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Calibrating the analog voltmeter on a PS

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Hello guys,

Yesterday I've purchased a used bench power supply. Its not the best in the world, but it'll do for my needs (2 channel with 0-40V and 1A). Its also build quite well (I've had a little look inside). However, one of the voltmeters isn't accurate, and shows about 4-5% higher than the actual voltage (measured with the Agilent U1253A). I would like to recalibrate it so it'll be closer to the actual voltage, but I'm unfamiliar with these analog voltmeter, and can't find the schematic or manual of that power supply online. Can it be done? Can someone point in the right direction?

BTW, other than that it works great. Both channels were tested all the way from 0V (actually about 0.05V minimum) to 40V, and work great even under load. I've also tested the current limiting circuit and it works just fine on both channels. So all that's left to do is calibrate the voltmeter if possible, and its as good as new :)

Problem solved :)
After reading a bit about these analog voltmeters I've understood that the resistor that's put in series with them it setting the scale. The resistor in these are internal so instead of taking it apart, I've hooked a pot in series, and turned it until the reading consists with my DMM. Thankfully it drifted to a lower value so I can just add a resistor (with a value of 2K) in series to the meter, and there's no need to open the plastic case of the voltmeter to replace the internal resistor.

Edit 2:
In case anyone in the future will read it and will need to recalibrate one of these on their own. The solution I've used (changing the value of the series resistor) is only good if the error is relative to the reading. If there's a constant error, it can be fixed even easier, there's a small plastic screw on the case of the meter that turns the needle. Just turn it a little bit to the direction you need and that's it.

Glad it went well.  It has a nice unique look, not many PSU use those types of dials, its worth upkeep for the look. Did you take pictures when you took it apart and can you share ;)

Yes I did. It was full of stickers and markings on the outside, and had quite a bit of dust on the inside, so I took it apart and cleaned it inside out.

Wow.  There's a lot of room inside, easy to repair, when needed.  From the size of the components and room, for an analog supply, it looks like it was designed with a lot of parts derating.  The heat sinks are large by today's standards even for just convection cooling and are they exposed to the outside?  Really easy to stay cool if your lab is comfortably cooled.

I'd be curious to know what its specs are, if you've the time to determine its ripple under maximum load, and output stability is in mV, that is can you dial out 10.00 stably? 10.000, 10.0000 etc.,

Lovely little supply. Never seen any like that.

I think this style of meters never became popular because of its somewhat highish parallax error. On classic meters the plane of the scale and also the plane the needle travels on are parallel and normal to the viewer. On these models the planes are arc shaped and the distance between the needle and the scale varies, being more severe at both ends of the scale, thus aggravating the parallax error. Notice how the upper left meter is not at zero. Anyways, they were superseded shortly by the numeric displays.

In the old days there were some light reflecting meters that projected the shadow of a needle to a semi transparent screen. I wonder how much resolution they got. Speaking of resolution, my professor of history of architecture had a Kern DKM3 theodolite, that baby could resolve 1/10 arc second! 12960000 divisions full circle. I believe they are still worth a few thousands dead.

Oh never mind me just lingering thoughts, I am so distracted....


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