Eric,

That is not a synchroguide, it's a diode phase detector afc circuit. In later models those diodes are solid state, and usually one is bad when this kind of problem occurs. Since you have checked the tubes, it has to be something else, but in the area between the sync separator and the H oscillator. I am suspecting C65, C66, C67, C68, C69, C70, & R76, R77, R78, R79, R80, R81, R82, R83, & R84. You may have already replaced some of those caps.

A pulse from the flyback is taken off at the width control and is fed back through several components to the 6AL5 phase detector diode cathode & the 6T8 diode plate. Sync pulses are taken off at the junction of R77 & R78 through C65 to the 6T8 phase detector plate. Sync is also supplied from the 12AU7 sync sep cathode thru C66 to the 6AL5 cathode. The phase detector circuit compares the flyback pulses to the incoming sync pulses, and develops a correction signal if there is a timing difference, which is supplied to the oscillator through R63, R64, & C69. So, there are four paths to be concerned with: the two sync paths, the flyback pulse path, and the correction path. I'll bet the problem is that one of these four signals is missing, or low in amplitude. A scope should locate the problem in short order. Good luck!