Im modifying this original post to include the 3 part series of the troubleshooting I went though to finally find the solution to this 40 year old issue. What an event indeed!!!
Original content directly below the three videos...
Ive been repairing a Heathkit IT-1121 Curve tracer.
Anyone hear familiar with the IT-1121 inner workings? Notably the Step generator, step number control circuit?
Step generation works fine, and if I clamp IC6 [decade counter SN7490] PIN 2 (reset) to ground I get all steps 0-9 at 1 volt at 83ms without issue. In normal operation the knob only works in a short range and goes from 0 to 4 to 8 (or 1,3,7 depending on how you want to read this from null).
All resistors checked, 5v and 15 v checked. only thing that still bothers me is if I lift connection B away from Q17 I get a good voltage swing when I turn the step knob, when connected there is a considerable voltage drop. Im thinking I shouldn't expect a considerable voltage drop on an input of a transistor. Tested fine though using basic transistor diode test method.
I ordered some NOS transistors anyway.
Any thoughts.....