Odd Moving Meter Failure on Farnell LT30 PSU - Hand Changes Movement - Page 1
Forum section: electronics / repair

author: killingtime
author_rank: Regular Contributor
timestamp: April 15, 2025, 01:29:37 pm
content: Hi,I've got a Farnell LT30-2 bench power supply. The ones with two large moving meters. Linear and old.Noticed that one of the meters was reading around 3v with the power off (30v FSD).While I was trying to get the clear meter cover off, I noticed that when I placed my hand about 0.5cm from the meter (with the clear plastic cover on) the meter pointer moved about 10mm. Moved my hand away and the meter pointer fell back to 3V. All this with the psu unplugged. To be clear, my hand was outside the meter with the clear cover on.Waved a magnetic tip screwdriver near the meter in case it had become magnetized somehow, hardly any deflection. It's just my hand that deflects the pointer. Grounded the meter and myself. Same issue.Pulled the meters off and removed the clear plastic cover. Meter falls back to 0.Put the clear cover on again and it jumps to 3v.Put the clear meter cover about 5mm near to the meter movement (but not on the meter) and it jumps to 3v.Move my hand near the meter with the clear cover off - no deflection.So I'm thinking electrostatic charge on the clear cover.Wash the cover with soap and water, dry with a towel and replace. Movement stays at 0v. Hand movement near the meter does nothing. Come back 30 minutes later and the issue has returned...This is a new one on me.I don't think it's the movement. There are two meters. One with the issue and one without. Powered both meters with a DC bench PSU via a 10K resistor and both read the same voltage - if you leave the cover off the problem meter. Both return to 0 when disconnected. Repeatable.Pictures below showing deflection with the clear cover on, and resting at 0 with the cover off.Meter hit FSD every time when connected to the psu, and returns to 0 when discon - cover off.Any ideas?Thanks.

author: TimFox
author_rank: Super Contributor
timestamp: April 15, 2025, 03:49:35 pm
content: As I remember, that problem was endemic with plastic-faced sensitive microammeters.Glass-faced units were less prone to the electrostatic charge issues.

author: Gyro
author_rank: Super Contributor
timestamp: April 15, 2025, 04:04:45 pm
content: Yes, static buildup on those plastic meter fronts can be a real pain if they've been rubbed/polished clean. Try removing and washing with some dilute fabric conditioner (the sort that stops static cling). It leaves a conductive enough film to discharge and prevent static build up. Dab dry / air dry rather than rubbing.

author: killingtime
author_rank: Regular Contributor
timestamp: April 15, 2025, 04:15:48 pm
content: OK.Put a ground wrist strap on while washing, drying and assembling the clear cover. Seems to have worked. I was also walking around with the cover between drying and assembly before, as the sink wasn't near my bench. I must have been picking up static charge while walking about.I still don't know how electrostatic charge got onto the meter cover in the first place. Only cause I can think of is the supply is stored in a room with wool carpet wile not in use.I've blown mosfet & cmos components while handling without a ground strap before, but never a meter. A 3v deflection on a 30v scale is 10%.Learned something new today.

author: Poroit
author_rank: Frequent Contributor
timestamp: April 16, 2025, 09:21:05 am
content: G'day killingtime,I worked at a company that manufactured those type of meters. Definitely Static as the other Members have pointed out.We used "Anti Static solution" which was basically a mix of Washing Up Liquid and water.Wipe it on sparingly inside and out but don't polish to hard.