Shielding the UT-81B oscilloscope

About three years ago I brought an UT-81B multimeter/oscilloscope. It is not a high range oscilloscope, it is more like a low range / "toy" oscilloscope but it has worked very well for me, making a great job solving problems with my Geiger counters and gamma spectrometers.

But it had a small and somewhat annoying problem: While in stand by, a pulsating signal appeared on the screen.


The signal was weak and it was reduced when the probe was connected to the multimeter, but sometimes, when searching for small signals, it was a big problem. I searched a lot in the net for other people having the same issue, but it seems I was the only one person having this problem.

A few tests proved the problem comes from the LCD screen, a Hai Nan HQTG160160A1. It uses some kind of DC-DC converter and its noise was picked by the oscilloscope's input. At first I thought in the back light circuitry, but turning the back light off did not solve the problem. The problem seems to be a DC-DC converter in the LCD panel used to get some negative voltage.

Once opened, I tried to add shielding to the back case using some adhesive aluminum foil, but nothing changed. Then I tried to solder meter's GND and LCD's GND directly with a sort piece of wire, but results were worse indeed: Now a big noise were picked by the oscilloscope. I suspect I did a ground loop.

The next thing I tried was to add shielding between the LCD and the meter's main PCB, but again nothing changed.

At last, I tried to shield the flat cable that connects the LCD to the main PCB. I shielded it just by adhering the aluminum foil tape along it only by one surface, while making electrical connection to the LCD ground at the LCD side only:

All the aluminum foil you see is connected to LCD ground at the upper left corner only. Surprisingly this worked just fine:

So I'm happy. The UT81B is not a top range oscilloscope, but works fine for signals up to a few MHz (and hundreds of volts!), and as a bonus, it is also a complete multimeter. You can't ask much more for about 100 euros, shipping included.