Agilent E4980A Precision LCR meter repair - Page 1
Forum section: electronics / testgear

author: Finderbinder
author_rank: Regular Contributor
timestamp: February 19, 2025, 09:32:04 pm
content: Got one nice beast but in not nice working conditionAs of previous owner the meter has got a discharge from capacitor, likely a few hundreds of volts.Simple things were already done by me - in analog board and daughter analog board few through hole 1N4004, some smd BAV99 were replaced, one through hole TVS diode removed (I'm not sure should it be unipolar or bipolar, other TVS's are unipolar and this one shows signs of been replaced earlier and is bipolar, so makes me doubt regarding polarity).Another mystery is Hcur port. I was unable to trace it to anything on the board. At best it shows 10pF from center pin to shield. Would be nice if someone who own functional E4980 may test at least all ports for resistance/in_diode_mode between center pins to any other points (for easier troubleshooting).As of current condition it shows OVERLOAD if all ports are open and nF greatly changes if Lcur shorted to Lpot and Hpot shorted to Hcur.I assume schematics are absent for this device? Even pictures of internals, boards was unable to find

author: Finderbinder
author_rank: Regular Contributor
timestamp: February 21, 2025, 07:44:28 am
content: It looks like conductor between board layers is broken. A short piece of wire from Hcur center pin to the daughter board's coax cable Hcu center pin cured this.Now instrument works properly, at least with capacitors I tested so far.Quote from: Finderbinder on February 19, 2025, 09:32:04 pm

author: Kean
author_rank: Unknown
timestamp: February 21, 2025, 07:57:57 am
content: Congrats, but we need to see a photo!

author: Finderbinder
author_rank: Regular Contributor
timestamp: February 21, 2025, 08:37:15 am
content: Photo of what?Screen of operating, repair process or inside/boards?Actually not too much repair photos took, as process was rather simple.Also one correct TVS diode is to be ordered (unipolar or bipolar?).

author: Kean
author_rank: Unknown
timestamp: February 21, 2025, 09:17:06 am
content: I am just teasing, but a photo of it working again is always nice. Share the repair joyI am a bit surprised that it sounds like the cap discharge burnt out a via rather than a trace on an outer PCB layer.

author: Finderbinder
author_rank: Regular Contributor
timestamp: February 21, 2025, 10:03:48 am
content: I'll take some photos of internals/boards the next time I tear it down (probably for TVS diode soldering). Some photos of operation will be added soon, stay tunedRegarding broken inner trace, I also was surprised it didn't ping to anything on PCB. No any evidence of fault visually too. But fact it just shouldn't be left float made me search further. And "Hcu" marking on daughter board's coaxial port suggested me to associate it with instrument's input port "Hcur". And bingo!Wondering if someone may offer a quick and dirty method to check overall proper functionality to reveal any potentially residual problem may be left after that cap discharge accident.

author: Finderbinder
author_rank: Regular Contributor
timestamp: February 21, 2025, 02:35:22 pm
content: Some pictures of measurements of new parts.Any comments? Should I check something particular?BTW: a test fixture received with instrument (HP 16034B) needs attention - wires are basically torn

author: Finderbinder
author_rank: Regular Contributor
timestamp: February 21, 2025, 02:37:11 pm
content: continued:

author: Kean
author_rank: Unknown
timestamp: February 22, 2025, 12:05:58 am
content: Looks pretty good. I'm a bit jealous. I have many HPAK instruments, but no LCR. My main LCR is a CKT5000.Do you have any high value (100M+) resistors, or low value (single pF) capacitors to check? Any other equipment to do a cross comparison?If there was any additional damage I would expect to see some leakage affecting those measurements. It sounds like the BAV99s & TVS sacrificed themselves as designed to protect the other parts.

author: Finderbinder
author_rank: Regular Contributor
timestamp: February 22, 2025, 10:04:52 am
content: My Gohm glass resistors are on the way now. And some precise capacitors sits in a mouser basket to reach 50 euro free shipping.Other available instruments to check capacitors are Fluke 189, Metra Hit 29S and Peak Atlas ESR70. Have nothing to test inductors.btw, I get negative capacitance if measure in higher frequencies. Is it normal?

author: Kean
author_rank: Unknown
timestamp: February 22, 2025, 10:47:41 am
content: Quote from: Finderbinder on February 22, 2025, 10:04:52 amWhat capacitor value and what frequency?For more detail regarding reading negative capacitance:https://www.eevblog.com/forum/testgear/negative-capacitance-on-hp4194a-lcr-meter/msg3847559/#msg3847559https://electronics.stackexchange.com/questions/611003/what-does-negative-capacitance-meanAre you doing a calibration before measurement to eliminate effects of your connections (residual L/C/R).You really want a proper test fixture rather than sticking wires into the BNC connectors, to ensure repeatable measurements and valid calibration.Maybe a TH26001A for leaded parts, and a TH26008A for SMD parts. I'm not a fan of tweezers like the 16034B on a bench LCR - I'd rather use something like Shannon Tweezers.mwyatt did a good post about test fixtureshttps://www.eevblog.com/forum/testgear/cheap-smd-lcr-fixture-the-good-bad-and-ugly!/

