TDS 744A Attenuator Problem

Hey,

first, I want to apologize, if I missed a thread, that already addresses this problem, but I had a hard time to find one.

Two of the four channels suddenly stopped working correctly. A very careful guess is over voltage: Might be, that the combination with a hidden inductance and a 1x probe caused the issue, but that's not sure.

See the attached images. Channel 2 and 3 should show the same test signal as channel 1. So only a weak AC signal remains.

I switched the attenuator modules (H2462G3). After that, all channels run correctly. Another day the error came back and showed the symptoms, but the affected channels have been switched according to switched attenuator modules. Visual inspection did not show a problem on the attenuator modules.

Does anyone know this kind of symptom and what could cause this problem?

Might be, that the 50 Ohms series resistor is the problem:

https://w140.com/tekwiki/images/f/f9/H2462_schematic.png

This would explain, that an AC signal is getting through?

Is this resistor placed behind the shield?

https://w140.com/tekwiki/images/e/e5/Tek_h2462g.jpg

Hi,

I've seen similar failures before, the contacts go bad and it leads to incorrect configuration (wrong amplitude) or AC coupling, like you see there.

It's just the relays of the attenuator network wearing out and going bad, a replacement would be ideal but if that's not possible, you could try very carefully cleaning them with alcohol or even just cycling them a few times. Tapping on them works as a temporary fix often times.