Here we have a JDR 3500 35 MHz 2 channel scope. This unit came in with no trace. After looking I found Q8 (S2D401) shorted.
This was replaced and other components tested. This unit uses some type of switch mode power supply for the high voltage and heater. Power comes from 15 volts DC into the circuit.
I really do not know that much about these switching high voltage supplies
Manual:
http://nebula.deanza.edu:16080/~norona/3500_oscilloscope_files/JDR3500%20oscilloscope.pdf
