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 Post subject: Simpson Model 383-A Capacohmeter running - CAUTION
PostPosted: Oct Mon 28, 2013 5:40 am 
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It took a previous thread to get a working meter movement on this unit that came through epay;
viewtopic.php?f=8&t=234237

Actually it took a bit more work than that; it had been re-cap'ed when I got it and several high value resistors had been replaced, but there were wiring errors I had to discover and fix.
A couple of the replacement carbon-comp resistors (marked 430K) had doubled and tripled in value. This was knocking the pulser B+ down from the 1600 - 1900 VDC it runs at now because they were in series from the power supply to the pulser plate circuit.
Two filter caps, 25 mfd at 25 vdc, that smooth the -dc (variable 0 to -15vdc) used to control the grid of the thyratron (2050) had been wired in reverse polarity.

Attachment:
File comment: My Capacohmeter works now.
capacohmeter.jpg
capacohmeter.jpg [ 73.46 KiB | Viewed 4745 times ]


So now, all three sections of the unit are running, but I have not done calibration yet. :D

(1) It has the capacitance measurement section that uses 60Hz ac through the Cx capacitor and a rectifier bridge on the meter to indicate the capacitance value. The meter has several switchable ranges.

(2) It has the meg-ohm leakage section that uses about 300VDC on the Cx capacitor and reads Meg-ohms on the meter.

(3) It has the "PULSE" stress test section (that I got working tonight) which uses a controversial method unique to Simpson, I think, to weed out capacitors with weak or deteriorating dielectric. Not for electrolytics.
The tester generates narrow spike pulses with up to 900 Vp-p that are passed through the Cx (under test) and into a 1000mfd cap across the meter. The idea appears to be that if there is a breakdown at peak voltage, that will pump some current into the meter and give it a net deflection off of the "GOOD" mark. DC leakage is not being measured because there is no DC applied with the pulse. A purely good capacitor would pass the pulse equally during the rise and fall of the spike, and not put a net charge into the meter capacitor.
They call for turning the pulses up to 150% of the Cx rated working voltage, 900V for a 600WVDC capacitor.

Many say that this stress test is strong enough to damage capacitors, without the failure being noticed, leading to later call-back service when the cap fails in normal operation. Others say that this only happens with operator error in testing.

I am simulating a capacitor going into breakdown with three NE-2 lamps in series, with 100K ohm limiting resistor, all across a 10000pf cap; I get to watch the neons flash along with the thryatron and see the meter move from the GOOD tic mark into the BAD range on the meter scale.

I will need to rig a high-voltage probe for a scope to calibrate the pulse peak voltages against the scale on the control pot. Many scope probes, even 10X or 100X may not be rated for the 900 Volt peaks this Capacohmeter is supposed to generate. I need 200 v/division or more.

Wish me luck, it has not 'bit' me yet; not even when there was that tiny little white-hot arc that may have damaged those carbon comp resistors that went way up in value.
It has also proved that the protection in my Triplett 630-PLK works, a couple of times.

Warnings and suggestions welcome.
Edit: added CAUTION to title
See Also Posted: Wed 27 Nov, 2013 7:04 pm for an update on the arc location

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Last edited by Willitwork on Nov Fri 29, 2013 7:55 am, edited 2 times in total.

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 Post subject: Re: Simpson Model 383-A Capacohmeter running
PostPosted: Oct Mon 28, 2013 12:05 pm 
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Suggestion: Don't use it.


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 Post subject: Re: Simpson Model 383-A Capacohmeter running
PostPosted: Oct Mon 28, 2013 6:31 pm 
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Willitwork wrote:
I will need to rig a high-voltage probe for a scope to calibrate the pulse peak voltages against the scale on the control pot. Many scope probes, even 10X or 100X may not be rated for the 900 Volt peaks this Capacohmeter is supposed to generate. I need 200 v/division or more.

My cheap aftermarket 100X probe is only good for 600 volts. I think I'd rig up a peak detector out of a pair of 1n4007 diodes and a cap and use a DMM for reading the peak voltage. If you want to examine the waveform, I'd stick with the lower voltages.

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 Post subject: Re: Simpson Model 383-A Capacohmeter running
PostPosted: Oct Mon 28, 2013 11:42 pm 
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Just in case anyone else is following this thread and might try a peak detector for a DVM on a Capacohmeter, Don't do it! At least not until we figure out why I got an arc across 3/8" of free air at the leads of a 0.1mfd 1KV cap, along with killing the cap (the ceramic is now a 150 ohm resistor) and killed the DVM too.

I think what I lashed up must have actually created a voltage multiplier, or a charge-pump integrator; I put in series a 1n4007 with the 0.1ufd across the Cx (#2 - #3) test terminals of the 383A, with the DVM across the 0.1ufd set to 1000VDC.

