VOGONS


First post, by squelch41

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Hi,
I've got quite a few old computers from the 1990s whose capacitors are obviously now rather mature.

My soldering skills are adequate but certainly amateur.

I was looking at LCR meters to check the capacitors on the boards to identify ones that are near failure. I dont really want to replace all the caps on everything as a) I might just break the board b) I just dont have time.

Will sub £30 LCR meters do that?
Had seen ones like this on amazon
https://www.amazon.co.uk/Capacitance-Inductan … fix=Lcr+&sr=8-3

I don't want to spend loads - just want something that I can use to say 'fine for now' vs 'about to fail'

Thanks

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Reply 1 of 12, by Tiido

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You cannot test power related parts in circuit for the most part, since they are in parallel with many other capacitors on the same power rail, giving you bogus results. You can get a collective reading of all the parts on that same power rail but not of a single part without separating from the rest.

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Reply 2 of 12, by squelch41

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But I think you can use esr meters in circuit to get an idea of the health of the capacitor in circuit. If you want an absolutely accurate measure of it's capacitance then you do need to remove it, but I don't need that
Eg
https://youtu.be/H9_CDPD4sh0

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Reply 3 of 12, by Miphee

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If you have 3 blank resistors in a circuit, can you measure R1 and determine it's exact value with a multimeter?
You can't unless you remove at least one leg of R1.
Repairing electronics is often a trial and error thing.

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Reply 4 of 12, by squelch41

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True for resistors, not true for esr measurements (with a few caveat) for capacitors- as demonstrated in the element 14 video above and instructional videos on esr meters in general.

My issue is whether cheap meters can do it ot whether only the £80+ meters can

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Reply 5 of 12, by mkarcher

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Tiido wrote on 2020-08-09, 13:21:

You cannot test power related parts in circuit for the most part, since they are in parallel with many other capacitors on the same power rail, giving you bogus results.

This is generally true. If a single electrolytic failed on a power rail, and all the other ones are fine, you are unlikely to find it in-circuit.

In practice, if only a single of many electrolytics (e.g. tantalum caps) failed (and did not fail short!), the system usually keeps working fine. It's not the healt of every capacitor that keeps the system working, but the ESR at any point of the system must not exceed a certain threshold. While it is true that stray inductance makes capacitors far away less effective, this applies to ESR measurement in a similar way to system stability. Again, it is true that ESR measurement is usually done around 100kHz, whereas high-frequency spikes have spectral components up to 50MHz, but on the other hand, these high-frequency components are not bypassed by the electrolytics, but by the 100nF ceramics scattered over the whole board. These rarely fail.

If many electrolytics are paralleled next to each other (like on PSU output or VRM input), they are likely to fail at the same time, as wear and tear is similar to all of them. Even if one fails slightly, it increases stress to the other capacitors, so they will soon follow.

So while it is definitely correct that you can not probe a single capacitor in-circuit, measuring ESR in-circuit usually is a sufficient indicator whether there might be ESR-induced problems in the circuit. The cheap and handy all-in-one component testers seem to be quite useless for that, though. The interesting measurement is the ESR (or often the impedance which is assumed to be due to the ESR), which is displayed on all-in-one-testers only if the tester deems the circuit behaviour to be capacitor-like. More often than not, you get a "1.5V diode" or some resistance reading when you probe a power rail on a PC component. Dedicated ESR testers, like the Bob Pease one or the EVB one (I own the latter) work fine for the first-level in-circuit capacitor inspection in my experience.

Reply 6 of 12, by squelch41

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mkarcher wrote on 2020-08-09, 16:23:

So while it is definitely correct that you can not probe a single capacitor in-circuit, measuring ESR in-circuit usually is a sufficient indicator whether there might be ESR-induced problems in the circuit. The cheap and handy all-in-one component testers seem to be quite useless for that, though. The interesting measurement is the ESR (or often the impedance which is assumed to be due to the ESR), which is displayed on all-in-one-testers only if the tester deems the circuit behaviour to be capacitor-like. More often than not, you get a "1.5V diode" or some resistance reading when you probe a power rail on a PC component. Dedicated ESR testers, like the Bob Pease one or the EVB one (I own the latter) work fine for the first-level in-circuit capacitor inspection in my experience.

Thanks - was worried that the price of those would be too low to do the signal generation that is needed for ESR etc. Oh well!

There are a few guides to building esr meters with Arduino. Couldn't find the ones you mentioned with a Google search - do you know if they are still made?

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Reply 7 of 12, by gdjacobs

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squelch41 wrote on 2020-08-09, 12:17:
Hi, I've got quite a few old computers from the 1990s whose capacitors are obviously now rather mature. […]
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Hi,
I've got quite a few old computers from the 1990s whose capacitors are obviously now rather mature.

My soldering skills are adequate but certainly amateur.

I was looking at LCR meters to check the capacitors on the boards to identify ones that are near failure. I dont really want to replace all the caps on everything as a) I might just break the board b) I just dont have time.

Will sub £30 LCR meters do that?
Had seen ones like this on amazon
https://www.amazon.co.uk/Capacitance-Inductan … fix=Lcr+&sr=8-3

I don't want to spend loads - just want something that I can use to say 'fine for now' vs 'about to fail'

Thanks

In circuit measurement will tell you the lowest possible ESR in any given parallel arrangement from the point of view of your test leads. All parallel legs of the circuit will have an individual ESR higher than the measurement. Not very revealing of the health of the single capacitor.

