502/Repairs

Supply out of regulation

Symptom: +350 V supply out of regulation (~300 V)

Root cause: C751 10 nF "Bumble Bee" leaky

Solution: Replace C751

HV very low

Symptom: HV very low, oscillator tube V800 gets very hot

Root cause: Black Beauty capacitors in HV circuit leaky

Solution: Replace all 6.8 nF HV caps

No sweep at slower speeds

Symptom: In 1 ms/Div and slower, no sweep − trace stationary on left

Root cause: Bumble Bee capacitors in holdoff circuit leaky

Solution: Replace C180B-D

Burned-out HV transformer

Symptom: HV transformer showing burns

Root cause: overload by earlier V800 failure caused by leaky HV caps

Solution: Replaced by 120-114 transformer (502) with 120-150 (from 502A) using only one cathode secondary. Note the grid winding has more turns on the 120-150, so a dropper resistor needs to be added to prevent the grid going too negative (no beam).