Fluke 77 diagnosis and repair

This Fluke 77 DVM was giving incorrect readings by around half. I open it up and after some wrong turns find the problem. This is kind of an old meter but it was nice to get this classic working again!
Here's the documentation I used:
1drv.ms/b/s!AgRy1tCE6lTd9E4Hl...

[@17630973]

It was an error in judgement not to change the battery in response to the low battery annunciator. The calibration is now suspect when a good battery is installed.

[@lucman22aa]

The majority of the various problems with the old Fluke 77 meters is simply to do with dried-out contacts on the rotary switch inside. You just need to use a sharp pointy X-acto blade and place a tiny, tiny amount of household oil to each track as you rotate the various switch positions. You should use magnification when doing this so that you don't over apply the oil. Basically, the oil just re-wets the tracks and combines with the original dried-out factory grease that remains on the tracks and contacts. Good Luck! These Fluke 77's are one of their most reliable DVM's. They'll last a lifetime if you take care of them.

[@francoisguyot9770]

You're very tenacious and hopeful. That's very encouraging for the rest of us. I guess I'll give it a try too. I've got the same meter and it gives me totally random readings.

[@RobertShrimpton]

Did you check the 9v battery???
I have one of the original Fluke 77 units, which I've owned since new (around 1987). Still works fine - just change the 9v battery every 3 or 4 years!
If you see the battery symbol (top left of display) on during normal operation (i.e. after the first 2 seconds of power-up), then the battery should be replaced.

[@Sagittarius-A-Star]

My Fluke 75 behaved in the same strange way.
I first thought the battery (expired 2001!) was empty because the indicator was visible - but it was not(!).
In the end I just had to clean the contacts of the rotary switch with a toothbrush and
replace the smaller of the two fuses which was blown (tried to measure the voltage of a mosquito swatter in DC mode - maybe a mistake).
Although the battery is so old and the indicator shows up (which - according to the manual, which is for models 75 and 77 - means that 100 hours of operation remain)
it works again.
Even when I use a brand new battery, the indicator shows up.
Only when I use a rechargeable battery (which is not even fully charged) the indicator does not show up.
The voltages of the three batteries:
Old one: 8,83 V
New one: 9,68 V
Rechargeable: 8,72 V
A mystery.

[@ntag411]

I think it's best to press using a stiff plastic Spudger tool. A finger might be a kind of antenna. The newer designs have the electrical contacts of the rotary switch right at the board level rather than a switch that is elevated from the board that can pickup noise. Even the cheaper scopes of the past used wafer type switches while something like a Tektronix 7000 series scope has the contacts at board level using finger type contacts actuated by a cam of the rotary switch.

[@youtuuba]

Rob, you might want to look into devising a way to check that your camera is seeing....most of what we should be looking at is glared out by your overhead light(s) in this video.
I was also cringing during a lot of this video as you came close to nailing the problem then got derailed by some inconsequential tangent....(but it happens, I know).
As some others have commented, the 77 was practically screaming at you to change the battery the whole time. When the display shows the LOW BATTERY icon, all bets are off as far as meter operation, not to mention performance, and although that was apparently not the only problem, you could have saved yourself a lot of time and grief by dealing with that obvious thing first.
You spent some time wrestling with the reference voltage into U1 pin 14. which is supposed to be dead-nuts-on 1.000V. This is simply derived from the internal 1.22V precision reference chip VR1, which does not look like an IC but is, then the excess 0.22V is dropped by R15, the 56.2k resistor, leaving 1.000V across the series combination of R16 (205k) & R8 (100k trim pot "3V Cal"). The trim put just adjusts the current through the voltage divider so that you get the proper voltage drop for the pin 14 input. But if the battery is low, the VR1 voltage might be below specs and this can throw off the reference voltage at pin 14, which can cause more significant errors and instabilities that look like something else is causing them.
You also appeared to just de-ox the front side contacts of rotary switch S1, and that side does manage the reconnections of the input signals to the U1 input pins. However, there is a set of contacts on the rear of S1, hard to access, and those handle two things: they are the power switch for the meter, and they control the three "function code" input signals to U1 that tell it which function is selected. Unlike earlier DMMs, with their complex input circuit switching, the model 77 switch S1 is pretty simple and once it tells U1 which function is selected, the chip does the rest internally. If the function switching contacts are iffy due to dirt or corrosion, the chip might be randomly changing from function to function, making confusing displays that look like something else is causing them, and this can lead to unnecessary component replacements, re-soldering joints that are not bad, etc.
I have a few 77s, bought back in 1986 and used almost daily since then. They never miss a beat and are as good as new. I obtained a service manual way back when, and studied the same schematic you have, and there is really not much to go wrong on these, as long as the meters are used enough to prevent switch issues, the battery is kept fresh, and they are not abused.

[@RobsFixitShop]

Hey, thanks for the great comments. I do struggle with reflection from the overheads. Meter faces are the worst. I did use a power supply for some of the video which took the battery out of the loop. My channel is mostly about how I figure something out which can be pretty frustrating at times for people that actually know what they're doing!

[@johnlucier8572]

Should always use plastic and not metal to prob around in there and what makes it worse you have the power on. Had the same problem, bad selector switch. Ordered a new one..works fine.

[@helifynoe9930]

The layout looks the same as in my ancient Fluke 75. They of course have the grey push button (Range Hold ), rather than it being yellow. The display plastic cover was scratched up, so I took it out, sanded it down, and then buffed it back to being shiny and transparent once again. So it is working and looking as good as new again..

