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!