Philips GM6012 AC millivoltmeter

On the quarterly radio flea-market of the NVHR in december 2001 I bought this GM6012/2. It looked in good condition. Not much scratches, clean. This AC millivoltmeter was built in the early sixties. It sports a frequency range of 10 Hz to 1 MHz. It has 12 ranges, from 1 mV to 300 V.

The GM6012 has a 6-stage wide band amplifier. The first stage is a cathode follower, so that the attenuator after it can have a low impedance, which makes frequency compensation for the attenuator much easier. The GM6012 contains a total of 10 valves:

• 4 EF80 penthodes in the pre-amplifier
• 2 EF80 penthodes in the output amplifier
• E88CC as oscillator for test signal
• EZ80 rectifier
• 85A2 as voltage reference
• PCL82 as voltage regulator

When I tried my new asset, there were a few surprises: the meter showed a lot of signal on the lower ranges - about full-scale on the 1 mV range. This probably indicates a bad hum somewhere. Furthermore, after swinging to the right, the needle of the meter got stuck at about 80% of the scale. That looked like a mechanical failiure.

I decided to try to fix the meter first. After all, if that would fail, there would be no point in fixing other faults. I had to disassemble the case first. T he case consists of a number of aluminium panels with a grey leather look plastic coating. One of the sides was jammed. It looked like the front had shifted a mm, maybe as a result of a fall. I was able to take the case apart after loosening a few screws on the frame. After that, I took off the knobs and the front. Then I could take out the meter. I unsoldered it and took it in my hand. Nothing to be seen. So now I had to open the meter. I cleaned up my workbench, remembering an occasion where I got some iron filings in a meter, that would stick to the poles of the magnet and were impossible to remove.

Getting the meter movement out of its bakelite case, I still couldn't see anything that kept the moving coil or the needle from moving back to the zero position. The needle didn't touch the scale. The coil seemed to be in the right position in its bearings, there was no dirt or iron particles in the space where the coil is. I thought there was a lot of room in the bearings, but I couldn't decide if this was too much. Anyway, judging by the position and form of the control springs, the meter coil was in the right position. The distance between the coil and the cylindrical core within the coil was slightly asymmetrical. Maybe the core was out of position. But there was nothing to adjust it.

It was supported by an aluminium support, pressed between the pole pieces. I decided to loosen the screw that keeps a small piece of metal in place across the magnet pieces. I thought this was a magnetic shunt, to set the meter sensitivity or linearity. Strangely, now the meter could move more freely. I screwed the magnetic shunt back in place, but the needle still was free. Even after reassembling the meter, it was ok. Apparently the meter has suffered from a fall. My loosening the screw may have released some mechanical stress and allowed something to fall back into place. Information on moving coil meters on Integrated Publishing or Open Door Physics website

But when I tried the instrument again, the meter still had something that disturbed the movement of the needle. And that terrible hum was still there. I'll fix it later.