How does the Fluke 789 recognize banana plugs?

[r0tati0n]

Hello all,

I just had to repair a Fluke 789 my company threw away because the already damaged plug now broke.

I noticed when testing the meter on Ampere measurement:

When nothing is plugged in, the meter displays mA with -25% on the lower half. pressing the blue button does nothing.

When test leads are plugged in (not connected to anything) the meter displays Amps and I can press the blue button for switching between AD and DC.

How does the meter actually detect if something is plugged in and switches the display?

Asking because I tested the replaced input jack receptacle by holding the tips of test leads into the port connected to a low ampere limited power supply and got no reading.
Only after plugging in proper banana plugs and holding the tips into the power supply I got the correct reading.

[wasedadoc]

I fo not know for certain how that meter does it but a simple, common method is that the socket is not a single contact. A split ring or similar which is bridged by the metal plug.

[Xena E]

Optical test lead detection is used on several Fluke meters.

X

[coromonadalix]

if the barrel socket jack is not split in two  if you see it directly, it is optically sensed ...

[timeandfrequency]

Hello r0tati0n,

I don't know how exactely how it works on the 789, but Fluke also uses 4 mm banana posts/socket with two separated contacts.
Here attached is an excerpt of the Fluke 1653 schematic.
The two contacts of the female 4 mm connector are identified as J1 & J2 and the detection sub-system is on the left.
When a 4 mm banana plug is inserted, the secondary of the T2 transformer is shorted.
Another sub-system of the meter provides the oscillator signal JOSC.
The JPD signal provides the information about an inserted banana plug.