Wallis high voltage power supply - repair, teardown, reverse engineering

[Wim_L]

A repair of a Wallis VCS 103/3 10 kV 3 mA power supply, a rather old device, with some design choices you aren't likely to see in more modern equipment. It turns out that there wasn't all that much wrong with it, and it was easily restored to working condition. As a bonus, a fairly in-depth look at how it works, and some corona discharge at the end.



Lack of a service manual forced me to reverse engineer part of the main PCB. And, as I had plenty of time, I did the entire thing. There are a few unknowns in it, because I didn't feel like desoldering components which were mounted with their labels towards the board, but I'll attach my working schematic. Perhaps it'll be useful to the next person to encounter one of these.


Sidenote:

It's my first Youtube video (and first video editing experience), and it has been a learning experience... Audio recording was a challenge, being limited to a camere with built-in microphone, and a noise-cancelling headset that was remarkably noisy itself (though the onboard audio chip in the PC may be to blame for that). Some filtering in Audacity took the worst edge off, but it still isn't fantastic.

It taking about 5 hours to render on my i5 was a final surprise - in retrospect, recording everything at 1080p50 may have been overkill. It ended up being fairly long too. I tried cutting a lot, speeding up the slower parts... But there was so much interesting stuff to examine!

[Muxr]

Very meticulous analysis, nicely done! So was it just some proprietary (remote control) plug that was missing?

[OldNeurons]

An unusual device, and, for a first You Tube video attempt, very nicely done

[Wim_L]

Thanks, and indeed, all that was missing was that remote control plug.

It's a relatively versatile way to do things in an analog power supply, and probably the best if there's no built-in microcontroller: the setpoint is determined by a voltage input which must be applied to the remote control input. If you want to use the built-in frontpanel controls they have to be connected through the remote control connector (and powered from it) just like an external remote control signal might be.

That they also routed much of the main positive power rail through the remote control connector probably was a safety feature (unsafe as it might seem to put about a 35 V rail that can deliver multiple amps on such a connector). If the power supply could be powered up without a proper connection to the remote, the voltage and current setpoints would automatically be set a bit above their official maximum values.

The downside is that, even for local operation, you need that little plug. The power supply probably has been used with remote control in some larger system. Then, when it was put back in storage, nobody might have remembered to put the plug back in. The next person to try it finds it's broken, labels it as such, and it spends a few years in a closet, unused.

This power supply probably is over 3 decades old (not so familiar with those old ones, but the datecode on the single opamp suggests it has been made in 1982). Plenty of time to forget where such a loose part might be, or even that you need it.