I’m dabbling in the nostalgic or legacy end of the spectrum here, trying to repair a late 70’s panel meter off an old trawler. It’s the indicator dial for a rudder position sender (ComNav) and as far as I can tell it’s just a galvanometer reading +/- 0 to 12 vdc. It stopped working a while back; pulled it out of the boat and onto the bench where I’m getting weird results testing it – when I gently apply 0-10v using my bench supply I get (apparently at random) either no response at all, or wild needle excursions not corresponding to the input voltage. Interesting
So there are two possibilities here: one is that the galvanometer itself is failing (and if so, what’s the failure mode?); the other is that the signal that feeds it is actually a signal of some kind and not a straight voltage. I’ll visit the boat and check on the output from the sender, but in the meantime, I’m wondering if there’s any source for analog – needle dial – galvanometers that read +/-. I’ve checked Ebay and done some googling with little success.
IIRC these analog voltmeter doodads use a moving coil to push/pull the needle against a weak spring. It seems pretty much foolproof – magnets and coils, brute simple – so I’m wondering if the weird behaviour I’m seeing indicates something as simple as mechanical stiction. I haven’t yet pulled the actual indicator needle assembly out (the unit was sealed so it’s been hacksaw work just getting into it) to inspect, but there’s absolutely no sign of corrosion or water ingress anywhere in the canister.
ComNav wants over $300 for a replacement gauge so I’m determined to fix it myself if I can – also partly for the fun of it – but am not sure what the failure mode is or whether I’ve any hope of finding a replacement galvanometer mechanism to fit into the housing. I’m not an expert electronic tech, just someone who tries to fix things instead of tossing them; so if anyone has advice (or a favourite supplier of analog gear) I’m all ears.
looks like the guy who owns the boat gave me some bad data the voltage range on the galvanometer is more like +/- one half volt. So yikes, I may have fried it. My only working bench supply at this point is a crude sledgehammer of a device so I’ve had to order a more precise model with sub-1-volt resolution, and further diagnosis will have to wait till it arrives. If I haven’t fried it, I’ll be relieved.
The interesting thing is that the meter didn’t reliably peg when receiving the massive overvoltage; behaviour of the needle was more random, with excursions at the very low end of the range when. So it may have a protective circuit of some kind built in.
At any rate, whether it died of natural causes or I finished it off, if I can’t revive it I’ll be seeking a source of galvanometer guts, so the original appeal remains… anyone know where I can get the moving coil/needle core of a galvanometer with +/- .5vdc range?
RootlessAgrarian:
… anyone know where I can get the moving coil/needle core of a galvanometer with +/- .5vdc range?
Strictly speaking you don't need one. The needle motion is actually controlled by the current through the meter, voltage is one part of the current equation, the resistance (of the coil, sending unit, and anything btw the two) also factors in. Read Ohms law.
So if you want to use a “high” voltage to test your meter you can but you’ll need some “high” resistance in between the two. Does the meter have some current or resistance markings on it ?
Also, do you know that the sender outputs -12v to +12v ? Do you have a voltmeter you can measure it with ? I’m sure you can find a replacement meter, though it probably won’t be the same physical size and will need a paper background change. Then again you could go digital.
Boats, one of the few places where you can spend so much for so little. :twisted:
I agree with the Mee_n_Mac. Just about any analog meter will work. All measure current flow, not voltage. The basic movement rotates by the pointer by an angle proportional to the current flow. Meters range in full scale sensitivity from about 10 microamperes to a few milliamperes, with 1 milliampere for a full scale reading being fairly typical. Some meters have a centered scale and measure + or - current but the principle is exactly the same.
To set the voltage range, you must use a series resistor that controls the current, so that applying full scale voltage results in full scale current. The resistor value includes the resistance of the meter movement, which may be a few hundred ohms.
Analog voltmeters are just current meters with an included resistor.
So, find a meter movement that will fit in the case and then work out what resistance is needed. Since you probably can’t buy the exact value you need, you may need to fit a small variable resistance (potentiometer) into the case and adjust it as needed.
Hey, thanks for the replies! @jrem, yes, that ComNav part is the one I’ve just dismantled – almost $300 just for an old school voltmeter, go figure! Anyway, @Mee, I went down to the guy’s boat and measured the voltage being put out by the sender – he was a bit confused about the decimal point and it seems to vary lock2lock from -.5v to +.5vdc. So that is what the meter was calibrated for.
The meter has no markings of any kind though there is a circuit board attached to the back of it, it’s unpopulated except for one zero-ohm resistor (a jumper in fancy dress). Not only that, but one of the traces has been cut (at the factory obviously, since I had to use a hacksaw to open the case)… interesting. Anyway, not having any big honkin’ resistors kicking around I’ll wait till a more precise bench supply arrives in the mail and then try it in the recommended range. Still hoping I didn’t blow it up by applying 10v!
Happy ending – meter not fried, responds properly to the correct voltage range; the issue all along was an iffy solder joint (shame on ComNav, it failed after a mere 25 years in service!). Glued back together, aside from the Frankenstein scars it’s working fine and handed back to owner for re-installation.