The arc was not actually as loud as a 22 cal pistol, but it was at least as loud as a cap pistol (remember when they were not regulated?). The other two clicks I heard were probably arcs inside the DVM before the big-bang.

The Simpson 383-A Capacohmeter survived just fine and is working as before with the cap and neon bulbs on it.

Who can tell me what the voltage got up to so it could jump the 3/8" (10mm) gap?

Maybe I'll work on this again tomorrow......
EDIT: See also Posted: Wed 27 Nov, 2013 7:04 pm for an update on the arc location.

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Last edited by Willitwork on Nov Fri 29, 2013 7:56 am, edited 1 time in total.

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 Post subject: Re: Simpson Model 383-A Capacohmeter running
PostPosted: Oct Tue 29, 2013 12:10 am 
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Willitwork wrote:
not until we figure out why I got an arc across 3/8" of free air at the leads of a 0.1mfd 1KV cap, along with killing the cap (the ceramic is now a 150 ohm resistor) and killed the DVM too.

Yikes!

Sorry about your DVM.

Hey, at least it's a good circuit analysis puzzle :-)

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 Post subject: Re: Simpson Model 383-A Capacohmeter running
PostPosted: Oct Tue 29, 2013 12:20 am 
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Willitwork wrote:
Who can tell me what the voltage got up to so it could jump the 3/8" (10mm) gap?

Depends on whether there were any pointy electrodes. If not, Wikipedia sez:

"The dielectric breakdown strength of dry air, at Standard Temperature and Pressure (STP), between spherical electrodes is approximately 33 kV/cm."

33 kV!!!

Edit --
I don't think it could anywhere near that high. Must be something pointy, or maybe the cap leads are small enough in diameter.

I'm trying to decipher the circuit. The schematic from Simpson260.com is pretty hard to read. I can't figure out what the second and third poles of the switch connect to; on the schematic I can see they end in arrows, but I can't read the designator for where they connect.

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Last edited by stevebyan on Oct Tue 29, 2013 2:22 am, edited 1 time in total.

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 Post subject: Re: Simpson Model 383-A Capacohmeter running
PostPosted: Oct Tue 29, 2013 12:40 am 
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Willitwork wrote:
I think what I lashed up must have actually created a voltage multiplier, or a charge-pump integrator; I put in series a 1n4007 with the 0.1ufd across the Cx (#2 - #3) test terminals of the 383A, with the DVM across the 0.1ufd set to 1000VDC.


The only mechanism I can see for generating a high voltage would be inductive kick from the power transformer high voltage secondary winding; if the thyratron tries to quickly shut off the current flow at a current peak, maybe there's enough leakage inductance in the power transformer to generate some high voltage. Seems like a stretch to me. But that could explain the reputation this circuit has for inducing latent faults in good capacitors.

Hmm, maybe a 1 Meg resistor between the test terminals would help tame things?

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 Post subject: Re: Simpson Model 383-A Capacohmeter running
PostPosted: Oct Tue 29, 2013 10:33 am 
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Maybe I should listen to Johnnysan.
Quote:
Suggestion: Don't use it.

Well, strictly speaking, I was only tinkering with it, not USING it.

The cap leads are small wire, so not sharp, but not at all spherical.

The circuit is kind of like a Capacitor-Discharge Ignition unit.
The high voltage B+ (1600-1900VDC) charges up the output capacitor (1ufd, 1kv) which connects from the thyratron plate to the #2 test output terminal.
The charging voltage ramps up to the firing point, then the thyratron conducts switching its plate down to near ground. So the other side of the output cap goes down the same amount at terminal #2.
That went through the 1n4007 to the 0.1 @ 1kv peak-detector and DVM and then back to test terminal #3 which internally connects to the meter in parallel with the non-polar 1000mfd cap(s). Those are tied to the power supply common return (test terminal #1, with the fuse).
There were several pulses at about two per second before the DVM quit and then the big-bang.

I have no idea why the output pulse is across a 600 ohm 5W resistor that is returned to the thyratron filament on the 6.3 vac side, not the power common side where the cathode is connected. They couldn't be doing something weird with the power transformer, could they?

More later... I'm working 2000-0400 shift this week.

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 Post subject: Re: Simpson Model 383-A Capacohmeter running
PostPosted: Oct Tue 29, 2013 12:07 pm 
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My 383a literally almost killed me. The only reason I'm still here is sheer luck. I now know exactly what it feels like to be (nearly) electrocuted. While it did satisfy a long-standing curiosity of mine, it wasn't something I ever planned on experiencing.

Anyway, where did you get the high voltage carbon comps? I need two 510k to get the pulse section of mine going again. Also, is it normal for the pulses to slow down as you increase the pulse voltage? Does yours have a resistor in series with the pulse calibration pot? My unit and two schematics all have discrepancies between each other...

