Repair Help needed for Fluke 177 dmm

[nikosgr]

Dear Community Hi this is my first post here.

I am trying to troubleshoot a used Fluke 177 dmm with no visible damage on the input front end stage (ptc is ok. (measuring 1,017 kohm) fuseble resistor is OK (measuring 1,0 kohm) mov"s are also OK (measuring OL in both directions after been de-soldered from the pcb)) additionally mode switch has been cleaned & new battery has been used.

Problem description:
Unit wont pass initial start up status (all digits and operation indicators in the LCD display "lets say" that are stack) and the meter is not passing to any measuring mode by rotation of the switch to any measuring mode.


Any suggestions on how can this be fixed?         
   

[shakalnokturn]

Have you been through the usual first steps?
Power, reset, oscillator, I²C bus...

[nikosgr]

Dear shakalnokturn Hi,
 
Thanks for pointing to the right direction.
 
Power: any particular point that i can check on the pcb?
Reset: if i am not mistaken this is done via the press of a dedicated baton located on the upper back side of the pcb.
Oscillator: to check if the oscillator on the pcb located on the bottom of the lcd outputs a signal or not?
I2C: i do not have a logic analyzer option on my oscilloscope.

Thanks for your help and support

[shakalnokturn]

I've never had to repair a Fluke 177 so I'll only be giving outlines.

Unortunately I don't know of a schematic available for this model or any other Fluke using the same IC's.

For power in most cases you can expect a +5V or +3.3V for the digital side of things. (CPU pins 38, 87 download HD6433844 datasheet)
The analog (smaller QFP IC) will usually require an additional symmetrical supply somewhere between +/-1.5V and +/-3V.
The quick way to look for the power rails is to measure on the tantalum capacitors and larger SMD ceramics relative to the COM input socket.

As for reset I was not clear enough: I meant Power On Reset to the CPU. (Pin 15 must not be stuck low.)

Oscillator check with an oscilloscope (and suitable probe) for right frequency and sufficient amplitude.

For I²C if all previous points seem good, a sanity check with the oscilloscope should be good enough, you're mainly looking for bus activity on SCL/SDA with sufficient amplitude  for CPU.