HELP! -> Fluke 88v AVG meter to TRMS by simple mod? - Page 1
Forum section: electronics / repair

author: Scherms
author_rank: Regular Contributor
timestamp: April 02, 2022, 02:00:44 am
content: Fluke 88v AUTO averaging meter to TRMS by simple mod?I mostly do DC vehicle checkings so the 88v meter's specific functions really suit me but I want it to be RMS...Fluke 88v multimeter:I have a Fluke 88v which is an automotive averaging type as opposed to the 87v RMS type. Dave seems to have found that the main boards of the currently available 80 series are all populated with the same components*. The only difference between models and markets are strategically placed caps to enable certain functions for certain models. In the case of the AD737J chip that controls the RMS, it has been enabled in the 87v with the inclusion of a 33uf cap on C7 but am I to assume it has been disabled on the 88v by leaving those pads vacant for the 33uf cap at C7?Fluke 87v main board:VSFluke 88v main board:Quote from: shakalnokturn on April 05, 2022, 12:23:51 pmIf this is true (unverified) then this simple cap inclusion (TRMS) will bring the accuracy of the 88v to that of the 87v without loss of specific model functionality, great! But can it be done?Maybe we could also reinitiate the 'Lo pass filter' disabled in the 88v as well in a similar way???...Does the 87v and 88v share the same board and can I get it to RMS by soldering on the C7 33uf averaging capacitor and then recalibrating it like what Dave did with the Fluke 77 to 177 hack?*https://www.eevblog.com/forum/blog/eevblog-1448-convert-a-fluke-77-iv-to-true-rms-for-10-cents/It would be the perfect Auto DVOM if it did TRMS by default...PS: I can't find any specific 88v service manual or main board pics, if anyone has them then post links and pics here please as I only have the 80 series calibration PDF manuals!https://dam-assets.fluke.com/s3fs-public/8xv_____cmeng0100.pdfhttps://dam-assets.fluke.com/s3fs-public/8xv_____cmeng0109.pdfQuote from: Aqunity on January 13, 2022, 03:21:33 amQuote from: EEVblog on January 12, 2022, 09:53:44 pm

author: alm
author_rank: Super Contributor
timestamp: April 02, 2022, 01:30:05 pm
content: Quote from: Scherms on April 02, 2022, 02:00:44 amI would say the AC functions would need to be recalibrated before you can make any claim about accuracy after such modification.Quote from: Scherms on April 02, 2022, 02:00:44 amYou seem to have a Fluke 88V, so what's stopping you from opening it up and taking high-resolution pictures from both sides of the main board, and compare them to the 87V for which you can find some medium-resolution pictureshere, and Dave'svideoandpictures.I could imagine the 88V having a different firmware, so I wouldn't count on getting other features like the low-pass filter.

author: Scherms
author_rank: Regular Contributor
timestamp: April 02, 2022, 06:09:26 pm
content: Quote from: alm on April 02, 2022, 01:30:05 pmCheers, no 'counting' on anything here... My 88v has a calibration warranty seal on it so I don't want to void the warranty before I know the mod can be done and absolutely a recal would be required after the mod. See video for recal verification...

author: shakalnokturn
author_rank: Super Contributor
timestamp: April 03, 2022, 11:18:54 pm
content: Here's what you get in the 88V:A 4.99 Ohm resistor in series with a small ceramic cap that wouldn't measure in circuit and a spare footprint for your missing tantalum.

author: Scherms
author_rank: Regular Contributor
timestamp: April 05, 2022, 03:34:21 am
content: Quote from: EEVblog on April 02, 2022, 12:06:22 amQuote from: shakalnokturn on April 03, 2022, 11:18:54 pmHere is an updated picture of the area in question on the Fluke 88v, there are differences than the 87v! What are the cap and resistor (blue box) doing there between the blank pads and the AD737 chip? Would they need to be removed for the mod?

author: shakalnokturn
author_rank: Super Contributor
timestamp: April 05, 2022, 12:23:51 pm
content: Sorry I did not go to the trouble of measuring the input coupling, RMS averaging and output filter capacitors on my meter, didn't want to go to the trouble of desoldering them. I expect them all to be in the 1-10nF range.Have yo read the AD737 datasheet? I have partly read it, as I understand, in the 88V they have compromised for mainly average AC measurements but with a little twist to aid in catching fast pulses. (Common-rail injection voltage or current pickup on ignition maybe?)I couldn't really make sense of this phrase on page 67 of the 88V user manual though: "AC conversions are ac coupled and are average responding, rms indicating."I wouldn't go to the trouble of removing the existing RC if you want to change to true RMS, they're not going to have a notable effect parallel to 33µF. I'd be worrying more about being set for the readjustment.

author: alm
author_rank: Super Contributor
timestamp: April 05, 2022, 11:23:01 pm
content: Quote from: shakalnokturn on April 05, 2022, 12:23:51 pmAC coupled = any DC components in the signal will be ignored for the measurementaverage responding = the results will be proportional to the average of the (absolute value of) the signal: if the average value is twice as high, the reading will be twice as high.RMS indicating = although the value is proportional to the average value, it's adjusted so it matches the RMS value for sinusoidal signals.So for a sine wave with a 1V amplitude, the average value is 0.637V, which the meter multiplies by 1.111 to give 0.707V (the RMS voltage) a true RMS meter would read the same. If you measure a square wave with the same amplitude, the average value is also 1V, which the meter will multiply by 1.111 to give 1.111V. But the RMS voltage is also 1V, so the meter would read 11.1% high. A true RMS-responding meter like the 87V would indicate 1V.

author: shakalnokturn
author_rank: Super Contributor
timestamp: May 15, 2022, 04:50:29 pm
content: Did OP go to the trouble in the end?