RCA WV-38A(K) multimeter - remagnetize movement?

[snpaine]

Hi All,

My grandfather's RCA WV-38A(K) multimeter, which he built from a kit in 1961, sat on a shelf in my office since he passed away years ago. At last I brought it home to check it out, and found it reading low on all scales. Turns out the 50 uA meter movement requires 66 uA to reach full scale, which is too far out of range for the series pot that adjusts the overall calibration. Linearity is good, though.

I'm guessing the magnet has weakened over the decades. The meter has a sealed clear plastic housing (photo) which I wouldn't risk opening up, but I'm wondering if it might be possible to remagnetize the magnet from outside the case with a sufficiently powerful permanent magnet.

Before I try anything, has anyone here ever tried to do something like this? Any advice?

Thanks,

Scott

[Silicon]

Have you measured the Full Scale Deflection of the meter when it is isolated from the multimeter components?

[snpaine]

Thanks--I have indeed. That 66 uA full scale deflection was measured on the bare meter.

[The Philpott]

I know this isn't what you want to do, but....my first thought would be cutting into the appropriate rear part of the housing to get access to the magnet so that one or perhaps two very small neodymium magnets could be experimentally located to boost the overall flux.

The cutting is most likely to be the problem- my only reasonably reliable method of cutting into plastic without creating debris is the smallest soldering iron available, with a small blade tip. In this application you'd want a vacuum cleaner on hand to avoid smoke deposition on the inside of the plastic. Needless to say the iron tip should not touch the magnet, but that's relatively easy as the melt of the plastic damps out any sharp movements of the iron tip. A new cover could be made to glue over the hole once calibration is restored.

All this having been said, it's quite a brutal idea and i don't know if you have room the add a rare earth magnet in the space provided.

Dave

[Craig Sawyers]

A relatively non-invasive method is to add an opamp with sufficient gain in series with the movement to correct for the low sensitivity.

This of course needs a bit of surgery, a connector on the rear panel, and a small external power supply to power the opamp.

See this thread https://www.vintage-radio.net/forum/showthread.php?p=1449662 post number 8

Good luck!

Craig

PS I found the manual for this unit here http://bee.mif.pg.gda.pl/ciasteczkowypotwor/%23ham_radio/Other/ACC_instrument/RCA_WV-38A_user.pdf

[markgengine]

Might be worth trying some strong magnets as Dave suggests, but without cutting into the plastic. A sort of non-invasive approach that won't damage anything.
Mark

[snpaine]

Thanks, Craig, for that link to the 1971 version of the manual. Its redrawn schematic is much easier to follow than the 1961 version I have. Interesting, too, that this model stayed in production for at least 10 years. In 1961, it cost my grandfather $29.95 in kit form, equivalent to about $325 nowadays.

Mark, I'm definitely leaning towards the approach you suggest. I've got a working analog meter and several DMMs, so I'd rather leave my grandfather's handiwork undisturbed. The construction of the meter's C-shaped field magnet (photo) is a rough rectangular permanent magnet soldered to two more finely-machined pole pieces, possibly soft iron, that carry the flux down to the cylindrical gap around the movement.

The goal is to apply flux to the permanent magnet via these pole pieces, through the housing, in the hope of remagnetizing it. I'm guessing the pole pieces themselves won't retain much magnetization from this process, and with the external field removed the shape of the field in the pole pieces will remain as designed.

I experimented with a couple of small but strong neodymium button magnets I had around. These readily snapped to the the right polarity near the ends of the pole pieces on each side of the movement, and I closed the loop behind them with a strip of steel. With this arrangement in place, the full scale sensitivity dropped from 66 uA to 77 uA. On removal after a few hours, the meter full scale sensitivity was unchanged at 66 uA.

Still hoping this might work with some stronger magnets of the right size and shape...

Scott

[markgengine]

Good luck Scott, if you get the drop in sensitivity as observed it sounds like you might need to "force" north to north mag poles, that is the reverse of what you have tried.
I'm always really nervous fiddling with meter movement guts, the magnetic fields are so easily upset in high sensitivity movements.
Mark

[The Philpott]

I don't think you can beat magnet to magnet direct contact. I don't believe you'll be able to apply a permanent boost using magnets which are then removed.
The advantage of an applied magnet is it can be shuffled around to achieve fine adjustments...then secured with a blob of glue.

Mark's comment has made me ponder though...when i had success it was with a low sensitivity movement with FSD of around 2.5mA. The magnet was an old horseshoe type and the Neodym. was applied to the concentrator.

Dave

[snpaine]

Well, I'm amazed. After fiddling with a neodymium button magnet oriented edge-on to the gap around the moving coil, flipping between attracting and repelling orientations, I found that the meter sensitivity was restored to its nominal 50 uA full scale (photo). I'm pretty sure this means that the meter's internal permanent magnet had never changed, and that this operation either demagnetized something that wasn't meant to be magnetized, or else dislodged some hidden bit of magnetic debris in the gap area.

The multimeter is back to together and reasonably well calibrated, although the zero setting is touchy. Here it is monitoring supply voltage to a QRP Labs QMX transceiver. Heartwarming to have three generations spanning 60 years working together.

Scott

[markgengine]

Thars a great result Scott. It was making me wonder because out of my collection of atound 50 analogue meters I've not really noticed the sort of demagnetising you found, and some are approaching 100 years old.
Mark

[The Philpott]

Well done Scott. I suspect that you may indeed have forced a piece of ferrous debris into a position where it's no longer having an effect. i hope this remedy 'sticks' but if not, then i'm sure we can revisit the gremlin and try to sort it out.

Dave