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 Post subject: Open meter movement coil - Simpson 50uA
PostPosted: Oct Sat 19, 2013 4:28 am 
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It looks to me like I have an open meter coil.

The meter is from a Simpson Capacohmeter model 383-A, and rated at 50uA fullscale.
The whole meter face bezel is 4.5" wide and 4" high, "modern design", and naturally the scales on the face are non-linear and custom.
It swings very freely, no binding.

There is no continuity from terminal-to-terminal on the outside of the meter housing.
I removed the bare movement from the meter housing and have continuity from the front wire lead through the pivot, frame, (and magnet) and hair spring to a soldered terminal on the front of the moving coil.

There is continuity from the rear wire lead through the rear hairspring to a soldered connection on the moving coil, but it appears NOT through the moving coil.

My continuity tester only pushes 39uA, and when I was using an ohmmeter it had 800K in series.

How should I proceed? There is no way I can work under a microscope on the winding of the coil.
(I shake too much, and my sight is not that good.)

I don't know what to look for in a panel meter that MIGHT use the same movement, unless it was labeled as 50uA. But that severely restricts any choices I might find online. The "modern" housing style was listed as "Custom order" in a Simpson ad I found.

Is my only chance to get a full parts-donor Capacohmeter? (And have better luck with that movement?)

Simpson Capacohmeter model 383-A:
Measures capacitance at 60 Hz AC using a rectifier for the meter movement (Reactance meter, not bridge).
Measures leakage Resistance Meghoms at 300 VDC.
Applies a "Pulse" voltage stress test to capacitors, intended at 150% of WVDC (up to 900V peak), to determine 'Good' / 'Bad'.

Bill

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 Post subject: Re: Open meter movement coil - Simpson 50uA
PostPosted: Oct Sat 19, 2013 5:39 am 
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Why don't you contact Simpson and see if they can sell you a new bare meter?

The basic designs (size, mounting holes, etc) are quite long-lived, so they may have a good replacement.

Given a proper movement you should be able to transplant the faceplate.

Can you post a photo of the meter with some basic dimensions?
I have quite a few new Simpson panel meters.

- Leigh

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 Post subject: Re: Open meter movement coil - Simpson 50uA
PostPosted: Oct Sat 19, 2013 6:08 am 
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No attempt to contact Simpson yet.

Beside the 4.5w x 4h inch outside face dimensions, the round panel mounting hole is 2.75".
The width/'diameter' of the movement magnet is 1.9" and the movement mounting screws are 1.05" oc.

Yes, I believe I could do a movement transplant into the existing housing and scale face plate ok, it came apart easily.

This is my first picture posting.....
Attachment:
File comment: Outside dimensions of the face are 4.5" wide and 4" high.
Little-190930487068_1.jpg
Little-190930487068_1.jpg [ 4.93 KiB | Viewed 3220 times ]

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 Post subject: Re: Open meter movement coil - Simpson 50uA
PostPosted: Oct Sat 19, 2013 6:42 am 
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There are 2 383a on fleaBay right now. This one looks good:
http://www.ebay.com/itm/VINTAGE-SIMPSON-383-A-CAPACOHMETER-HAM-RADIO-TUBE-AUDIO-CAPACITOR-TESTER-WORKS-/380747792702


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 Post subject: Re: Open meter movement coil - Simpson 50uA
PostPosted: Oct Sat 19, 2013 6:52 am 
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True, parts donor units are available.
Both of those on epay at this moment do not show any meter deflection in any 'test'.

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 Post subject: Re: Open meter movement coil - Simpson 50uA
PostPosted: Oct Sat 19, 2013 7:32 am 
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Location: Albuquerque, NM 87123
I was going to offer my meter, but it too is open. I will dismantle it to see what the problem is.


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 Post subject: Re: Open meter movement coil - Simpson 50uA
PostPosted: Oct Sat 19, 2013 10:54 am 
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Given that all four of the instruments currently under discussion apparently have open meters, I'm wondering if these suffered from people connecting them to electrolytics that were still charged.

