Fluke 75 drifts

[Http 418]

Hi all

So managed to dust up 2 old fluke 75 dmm's at 40$ a pop 

They were advertised as have drifting vdc messuraments, so in my head worst case I get one Worning out of the del or 2 that has some problem areas but besides that working.

Get Them home and power them on and they are all over the place in all ranges  might just have bought two pairs of really expensive testleads.

I ohmed most of the resistors and none are broken, dono the correct values on them. Then i cleaned the pcb with IPA and put in a new 9v battery. Still all over the place 

Looks like the there is some charge building and tjen dropping off.

Any one have any ideas.

The is a big ic on the back that say's fluke on it, please dont tell that have released the magic blue smoke

Lars

[Vgkid]

I assume you have another multimeter, check the 1.000Vref, R8, 14-16, and CR2 as a start.
Then trace the input voltage.    * both on pg37 of the manual.
http://assets.fluke.com/manuals/77______smeng0100.pdf
After reading the manual, can you take a picture of the U1 chip.

[LaurenceW]

Yes, my money is on the integrity of the small variable resistor that fine-tunes the voltage reference that goes to the main chip (which i suspect is OK itself). Problems with this variable resistor would effect ALL measurements, however.

[lowimpedance]

Does the ohms range actually try to measure any known value? .Could be damaged input protection R1 is 1K?. Maybe give the switch another careful clean. The manual link posted contains the schematic with component values. Hopefully you still have a little more than test leads to show for it.
 As LaurenceW said be suspicious of the reference adjust pot.
 

[Http 418]

I assume you have another multimeter, check the 1.000Vref, R8, 14-16, and CR2 as a start.
Then trace the input voltage.    * both on pg37 of the manual.
http://assets.fluke.com/manuals/77______smeng0100.pdf
After reading the manual, can you take a picture of the U1 chip.

So i ohmed out R8 and i get from 1.9k to 74.1k ohm on that, and from measuring the 1.000Vref witch I can get to 0.971v turning R8 to max.
Measuring the voldtage signal path I keept comming up a bit short on the input divider, it should be 200mV but i only got 181.9mV both on Z1 pin3 and U1 pin 26 and 27, but my fluke 87 probally out of spec, but seems a bit much.

When measuring a resistor it seems to hit the Rights ohm value initial but then it starts dropping allmost to 0 then up and down in the low end. Seems to be the same for all measurament

I ohmed most of the resistors and they all check out, but there is some differences the lower section of the pcb, but seems to be ok.

Lars

[ModemHead]

Your 181.9 mV reading is essentially perfect.  Remember that the DMM you're using has a 10Meg input impedance which will go in parallel with the 1.111Meg bottom leg of the divider.  Do the math and 2.00V input gives 181.8mV.

I've worked on a number of these meters and this kind of problem is very often due to contamination.  I know you said you cleaned the PCB, but do it again.  Scrub it with a stiff brush and immediately blot the used IPA away with a paper towel.  Once you float the contaminates, you don't want them to dry back on.  Drag something sharp between the pins of U1, and scrub it again with IPA.  Allow plenty of time to dry.

[retiredcaps]

For cleaning, I use a generous amount of IPA and generally drench/douse the board.  For a Fluke 17x, I had to do it twice before the ohms worked properly.

I use a sharp multimeter probe to clean out all the pins on the big chips.  I then use a 10x jewelers loupe to inspect.

[Http 418]

Your 181.9 mV reading is essentially perfect.  Remember that the DMM you're using has a 10Meg input impedance which will go in parallel with the 1.111Meg bottom leg of the divider.  Do the math and 2.00V input gives 181.8mV.

I've worked on a number of these meters and this kind of problem is very often due to contamination.  I know you said you cleaned the PCB, but do it again.  Scrub it with a stiff brush and immediately blot the used IPA away with a paper towel.  Once you float the contaminates, you don't want them to dry back on.  Drag something sharp between the pins of U1, and scrub it again with IPA.  Allow plenty of time to dry.

Hi ModemHead

So I cleaned it one more time, fresh IPA, swap and 1½ hours, and now I got one of the 75 to work so the other one is getting the same treatment
It meassures perfectly, but it is still abit itchy when "idling"?

Thanks for the advise all so happy that I got more then test leads out of the deal.

Lars

[retiredcaps]

It meassures perfectly, but it is still abit itchy when "idling"?

Please explain the above with an example.  For instance, when shorting out the probes, I get a reading of 0.2, 0.1, 0.2, 0.1 in 10 seconds.

[Http 418]

It meassures perfectly, but it is still abit itchy when "idling"?

Please explain the above with an example.  For instance, when shorting out the probes, I get a reading of 0.2, 0.1, 0.2, 0.1 in 10 seconds.

I borrow a fluke 75, that be really well look after, mint condition when I put them side by side, the mint one jump one number on the last digit when not shorted, sitting on the desk. Where my 75 goes up and down alot quicker and it goes alot  further, all the way to the second to last digit jumps up and down.

I dont really know if it is a problem, but the mint one just seems more relaxed mellow but if I start measuring they both are dead on and swings the same amount.

Lars

[ModemHead]

Sounds like progress!  It is normal for the display to "wander" a bit with open leads while on a voltage range.  But if the leads are shorted, it should zero out.

There is a shield on the back of the meter that is grounded to the board via a little spring around the small screw that is underneath the fuse.  Make sure the spring is there and the contact surfaces are clean.  If not, it might be picking up noise.