Fluke 77 problem - all characters on LCD stay lit

[Tallie]

I know of the notorious faded LCD issue... what could cause all the characters to stay lit?

[retiredcaps]

what could cause all the characters to stay lit?

1) It sounds like it is not coming out of self test mode.  The service manual  (page 3-10) has 5 tests. 

2) Clear focused pictures of your pcb board (both sides) with the lcd removed will also help.

3) If you haven't tried a fresh 9V battery, do so.  The 77 has an outstanding 2000 hour battery life, but when it gets low, it might cause funny readings and/or non start behaviour?

4) Excavatoree, a member here, has seen poor soldering on U1 causing all sorts of problems.  You may want to examine U1 under magnification.

[Cubdriver]

I'd start with a new battery and go from there.

-Pat

[Tallie]

Thanks for the advice. I've downloaded the service manual and will try all the things you've said. I'll report back... eventually...

[Excavatoree]

You might want to gently press on the SMD IC near the bottom of the PCB.  Some of these had poor soldering on this IC, and I've fixed a few by re-soldering.

Sometimes, that IC gets damaged and all you get is the self test, but first perform the tests that retiredcaps mentioned.

[Tallie]

TP1 = 4.55 - 4.58V
TP2 = 3.0V avg, 34.1Hz @ 50% duty
C5, 6, 7, and 8 are good
R15 to R16 reads .858V reference, not 1.00V
Y1 reads correct frequency
Display voltage is good... reads 1.6V

I'd could probe the pins if I knew how to count them... pin 1 isn't marked on the either chip; U1 or U2... on U1, checking for 1V reference, if I start at the bottom left corner and count counterclockwise to 14 - counting every pin, I get nothing on the scope. If I count to 14, counting only the pins that are soldered to the board, I get 3.18V on the scope.

Something interesting happened while I was trying to probe the pins on U1... when I flipped the meter back over, it was working... all ranges were working. I don't know what I did... so I turn it off, reassemble everything, turn it back on, and now it's back to not leaving the self test screen... any help is welcome!

[ZeroTolerance]

Sounds like a bad solder joint

[Tallie]

Well, it's working consistently now. No clue how I fixed it... I did power it on slowly a couple of times(turned the knob slowly)... I noticed it would pass self check when I did that, now I can turn it on normally and it will operate. A switch problem? I've got a can of DeOxit on the way...