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 Post subject: Re: Open meter movement coil - Simpson 50uA
PostPosted: Oct Sun 20, 2013 10:32 pm 
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Looks just like mine, except on the back; that must be the diode bridge for the meter to measure AC, which was not on my meter (the diodes are on a terminal strip under the chassis).

I'll send a pm.

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 Post subject: Re: Open meter movement coil - Simpson 50uA
PostPosted: Oct Mon 21, 2013 2:15 am 
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Location: Long Island NY
Quote:
Do you have a better manual copy with more info?


I have an original manual but no means to scan. It probably does not have much more info than what is already on line; there's only a part number for the meter. The 5-0-50 uA notation only appears on the schematic; the text describes it as a 50-uA meter, as you say.

The pulse measuring circuit is the height of simplicity. The pulses from the thyratron stage are fed (through a blocking capacitor) to the red test lead; the green lead returns them to the meter and a pair of series-connected 2,000-uF @ 6-volt capacitors in parallel. So if a capacitor under test started breaking down, the internal capacitors would store a charge and you'd get a reading. Normally the meter would deflect upscale if there's a problem, but they use the third (black) lead to tie the circuit common of the tester to the chassis of the set being repaired when pulse-testing coupling capacitors in circuit. Depending on how the capacitor fails, the meter could deflect backwards, so that's why they offset the zero and have a "bad" zone on either side of the "good" mark.

This reverse engineering reveals a definite flaw in the design of the tester. If the red and the green test leads are shorted together in "Pulse" mode while the pulse voltage is full up, the resistances in the circuit will limit the current to about 1 mA--through that 50-uA meter! What's worse, the user is instructed to short the leads together and set the "meter adjust" control to full scale on the capacity and leakage resistance tests, but not on the pulse test. What an obvious "booby trap" for somebody who didn't read--or forgot--the instructions! Presumably this is why so many of these testers turn up with dead meters.

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 Post subject: Re: Open meter movement coil - Simpson 50uA
PostPosted: Oct Sun 27, 2013 1:04 am 
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Location: South New Jersey 08077
Solved with wonderful generosity from mbear2k providing me a meter (and a parts-donor 383-A from epay) I do now have a Simpson Capacohmeter that gives meter readings of capacitance and megohms leakage.

Beside the working meter, and the original 'open-coil' meter, I also have a 'spare' movement, but it has mechanical problems, mainly what looks like spider webs in the gap where the coil is trying to turn. Interestingly, the spider webs all align radially with the magnet. Yes, I am sure they are hair fine bits of iron wire. This movement will 'hang' or 'stick' almost anywhere on the scale and also goes way off the left end of the scale and points in different directions depending on which way is up.
I have read some posts about using stickey tape to fish debris out of the gap, maybe that would salvage this meter as a back-up spare.

Leigh said
Quote:
Simpson has pretty much standardized their 50μA movements at 1800 Ω.

I agree with that value; I did a quick Lo-Power analog ohms reading and got about 1910 Ω on one movement, but have not double checked that ohmmeter. (No resistors inside the meter.)
Using the 20K ohms scale of a DMM (only 23uA) it read 1.80 / 1.79 Kohms.

Thank you to the numerous ARF members for assisting this repair project.

It will be a new thread when I do more work on the Capacometer high-voltage pulse circuit to get it operational beyond just the HV rectifier.

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 Post subject: Re: Open meter movement coil - Simpson 50uA
PostPosted: Oct Sun 27, 2013 2:35 pm 
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Willitwork wrote:
... and points in different directions depending on which way is up.
The balance weights are out of adjustment.

Don't know how that could happen; never seen such a problem. But apparently it did.

Those are the little coils of wire wrapped around stems set in 90° increments around the front.

They are set at the factory so the needle stays on zero regardless of the position of the movement.

- Leigh

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 Post subject: Re: Open meter movement coil - Simpson 50uA
PostPosted: Oct Sun 27, 2013 9:10 pm 
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About the counter-weights and balancing the meter movement:
I THINK the weights are ok; I have adjusted the meter and it is now closer to balance.
However, what I adjusted was the pointer.

The pointer is made "cranked", I'll call it. It is made with two 90 degree bends, it comes from the pivot axis out, then bends forward (toward the glass) then bends back toward the scale.
I observed that the pointer was out of line with the main counter-weight.
My guess is that this mis-alignment could have been caused by slamming the pointer against the right end peg with an overload.

I adjusted the pointer, on two axes, and with better alignment the balance got a lot better. This was all 'by eye' but with a magnifier.

That "crank" had to be rotated to make it perpendicular to the scale face (=parallel to the pivot axis).
I was probably lucky that neither pivot jumped out of the jewel since this was twisting the part of the needle that goes to the pivoted coil.

Then I bent the main needle slightly clockwise (on the meter scale-face) so it looked like it was straight across the pivots from the main counter-weight. This 'bend' was done by a 'twist' in the part of the needle that is 'cranked' in the 'forward' direction.

So far, so good.

The tool I used for the twisting of the pointer was an aluminum Weller solder-aid, the end with the slotted tip, the slot fit over the pointer and it was non-magnetic (and clean).

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 Post subject: Re: Open meter movement coil - Simpson 50uA
PostPosted: Oct Sun 27, 2013 11:29 pm 
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Location: Pocasset, Cape Cod, MA
The general procedure for balancing meter pointers: hold the face horizontal and adjust the hairsprings/adjusters for zero (set the front adjuster to center, and move the rear one for zero). Then hold it with the face vertical and the pointer horizontal and adjust the tail weight for zero (like balancing a seesaw). Then hold it with the pointer vertical and adjust the side weights for zero. Repeat the last two steps as required, and the pointer should hold zero whatever the meter position.

As you found, if the pointer and tail weight are not exactly in line, they will have the same effect as a change in the side weights.


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