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  #1  
Old 02-02-2017, 11:37 PM
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What's this thing do?



Simpson Capacity Bridge 381. Guess where I got it.

Can I... measure the capacitance of capacitors with it? O_o I mean, that's kind of what it looks like. Can't really find any info on these.

Last edited by MadMan; 02-02-2017 at 11:58 PM.
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Old 02-03-2017, 09:36 AM
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Yes. Here's an article:
http://www.rfcafe.com/references/qst...l-1944-qst.htm

I never used one, but I believe this one needs an external signal generator, indicated by "1000~" in the article.
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Old 02-03-2017, 10:06 AM
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'tis from 1950 - here's some "new product" listings..
Attached Images
File Type: jpg 381-2.jpg (29.3 KB, 20 views)
File Type: jpg 381.jpg (39.4 KB, 16 views)
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Old 02-03-2017, 11:01 AM
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Works a lot like my Heathkit C3...Only swap a meter for the eye, remove resistance and leakage testing.

Might be able to mod that to be a RLC bridge with some engineering work...
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Old 02-03-2017, 09:07 PM
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oops - didn't realize the power cord was attached, so now guessing this has an internal oscillator, so nothing more is needed.
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Old 02-04-2017, 12:19 AM
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Thanks for the help guys!

Quote:
Originally Posted by old_tv_nut View Post
Yes. Here's an article:
http://www.rfcafe.com/references/qst...l-1944-qst.htm

I never used one, but I believe this one needs an external signal generator, indicated by "1000~" in the article.
Very helpful! Thanks

Quote:
Originally Posted by old_tv_nut View Post
oops - didn't realize the power cord was attached, so now guessing this has an internal oscillator, so nothing more is needed.
You know... there's nothing in it, really. Which is a little weird. It would need a tube for an oscillator, right? Must use line 60Hz.

>Pic<

Quote:
Originally Posted by Findm-Keepm View Post
'tis from 1950 - here's some "new product" listings..
Cool. I love all this old Simpson stuff.

Quote:
Originally Posted by Electronic M View Post
Works a lot like my Heathkit C3...Only swap a meter for the eye, remove resistance and leakage testing.

Might be able to mod that to be a RLC bridge with some engineering work...
So you mean I don't have to buy a big expensive thingy? Sweet.
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  #7  
Old 02-04-2017, 09:39 AM
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It's hard to believe it could operate at 60 Hz for small capacitors, especially the mmf scale.
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Old 02-04-2017, 09:44 AM
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By the way, do you notice the meter movement is mounted upside down? That's because the instruction to tune for "maximum" is really tuning for a minimum.
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Old 02-04-2017, 01:10 PM
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Quote:
Originally Posted by old_tv_nut View Post
It's hard to believe it could operate at 60 Hz for small capacitors, especially the mmf scale.
The Heathkit C-3 just uses 60 Hz from a 55 Volt winding on the transformer... reads down to 10 mmf.

jr
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Old 02-04-2017, 04:04 PM
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Quote:
Originally Posted by jr_tech View Post
The Heathkit C-3 just uses 60 Hz from a 55 Volt winding on the transformer... reads down to 10 mmf.

jr
Interesting - I believe you, but still hard to -

Reactance of 10mmf (10pf) at 60 Hz = 265 Megohm
Maximum current at 55 volts is 0.2 microamps

Reactance goes down to 16 Megohm at 1 Khz, still a pretty big number
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Old 02-04-2017, 08:55 PM
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Just for grins, I just measured the smallest cap that I could find in my (junk) assortment that had leads... (7.7 uuf according to my DE-5000)... the old Heathkit C-3 produced a sharp null on the eye tube at slightly below 10 uuf, so I believe that this bridge circuit is indeed capable of measuring small values of C using a 60 Hz source.

jr
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  #12  
Old 02-28-2017, 12:28 PM
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Quote:
Originally Posted by jr_tech View Post
Just for grins, I just measured the smallest cap that I could find in my (junk) assortment that had leads... (7.7 uuf according to my DE-5000)... the old Heathkit C-3 produced a sharp null on the eye tube at slightly below 10 uuf, so I believe that this bridge circuit is indeed capable of measuring small values of C using a 60 Hz source.

jr
Interesting. I never could get the lowest range on my C-3 to work. Thought it was because of the reason Old_TV_Nut mentioned. I tried another eye-tube.
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Old 02-28-2017, 01:10 PM
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My low range works somewhere below 100pF (lowest cap I recall trying), but I did recap my unit with precise value caps.
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  #14  
Old 03-01-2017, 02:03 AM
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Quote:
Originally Posted by Notimetolooz View Post
Interesting. I never could get the lowest range on my C-3 to work. Thought it was because of the reason Old_TV_Nut mentioned. I tried another eye-tube.
Perhaps the range selector (wafer) switch was just dirty. Most are after 60 years. if not that then that range's reference cap was likely bad. My experience with any of that type (many similar) cap checker is that almost all of the original caps have to be changed out. Especially if you are actually going to use it for value measurements, which many people don't even care about due to the ease and precision of a DVM.

But these old units IMO are invaluable for the leakage function at rated voltage. Simply nobody today makes anything that does this EXCEPT Sencore to my knowledge. Ebay has a couple of those modern units right now. An LC102 for a grand or two and even the old Z Meter LC75s from a suspect the 70s are usually a few hundred.

So the old Heathkits are to me definitely the way to go hands down. Simple in construction, yet durable enough, and they get the job done well with the added attraction of vintage looks and a magic eye. Some don't like the magic eye and want a milliammeter. Indeed more accurate, so you step up to a Sprague TelOhmike, but they are much more complex. There is a Pyramid model that is like a C3 only with a milliammeter. That's what I want! Oh and never change out you magic eye tube unless it gets dim or shorts.
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  #15  
Old 03-02-2017, 01:05 PM
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Quote:
Originally Posted by Notimetolooz View Post
Interesting. I never could get the lowest range on my C-3 to work. Thought it was because of the reason Old_TV_Nut mentioned. I tried another eye-tube.
I would check the 200 pico farad (pf or uuf) cap on the range switch and make sure that it is correct... you might have a 200 nano fafad (nf) by mistake in that position.

jr
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