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 Post subject: Re: Simpson Model 383-A Capacohmeter running
PostPosted: Oct Tue 29, 2013 3:23 pm 
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Willitwork wrote:
I put in series a 1n4007 with the 0.1ufd across the Cx (#2 - #3) test terminals of the 383A, with the DVM across the 0.1ufd set to 1000VDC.


Edit - Ignore the paragraph in italics below. I mis-read the schematic terminal labeling.
The pulse test circuit uses terminals #1 and #2. Using #2 and #3 shorts the test pulse to ground without putting it through the metering circuit. I don't think that should make much difference with respect to the arc-over and the dead DVM, though.

OK, now I understand the schematic. It's telling me that the second and third poles on the function switch short adjacent contacts together.

Yeah, the resistor to the thyratron filament supply is a puzzle.

Is C5 (the 1 µF relaxation oscillator cap) at all leaky, along with R6 (the resistor to the thyratron filament circuit) being open? That might create a voltage doubler with your peak detector. The peak detector cap would charge up to B+ from the leakage voltage, then when the thyratron fires and brings C5 down and puts the #2 terminal at a negative voltage equal to B+. The 1N4007 then would avalanche as it has about 2 kV or so reverse voltage across it, or maybe it would hold up to that peak reverse voltage, and then you've got 2 kV or so to make the spark.

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Last edited by stevebyan on Oct Tue 29, 2013 9:21 pm, edited 1 time in total.

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 Post subject: Re: Simpson Model 383-A Capacohmeter running
PostPosted: Oct Tue 29, 2013 3:49 pm 
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Great read - I'm learning a few things here. And this will be an excellent thread to read for those that follow us!


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 Post subject: Re: Simpson Model 383-A Capacohmeter running
PostPosted: Oct Tue 29, 2013 4:14 pm 
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anhero wrote:
Anyway, where did you get the high voltage carbon comps? I need two 510k to get the pulse section of mine going again.

The original components look like ordinary 1/2 watt carbon composition resistors, good for 500 volts each. I did a search at http://www.mouser.com for in-stock 510K resistors of 1/2 watt or larger with axial leads. The data sheets for the first four that turned up show they can handle 500 volts or more. Stay away from the Xicon metal oxide resistors, though, they're only good for 350 volts. But the Vishay metal film 1/2 watt, Vishay metal film 2 watt, IRC carbon composition 1/2 watt, and KOA Speer carbon film are all rated for 500 volts or more.

anhero wrote:
Also, is it normal for the pulses to slow down as you increase the pulse voltage?

Yep. As you increase the pulse voltage, it takes longer for the cap to charge up to the higher threshold voltage.

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 Post subject: Re: Simpson Model 383-A Capacohmeter running
PostPosted: Oct Tue 29, 2013 9:11 pm 
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anhero asked:
Quote:
Does yours have a resistor in series with the pulse calibration pot? My unit and two schematics all have discrepancies between each other...
No resistor on the R2 pulse cal pot. That is true on two units, one looks original, second had been re-cap'd and re-wired, not always correctly.
Quote:
where did you get the high voltage carbon comps?

I didn't. :oops: In the modified unit that I have been working on, those two 510k's (PL says 1/2w 10%) were substituted with six (6) series common (looking) carbon comp 1/2 w. resistors (had bad high values); I used three 1/2w carbon comp 10% from my NOS bins, 220K + 330K + 470K (actually close to 1020K).
Quote:
My 383a literally almost killed me. The only reason I'm still here is sheer luck.

Can you help us have some luck? Did you find something everyone should be aware of to avoid the learning experience?

About the #1-Black, #2-Red, #3-Green test terminals as I see them:
#1) Black - Through the front fuse, is the power supply common return, labeled for "Coupling Capacitor Test".
Manual page 9 says "Jack 1 is used for the third lead when testing coupling capacitors in-circuit." Then Figure 2 on p.14 shows that "Lead from Black Jack #1" goes to the set under test ground.
#2) Red - Is the 'Hot' side of all measurements: Cap, Meg-ohm, Pulse.
#3) Green - Is the input to the metering circuit, for all measurements: Cap, Meg-ohm, Pulse.
The manual says #2 & #3 are used for all measurements.

If the capacitor leads that arc'ed were considered sharp, spaced 3/8" or 10mm, is there a rule of thumb for the voltage that broke-down the air? (That was no little static spark; it was fat, bright, loud, and scary.)
On the other hand (bad choice of words...) The good thing is, I bet the capacohmeter is NOT ESD Sensitive.

Maybe we should make this thread a "CAUTION!" or even a "WARNING!" thread?

