EEVblog 1448 - Convert a Fluke 77 IV to True RMS for 10 CENTS

[EEVblog]

How to convert a Fluke 77 IV average responding multimeter into a True RMS model 177 for 10 cents.
Should also work for converting a Fluke 83V to 87V
True RMS measurement vs Average Responding
And calibration of a Fluke meter.

00:00 - Hacking a Fluke 77 IV Multimeter
00:35 - Is the Fluke 70 series discontinued?
01:15 - Fluke History
03:10 - I noticed something...
04:49 - The AD737 True RMS converter chip
06:04 - Theory of Operation
07:21 - What is Crest factor
08:58 - How do they do it?
09:25 - Fluke 87V Schematic and the custom ASIC
10:53 -  Let's modify it!
12:58 - A sine wave is NOT a crest factor of 1
14:01 - Soldering
15:15 - Will it work?
16:46 - Calibration procedure
20:09 - Winner Winner Chicken Dinner

[bigsky]

Dave, you said in the video you'd include a link to the 87V service manual - I couldn't see this - did you forget?

[Aqunity]

Did a little digging and found it here! Downloaded it myself too.

https://xdevs.com/doc/Fluke/87V/Fluke_87V_Full_service_manual_with_schematic_and_electrical_parts_list_Service_Manual-87VANFullService.pdf

[golden_labels]

• Buy a DMM where ¹/₃ of the price is calibration.
• Pay 10¢ to modify it.
• No longer have calibration

Sorry, I just had to make that comment . Don’t take it too seriously, I am fully aware people would buy Fluke for reasons other than calibration.

[CatalinaWOW]

yeah.  10 cents.  It is kind of like when I do my own auto repairs.  I save hundreds of dollars over taking it into the shop.  But that ignores the literally thousands of dollars of tools I have accumulated over the years (and the not insignificant amount of blood donated to various vehicles and soap used in cleaning up).  The accounting does come out in my favor over all of the repairs that have been done and I know the quality of the work (both good and bad) so it works out, but it sure isn't for everyone.

On the other hand, it is really neat to have discovered this and to be able to do it.  Good on you, Dave.

[ejeffrey]

• Buy a DMM where ¹/₃ of the price is calibration.
• Pay 10¢ to modify it.
• No longer have calibration

Sorry, I just had to make that comment . Don’t take it too seriously, I am fully aware people would buy Fluke for reasons other than calibration.

It's also not as if you are buying a cheap meter and upgrading it to be equivalent to a much more expensive model, at least if you are talking new meters.  It mostly just cool to see how it works, not a really practical upgrade.  I guess if you find a cheap 70 series at some equipment sale you might do this as a fun way to get a fluke meter but mostly it's just fun.

[Fungus]

• Buy a DMM where ¹/₃ of the price is calibration.
• Pay 10¢ to modify it.
• No longer have calibration

You forgot to mention that the difference in retail price between the two meters is only $10. 

[M0HZH]

Dave, you mention in the video the 77 IV might be discontinued. I work for a Fluke authorised distributor, it's still on the latest price list (October) and we didn't get any notice about it being discontinued, so I guess it's still available (although that's a vague term with lead times being what they are nowadays).

[GnatGoSplat]

Interesting video.  My decades old Tenma multimeter uses an AD737 also.  This would be fun to try if I had any info on how to calibrate it.

[Ed.Kloonk]

• Buy a DMM where ¹/₃ of the price is calibration.
• Pay 10¢ to modify it.
• No longer have calibration

You forgot to mention that the difference in retail price between the two meters is only $10. 

I can see his point, but if you have one meter and suddenly need RMS measurement often (as I did once), the calibration would be close enough compared to something that doesn't give what you want to see.

 

[EEVblog]

Dave, you mention in the video the 77 IV might be discontinued. I work for a Fluke authorised distributor, it's still on the latest price list (October) and we didn't get any notice about it being discontinued, so I guess it's still available (although that's a vague term with lead times being what they are nowadays).

Yes, Fluke responded and said it's not discontinued. It's just not on the Australian website.

[dardosordi]

What if you modify the rom data only for the rages that needed change ? Would be interesting to look at a rom dump.

[scopeman]

Dave,

How about a Fluke 37 style version of your Bryman meter?

Sam
W3OHM

[EEVblog]

How about a Fluke 37 style version of your Bryman meter?

That would be cool, but I doubt any manufacturer would be interested.
Nearest is this:

[Scherms]

Hi Dave, saw this on Bitchute...

