Low-ripple HP power supply Jun 21, 2017

I got a power supply from eBay a while ago:

That’s an HP 6255A: 2x 0..40V @ 1.5A, max 80V in series. But the interesting spec is its very low noise: <0.5 mV and <0.2 mA.

These supplies are very clean - and it’s all done with old-school linear regulation. So there can be a lot of heat to get rid of:

That back section is painted in RED for a reason: it contains some resistors which act as energy radiators. The other section is a heat sink with fins for the trusty 2N3055’s.

This was sold as US unit (with substantial shipping costs, alas), so it came set for 110 VAC, and initially I didn’t even realise it could be adjusted. Luckily, these units can in fact be modified to operate at 220V and will support a line frequency of 50..400 Hz (the latter is common aboard airplanes).

As it so happens, the manual can be found online and describes every detail, including schematics and adjustment info. Turns out that you have to cut some traces and solder a jumper inside to convert it to 220V:

Next problem was badly dislodged meters:

For one of the meters, the plastic mounting bracket was in fact missing. And as you can see, this is all really old-school electronics. Discrete components, and it’s basically just a doubled-up unit, with all the front panel elements mirrored for more natural use.

Here’s another close-up - neat and tidy:

I fixed some bad connections on the pots and after that, everything started working!

So now I have a shiny, as-new, heavy, hot, eh… brick to power circuits with very good regulation and extremely low noise:

I’m considering replacing the meters with V+A LED displays … some other day.

There’s also a connector strip on the back, which can be re-wired to support a form of “remote control”. Analog remote control that is, i.e. external voltages can be used to drive the settings of this supply. Whatever.

I have more old gear here, all surprisingly operational and useful, which I’d like to mount in some sort of scope trolley cage:

That way it could be carted under the desk when not in use. Because, let’s face it, how often do you really need to use old & bulky electronic gear, when a small LiPo-based regulator is often more than enough for (digital) projects, many hours on end?

Still, I’m happy with it. Quality equipment deserves a second chance, even if it’s old.

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