Keysight U127A faulty MegaOhms mode

[KamKong]

Hey guys,
Long time lurker 1st time poster.

I recently purchased an Agilent U1272A (pcb Rev 005) from ebay. I didn't carefully read the add, the seller didn't offer returns of the unit and I was away for work when the unit was delivered.
When I started testing I realized there is only 1 (so far) issue with the meter. In Ohms mode it won't read anything on the Megaohms range and it won't auto-range out of the Megaohms range.
At first i thought the ohms mode was faulty but it seems to just be the Megaohms range.
When the meter powers on and I short the probes together the meter displays Overload, but if I manually range the meter to any range besides Megaohms it will correctly displays 0 ohms (or the correct resistance value).

I dissembled the meter and found residue from battery leakage, i cleaned the board with vinegar, isopropyl alcohol.. nothing doing. (attached picture is of battery leakage residue)

The meter works in continuity and smart ohms mode.
I checked the spec sheet and measured the test current for each range of ohms mode and everything seems ok (the current for the Megaohms range is 93 nA, my other meter is a Keysight U1242C and doesn't read that low but it seems to pick up something...).

I traced the signal path on the board and if I short some test points (bypassing the input jacks) I can get the display to flicker to 0hms but it doesn't stay there and doesn't seem to be conclusive.

I don't know if anyone has seen/had a similar issue? Thanks in advance.

I also messed up a solder job on the meter and need to replace C35, I read around and it seems to be a ~10pF, 3 kV, 1808 case size can someone confirm?

[jchw4]

That's some serious corrosion here and it looks like an exact reason for the fault.

So it could be either a leakage (i.e. a new current path) or a corroded trace or via (i.e. a missing path).

I would first clean up all the corrosion making sure there is none left under the ICs.

Personally I would submerge the PCB in vinegar (regular 5% or so) and leave it there for a 2-3 minutes. It's risky, though, but the corrosion is also pretty bad.

Then blow the liquid away with compressed air. Wash in tap water, blow with compressed air again. An then leave fully submerged in pure 99% alcogol for a few days. I personally found kitchen ziplock-style bags to be very useful here.

If it does not help, I would start hunting for some coroded trace. I.e. taking photos, tracing as usual...

[KamKong]

Alright thanks for the tips, it's soaking in the alcohol now.

Well see how it goes!