BM235 - Input error on Amps even with no probes connected

[Bacon]

Hello everyone. Having an issue with my BM235. About a week ago it began to signal an Input Error on every mode selection except for A/mA. This is regardless of the probes connected. If I remove the fuse for the Amps range, the error goes away. With the fuse removed, all other functions operate correctly. With the fuse in place, the A/mA measurement also appears to work without issue.

Any suggestions on diagnosis here? I'm very much an amateur and not familiar with jack detection circuits. I originally bought this meter from Dave directly, would love to avoid returning it!

Many thanks in advance.

[IanB]

The input error occurs because the meter thinks a probe is plugged into the A or mA socket. It seems there is some kind of internal short that is causing this to happen (maybe some debris lodged inside somewhere?). Have you taken the meter apart and examined the probe sockets? Try cleaning around them with a puff of compressed air and examine carefully for anything that might be shorting across the detection contacts.

[alm]

Could be that they use a photo diode to detect a probe plugged in. In that case, it could just be debris that is blocking the light. I second the opening up and cleaning suggestion. Pushing debris deeper inside the meter may not be ideal.

[Bacon]

Thanks for the suggestions. I have had it apart, but didn't remove the input board. Nothing obviously amiss. I'm surprised that the error goes away with the fuse removed, however. Makes me think that the issue is further up the circuit.

When measuring resistance between the Amps jack and common, what should I expect? Appears to be a dead short.

I'll take the input board out and see if there's anything obviously amiss.

[alm]

If the sensing is electrical, then the jack will likely consist of two isolated parts that are connected by the plug. Removing the fuse would disrupt the connection to one of the parts.

[floobydust]

Jack-detect seems to be done using a split banana jack, note there are 2-pins on the A and mA jacks. <I don't own a BM235 not super familiar with it.>

I'd take a magnifier and look down the tube for metal strands etc. shorting the sense. Maybe a PCB bridge or bad connection. BM235 teardown pics

[Bacon]

Solved!

I didn't see any obvious metallic debris in the jack, but a bit of suction and some alcohol and the issue has been resolved.

Thank you all for your help and suggestions!

[cs.dk]

Also note, that you can't use lithium batteries in the meter.. The 2x0,2V more is enough to upset the comparator; https://www.eevblog.com/forum/testgear/the-bm235-does-not-accept-lithium-batteries/