Repairing a Mettler PC440

2025-03-27

Mettler PC440

After having a small collection of various old Mettler scales, I recently decided to get this Mettler PC440 DeltaRange series scale that was declared broken.

It didn’t turn on, but I could hear humming out of the case. After leaving it running for a bit and warm up it suddenly started working. So clearly whatever the issue is, it solves itself with temperature or power-on time - intriguing.

Troubleshooting

So I opened the balance up and checked the power rails, which looked fine on the multimeter. Then I started probing around with my oscilloscope and noticed the +5V rail dropping by 2V regularly. The longer its powered on the less the drop and once the drop was at 0.5V-1V it started working. So we are looking at a decoupling issue. I then noticed the ancient 2200uF ROE storage capacitor from which the +5V rail is derived getting a little warm. Bingo!

I replaced the 2200uF capacitor temporarily with one I had sitting around and the balance fired up right away when turned on.

Working balance

Mechanical fix

Next thing I noticed was the guide plate for the solenoid was broken off. I put a little bit epoxy on it. It does seem to hold for now.

Mechanical Fix

Proper repair

Now that I identified the electrical issue I decided to repair it properly using more quality parts.

I replaced the two storage capacitors with new ones from Vishay. The black film capacitor (2.2uF/63V) had a white powder stuff on it, so I replaced that one as well.

I accidentally touched one of the 15uF tantalum capacitors with my soldering iron, so I replaced that one and the other one as well just to be safe (with electrolytics). Since these are connected to signal lines of a 7400 series chip and I dunno whether it likes those being shorted out (probably not).

I also added some thermal paste between the heatsink and the case.