317/Repairs

317 serial number 5014

When powering up after a long slumber (possibly decades), R660 and R630 burnt.

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Assuming a shorted (or high DC leakage) capacitor, I reformed all of the electrolytic capacitors in the power supply section. The method was to disconnect each capacitor at its terminal and apply DC using my 576 curve tracer, monitoring for current, and keeping the dissipation in the capacitor below two watts. All of the capacitors successfully reformed after a few minutes, with leakage currents under 1mA at 80% of rated voltage. Then I replaced the burnt 10-ohm resistors. That got the scope running again. It is possible that powering up the scope very slowly on a variac would have avoided blowing the 10-ohm resistors.

The horizontal amplifier gain was too high, and outside the range of adjustment using R325, HORIZ GAIN ADJ. Furthermore, adjusting the intensity control upward caused the trace to move outward. These are symptoms of an HV power supply that is too low and out of regulation. I measured the HV test point. It was around -800V instead of -1300V. A common cause for this is a lossy capacitor C806 in parallel (resonating) with the primary of the HV transformer.

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I replaced C806 with a 1000pF 1kV ceramic capacitor. Note: Both the old and new capacitor are within 5% of 1000pF and both show a dissipation factor of between 0.024 and 0.027 at 100kHz on my DER DE-5000 LCR meter. Furthermore, both showed negligible leakage at 800V. And they look similar on the curve tracer. But clearly the old capacitor is misbehaving.

Replacing the C806 eliminated the problem of the trace moving when the intensity control is rotated. The HV power supply's regulation is restored. I adjusted the HV ADJ so the HV test point is exactly -1300V. And this voltage is now unaffected by the intensity control. Then I was able to calibrate the horizontal gain. And the vertical gain is correct, as well.