Steppers only work intermittently if I jog two directions at once

I was attempting to add a simple tachometer today. It is a photo interrupter and I tested it and it worked fine. I was running it off 5v power from my mesa 7i76 and it was inputting into A+. I also added a volt/amp meter (crappy ebay one) on the same 5v and gnd. I installed it and tied the cable to the spindle motor cable. Everything looked good until I turned on AC power to the motor controller and got the pop of a short circuit. The screen went dead on the volt/amp meter and I shut down the power. I pulled the cables all apart to see if I could get power back. While I get 5v power off the 7i76 and all my LEDS on the board and stepper drivers indicate all is well but.....

When I attempt to jog any axis I get nothing. If I attempt to jog 2 axis at the same time I will get intermittent activity from both but it is jerky and a quarter turn at best of the lead screw. Any thoughts on where to get started with the diagnosis would be great. I don't care about the tach or the meter, they were all after thoughts anyways.

Here is what I know:
Stepper power supply is fine
When I jog an axis the voltage on pul+ to pul- goes from .6v to 1v in one direction and .6 to .8v on the other
When I jog an axis the voltage on dir+ to dir- goes from .6v to 1v in only one direction
I have 5v power off the original pin I was using for the tach
The limit switches all work fine also the E stop

I am going to continue to dig around but would appreciate any advice pointing in a more specific direction.

Thank you

Just so I follow correctly. I want to measure pins on the db25 of the 5i25. And I am referencing direction pins 1-5 and step pins 6, 14-17 to any one of the ground pins 18-21?

And pins on the female 25 read right to left correct?

Too small for my eyes to tell that was there. Linux CNC would not boot without that 7i76 (which was out for repair) so I never got around to testing this. Fortunately the 7i76 came back this afternoon from Mesa repaired. They replaced 74ACT540 and ISL32273 and now everything works. Thank you for all the help and suggestions.