Fluke 27 / 27FM side by side tear-down

[idpromnut]

Hello, I'm sure this has been done before, but since I just got my Fluke 27FM edition, I decided to tear it down and see what was different inside. Turns out there are a couple of mechanical and electronic differences between the two!

Of particular note are:
- The battery compartments are slightly different, with the 27 having a cover over the fuses, and the 27FM having a battery holder (which acts as a kind of fuse cover I guess)
- The 27FM has an additional DIP, the SC77174 (marked "RMS" on it...)
- The 27 has a PTC where as the 27FM does not (but both have the limiting resistor and MOVs)
- The 27 has what looks like a high-precision resistor next to the limiting resistor; the 27FM has what looks like a "regular" resistor in its place.

Enjoy!
Chris

[retiredcaps]

- The 27 has a PTC where as the 27FM does not (but both have the limiting resistor and MOVs)

Thanks for the side by side teardown photos.

I believe the 27/FM does have two PTC (circled in red)??  It should measure around 1k ohm.

[Marvin]

The 27/FM has the same input protection fusible resistor (the brown one) as described in detail in http://mrmodemhead.com/blog/fluke-87-fusible-resistor/

And a word about that SC77174 - it is a True-RMS converter chip designed/engineered by Fluke for the 8060A. It was manufactured by Motorola and replaced by Analog-Devices AD636 in later revisions of 8060A (as seen on service manuals that are available from Fluke). It was resurrected for the 27/FM in 1987 and from the pictures on the net it was manufactured until 1991 (the latest 27/FM were manufactured then, '87-'89 have same markings and are much more common but I stumbled on one picture from the chip that had '91 date on it and the Motorola logo was placed differently). There is a lot more information about that in https://www.eevblog.com/forum/testgear/old-fluke-multimeters/ by drtaylor who was the designer of the 8060A. It is only present in the gray 27/FM meters - some newer yellow ones have 27/FM marked on the sticker on the back side, they are just usual 27 series meters with an average responding AC.

One thing to note is that from a newer revision they moved the 27 series from the old 4-bit SM4 microprocessor to a much newer and universally used in Fluke TI M430 series 16-bit microprocessor. 27/FM is always with the old SM4 one. The 27 service manual has schematics/BOM for both versions of the controller board.

Edit: added a pic of some of the datecodes on SC77174 in 27/FM

[daddario]

Since I failed to come to any documents regarding the 27/FM, nor information about its bandwidth, I just decided to acquire one to satisfy my curiosity.
Somewhat hoping for the figures present in the 8060A using the same converter I was not elated, as it appears not to be the case. Measuring a 100mV 60kHz sine it seems to read about 50 counts out and worsening as frequency is increased. So in that sense it seems to have been designed to be on par with the regular 27.

Also, I came to this surprising creature: http://www.ebay.com/itm/NEW-IN-BOX-Fluke-Model-27-FM-Multimeter-Newest-Version-/321148890625?pt=Desktop_PCs&hash=item4ac5f72e01

Appears to be a genuine yellow FM.

[MarkPalmer]

I grabbed a couple of the military 27/FM's a couple of years ago when large quantities of them were released as surplus and they were practically giving them away.  They are built like bricks and nearly indestructible.  I see the prices have been going up as the quantites of them begin to dwindle.  Great handheld DMM's that I got for the price of a Harbor Freight job 

-Mark-

[idpromnut]

Hmm!  That meter looks in excellent condition... the one I have I picked up for about 80$ shipped to Canada from Florida (minus all the awesome accessories!). That 27 is my daily use meter, rugged, good accuracy, fast, and a nice big readout.

[Napalm2002]

I bought my 27fm 2 or so years ago. Wish I picked up a few more.  Very nice meter. Its my go to beat it up outside meter. Fluke = great