- Leigh

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 Post subject: Re: Open meter movement coil - Simpson 50uA
PostPosted: Oct Sat 19, 2013 4:57 pm 
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Quote:
I'm wondering if these suffered from people connecting them to electrolytics that were still charged.


Or maybe testing the meter for continuity with a VOM? A 50 uA meter won't take much current before dying?

Rich


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 Post subject: Re: Open meter movement coil - Simpson 50uA
PostPosted: Oct Sat 19, 2013 5:19 pm 
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I did not mean to say that I could tell that the two on-line at this moment had bad movements, only that nothing showed they were operational. So, it sounds like Johnnysan and I have the same failure, and the other two might also be the same.
These meters are called the 'MODERNISTIC "CLEAR-VUE"' style in a catalog page of the 383-A manual. Simpson lists a 4.5" nominal size (which is what I assume is the width) that matches the Capacohmeter meters.
I do not see any bending or other mechanical 'pegging' type damage to the movement.

The one I got had already been re-caped. Unfortunately, there were wrong connections so it is a troubleshooting project. So far, I found the circuit for adjustable bias voltage to the 2050 thyratron grid with two small electrolytics reversed. and a mis-connection on a terminal strip.
Mine has only been on at reduced (and isolated) line voltage momentairly up to about 50 volts and the tubes are removed.

Although there is a scale for measuring up to ten ufd, there is no mention made of testing electrolytic capacitors, (it does not say NOT to, and does not say you can) but the manual does have warnings to have the unit under test powered off and the capacitor discharged.

The manual is very poor. I was surprised that there was so little information.

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 Post subject: Re: Open meter movement coil - Simpson 50uA
PostPosted: Oct Sat 19, 2013 6:02 pm 
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I have two older Simpson 50µA meters, with black Bakelite cases, and a newer style, but the only one with a case like yours is 1mA FS. No idea if the movements and scales would interchange.


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 Post subject: Re: Open meter movement coil - Simpson 50uA
PostPosted: Oct Sat 19, 2013 6:49 pm 
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Quote:
No idea if the movements and scales would interchange.

The whole movement has to fit back in a 2.5" inside diameter chamber in the meter housing and match the movement mounting screw spacing 1.05" oc (which is the same spacing where two tiny screws mount the scale face on the front).
Front-to-back the movement pivots make it 1.2" thick.
The needle extends 2.25" from the pivot.

Any chance the sizes match a 50uA?

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 Post subject: Re: Open meter movement coil - Simpson 50uA
PostPosted: Oct Sat 19, 2013 8:21 pm 
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In addition to size matching, the resistance of the meter's coil is important to match.


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 Post subject: Re: Open meter movement coil - Simpson 50uA
PostPosted: Oct Sat 19, 2013 9:24 pm 
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hawaii.ken wrote:
In addition to size matching, the resistance of the meter's coil is important to match.
That's true.

Simpson has pretty much standardized their 50μA movements at 1800 Ω.

I checked the datasheets for the Wideview, Trueview, Designer, Century, and Rectangular
series, and they all show that value.

- Leigh

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 Post subject: Re: Open meter movement coil - Simpson 50uA
PostPosted: Oct Sun 20, 2013 12:28 am 
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I pulled my meter apart and it appears that the coil is open; I can't see anything else wrong.
Before you spend lots of time on this project I think it should be said that this piece of equipment is NOT well thought of; it was generally agreed that using this cap tester would actually cause capacitor failures (the high voltage pulse would damage the dielectric and the cap would fail later). Supposedly many techs would just stop using the 383 because of all the call-backs.


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 Post subject: Re: Open meter movement coil - Simpson 50uA
PostPosted: Oct Sun 20, 2013 1:25 am 
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I believe that any analog voltmeter that is rated at 20K ohms per volt will use a 50uA movement by necessity, but you may need a resistor in series or parallel to match resistance.


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 Post subject: Re: Open meter movement coil - Simpson 50uA
PostPosted: Oct Sun 20, 2013 4:22 pm 
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It is absolutely true that the Simpson 383A was blamed for causing capacitors that were okay to fail soon after testing, and the testers soon became despised in the TV repair trade. But Simpson made no bones about it in the manual, the whole point of the pulse test function in these units is to turn weak or maginal capacitors into bad ones so they could be identifed and replaced! The capacitance and leakage resistance tests are done in the usual fashion (capacitance is measured as impedance with AC, and leakage resistance with DC) so they did not cause any harm. One cannot blame Simpson if the user did not read the manual or understand it and the tester worked as advertised!

