Ok so I tried adding a 10uF capacitor in parallel to the C444 to bring it up to spec, and the measured waveform did not change. Still too big in p-p and without the curvature. Same thing with adding a 0.1uF cap to the C442.
Could there be a problem with the transistors amplifying the signal too much and causing the too large p-p voltages?
Tomorrow I'll remeasure all the power supply voltages and every DC voltage in the trigger circuit to make sure its not that. Any good way to check the proper operation of a transistor?
No.
Go to a point earlier in the signal path and check for the correct waveform, say C405.
Component failures to expect when troubleshooting: (my order of suspicion)
Resistors OC or drifted high.
Caps, electrolytic, other dipped types, cracked coatings allowing moisture ingress.
Signal diodes
Power transistors, but more so HV output types
Signal transistors
Bridge rectifiers
Most stuff is easy to check with a few tools and datasheets, however in circuit measurements can be deceiving, thought and experience must be applied.
A cheap DMM HFE tester is usually all that's required to check transistors against datasheets for correct gain.
Same with diodes, cheap DMM diode tester.
There are many guides for transistor tests in circuit, a quick look found this one:
http://www.electronics-radio.com/articles/test-methods/meters/transistor-circuit-fault-finding.phpFind one you like, print it and keep it handy.