[@RobsFixitShop]

They're a classic!

[@bonhof69]

Hi, I accidentally measured AC voltage on resistance mode. The Fluke 77 now doesn’t function. Could you pls help me with the troubleshooting & circuit diagram?

[@pup9et]

Ok, so i bought one of these like 5yrs ago at a garage sale for $1.00. At that time i could tell the battery was dying. But i figured id pop a new one in it. Well, all this time later (actually forgot i had it) i dug it out and of course the battery is dead. As i was about to throw it in the garbage (Because i already had 2 other ones) I noticed it said "Made in the USA" Which surprised me, and how rugged it was. I decided to come inside and search online. After seeing what this thing cost new as of 2019 the new model 77 is ($360) I was glad i didnt throw it away. Mine is exactly like the one you have on the left, the older model. It has a orange stick on the back that says (Calibrated), then under BY: it says New Equip, Date: 1-89, DUE: On Reg. So i am assuming it was last calibrated in 1989. Thank you for making this video. It helped.

[@s-max3277]

hello Rob I have problem with display LCD... and need replace the CPU in the back lcd. is possible found this part?
I have this problem after using multimeter with low battery and have caused this problem 😭😭😭
dig2 c dig2 d in test 3.7this 2 line not working
and have test pin out voltage have +0.30 v respect other pin.
if connect this pin to ground with one resistor 1000ohm I have visible this line for 2 second and after 2 second this two line ghost

[@elceibo846]

Hi how i get the trobleshted that you are showing,,,, do you have a pdf...... thanks .

[@rogeroliveira7302]

Hi there Rob I hope you still have this Fluke 77 with you. I need spare parts

[@luis11347]

Hello I have a multimeter Fluke 70 . doesn’t power on, I reemplace a diode 1N4448 (CR1) (The original diode is short) . Is turned on, but no have lecture in diode and ohms Scale and other issue i have is the multimeter power off in 300mv Scale like a short, bud does find other elements damaged in the motherboard. Greetings

[@s-max3277]

hello Rob I have same multimeter do you have manual for check lcd segment ?
I have two segment with problem.

[@zafarhussain6696]

For Fluke 77 Service Manual You can download it from: www.manualslib.com/manual/1006231/Fluke-77.html
For Fluke 77 Series II service manual you can download from:
manualzz.com/doc/6275276/fluke-77--75--73--70--23--21-series-ii-service-manual
Zafar Hussain Karachi Pakistan

[@s-max3277]

@@zafarhussain6696 yeah i have chek and suppose the mcu is damaged. My pin segment have 0.20v extra surplus.respect the other pin. If connect the damaged pin to gnd with one 1k resistor the segment turn on for 1or 2 seconds.
If turn on the multimeter in test segment..this segment not turn on.

[@s-max3277]

@@zafarhussain6696 and have fluke 10 used with low battery. With problem ghost display.
Battery with low charge. Damage your multimeter.
This is a very problem!
Two multimeter used with battery low ... burning... 😢😢😢😢😢😢😢😢😢😢

[@taian6257]

Have the Fluke 75 , checking voltage 575v 3 phase .After second phase display show number even no test. battery, fuse, rotary seem ok. even at ohm function should read OL but some number showing. thanks for comment

[@tracyangelo8642]

You are so GOOD!

[@ramonrgil9125]

Sería posible tener este video en español?

[@jesusisidrolopezbenitez]

Bien

[@cwrobinson6363]

Anyone know where I can get new or replacement test leads for my FLUKE 77 Multimeter?

[@RobsFixitShop]

I just got a new set of ProbeMaster probes that I really like. Here's the amazon link but I ordered them directly from probemaster. amzn.to/32mPUAv

[@vincentlegros9319]

i put these uni-T UTL-20 on y fluke 75. very well. www.aliexpress.com/item/32821721616.html?spm=2114.12010612.8148356.24.100a5ae2QgWZ0G

[@daviddevillers6790]

battery ok? I see the battery indicator - does that mean the battery is low?

[@RobsFixitShop]

I tried to read the user manual about what the display battery indicator was showing battery low or battery good but it was unclear. The battery was new.

[@w2dmw]

I'm thinking solder joints. retouch those in the area where you get variations where you press

[@chaianekruttanaphant1411]

Can u share f.77 schematic diagram ?

[@17630973]

It is available from Fluke to download.

[@sparkytod2]

Clearly the battery replacement symbol is top left of screen, the Fluke 77 will show low readings when this happens. replace battery job done!

[@AlainHubert]

Indeed. Whenever trying to fix any battery powered device, it's always wise to use a fresh one.

[@JoseFlores-tp1eo]

How you fix a low buzzer sound?

[@RobsFixitShop]

I don't know. I am thinking of doing something to my HP DVM for the same problem. I can never hear it!

[@NoferTrunions]

I've opened mine up and have NO IDEA how the buzzer works - it does not appear to contact any board circuits. Wireless???

[@pedromontescirne3705]

Podrías mandar tu manual x pdf ya que tu video es en inglés gracias

[@RobsFixitShop]

I put a link to the manual I used in the video description. Buenos dias!

[@zafarhussain6696]

For Fluke 77 Service Manual You can download it from: www.manualslib.com/manual/1006231/Fluke-77.html
For Fluke 77 Series II service manual you can download from:
manualzz.com/doc/6275276/fluke-77--75--73--70--23--21-series-ii-service-manual
Karachi Pakistan