Refurbishing/calibrating HP 6255a power supply

[JonHendry]

I recently picked up an HP 6255a power supply. As far as I've tested it looks okay, apart from some chipped knobs and a meter pushed in with a broken bezel.

I'd like to test and calibrate it more or less according to the manual, but they want you to use a differential voltmeter, which I don't have. What I do have are a new Mustool MT109 meter and an older Fluke 75. Can I get "home gamer" accurate without the differential meter?

In addition to calibration I'm thinking of replacing the two-part voltage and current adjustment knobs with ten turn pots, and also replacing one or both needle meters with a digital DC volt/current meter. The power supply was available with ten turn pots as an option, so I'm hoping that swap won't be too complicated.

I might just try modeling and printing a bezel replacement for the loose meter.

Is there anything else I should look at?

Near-term I'll be using the power supply to provide voltages to perform external calibration on a Tektronix 2430a scope I picked up on eBay which needs some work.

[james_s]

I've never found a need to calibrate a power supply. Just check that the output agrees with what is on the meters. Personally I find good analog meters to be superior to digital for this application. I also prefer fine/coarse knobs over 10 turn as it's much quicker to set it. That's a matter of preference though.

If it were me I'd clean up the cosmetic bits, give it a quick check under load and then use it.

[kj7e]

No need for a differential voltmeter, just use a any decent DMM to measure the current and voltage, the only adjustments inside will be for the current and voltage sense for the analog meters.  Just need to check the regulation under load and for AC ripple, this will tell you if you need to look the caps.

[bob91343]

It depends on what they mean by a differential voltmeter.  Strictly speaking any voltmeter that can be 'floated' is a diifferential voltmeter.

However, some refer to one of those slideback voltmeters such as the very old Fluke units.  Those were notable for their accuracy.  I have one, but it's not a Fluke.  The original ones had a series of knobs that were turned to get a null and the value was read by the knob positions.  Leter, meters were automated so you didn't have to twist knobs.

Today things are different, and a very accurate voltmeter that can be floated is easy to find.  One characteristic of these is that, when a null is obtained. the burden is extremely light.  In other words, very high input resistance.

[JonHendry]

They're talking about those older high-precision ones with a null detector and a Kelvin-Varley divider. The power supply came out in 1966. The manual suggests using an HP 3420A Differential Voltmeter / Ratiometer.

[bob91343]

Yes those were fun to use.  Kind of like tuning in a short wave radio.  I like mine although don't use it any more.  It measures up to a kilovolt and its most sensitive range is, I think, 1 mV full scale.  Fairly compact, too.  Made by some long forgotten company in Pasadena California.  It's all solid state and uses a pretty good zener as reference.  Now it's in the garage ready to take to a swap meet.

I had planned to build one at one time.  I got the helipot and turns counting dial and was going to use VR tubes as reference, double regulation.  Never did it, though.

[kj7e]

The only reason they recommended such equipment is, in 1966 they didn't have 0.1% handheld DMM's laying around.