Fluke 8846A Repair

[mdszy]

I've recently acquired a broken Fluke 8846A that I'm trying to repair. The failure mode is as follows:

The front panel inputs don't work at all. Either they show "overload" or the screen is simply blank. The rear panel works, but only kinda. It has odd behaviors such as showing double the voltage that is actually existing at the input (shows 50V when 25V is being applied, for example) and showing very strange results for the diode measurement, like showing 4V across shorted probes.

I've been checking out some other repair threads of similar units, and all the opamps on the unit seem to be in working order, and the LM399 is working fine. However, the LT1394 on the unit (U50) is behaving strangely. In the units current setup, 1V test signal being applied, manual triggering, during idle the IN+ of the chip is at 15mV, IN- is at 0V, Q is at 3.6V and Q' is at 150mV. The chip is being supplied with +/-5V at its +/- voltage inputs. This 150mV as a "low" output seems high to me.

Is this chip a potential culprit? Or is it more likely something else?

EDIT: Through further testing, I've found out that this chip isn't the problem. Shorting its inputs makes the output change to this "low" 150mV value.

I've also tested the Op-amps on the unit and they all seem to be functioning.

Thanks!

[dacman]

Try cleaning the Front/Rear switch contacts.  It is about the only "repair" I've done to 8845A/8846A DMMs.  A quirk with these DMMs is that they will continually flash "Overload" in the default power-on state.

[mdszy]

Try cleaning the Front/Rear switch contacts.  It is about the only "repair" I've done to 8845A/8846A DMMs.  A quirk with these DMMs is that they will continually flash "Overload" in the default power-on state.

Well I'll be darned if you aren't a wizard. I don't have any contact cleaner with me, but I just disassembled the switch, cleaned up the contacts a little and lo and behold it works!

Thank you so much! I'll still hit it with some cleaner to make sure it stays working, but I'm super happy it was such a simple fix!

[coromonadalix]

lucky you to have found a beast like this, i hope you saved lots of cash.

[mdszy]

lucky you to have found a beast like this, i hope you saved lots of cash.

Yep, got it for 78USD shipped, actually! An absolute steal!

[coromonadalix]

arrrgh    lucky b@$@%  loll 400$ offer  loll

[dacman]

Is the front panel of the 8846A labeled with 100 mA or is it labeled 400 mA?  Fluke used to offer a firmware upgrade that would give the original 8845A/8846A a 400 mA range (and a Ratio function).

[mdszy]

Is the front panel of the 8846A labeled with 100 mA or is it labeled 400 mA?  Fluke used to offer a firmware upgrade that would give the original 8845A/8846A a 400 mA range (and a Ratio function).

It's labeled 400mA and also has a ratio function!