Analogue meter - suspension repair

A couple of years back, a colleague gave me a professional Ferrograph audio test set. This has a large format precision meter on the front, which has the same vertical 'twisty' ribbon as the coil / needle suspension scheme. It was broken when I got it. As there was little chance of being able to get a replacement movement, I figured that repair was worth a go. As I recall (it's been a while ago now) I simply fed the wire back through the hole in the U-frame and held the frame in tension pushing down on pointy tweezers in one hand, whilst using the soldering iron in the other hand. I had pre-tinned both the wire and the frame, and simply bent the end of the wire over with the tip of the iron, whilst letting a small 'blob' of solder previously fed onto the iron's tip, flow into the joint. I was absolutely amazed when I let go of it all, and blew on the pointer. It went from one end of the scale to the other, as smooth as you like. No issues with centreing or sensitivity or whatever. I seem to think that I struggled slightly to get a good zeroing action from the zero offsetting screw in the perspex cover, but I did manage it in the end. I can't remember why, or how I got it to zero in the end, but I did, and it has remained correct since. The unit is in daily use, so it has been an effective and long-lasting repair.

Make no mistake, it was a delicate and fiddly procedure, with lots of holding of breath and magnifying glasses and so on, but quite do-able for the likes of you who enjoys challenges like this. Go for it. You've nothing to lose.

Arfa

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Likewise no chance of a replacement . Reminds me Test Instrument Services, Totton Hampshire no longer seem to do this and meter rewinds , keeping ancient ship-bourne engine/generator monitoring meters of the world's merchant ships going Certainly a job fully under a x4 or so illuminated inspection lens. This Sifam one you can rotate suspension anchors at either end , but there is no user accessible adjustment of zero once the meter scale is set in the housing.

I think the sensible route would be . A loop of copper wire around each U spring and twisted ends until parallel spring arms. Offer up the ribbon and solder up and then while a finger over each spring, in turn, cut the copper wires and gently release finger pressure. I'd rather have too little tension in the U sptrings and so the fine ribbon, than too tight.

I've never needed to repair that type of meter movement, but it sounds as though what has been described is likely to be the Taut-Band type of meter movement.

-- Cheers, WB .............

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I never looked inside one but all those broken Pye/Cambridge? moving spot galvanometers presumably had that system. While at it anyone know what the suspension is in Megger insulation tester? that dead springless movement, jewelled without hairspring? - I've never looked insiside the meter movement of one

Am 02.05.2011 11:30, schrieb N_Cook:

First try to attach the ribbon to the frame. You might want to remove the magnet first (if it isn't a core magnet type meter).

Remove the solder from the U-spring, especially near the open end with solder sucking wick.

You have to press down the spring at the broken side more than the final position so it will pull the other side into position one it's released. You have to try that out. Too few tension is just as bad as too much tension, because the frame wouldn't be in the right position then.

Solder the ribbon to the U spring near the turn of the U. Release the spring, check if the tension is right (axial position of the frame). If it is, you can now slightly adjust the axis of the frame now by pushing the ribbon left or right. Now fix the ribbon near the open end of the U-spring. Check that the pointer moves smoothly over the whole range by slightly blowing at it.

Re-attach the magnet/scale (if removed).

Check equilibration (set the pointer to about 25% of the range and check that it doesn't move when the meter is tilted back/forth; repeat for 75%).

Set the pointer to zero. Check sensitivity. If you can adjust sensitivity at the electronics, do so. Else you have to move the magnetic shunt (small iron strip at the magnet) to adjust sensitivity.

Markus

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I never looked inside one but all those broken Pye/Cambridge? moving spot galvanometers presumably had that system. While at it anyone know what the suspension is in Megger insulation tester? that dead springless movement, jewelled without hairspring? - I've never looked insiside the meter movement of one

I used to work on meter movements from time to time. Some of the Meggers I worked on had a dual coil which worked as a balance (between currents in each coil). One or both of the jewels had a spring under them, I remember putting the coil assembly carefully back into the jewels just with a pair of tweezers.

Shaun

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2 , 3 or 4 conductor wires going to the coils frame? 2 hairsprings in opposistion ? why not no springs

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2 , 3 or 4 conductor wires going to the coils frame? 2 hairsprings in opposistion ? why not no springs

I don't remember, it was 13 to 17 years ago that I did that type of work.

Shaun

But your temper.....

Jeff

--
Jeffry Wisnia
(W1BSV + Brass Rat '57 EE)
The speed of light is 1.8*10e12 furlongs per fortnight.