author: Finderbinder
author_rank: Regular Contributor
timestamp: February 22, 2025, 11:23:53 am
content: For sure I need a test fixture without leads at least. There are Chinese ones for attractive price but I'm unsure I may rely on them. At the moment I just gently use these BNC connectors to troubleshoot instrument.Yes, I did Open and Short check, then apply corrections before measurements. The capacitor with negative capacitance is the same Nichicon 1500uF 25V as pictured above.Below you see more pictures of measurement.Quote from: Kean on February 22, 2025, 10:47:41 am

author: mawyatt
author_rank: Super Contributor
timestamp: February 22, 2025, 12:08:57 pm
content: Those negative capacitance readings are because you've passed Capacitor DUT and setup series resonance. As mentioned by Kean, you need to get some proper fixtures before moving forward with any serious measurements, and there are a lot of DIY fixtures and adaptations awaiting youNice instrument & repairBest

author: Finderbinder
author_rank: Regular Contributor
timestamp: February 22, 2025, 01:19:24 pm
content: Thanks MawyattNow when my instrument does it's job as expected for sure I need decent test fixture for leaded parts. And for dirty general electrolytic capacitors I assume TONGHUI TH26004S-1 should be enough? At least shielding goes up to clips as I assume from pictures? Although TH26011 is more expensive, have box, a rather long wires goes unshielded (also assumption from pictures).Quote from: mawyatt on February 22, 2025, 12:08:57 pm

author: Kean
author_rank: Unknown
timestamp: February 23, 2025, 02:30:43 am
content: Keep in mind the practical measurement ranges of the instrument.There is some charts in the Keysight manual that show you the effective measurement ranges at different frequencies and test voltage levels.The charts given in the manuals for some other brands actually show the expected accuracy in the various ranges plotted against DUT value vs frequency. I didn't see that in the Keysight manual.The impedance of your 1500 µF cap is approx 100 mΩ at 1kHz, 10 mΩ at 10kHz, 1 mΩ at 100kHz, and 100 µΩ at 1MHz.Similarly a 1 pF cap at 100 Hz is approx 1.6 GΩ.Even with a much better test fixture it becomes impractical and generally meaningless to try measuring high value capacitors at high frequencies, and conversely low value capacitors at low frequencies.

author: Finderbinder
author_rank: Regular Contributor
timestamp: February 27, 2025, 10:17:57 am
content: That's fine and understandable for known/predictive DUTI wonder what should be a measurement strategy for completely unknown DUT, say you have a "black box" with 2 contacts?Quote from: Kean on February 23, 2025, 02:30:43 am

author: Kean
author_rank: Unknown
timestamp: February 27, 2025, 12:05:44 pm
content: Quote from: Finderbinder on February 27, 2025, 10:17:57 amIf you have no idea what it is, then you might want to start with an I-V Curve Tracer as that can quickly help identify if it is a cap, inductor, resistance, diode, zener, etc. If it obviously an RF part (based on connectors, etc), then I'd maybe want to switch to using a VNA. If it is clearly not a diode or something more complex, then on the LCR meter I would start with a low signal level and do a frequency sweep, see what peaks there are, and adjust parameters before running more sweeps. Not all bench LCR meters can do automated frequency sweeps like yours can.I'm guessing you know the basics of how LCR measurements are done. If not check out these videos where he shows some similar measurement problems:FYI, I found the measurement accuracy chart I was looking for - it is on page 19 of the E4980A data sheet. I've attached a screenshot. This is for a 1 Vrms test signal. It is a little tricky to understand how to read this, but all the lines that meet in the center inside the dark bands are measurements that can meet a 0.1% accuracy spec. Next band out is 0.3%, and so on. Horizontal lines are for R, Z, etc values in logarithmic scales. Vertical lines are for frequency. Lines angled down from left to right are capacitance, and lines angled down right to left are inductance.For your 1500uF at 100kHz, start at the 1mF line from the left about 2/3 down and follow it down to the right until you hit the 100kHz vertical line. From this you can see that measurement is actually outside the worst case 10% accuracy band.If you haven't already, also check out the Keysight Impedance Measurement Handbook. I've only skimmed it myself as it goes into a *lot* of detail.https://www.keysight.com/au/en/assets/7018-06840/application-notes/5950-3000.pdf

author: Finderbinder
author_rank: Regular Contributor
timestamp: February 27, 2025, 03:20:14 pm
content: Those were useful videos.I have seen that measurement accuracy chart but did not dive deep enough yetYes it matters what frequency was chosen to measure.I got into Agilent Impedance Measurement Handbook (a version of the same document) few days before. Useful material. Particularly was trying to sort out Test fixtures question. But would be useful to read it through.

author: mawyatt
author_rank: Super Contributor
timestamp: February 27, 2025, 03:33:43 pm
content: Quote from: Kean on February 27, 2025, 12:05:44 pmThat Keysight Accuracy Graph and Measurements Handbook should be required reading and understanding for anyone involved with precision LCR Measurements, tweezers includedThanks for the linksBest

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 27, 2025, 03:59:02 pm
content: We have a couple of those meters at work. I've set one up with an external program so I can do a 200 point measurement at various frequencies and pretend it's a network analyzer. Works great and they must be pretty reliable as at least one has been running 24/7 for the last decade or so. The downside is support, and especially calibration, is crazy expensive, so much so that we just check it against some bench standards and call it a day. The local cal labs won't touch it because I think they use the exact same instrument for their calibrations and have no better standards. FWIW, those cheap import adapters work just fine and can be customized with less guilt.