Fluke 77 oddity

[TimeBandit]

I have a Fluke 77, and just picked up a second one today. Both are Series I and look identical in every way, including the note on the front that the 10A input is unfused. So I was a little surprised when I popped both open and found that their fuse setups are different. The one on the left, without the HRC fuse, is the one I just picked up.

[coromonadalix]

Its not rare to see pcb revisions / design changes  etc .... i would use the ( 2 fuses) model for everything and use the other but on 300vdc and 300 vac and lower voltages ...  sure they are not to par with the most recent ones with mov and other input parts protections.

But if they work  eh   have fun with them.

[retiredcaps]

Some people who do DIY repairs or "professional" repairs, they mix and match cases, etc to make a working unit.  This can be seen here on the forums, ebay, and other places.

[Nusa]

That large fuse is 3A, is it not? I think you'll find it's ALSO protecting the mA input if you do a little continuity checking. Which means you can believe the unfused label for the 10A input.

You'll find this enlightening. See section 8 in particular, since I think it applies to both of your meters:
http://bee.mif.pg.gda.pl/ciasteczkowypotwor/Fluke/70%20Series.pdf
Remove the battery to see the board version info.

Note that R1 and R20 are fusible resistors. They look original, which is good; sometimes you find R1 has been replaced with a resistor that isn't fusible.

Both meters should be equally safe for whatever you plan to use them for. How safe that is is for you to decide. Everything of that vintage predates modern CAT ratings, so if that's what you personally require, use a more modern meter for mains work.

[TimeBandit]

Thanks everyone.

@Nusa - Yes, the larger fuse is a 3A. Sure enough, you're right about it being in series with the 630mA. That section in the service manual helped clear this up. Thanks for linking to that.

I use my 77 as a second meter for audio equipment repair work, and have a basic Cat III 600V job that I'll relegate to mains duty.

[TimeBandit]

That large fuse is 3A, is it not? I think you'll find it's ALSO protecting the mA input if you do a little continuity checking. Which means you can believe the unfused label for the 10A input.

You'll find this enlightening. See section 8 in particular, since I think it applies to both of your meters:
http://bee.mif.pg.gda.pl/ciasteczkowypotwor/Fluke/70%20Series.pdf
Remove the battery to see the board version info.

Note that R1 and R20 are fusible resistors. They look original, which is good; sometimes you find R1 has been replaced with a resistor that isn't fusible.

Both meters should be equally safe for whatever you plan to use them for. How safe that is is for you to decide. Everything of that vintage predates modern CAT ratings, so if that's what you personally require, use a more modern meter for mains work.

One related follow-up question regarding the 630mA fuse in both meters. The one with the second 3A fuse had a regular glass type in the 630mA spot when I first got it. The other 77 that does not have a 3A fuse had an HRC type in the 630mA spot. When I saw this I tried swapping in an HRC for the glass 630mA. Right after doing so, I got weird fluctuating readings when measuring resistance. Switching back to the glass 630mA seemed to correct the issue. I have to repeat this swap to confirm the fuse connection to the wonky resistance readings, but does this make sense to anyone?

[TimeBandit]

Ok, I swapped the HRC back in and the meter functions fine on resistance readings. So forget my theory. Whatever was causing those wonky readings, it isn't happening now.

[TimeBandit]

Bringing my own thread back from the dead.

Still own the Model 77 Fluke discussed above. And it's been displaying the same issue with the resistance, though now it's bad enough that it doesn't measure accurately at all. I opened it up and reflowed the solder on the input jacks, but no change. So I checked R1, the fusible resistor (tests ok), and RT1, the 1.1k PTC thermistor. My understanding is that this should measure 1.1k ohms at 25 degrees C, which is just a bit warmer than room temp. I'm measuring 0 ohms. Wouldn't this measure open if it had failed?

[TimeBandit]

Never mind. I pulled the thermistor and it tests fine out of circuit. 

[Nusa]

Intermittent problems often come down to the selector switch. Try a little contact cleaner and exercise the switch several times. See if that helps.

[Shock]

They both look like legitimate Fluke 77. I think it's only the Fluke 75 pcbs that will fit in the Fluke 77 case anyway. The early Fluke 73 has three sockets the Fluke 70 has two.

[TimeBandit]

Intermittent problems often come down to the selector switch. Try a little contact cleaner and exercise the switch several times. See if that helps.

I believe that was probably it. After reinstalling the thermistor, I had cleaned the switch a second time and then applied a tiny amount of some switch contact oil. Now it appears to be operating normally in the ohms setting. Time will tell.

[Axtman]

Just curious, what do the circuit boards say?

[Shock]

They say 7X under the battery.