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 Post subject: Re: Simpson Model 383-A Capacohmeter running
PostPosted: Oct Tue 29, 2013 9:25 pm 
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Willitwork wrote:
About the #1-Black, #2-Red, #3-Green test terminals as I see them:

Yeah, sorry, I was confused about the numbers on the schematic. I agree with you.

I don't know how to calculate the e-field for your capacitor leads. I'd have to break out my old E&M text.

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 Post subject: Re: Simpson Model 383-A Capacohmeter running - CAUTION
PostPosted: Oct Tue 29, 2013 9:50 pm 
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It certainly sounds like 30kV to me. But I have no idea where it came from (haven't see the schematic either).

You can measure it with an old high-voltage probe of the sort that used to be sold for VTVMs, a 1000 meg resistor in a long plastic probe. The resistor will have drifted by now but you can calibrate it for any DMM or scope by measuring a lower voltage and setting the meter to millivolts.

Simpson made the probes too and that's what I use. This one has a 500meg resistor in the probe. If I parallel my Fluke 79 with 508k this probe comes out to 1000x. Says its rated for 25kV. I believe they stopped making them because people could use them for HV power lines which would be fatal if the ground clip slipped off.
Attachment:
Simpson model 0508.jpg
Simpson model 0508.jpg [ 102.99 KiB | Viewed 4629 times ]


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 Post subject: Re: Simpson Model 383-A Capacohmeter running - CAUTION
PostPosted: Oct Tue 29, 2013 10:14 pm 
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BAMA has the manual with schematic.
http://bama.edebris.com/manuals/simpson/383a/

Attachment:
Schematic_Page31.pdf [205.09 KiB]
Downloaded 96 times


That should be the schematic only above. Never saw that before, had to click the link, then see the schematic.

There is also a re-drawn schematic by McClellan who says it is not totally correct.
(Mainly around R1, R2, C1, C2 I see problems.)
The pulser thyratron circuit looks true to me.

Quote:
Here's a schematic if anyone needs one. The ones in the manuals are pretty much unreadable. I did the best I could with it, so I hope someone finds it useful.

http://i52.tinypic.com/23jho1x.jpg


Last edited by anhero on Jul Tue 05, 2011 4:14 pm, edited 1 time in total.




ADDED: The test leads in use during the high-voltage arc event measure about 1 uHy (10^-6) inductance.
The circuits, including the meter, in the capacohmeter in the Pulse hookup measure 20 mHy (10^-3).
This is using a handheld DVM measuring inductance.
If I managed to generate something like 30KV with a few dozen pulses, it feels like there had to be some inductance somewhere in there.

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 Post subject: Re: Simpson Model 383-A Capacohmeter running - CAUTION
PostPosted: Oct Wed 30, 2013 12:16 am 
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I generally like Simpson items, but this thing doesn’t look to have any redeeming value as a safe, or even legitimate piece of test equipment. I don’t think I would delve too deep in to damaging other pieces of test equipment (or your body), or in to too much hypothetical theory in attempt to calibrate it. Another way of saying it is there are better, safer and easier to use pieces of equipment available today to do the jobs this tool was originally intended to do. It is best left as a shelf sitter or something to bring out when you want to purposely blow things up.

-Mark-


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 Post subject: Re: Simpson Model 383-A Capacohmeter running
PostPosted: Oct Wed 30, 2013 12:26 am 
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Willitwork wrote:
those two 510k's (PL says 1/2w 10%) were substituted with six (6) series common (looking) carbon comp 1/2 w. resistors (had bad high values); I used three 1/2w carbon comp 10% from my NOS bins, 220K + 330K + 470K (actually close to 1020K).
Half-watt carbon comps are only rated 350 volts.

When you connect dissimilar resistors in series, the voltages across them vary with the resistance.

Given your 470K + 330K + 220K string, the voltage across the 470K resistor will be 46% of the total applied voltage.

Assuming that is 1000 volts, the 470k will have 460 volts across it, which is way above its 350 volt rating.

- Leigh

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 Post subject: Re: Simpson Model 383-A Capacohmeter running - CAUTION
PostPosted: Oct Wed 30, 2013 1:03 am 
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Out of curiosity, what make / model was the DMM that you managed to fry? Hopefully not a nice meter like a Fluke...

-Matthew

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 Post subject: Re: Simpson Model 383-A Capacohmeter running - CAUTION
PostPosted: Oct Wed 30, 2013 1:06 am 
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Willitwork wrote:
The test leads in use during the high-voltage arc event measure about 1 uHy (10^-6) inductance.
The circuits, including the meter, in the capacohmeter in the Pulse hookup measure 20 mHy (10^-3).
This is using a handheld DVM measuring inductance.
If I managed to generate something like 30KV with a few dozen pulses, it feels like there had to be some inductance somewhere in there.

That 600 ohm 5 watt resistor to the thyratron filament circuit is probably a wire-wound. What's its inductance?

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