily watcher). It recieves OTA UHF channels clearly:



There are no OTA VHF channels in range, so the VHF tuner hasn't really ever been used. I tried out the Insignia DTV converter today with really poor results. The converter works fine on other TVs. On either channel 3 or 4 I get almost no audio and these curious white spots:


Any expert-TV-guru thoughts on this?