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Reply 8 of 12, by mkarcher

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squelch41 wrote on 2020-08-09, 17:56:
mkarcher wrote on 2020-08-09, 16:23:

The cheap and handy all-in-one component testers seem to be quite useless for that, though. The interesting measurement is the ESR (or often the impedance which is assumed to be due to the ESR), which is displayed on all-in-one-testers only if the tester deems the circuit behaviour to be capacitor-like. More often than not, you get a "1.5V diode" or some resistance reading when you probe a power rail on a PC component. Dedicated ESR testers, like the Bob Pease one or the EVB one (I own the latter) work fine for the first-level in-circuit capacitor inspection in my experience.

Thanks - was worried that the price of those would be too low to do the signal generation that is needed for ESR etc. Oh well!

While the cheapness is the reason for the mediocre resolution for ESR measurements (IIRC 0.1 Ohm), the main problem is not the cheapness, but the "smart"ness. These all-in-one testers are usually unable to find out that you might be interested in ESR when you connect them to a complex circuit, so they don't deliver. If you could force them into a " capacitor" mode, they would possibly work well enough.

squelch41 wrote on 2020-08-09, 17:56:

There are a few guides to building esr meters with Arduino. Couldn't find the ones you mentioned with a Google search - do you know if they are still made?

I mixed up names. It's not Bob Pease, the well-known analog electronics engineer, but Bob Parker. His ESR meter is also sold with the EVB label:

https://evbesrmeter.pt/information.htm

If you find a dedicated ESR meter based on an arduino-like atmel controller with some transistors and resistors to be used as current source, it might work as well. The EVB meter delivers its 0.01 ohm resolution up to 0.99 ohms just fine. You can use this resolution as benchmark to classify other ESR solutions

Reply 9 of 12, by gdjacobs

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mkarcher wrote on 2020-08-09, 23:20:

If you could force them into a " capacitor" mode, they would possibly work well enough.

It won't because in circuit measurements won't show you if a capacitor has a higher ESR. It'll show you if at least one of the parallel legs of a circuit has a low ESR. Not very useful at all.

All hail the Great Capacitor Brand Finder

Reply 10 of 12, by mkarcher

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gdjacobs wrote on 2020-08-10, 03:38:
mkarcher wrote on 2020-08-09, 23:20:

If you could force them into a " capacitor" mode, they would possibly work well enough.

It won't because in circuit measurements won't show you if a capacitor has a higher ESR. It'll show you if at least one of the parallel legs of a circuit has a low ESR.

Thanks for pointing the limitations of in-circuit measurement out again. I already discussed them in my previous post. For a more details discussion, please find the details in that post.

mkarcher wrote on 2020-08-09, 16:23:

So while it is definitely correct that you can not probe a single capacitor in-circuit, measuring ESR in-circuit usually is a sufficient indicator whether there might be ESR-induced problems in the circuit.

gdjacobs wrote on 2020-08-10, 03:38:

Not very useful at all.

And that's where my opinion differs. If I want to fix equipment, I am primarily interested in whether high ESR in causing the problems I'm currently seeing, not whether every single capacitor is fine. The first question can usually be answered well enough by in-circuit ESR measurement (granted, some experience helps recognizing cases where the measurement might be bogus), whereas the second question can not be answered by in-circuit measurement.

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Reply 11 of 12, by squelch41

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mkarcher wrote on 2020-08-09, 23:20:
While the cheapness is the reason for the mediocre resolution for ESR measurements (IIRC 0.1 Ohm), the main problem is not the c […]
Show full quote

While the cheapness is the reason for the mediocre resolution for ESR measurements (IIRC 0.1 Ohm), the main problem is not the cheapness, but the "smart"ness. These all-in-one testers are usually unable to find out that you might be interested in ESR when you connect them to a complex circuit, so they don't deliver. If you could force them into a " capacitor" mode, they would possibly work well enough.

squelch41 wrote on 2020-08-09, 17:56:

There are a few guides to building esr meters with Arduino. Couldn't find the ones you mentioned with a Google search - do you know if they are still made?

I mixed up names. It's not Bob Pease, the well-known analog electronics engineer, but Bob Parker. His ESR meter is also sold with the EVB label:

https://evbesrmeter.pt/information.htm

That's very helpful - thanks.

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Reply 12 of 12, by gdjacobs

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mkarcher wrote on 2020-08-10, 07:01:

And that's where my opinion differs. If I want to fix equipment, I am primarily interested in whether high ESR in causing the problems I'm currently seeing, not whether every single capacitor is fine. The first question can usually be answered well enough by in-circuit ESR measurement (granted, some experience helps recognizing cases where the measurement might be bogus), whereas the second question can not be answered by in-circuit measurement.

That's absolutely not true in the general case. ESR measurements are generally accurate for only around 100khz. They will be unrevealing of problems at higher frequencies (including higher harmonics of a switch mode supply) which distributed capacitance is intended to address. On the other hand, knowing the majority of caps are in spec will result in design compliant performance.

Even knowing some of the caps are out of spec would be useful, but in circuit ESR measurements aren't even good for revealing that.

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