(I joined this site years ago but can't remember my old login details)

I have a Fluke 88v which is an automotive averaging type as opposed to the 87v RMS type. Oddly this one was even more expensive than the 87V.

Yes I mostly do DC vehicle checkings so the meter's specific functions really suit me but I want it to be RMS...

...Does the 87v and 88v share the same board and can I get it to RMS by soldering on the averaging capacitor and then recalibrating it?

It would be the perfect Auto DVOM...

PS: I can't find any specific 88v service manual, only the 80 series calibration manuals!

https://dam-assets.fluke.com/s3fs-public/8xv_____cmeng0100.pdf
https://dam-assets.fluke.com/s3fs-public/8xv_____cmeng0109.pdf








 

[Scherms]

• Buy a DMM where ¹/₃ of the price is calibration.
• Pay 10¢ to modify it.
• No longer have calibration

Sorry, I just had to make that comment . Don’t take it too seriously, I am fully aware people would buy Fluke for reasons other than calibration.

Not exactly a silly idea! I bought an 'as new' but damaged Fluke 88v a while back that had an Eprom Error from GSM phone frequency exposure and it was sold dead 'for parts'...

I paid under $50 US for this 88v and promptly sent it into Fluke Celemetrix in Sydney for a fixed rate $185 repair. What I got back was a new version 11 (GSM proof) main board + calibration (with certificate), new holster, port covers, battery and CDRom. An almost new and complete Fluke 88v for $230 AUD, not bad.

PS: this new 'free' mod makes my 'smart' purchase 88v now as accurate as the 87v " ... chicken dinner"!





 

[golden_labels]

That was a whimsical comment about the video. That was a different situation. You bought a broken DMM and got it fixed. Not bought a working DMM and made a $0.10 modification that makes you lose a third of the device’s value.

[bdunham7]

Yes I mostly do DC vehicle checkings so the meter's specific functions really suit me but I want it to be RMS...

I'm just curious why you want TRMS?  There's no shortage of TRMS meters, but my 'first grab' meters these days--bench and handheld--actually are average-responding. 

[Scherms]

That was a whimsical comment about the video. That was a different situation. You bought a broken DMM and got it fixed. Not bought a working DMM and made a $0.10 modification that makes you lose a third of the device’s value.

Not so, many previous owners of expensive older Flukes like me have often traded functionality for outright performance. If a simple mod can propel a compromise purchase into 'the best you can get' then WOW!

So my 'smart buy' just got even better!

Yes I mostly do DC vehicle checkings so the meter's specific functions really suit me but I want it to be RMS...

I'm just curious why you want TRMS?  There's no shortage of TRMS meters, but my 'first grab' meters these days--bench and handheld--actually are average-responding.

Some critical vehicle ECU and ABS components require AC RMS measurement. Since I have 'the best', why not make it better...

[bdunham7]

Some critical vehicle ECU and ABS components require AC RMS measurement. Since I have 'the best', why not make it better...

I've never seen that specified and I'm not sure how it would matter, but my go-to for such things was the Fluke 98.  The problem with TRMS is residual counts and small signals--you will lose the bottom 5% or so of every range if you convert.

[Scherms]

Some critical vehicle ECU and ABS components require AC RMS measurement. Since I have 'the best', why not make it better...

I've never seen that specified and I'm not sure how it would matter...

Automotive ABS signals are AC and are often complex... but this ain't 'rocket surgery'!

[EEVblog]

That was a whimsical comment about the video. That was a different situation. You bought a broken DMM and got it fixed. Not bought a working DMM and made a $0.10 modification that makes you lose a third of the device’s value.

Not so, many previous owners of expensive older Flukes like me have often traded functionality for outright performance. If a simple mod can propel a compromise purchase into 'the best you can get' then WOW!

It's a "simple" mod technically, but it requires a full recalibration of the meter and the gear to do that.

[Kleinstein]

Some critical vehicle ECU and ABS components require AC RMS measurement. Since I have 'the best', why not make it better...

I've never seen that specified and I'm not sure how it would matter, but my go-to for such things was the Fluke 98.  The problem with TRMS is residual counts and small signals--you will lose the bottom 5% or so of every range if you convert.

When using the TRUE RMS chip to calculate the simple rectified value the limitation at small signals may be similar. Part of the problem is the rectifier that just gets ever slower the lower the amplitude and there can be some offset in when to change the sign. The main way to get around that problem is to get a meter that does digital RMS with sufficient resolution of the ADC.  Some of the modern cheap meters do that, but with a limited bandwidth - but at least a constant bandwidth.