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Hi. This oscilloscope failed many years ago - I managed to get a new HT transformer and voltage tripler, but on re-visiting it today - found that the oscillator transistor Q458 had
shorted (B-E) and took the 51 ohm resistor out with a bang when I fired it up. So the recommended transistor is a TIP50 (NPN) - replaced that, but nothing happens. All
supply voltages seem to be correct. HT transformer seems OK - (resistances same as the new one, and I'm loath to take it out). Any ideas? Thanks.

While you were in there, did you check out the health of C458 (10uF cap)? Is the HV supply oscillator (Q458 drives the primary for the HV step-up transformer T460) oscillating at roughly 50KHz?

Neil

Check F722. That 62mA fuse is likely open. Also check VR746 to see it is open. If so, use BZX85B9V1.

Mark

Thanks - Cap seems OK, can't find VR 746 (as Tektonix seem to have their own way of assigning groups. Maybe I'd better get better close-up glasses!)
So voltages seem normal - and the fuse is OK. Maybe the Hfe of the transistor is too low - but it seems a very simple and robust circuit.

VR746 is in the power supply to the base of Q754.

Mark