Simpson 260 Meter Movement only works correctly when horizontal

[Zandor]

Hi,

I have a Simpson 260 series 6 and I have a issue with the meter movement only working correctly when it is lying on its back horizontally.  All voltage, current and resistor checks work great.

However, if I set it upright, the needle will move off of the full left and go to about the 0.6v mark if you would be measuring at a 10v scale.  And when it should be fully to the right at the 10v it is actually at 9.4v.  So it seems to be just shy of both ends about the same 0.6 when upright.

Has anyone seen something like that?  If so, is there any way to correct this? 

I am hoping to get some guidance here before I try opening it up.

Thanks,
-Z

[Gyro]

It sounds as if the movement is slightly out of balance. If you look at the moving coil assembly at the base of the needle, you should see a 'cross arm' with three stubs (the 4th arm being the needle itself). The stub opposite the needle has a (normally coiled wire) counterweight on it to balance the weight of the needle. The two side arms may or may not have much smaller weights to provide any additional balancing of the movement needs at 90' to the needle.

Most accurate meters are designed to read correctly in the horizontal orientation. Depending on the quality of the fine balancing, they may show some offset when positioned in other orientations. Balance can shift over time due to handling, eg. a sharp knock can bend the needle slightly at the cross arm (where the leverage is highest). Hitting the end stop hard can do the same thing (hence the terms 'wrapping the needle' and 'burying the needle').

My advice is to leave the movement strictly alone. If you start messing with it, trying to bend the needle etc, you are likely to make it far worse, snap the needle, get ferrous particles in the magnetic gap and so on. It is sometimes possible to improve the balance by adding small blobs of varnish to the adjustment arms, but first you need to work out exactly where this is needed - is it just one side arm of one side arm, or a combination of side arm and main counterweight? etc. If you get it wrong, the linearity will go off (under-read at the left hand part of the scale, over-read on the right hand side - or the opposite). Add to this the risk of damaging or contaminating the hairspring or pivots and it's just not worth the risk unless you are a watchmaker.

Be happy that the movement works smoothly and use it horizontally.


P.S. In your case it sounds as if the counterweight ls a little heavy. Maybe the very tip of the needle is missing? Trying to reduce the size of the main counterweight can only end in tears!

EDIT: Out of interest, I checked one of the Simpson 260 interest sites. I noted that the 260 has a taut band suspension, so no pivots and hair springs, but it will still have the same static balancing arrangement and danger of dust entering the magnetic gap. In addition, there is the real danger of snapping the suspension band if you start 'tweaking' things. I also noted that the handle is designed to allow the meter to be operated in a sloping position. Does it read right when it is sloping rather than upright?

[Zandor]

Gyro,

Thanks for the response. 

I messed around with starting to tilt it up from horizontal and as soon as the top started to come up, the needle began to move off of the left and continued inside to move until the 0.6 volt (again assuming 10 volt range) offset.  I also tried tilting it left and the needle again move up.  If I tilted to right, the needle moved down away from the scale.

I looked around for any info on internals of the movement, but didn't find a lot of details.  For now, I will just leave it alone and do like you said and use it horizontally.

[jdragoset]

Sometimes those cross arm balance weights slip and very close inspection (varnish stain on cross arm) will indicate original position.

[Gyro]

Yes, it's possible for a weight to shift - in fact it's the most likely explanation for an overly heavy main counterweight that I can think of. Keeping it horizontal is definitely the safest option though.