[Amps] Follow-up on Bouncy Meter

[Peter Chadwick]

Jerry said:
> Is this mechanical problem fixable? I assume there are dampening springs inside and somehow they are
not functioning properly? Replacing these meters with new, old stock costs about $70 - is it worth fixing the old one?<

Sounds a bit as if one or both hairspings have become unconnected. It's got to be worth opening it up to have a look, but whether or not it's repairable without special tools and equipment is another matter. For opening it up (this is probably teaching grandmother to suck eggs but........)
Use a table well away from where you ever done any mechanical work, because any trace for any reasons of steel particles and they'll be in there faster than a rat down a drainpipe! Spread out a sheet of clean white paper. Make sure you've plenty of light, and having a magnifier or even a jeweller's loup available is a good idea. Get a set of small jeweller's screwdrivers, and some stainless steel, plated brass, or plastic tweezers. (non - magnetic). This is ebst done when not tired, or suffering from a drop too much the night before, as a steady hand is required.
You'll likely see some tiny coils of wire wound around the end of the pointer that projects beyond the pivot. These are balance weights, so don't disturb them. Sometines they drop off, and if you find some floating around when you open the case, don't lose them. I've refixed them with 5 minute epoxy: superglue didn't work for me. But you really need the very tiniest and then smaller than that amount of glue. Don't get it in the wrong place or you'll literally gum up the works!
If the hairsping(s) have become detached (usually when that's happened to me - not that it has more than two or three times in the best part of 50 years - it's been at the outer end - if it was the inner end I'd give up!) it's usually possible with a very fine iron tip to re-attach them. Use very thin solder - if you haven't any, flatten out the round stuff and use the dikes to cut it into a narrow strip.
When re-assembling, set the zero setting arm (usually a slotted piece of metal sticking downwards from the pivot when the meter is held vertically) to mid position, and, looking in the meter case, set the pin on the zero set screw vertical. Make sure that you get the pin into the slot - you should be able to get 360 degree rotation of the zero set screw, withe pointer moving nicely over the range. If the pointer isn't in about the right palce with the zero set arm vertical before yu re-assemble, you'll need to adjust the position of the rear hairspring. That may well mean removing the scale so that you can get at it. Be careful not to bend the pointer or to put  strain on the bearings - sometimes, they have jewels, more usual I've found, is straight steel point in a steel cup, and there are limits as to how much that can stand.
Before re-assembly, you may wish to check that teh Full Scale Deflection (FSD) current is corect. You'll probably find a little pice of metal screwed to on of the pole pieces and extending some way across the gap in the magnet. That acts as a magnetic shunt, and if you've lost some magnetism, you can adjust that to get back to FSD. It can affect linearity, so don't touch unless you have to.
All in all, you may feel that all that effort isn't worth saving $70!!
Good luck
73
Peter G3RZP






========================================
Message Received: Dec 02 2006, 02:58 PM
From: "JOS Earthlink" 
To: amps at contesting.com
Cc: 
Subject: [Amps] Follow-up on Bouncy Meter

Hi Group,

Thanks for the advice on the bouncy meter. After removing it from the amp -
it is clear that this is a mechanical issue. When the meter is out of the
amp physically and gently turned right and left you can see that the meter
arm can bounce around wildly from one end of the scale to the next. I have
another identical meter and they are fairly resistant to this hysteresis
motion. When in circuit, this bouciness manifests itself by wide swings of
over-shooting and/or undershooting before it settles to the correct
position. Someone suggested placing a capacitor across the meter but this
clearly is a mechanical not electronic problem. Is this mechanical problem
fixable? I assume there are dampening springs inside and somehow they are
not functioning properly? Replacing these meters with new, old stock costs
about $70 - is it worth fixing the old one?

73 Jerry
K1JOS

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