Re: T912 storage oscilloscope - No 2.7 KV high Voltage

Roger Evans


I am writing this on the phone and I can't easily get schematics at the same time - so no part numbers!

When you disconnect the diodes that generate the CRT cathode and grid voltages (-2.7kV) you effectively disable the negative feedback from the end of the focus resistor chain to the power transistors that energise T460. So the oscillator is running at maximum drive trying to generate -2.7 kV and in the same way is producing too high values for -120V and -220V. When running normally with negative feedback the transformer turns ratio ensures that if -2.7 kV is correct then so are the other voltages.

To me it looks like T460 is OK and so is the circuitry that drives the power oscillator. You have a problem with a failed diode or capacitor on either the CRT grid or cathode supply. If you replace just one of the supplies then the voltage across the two neons causes them to conduct and a fault on either supply will cause the oscillator to stop working.

I am afraid you may have check individual components to find the failure. Quite often a leaking diode or capacitor looks OK with a DMM but the leakage shows up with a high voltage around 1kV. I bought an insulation tester as used by electricians for checking house wiring.

I can go into more detail when back at a decent sized screen!

Best regards,


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