Agilent U1272A DMM wrong resistance and temp readings

[andreathepsyco]

Hi everyone,

I recently strted having some issue with my U1272A. Unfortunately the battries leaked electrolites, i cleaned it up and put it back together but it started behaving oddly when measuring temperatures and resistance, other ranges seems fine.

Basically when measuring resistance with nothing connected it just show a variable high value (in the tens of Megaohmn) and seems to count down (see this video for the described behaviour ), when shorting the leads it seems to do the same but starting from about 3 Ohm and counting down. Checking with another DMM i can see the applied voltage during resistance measuring going down.

On the temp range it displays 46 deg C with 26 deg C ambient temp on a K thermocouple.

Should i consider it toasted or there's something i should try?

[EDIT]: Spelling

[mzacharias]

This would usually be due to contamination or damage near the input jack area. Sometimes a leaky or shorted protection device like an MOV for example. The meter is sort of measuring it's own input resistance, and since the temperature function operates in the millivolt range it does not surprise me that this is affected as well.

You can use another 10 megohm input impedance meter to measure the input resistance of the 1272; my guess it will be much less than the rated 10 Meg.

Obviously contamination and corrosion due to the battery leakage should be addressed if possible, sometimes the damage is simply too great.

[andreathepsyco]

Thank you for your reply, unfortunately that's not the problem. I measured the input impedance with the DMM in the dc volt range and it's greater than 10M, if this is really contamination causing a leakage current i'd have expected to see a lower value. Also this wouldn't explain whats'a happening when the leads are shorted.

I should add also that despite the problem with the batteries the pcb is in grreat shape with no signs of corrosion.

[pcwrangler]

Can you take good pics of the inside?

[andreathepsyco]

For some value of good... i only have my cellphone and not a proper camera.

Turns out i had missed a couple corroded spots, but reworking them made the only difference that now the displayed value without leads connected is stable (22M) and the shorted leads value is lower (1.5 Ohm). The two spots are the obvious one (back of ptc3) and one of the terminals of z02, near the contact spring. I attached photos of before and after.

[EDIT]: please tell me if the resolution is good enough

[andreathepsyco]

Up

[floobydust]

Check the PCB trace from BD1 going to the via near ZD2-K. It looks badly corroded and no solder mask left. The via goes to the rotary switch, it might be open-circuit too.
Did you check the rotary switch for corrosion too?

For a leaking battery mess I first use diluted vinegar (acidic) to neutralize the alkaline mess and then rinse and an old toothbrush or Q-tip and IPA to scrub off soolder balls etc.

[andreathepsyco]

Sorry if I disappeared, but work got the precedence. I didn't have the time to check the via yet, but I already cleaned everything with vinegar (I knew the electrolyte is manganese based and alkaline so anything acidic within reason works). The backside of the PCB seemed fine and the obvious signs of corrosion were one trace that I repaired and the area around the front shield contact spring. I'll try to check the via and related connection more carefully.