My MDO3014 oscilloscope seemed to work fine with the high impedance probes but when I checked the inputs at 50 ohm with the fast risetime pulse generator from Leo Bodnar (link) the traces did not look right… three of them clearly showed ringing:
Checking the input impedances
The first thing I did was to measure DC resistance with the multimeter, and that was enough to understand that someone had damaged the inputs, something pretty common. Both the 75 and the 50 ohm internal terminations were giving values far from the expected ones. Even CH3 was not 100% right !
I also wanted to look at the 50 ohm terminations up to 500MHz with the VNA… and it confimed the results. Channels 1, 2 & 4 were terrible, similar to this:
As expected, CH3 did not look as bad as the rest but still not as good as it should:
Replacing the resistors
Time to open the scope and find the damaged resistors. I was not expecting to find 50ohm and 75ohm resistors, but I wasn’t ready to find such a complex circuitry either (I took the picture after replacing the resistors). It was a lot of fun:
Just by visual inspection under the microscope, it was easy to spot the damaged chip resistors. Some were so badly burned that it was not possible to see the marking, but I was able to figure out all the values by looking at the corresponding place on the other channels:
The new input impedance readings
I repeated the resistance measurements of the four inputs with the multimeter and everything seemed ok:
I also repeated the VNA measurements on all four channels, and the results looked fantastic, in all of them:
Final check with the pulse generator, and all traces looked great!