Fluke 8840A/AF Blank display, 6 right side keys , no response, no relay clicking

[Paceguy]

Hi everyone. I have this Fluke 8840A/AF that when powered up has a blank display and the 6 keys on the right side for self test, rate, offset, etc. have no response since I don't hear a relay clicking when they are pressed. When range and function switches are pressed ,relay clicking can be heard. All supply voltages measure within tol and show no ripple when scoped, all power supply caps tested esr OK. Which chips could be the culprit? Oh yes, I reflowed all the transformer pins too. Thanks in advance !

[bdunham7]

None of those six buttons will give you a relay click right away.  If you hold the SRQ button down for 2-3 seconds you'll get a click and then a few more clicks over the next 20 seconds or so as it goes through it's self test.  The first things to check for no display is whether the VFD filament has voltage and whether the ribbon cables going to the front panel are connected correctly.  Do you have the front of the meter disassembled enough so that you can see the VFD tube?  The most likely problem by far for no display would be a bad (usually broken) VFD tube.

[Paceguy]

Even if I hold the SRQ for a few seconds, I don't get the relay clicks from it going into the self test sequence. The VFD tube looks good. All the connections to the display board have been checked and the ones that are soldered were re-soldered. Same with the two filament  pins on the power transformer. Could it be U212 ?

[bdunham7]

You need to do more diagnostics before you can conclude that the processor has failed!  Sure, it could be U212 since pretty much everything depends on that, but the fact that you do get responses to other buttons seems to indicate that it is not dead.  The display multiplex is also used to scan the keyboard.  If you look at the attached picture, on the right side there are the digit selection lines G0-G7 and then the segment lines Pa, Pb.....DP.  They both are active high (~30V) and if the digit and segment lines are both high, then that segment should light.  You need a scope to look at those lines to see if they are active or not.