Zenith 23G24 Chassis Help Needed

[classicradios]

Does anyone have a Sams 91-A folder 13 they could part with? My Zenith Porthole set has developed a few problems and I need to test voltages. The TV was runnning fine after replacing all paper caps, Micamolds, Lytics and out of spec resistors. A few days ago I started hearing a buzzing sound coming from around the yoke I think. Swapped the yoke with one from a parts chassis but the same result. I've attached a screen shot that occurs when the buzzing starts. The vertical is very unstable and adjusting the vertical hold on the front of the set is very touchy, If I adjust too far one way or the other I lose Vertical deflection. Any help or thoughts would be greatly appreciated.
Thanks
Jim