Hi David,
The half period oscillation is normal.
I would first test Q280 before bothering about a replacement type. They should be matched at Id about 1 mA each and Vds about 15 V.
Is there any need for a curve tracer? In the absence of triggering you can measure DC levels all around, drain of Q280B (should be 15 V mine 0.7 V), common source a few volts positive, difference between gates (2.2 V?). You can even probe these points during operation; a 10x probe won't disturb the results.
Maybe there is another same type dual fet in the 7T11 that can temporarily be used/exchanged?
Albert
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Yes, a slight inbalance I could understand - but +2.2V is a LOT.
Yes you can see the ramp very nicely at 100nS/div. But regardless of the sweep speed there's a very clear problem here. As the ramp stop voltage increases (more negative), the ramp start voltage moves from close to 0V until it reaches about +2.2V when the stabilised stop voltage (after the overshoot) gets to about 4V.
I've uploaded a 3 second video clip here: <>
Time to pull Q280 and put it on the curve tracer I think. 151-1011-00 seems to cross to dual 2N3822 or 2N5565, neither of which seem readily available - any suggestions on a suitable replacement dual FET?
Thanks
Dave
