Simpson 260 series 6 repair

[flashthunder]

Hi all....I have a Simpson 260 series 6 meter with a bad R1 (the rheostat that is across the meter). It seems that this part is not available or at least very difficult to find. Does anyone have a source for the part, or possible a work around solution? Thanks!

[jonpaul]

See service manual,

Show the phot the part.

Try to clean the pot forst.

use any trimpot same resistance not critical.

j

[flashthunder]

Hi guys...I am having an issue with calibrating a Simpson series 6. When I place 50ua across the movement the needle will deflect to @90 on the scale. I cannot seem to be able to get full deflection by adjusting the R2 trimmer pot. It takes @56ua to get the needle to deflect full screen. The movement seems to move freely with no binding. Were there different movements made between the different series...or should all the movement deflect with 50ua? Thanks

[wasedadoc]

Your meter's scale plate says 20,000 Ohm/Volt DC.  That means 50uA for full scale.

How are you measuring the current?

You do not "place 50ua across the movement ".  Volts across, current through any circuit element.

[floobydust]

If you have them, check the two clamp diodes across the meter movement are OK, not leaking current or one shorted.
Careful if you are injecting current directly to the meter movement, always include a series resistor like 10kΩ I think for PSU set to a few volts.

[flashthunder]

I am using a 50ua constant current source and injecting into the 50ua jack. I have checked the diodes (both good) and also disconnected the varistor. I have @260mv across the movement and I can change that voltage via the R2 potentiometer, however I just cannot seem to get full scale deflection with 50ua. At this point I am thinking that possibly the magnet has lost some of its magnetism as the meter will fully deflect, but only with 56-57ua.

[wasedadoc]

How do you know that the 50uA current source is 50uA?

[flashthunder]

Hi wasedadoc....I have a current meter in series with a constant current source. I am fairly confident of the 50ua as I have a second Simpson 260 that I have used the same current source on and I am able to calibrate it with no issues.

[floobydust]

What orientation (H or V) is the meter at? See if gravity or the zero cal is a problem. Otherwise, I would think the magnet has weakened or there is a mechanical issue with the main taut-band spring. Sometimes they get mangled inside, see if it has a cal mechanical adjust possible.
If the needle sticks or jams and does not move smooth and silky, there can be a contamination problem, I have seen metal chips in the magnet gap and used masking tape to stick them and get them out.

[flashthunder]

Hi floobydust....I have tried both H&V positions...same results. I don't believe there is any binding of the movement as it will deflect smoothly, and completely, it just takes more than 50ua to get full deflection. I am leaning towards the magnet is just not as strong as it should be...I suppose after 60 plus years and most likely a few bumps along the way the magnet has lost some of its umph.

[floobydust]

You're sure your doing it right? there are two CAL trimpots and the meter's internal resistors must not be in-circuit when doing that.
First is applying 50uA and setting the parallel trimpot R1 so it's at full-scale. I'm assuming this step is where the party stops. Did you try (it) full-scale current with the trimpot disconnected? Your trimpot might be the wrong value is it 100k or 200k etc. Lift a leg on it, to see what the raw meter movement can do. Army CAL manuals (bottom of webpage I linked) is applying to 50uA/250mV input jack.
(Second step is setting the meter circuit to 5,000 ohms with the second trimpot R2).
A few different Series 6 models. Not sure which you have.

Can you see the main spring calibration, they usually solder the one end into a U or fork.
Otherwise, I don't know how to re-magnetize it. I would blame the magnet last.