Extreme High Voltage: 15kV Supply Repaired and in Good Health

The 15kV supply that I popped a couple days ago is now repaired.

I first disconnected the HV section’s primary from the driver transistors, and powered it up by simply hooking the Interstate F43 Function Generator across its center-tapped primary and feeding it a 20 kHz square wave. At about 5 V in the HV electrode started indicating over 1000V so I knew the HV section was basically good. I checked both sides with the inductance meter and by the FG injection and no damage showed up.

Then I started troubleshooting the oscillator board. The 18V regulator was running hot and off-voltage, so I pulled it and supplied my own 18 V to the osc. section. The 555 timer’s output looked good on the scope, but it was the only chip not socketed, so I pulled it anyway, installed a socket, and a new 555 timer.

But the signal disappeared before it got to the PWM chip and of course there was no output from the PWM chip, which drives the pair of 2n2219s that in turn drive the power transistors driving the HV section primary.

To check the circuits downstream of the PWM chip, I simply pulled the chip out of its socket, hooked up the HV section back to the power transistors again, and used the Function Generator to inject a 20 kHz signal into the PWM chip’s socket, where its 2 outputs connected. One side worked great, a tiny signal from the FG made a great HV output, sparking across a little gap and making a good signal down to the base of the power transistor. But the other side didn’t work. I checked that side’s 2n2219 and found it leaky, so I replaced it and rechecked with the Function Generator injection. Perfecto! Now both sides of the flipflop worked symmetrically.

So I put in a new PWM chip, and promptly screwed up by accidentally hooking the input power to 40 volts instead of the spec 28. This caused a great arc, popped the 18 volt VR (which was known to be flaky anyway) and of course the PWM chip and maybe an electrolytic cap or two. Back to the parts store….. another new PWM chip and a couple of capacitors just in case, and a replacement 1458 dual opamp used in the current limiting section, and now it’s running all copacetic, with the exeption of the glitch I talked about in the video.

Burning in, running behind me now, it’s been on for half an hour and is spinning the motor at 5250 RPM, and isn’t making the funny sounds it always did before.

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