Rigol DS4014 mis-identifies probes

[sergey]

Hello everyone.

The symptom of the problem I am experiencing with my DS4014 is that sometimes it mis0identifies plugged probes. The probes are Rigol RP3500a (10:1, 10Mohm input impedance, for 1Mohm inputs). Sometimes the probes are identified with a wrong division ratio, sometimes scopes switches input to 50ohm termination.

The annoying part is that it is hard to see an exact pattern. Sometimes it is channel 1 which has the issue, sometimes channel 3, maybe others but it's been a while since I saw that happen. Sometimes there is even some "cross-talking' going on: for example, plugging probe to channel 1 makes channel 3 to detect a probe.

Currently it is the channel 1 which causes an issue, with few possibilities:
- Probe is detected as 100:1
- Probe is detected as 10:1, but into 50ohm input
- It also sometimes makes channel 3 to detect 10:1 50ohm probe, sometimes 20:1 into 1Mohm input

Plugging one probe to channel 3, then another to channel 1 makes both of them to be properly recognized. However, unplugging probe from channel 3 is not detected. So, something is still quite bizzare.

The probes themselves seems to be fine or, at least matched in behavior: currently all probes are properly detected in channels 2,3,4 and all probes are mis-identified in channel 1. The probes also work fine from the RF perspective.

Not sure if it is software related, or hardware related. The scope is running the latest available firmware. From the hardware part I hasn't been able to check: a big part of the board in under a heatsink/shield which seems tricky to be removed (possibly due to thermal paste). Didn't feel like prying it open just yet.

The scope is probably around 10 years past the warranty period, and is discontinued by now. Probably would not worth trying contacting Rigol for the repair.

The easiest fix is to simply cover the auto-detection ring with a kapton tape, and configure probes good-old manual fashion. But I wanted to see if maybe someone else experienced something similar? Or have some other ideas?

[sergey]

Managed to get some extra information about voltages on the identification rings under different conditions:
- Without any probe connected the voltage on the ring is about 3.485 volts
- On channels 2,3,4 the voltage with a probe attached is about 0.218
- On the channel 1 the voltage with a probe attached is about 0.437
- Connecting probe to channel 1 makes the voltage on the identification ring of the channel 3 to drop to 1.8 volts, and then it slowly recovers

There are also appears to be some ramp shaped (fast raise, slower decay) pulses appear on the identification rings when any probe is attached. Not sure if that's some sort of part of capacitance measurement, or pulses from DC-DC converter or something. They probably happen twice a second.

So, guess that could expand the question to the lovely community:
- Is the voltage on the identification rings supposed to be steady with a probe attached, or some pulses are expected?
- Did anyone looked into the related parts of circuits before? Maybe there are high-res photos or schematics available?