Simpson 260 repair

Oops, I meant to post this in sci.electronics.repair but here we are.

Leave this alone - it's normal for such meters. The 260 may need to be calibrated.

Joe

I did wonder! Simpsons are nice meters. Really nice and broadly the equivalent of the UK AVO brand (still in business AFAIK). I really like these old analogue meters. People say stuff like 'well, the trouble with them is they load a circuit too much compared to modern DVMS' but that *can* be desirable in many cases. Repairing vintage radios you find that oftentimes the servicing instructions have voltages printed on them which were read by meters with 20k ohms/volt or less, so at high impedance parts of the circuit a modern DVM will give completely wrong readings. Also I find it's much easier to peak signals with a moving coil meter than a DVM. And another thing - you want to monitor a voltage constantly over days and weeks (or a current) you can do this with an analogue meter without having to worry about the battery draining. And when reading high currents, there's no time limit like you find with modern DVMs. You can leave it hooked up to 10 or 12 amps for as long as you like. I could go on, but there's just a few of the reasons I like these old beasts.

Right. I've calibrated the DC voltage as best I can with what I have; get the meter as close to full scale at 50 uA into it as I can with the parallel pot, then set voltage across the meter movement and its series pot to 250 mV. On the higher voltage ranges (which is what I'd tend to use it for anyway) it definitely seems good enough for rock & roll.

Anything further with analog meters I believe require specialized tools which I ain't got.

Tape a slide rule to the side to apply cal factors.

,lol

Would I be showing my age if I said I always started on the highest voltage setting and worked down? It's a habit I got into with analogue meters and I still do it with an electronic meter, even though there is no need for it.

Sometimes there is a magnetic shunt which is used to set the sensitivity of the basic movement. If the magnet has aged, the shunt may need adjusting.

This ain't the National Bureau of Standards. Calibrate the 260 against a DMM.

Joe

After a "calibration" it still looks pretty good for rock & roll even with the movement slightly out, at least far away from the extremes. This is 25 volts at the terminals:

Also an excuse to check out my surplus 6177B whose output is still pretty stable and meter bang on after all these years, even though the meter doesn't go down anywhere near that far it's not hard to use the 10 turn pot to get a stable 50 uA out of it.

Pretty clean inside except for some dust:

Wait a minute, what the hell is this? That doesn't look factory :-|

I don't know how old this particular meter is, but you can't really be too surprised when you encounter something like this in vintage equipment. It doesn't look very elegant, but the important thing is: does it impair functionality? Someone who really knew his onions theory-wise may have done this and left the meter working perfectly. Judging the efficacy of a repair by the quality of the workmanship isn't a watertight approach. If the meter's accurate on all ranges I wouldn't worry about it.

Using a stiff voltage/current source.

Sorry for the confusion, those last two shots are actually of the HP current source! The 260 looks untouched inside.

For a meter that is over 50 years old that is probably not too bad. I have 2 of the meters and the book says the meter is 48 uA the best I recall and that parallel pot sets it to 50 uA. So that meter does not seem too bad. with digital meters getting much closer it is hard to get it in ones head that those old meters were field instruments and not super accurate. A 50 uA meter that has been banged around for 50 plus years is luck to be that close.

Digital meters aren't always honest either, especially on AC ranges. Some can't even cover the audio range. Some disappear the low 1% of readings to hide their internal noise floor.

The Simpson 260 6M uses a taut band meter movement. It shouldn't stick. Even though your gentle rocking "fixed" the sticking problem, you should take a look around the meter movement for some remaining debris. You might need a microscope:

I recently fixed my long forgotten Heathkit IM-105 VOM. It had a sticky taut band meter movement. Snooping around inside, I found that a tiny paper label inside the meter had fallen apart and jammed the movement. Compressed air didn't work too well, so I switched to a 20 gauge needle and an improvised vacuum pump. The suction vacuumed out the paper debris, which clogged the needle. That was acceptable because the movement was now clear of debris and moving normally.

I don't know what to suggest for fixing the high resistance movement problem. Good luck.

There's something very satisfying watching a perfectly-damped needle move across the face of a meter and come to a steady halt. I was given an Avo Multiminor almost 50 years ago and occasionally still use it.

Rather OT for this group I also have a couple of Post Office and British Telecom meters (ex my F-i-L. He started working for them in the early

Persistent megnetic metal debris can be removed with adhesive tape. Another method is to use a latex-water emulsion adhesive, which can be peeled off when dry, taking the debris with it.

I'll have to drag my 260 out and see how it's fared.

It probably does not require repair: 1880/1800= 4.4%, which is very good for a wound moving-coil meter. My recollection is that those coils were 5% or so, and the objective for lab instruments (versus field tools) was overall errors of 1% max.

Joe

Some meters have a magnetic shunt to account for the magnet strength loss over the years. Not sure about Avo, but the uk GEC Selectest, does have that and can be used to calibrate the meter. Only aware of it because I bought a Selectest a while back, reading around 5% low on all ranges, but was able to recal that using the shunt. Always thought Selectest a better better than the Avo 8, but ymmv.

Rebuilt a 260 a year or so ago. Dated Dec 1944 and bought by my father in the 1950's. Movement still good, but all over the place error wise. Have done all the dc ranges, replacing just about all with metal oxide, sometimes two or 3 in series to get the right value. The

Chris