SC502 still have 80% of display with HV cable unplugged, multiplier disconnected?

Yeun-Jung Wu

Yesterday my little cute scope SC502's trace disappeared after half an hour operation. Brief inspection revealed F800 fuse was blown. I removed AUX board near T800 to expose C850/C851; they were O.K. K6JCA had an interesting discussion on repair job of SC502/504. I followed his guidance and unplugged HV cable, unsoldered input to multiplier. Reinstall F800, I can measure +- 70V. Q860, Q855 were healthy, Q850 was hard to reach. The culprit could be the multiplier P/N 152-0634-00?

I had another SC502 with similar F800 blown fuse problem. Maybe I can retrieve its HV multiplier? I did the same to this SC502. Surprisingly, I can have a trace displayed on 80% of the screen; focusing worked as expected, Ch1/Ch2, triggering all work as expected! But it's CRT's red HV cable was disconnected, even multiplier's input was disconnected from T800! +-70V were correct too.

The other HV unplugged SC502 was as expected: no screen display. +-70V O.K.

Why CRT still have trace under control even HV cable was removed; even multiplier's input unsoldered? Leakage? Can its multiplier still be salvaged? How to restore both SC502's to working condition?

This is my first zombie experience of scope. It seemed that that scope was still usable, in certain sense.

Yeun-Jung Wu