Author Topic: Accurate Low Amp Current Probe Advice  (Read 17940 times)

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Online Martin72

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Re: Accurate Low Amp Current Probe Advice
« Reply #250 on: November 12, 2024, 06:48:04 pm »
I removed and tested the 3 caps from the PSU, and...it seems pointless to replace them. They look new, I don't know if they're original or not. I assume not original, because there was some flux residue on the PCB around them.

Also, the ESR is super low. I can't get anything lower, so what's the point if they're not bad.

Does anybody else have a TM502A they can check the PSU caps in? I'm curious what's normally in there.

Thanks,
Josh

The electrolytic capacitors in your power supply have definitely been replaced; I can see that these are (good) hand solderings on the circuit board.
I'll check my 502s to see if they still have the originals installed.
We still have three Tek systems in use at work that are calibrated annually.
But they are quite old, so I contacted Tektronix today to ask if there were any documents for the AM503B module and if we could have them.
They responded promptly and sent me links where I could download the original AM503 circuit diagram, among other things.
There is no circuit diagram for the AM503 B, I could just look at the linked diagram to see if there are similarities between the two versions, was Tek's answer. ;)

 
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Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #251 on: November 12, 2024, 06:56:49 pm »
The electrolytic capacitors in your power supply have definitely been replaced; I can see that these are (good) hand solderings on the circuit board.

Even better now that I reinstalled them after testing. 😉😉

Quote
I'll check my 502s to see if they still have the originals installed.
We still have three Tek systems in use at work that are calibrated annually.
But they are quite old, so I contacted Tektronix today to ask if there were any documents for the AM503B module and if we could have them.
They responded promptly and sent me links where I could download the original AM503 circuit diagram, among other things.
There is no circuit diagram for the AM503 B, I could just look at the linked diagram to see if there are similarities between the two versions, was Tek's answer. ;)

They're full of shit. There's no way they never had a schematic for the B version.
« Last Edit: November 12, 2024, 07:10:42 pm by KungFuJosh »
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Offline nctnico

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Re: Accurate Low Amp Current Probe Advice
« Reply #252 on: November 12, 2024, 07:04:10 pm »
Well, not for public release. Ofcourse Tektronix has a schematic. But for internal use only.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #253 on: November 12, 2024, 07:07:46 pm »
Of course, but it's still annoying. They supposedly declassified the whole system, but never released the B documents.
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Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #254 on: November 12, 2024, 07:54:07 pm »
To CAL we use TEK  PG506, (or similar,) and wind a  10, 100T magnet wire square loop  with a 50 Ohm term R on the loop in series with 50 Ohm on the gen.

Hi Jon,

Do you have any of the fixtures they say are needed for cal? I have the connectors to make them myself, but I can't find any info on the feed-thru fixture (015-0670-00) for pre cal degaussing. I'm hoping that it might be the same coding resistor as below, with no other connections, but I have no reference to go with.


The other info was on the tek wiki:

Quote
015-0598-00 Voltage adapter:

It is constructed from a female BNC terminated to pin 'M' on an Amphenol 165-9 or equivalent (Tek Part# 131-1854-00) connector.

The shield of the coax is connected directly to GND pin 'J' or 'K' in the Amphenol connector. There is no connection to the shell of the Amphenol connector.
A 1.13 kΩ coding resistor is connected between pins 'B' and GND (pins 'J' or 'K').

Could you please explain your calibration process in detail (with photos)? It sounds like what you do is different than the adjustment kit instructions.

Thanks,
Josh
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Online Martin72

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Re: Accurate Low Amp Current Probe Advice
« Reply #255 on: November 12, 2024, 08:09:18 pm »
I have now opened my TM502 after all.
The capacitors are still the original Sprague ones, date code 1987...
2 x 4700µF and one 18000µF. They obviously wanted to be on the safe side and didn't want to install too little. ;)
And while I was at it, I also opened the AM503 A, where all the capacitors are still original too.
 
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Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #256 on: November 12, 2024, 08:35:26 pm »
I bet those Sprague caps are worth some money! 😉

I think my 503B might have been all original on the caps too? I assumed the blue Nichicon caps were newer than the other caps (and also because a lot of manufacturers will use only the same brand for consistency), but maybe not.
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Online Martin72

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Re: Accurate Low Amp Current Probe Advice
« Reply #257 on: November 12, 2024, 08:48:41 pm »
Quote
I think my 503B might have been all original on the caps too?

You can actually always see that at the solder joints.
You can solder as well as possible, hand soldering always differs from those “professional” from the soldering machine.

Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #258 on: November 12, 2024, 08:55:24 pm »
Quote
I think my 503B might have been all original on the caps too?

You can actually always see that at the solder joints.
You can solder as well as possible, hand soldering always differs from those “professional” from the soldering machine.

I know, but I assumed when I saw the different brand of caps, so I didn't pay any attention when I replaced them. Oh well. 🤷
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Online Martin72

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Re: Accurate Low Amp Current Probe Advice
« Reply #259 on: November 12, 2024, 08:59:35 pm »
The electrolytic capacitors in my 503 module are all from Nichicon.

Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #260 on: November 12, 2024, 09:09:23 pm »
The electrolytic capacitors in my 503 module are all from Nichicon.

Mine only had Nichicon for the 220uF and BP 1uF. The rest were a different brand I can't remember right now. 3 letters, CFE or something like that. I think Mark's was the same with 2 brands.

My 503 is all Nichicon and Panasonic now (I went with whatever had better specs).
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Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #261 on: November 13, 2024, 07:50:30 pm »
Thanks to the Tek group, I found the correct values to make the pass through and voltage adapters for calibration (doc attached).

What I did was:
Pass through adapter:
Short M + N, also a 15kΩ coding resistor is connected between pins 'B' and 'K'.

Voltage adapter:
BNC terminated to pin 'M' on an Amphenol 165-9 or equivalent (I used 165-9-1000) connector. The shield of the coax is connected directly to GND pin 'J'.  A 1.13kΩ coding resistor is connected between pins 'B' and 'K'.

In my photo there's a 50Ω terminator at the end of the BNC cable, which is all it needs to avoid the 266 error code.

Degaussing was successful, I'll try doing the lower mV calibration later.

Thanks,
Josh
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Online Martin72

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Re: Accurate Low Amp Current Probe Advice
« Reply #262 on: November 13, 2024, 09:57:20 pm »
We do have some Tektronix stuff that was bought in the 90s.
What I didn't know, but discovered today, was that
we also have all the original documentation that was available at the time.
And the manuals look like they were filed yesterday. 8)
AM503B:
Tektronix Germany has no documentation other than the one already known.
I'll try again at Tek U.S.
 
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Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #263 on: November 14, 2024, 10:01:30 pm »
I was able to do the calibration. The lower levels both improved, but I made the 1A range slightly worse. I'm going to do a stability test now that I've done this and replaced the 1uF cap with a higher quality BP cap. When I do redo the calibration, I'll probably reverse the DMM setup so I have more digits for the spreadsheet.

I will probably toy with calibration more in the future, but the 1A+ range isn't that interesting to me, so  there's no rush.

Thanks,
Josh
"Right now I’m having amnesia and déjà vu at the same time. I think I’ve forgotten this before." - Steven Wright
 
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Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #264 on: November 15, 2024, 03:36:48 am »
I've been screwing around with calibration more, and have noted the following things:

- It's best to work on one range at a time.
- Getting the source current to a far higher accuracy than suggested is helpful.
- You have to reset back to 61A8 before readjusting, which makes the process slightly more tedious.

I've found, at least with my setup, getting the negative values more accurate to be more challenging. I'm going to try the sequence again and see if I can get my starting numbers better.

For the record, this is entirely silly. The goal is to be within 1.5% margin of error... I started with the 1mA range at 0.326%, and got it to 0.305%, but want it better than that.

I was able to get the 10mA range to 0.006%. Supposedly. 😉

Thanks,
Josh
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Offline MarkL

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Re: Accurate Low Amp Current Probe Advice
« Reply #265 on: November 16, 2024, 09:57:04 pm »
After being distracted by the AM503B internal serial port hacking, I got around to checking the stability of my second AM503B.  This is the one with the additional 100uF capacitor discussed in Reply #214.

It's slightly better than the first one, having a drift of about 0.4mA over 30 minutes as compared to 0.5mA.  Short-term variability and noise look about the same.

This is close enough for me to not start modifying the first one by adding the additional capacitor.  This capacitor is out for a reason, and without a schematic to understand its exact role there may unknown side effects.  And not to mention my long-standing mantra:  If it ain't broke...

I've ordered a pair of the Amphenol connectors to build the cal adapters.  I'll post calibration results for both units when available.
 

Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #266 on: November 16, 2024, 10:26:36 pm »
This is close enough for me to not start modifying the first one by adding the additional capacitor.  This capacitor is out for a reason, and without a schematic to understand its exact role there may unknown side effects.  And not to mention my long-standing mantra:  If it ain't broke...

I've ordered a pair of the Amphenol connectors to build the cal adapters.  I'll post calibration results for both units when available.

After everything I've swapped with mine, I would say the most value caps to replace are the 1000 and 2200uF caps. Maybe the BP 1uF also. Nothing else made much of a difference, though thankfully I have no regrets. 😉 My power supply caps had already been replaced with very high quality low ESR caps, so I didn't change those.

Where did you find the connectors? I found a decent deal on eBay for the 165-9-1000 variant.

Thanks,
Josh
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Offline MarkL

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Re: Accurate Low Amp Current Probe Advice
« Reply #267 on: November 16, 2024, 11:21:22 pm »
...
Where did you find the connectors? I found a decent deal on eBay for the 165-9-1000 variant.

Thanks,
Josh
On ebay.  They look like the ones you bought, maybe even from the same seller, but I don't have them in hand yet.  I offered $24 for a pair.  My backup plan was to find the right size male pins and (carefully) push the few connections that were needed into the AM503B connector for the calibration.  That would be ok given how often I'm likely to do this procedure.

Unfortunately I should have ordered 3 since I discovered the AM503 has a different pinout for its cal adapter.  Maybe I can make one of them do double duty.

