It sounds as though a pin, either the Reset or Power-On Self-test (POST), is now getting a strange signal since the repairs.
Look for:
* fractures in solder at pins of the main IC. A disconnected pin can be unpredictably affected by ambient mains hum, or static charges. A pin which is held continually positive or negative may prevent the meter from operating.
* Selector dial making faulty connect (or disconnect) to circular traces beneath it. May be due to dirt, fluid, misalignment, missing springs or ball bearings.
* Intermittent battery contacts or wiring.
Try connecting the red and black leads together as you switch the meter on. Or a stable voltage source as you switch it on. Or stable resistor as you switch to the ohm ranges.