The Zenith's that got recalled were the solid-state Chromacolor 2 chassis. On a 4 tube hybrid set the most likely problem is a bad horizontal output AND Osc tube. IIRC when the osc tube got weak, it would cause the output tube to fail. Those 20LF6 tubes are PRICEY. Check out these 20LF6's on eBay and see if the marks on your tube are like this.
http://cgi.ebay.com/ws/eBayISAPI.dll...sPageName=WDVW