7633 no high voltage -1475V on test point

Fabio Sturman

I just repaired the time base 7B53A and a new problem appears.
An acrid smell came from the oscilloscope so decide to test the scope
for 3 hours and the trace and readout disappeared. I measured the low
voltages and are all ok but is missing the high voltage (-1475V) on the test point.
Where is the problem?

Roger Evans


Check through the theory of operation in the manual. The high voltage is generated by the transformer T1199 driven by the power oscillator Q1195, the circuitry is protected by the fuse F814.

There are two common causes of failure, one is on the driver side, Q1195, Q1190 and C1198 / L1198. You will need to remove Q1195 which is on a sub-panel and heat sink in order to test it.

The other common failure is either the transformer T1199 itself or a failed capacitor or diode on the high voltage side of T1199. High voltage failures are more difficult to diagnose since leaky components often show as OK when tested with an ordinary DMM.