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Oscillating transistor in MK3C
Thought I'd pass along an interesting item:
Symptom: Mk3C. Oscillating crt patch in local only. Fine in remote.
supplies on V & H boards ok. Examining sigs returning from zoom board
indicate it is coming from that area. On zoom board, seeing
oscillations on +5V supply. Traction supplies Ok.
Cause: Stop lamps in local panel and power distribution panel burned
out. on 641 board, VT12 (lamp driver) oscillating like mad. Pulling 641
board stops oscillations. Even after re-seating interconnects, replacing
bulbs, scoping VT12 Collector would occasionally have a few cycles of
oscillation on logic level transitions. VT12 checks ok, as does VT7.
Oscillations only evident on VT12 output. Added .01 cap across R32 on
641 board. It appears that the resistance of the interconnects and
length of wire to the load (lamps) places the load far enough away so
that an occasional oscillation occurs. Only VT12 is a higher power
transistor. Other lamp drivers are lower power transistors... No
further evidence of oscillations... Are we having fun yet?