I think the real problem is that Simpson was way ahead of their time; the idea of "re-capping" an entire radio or TV was completely unheard of in the 1950s. Back then, a technician expected to find one or perhaps a couple of bad caps in a set, and after changing them, the job was done. They just didn't realize back in the day what a bunch of junk most paper capacitors were, because they didn't have anything better for comparison!

Note that the value of the meter in the 383A is given as 5-0-50 uA, meaning that zero is offset about 10% up the scale and it indicates both left and right of zero. This can easily be arranged by taking a standard 50-uA movement and offsetting the rear return spring tab (around the rear jewel holder on most movements) so the pointer zeros at about 10%. Since there are separate calibration adjustments for the various functions, it is probable that it's not too fussy about the DC resistance of the meter coil, and you could get it to work with a variety of 50-uA meters.

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 Post subject: Re: Open meter movement coil - Simpson 50uA
PostPosted: Oct Sun 20, 2013 7:22 pm 
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Having owned and used a 383A back when they were new I totally agree with Chris108. I have two positions in my shop and both have a 383A. If they have any weakness it is the power transformer. If still needed I can probably supply a good meter just let me know.

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 Post subject: Re: Open meter movement coil - Simpson 50uA
PostPosted: Oct Sun 20, 2013 8:57 pm 
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I've got a mostly working movement for the Simpson if you'd like it. It works, but needs the movement needs to be oriented horizontally to Zero the pointer. I believe this points to an internal balance issue with the movement? I have this second parts 383a if you need anything else!

Mark


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 Post subject: Re: Open meter movement coil - Simpson 50uA
PostPosted: Oct Sun 20, 2013 9:24 pm 
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Chris108 said:
Quote:
Note that the value of the meter in the 383A is given as 5-0-50 uA,


The on-line copy of the 383A manual I have only says that the meter "...has a sensitivity of 50 microamperes full scale." And the schematic shows "50uA". The parts list is silent about this. I had assumed that the movement was then mechanically biased up-scale (as you describe) to work as approximately -5 to +45 uA.
Do you have a better manual copy with more info?

Bruce Hagen; Thanks for the offer, but there is no rush; we'll see what happens.
For that matter, if one of your 383A power transformers dies, I could help, let me know!

I agree that there must be many-many 50uA movements in existence, but it would have to fit on the unique 383A scale face-plate. The megohm scale is a usual non-linear (0 right, 12 center, Inf. about 10% up from the left) and the Capacitance scale is non-linear (0 about 10% up on left, to '10' about 5% down on right) and the "BAD"-"GOOD"-"BAD" arc with GOOD only at the 10% left rest position. I do not think I am up to creating a new set of scales to put on a movement that is mechanically different.

Johnnysan; I guess your luck is as bad as mine as far as open meter coils goes. Did you have a special reason for trying to get that 383A going? Or just curiosity like me? Or ....

mbear2k; Thank you Mark, yet another generous offer for me to consider. Do you know if the counter weight is still there, hanging on the opposite end of the needle where it sticks out below the pivot?


Does anyone know what the Good-Bad Pulse test actually measured? I am guessing it was non-linear voltage dependent leakage current. A breakdown at peak voltage that went away as the voltage decreased.

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 Post subject: Re: Open meter movement coil - Simpson 50uA
PostPosted: Oct Sun 20, 2013 10:17 pm 
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I know when I was looking at this, someone asked if the counterweight was attached. It looks like it is to me.

Yours if you want it.

Attachment:
simpson1.jpeg
simpson1.jpeg [ 72.31 KiB | Viewed 3055 times ]


Attachment:
simpson2.jpeg
simpson2.jpeg [ 75.6 KiB | Viewed 3055 times ]


Attachment:
simpson3.jpeg
simpson3.jpeg [ 105.67 KiB | Viewed 3055 times ]


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