I also have an A6302 with a broken transformer that I was thinking about scrapping for the connector, but I haven't given up looking for a new transformer for it yet.

I did replace my TM503 caps some time ago, and that took care of a significant amount of line noise on an FG504 and a AM502.  It also helped both the AM503's, but made little difference on the AM503B's.  I think my AM503B's may be in healthy condition already.  When I get a chance I will poke at those caps with the LCR meter and compare to your measurements.
 

Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #268 on: November 16, 2024, 11:46:59 pm »
On ebay.  They look like the ones you bought, maybe even from the same seller, but I don't have them in hand yet.  I offered $24 for a pair.

Sounds the same as mine, I got 4 for $48 from here: https://www.ebay.com/itm/313033033242

I bought this cable and cut off the SMA end for the voltage adapter: https://www.amazon.com/dp/B0BBYTZM23

I posted the details previously, but in case you didn't see it, coding resistor is 1.13k for voltage adapter, and 15k for the pass through adapter.

Thanks,
Josh
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Offline MarkL

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Re: Accurate Low Amp Current Probe Advice
« Reply #269 on: November 20, 2024, 04:21:42 pm »
I took a look at the caps you found needed replacing in your unit, namely the 2200uF, 1000uF, and 1uF BP.  In my older unit they are clearly showing their age, but in the "new" unit they are still doing well.  All are still in spec using the EOL information from each series' datasheet.  Since both units are working fine, I'm going to leave the caps alone.

I'm still waiting for connectors for the calibration, and will post the results when done.  (And that was the same seller for the connectors.)

Edit:  Oops, fixed cut & paste error in spreadsheet image.
« Last Edit: November 20, 2024, 04:34:13 pm by MarkL »
 
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Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #270 on: November 20, 2024, 07:21:07 pm »
I assume the lower ESR & higher ripple handling of my newer caps helped filtering. Here's what mine were, all tested out of circuit on IM3570:

50V 2200uF    Old: 60mΩ ESR    New: 13mΩ ESR
50V 1000uF    Old: 120mΩ ESR    New: 11mΩ ESR

Thanks,
Josh
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Online Martin72

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Re: Accurate Low Amp Current Probe Advice
« Reply #271 on: November 20, 2024, 09:47:59 pm »
Quote
I assume the lower ESR & higher ripple handling of my newer caps helped filtering.

This would make sense for a switched voltage, but I cannot imagine that an auxiliary voltage is generated in a switched manner on the AM503B board.

 
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Offline MarkL

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Re: Accurate Low Amp Current Probe Advice
« Reply #272 on: November 20, 2024, 10:20:11 pm »
I assume the lower ESR & higher ripple handling of my newer caps helped filtering. Here's what mine were, all tested out of circuit on IM3570:

50V 2200uF    Old: 60mΩ ESR    New: 13mΩ ESR
50V 1000uF    Old: 120mΩ ESR    New: 11mΩ ESR

Thanks,
Josh
Yes, no doubt replacing those caps improved the ripple.

However, if the designers did their job, the worst case characteristics of the capacitors (including initial -20% of C, temperature effects, and end-of-life DF) would be taken into account.  The next stage of linear regulation would continue to reject enough ripple to meet a unit's specifications, or if no warranted noise specification exists as is the case here, to at least meet nominal characteristics.  Sure, that's putting faith in the designers, but I think it's a good bet from this era of Tektronix.

What was the capacitance of your old capacitors (@120Hz)?
 

Offline nctnico

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Re: Accurate Low Amp Current Probe Advice
« Reply #273 on: November 20, 2024, 10:37:40 pm »
Quote
I assume the lower ESR & higher ripple handling of my newer caps helped filtering.

This would make sense for a switched voltage, but I cannot imagine that an auxiliary voltage is generated in a switched manner on the AM503B board.
Yes. The 1000uf / 2200uf capacitors are all used for mains ripple filtering which means a frequency of 120Hz at most. At this frequency regulators and opamps have a very high attenuation. I don't think you can even measure the difference in ripple with new versus old capacitors. The series resistance of wiring, PCB traces, rectifier diodes and transformer windings is likely higher compared to the ESR of the original capacitors. Maybe the 1uf bipolar capacitor can make a difference; I have not checked where this is at in the schematics.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 
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Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #274 on: November 20, 2024, 11:31:06 pm »
Yes, no doubt replacing those caps improved the ripple.

However, if the designers did their job, the worst case characteristics of the capacitors (including initial -20% of C, temperature effects, and end-of-life DF) would be taken into account.  The next stage of linear regulation would continue to reject enough ripple to meet a unit's specifications, or if no warranted noise specification exists as is the case here, to at least meet nominal characteristics.  Sure, that's putting faith in the designers, but I think it's a good bet from this era of Tektronix.

What was the capacitance of your old capacitors (@120Hz)?

I don't recall the cap values being out of spec enough to take note of it, however, I was mostly focused on ESR.

I don't doubt the skills of the designers being far greater than mine; but I do know the tech we have now, along with what caps available, are much better than what was around 30 or 40 years ago.
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