> I have no choice in the
> oscilloscopes at my disposal at work.

Hmm, time to tell the boss that proper tools are needed to fix difficult fa=
ults ??? I wouldn't be surprised if a reasonable current probe for the scop=
e wouldn't help as well.

Have you checked the state of the tubes themselves? If they are going soft =
or somehow screwing the inverter around it could result in slower switching=
 of the transistors producing heating. What sort of heater wiring do the tu=
bes have? If one heater wire is open the tube may still fire due to voltage=
 peaks but may screw the transformer waveforms around due to not having the=
 proper heater current draw.

Any bypass caps that have gone AWOL with extended time at the elevated temp=
 - electros especially suspect, but could be any type of cap. Anything that=
 affects a bypass point could again screw up the waveform producing slow sw=
itching.


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