Tonight I fired up the high voltage power supply and had a go at getting this EM84 display tube going.

+250VDC on the anode, The grid is being operated between -20VDC and 0VDC. The heater on this tube is 6.3V.

Positive terminal on an isolated bench supply is connected to the ground of the +250V supply to give negative voltage required to drive the grid. The negative is fed into pin 1 via a 1meg resistor to ground.

As the grid voltage aproaches 0V, the shadow widens. I put up this video demonstrating adjustment of the grid.

Had a small win with some Logitec speakers today. They were exhibiting poor volume and distortion. My first guess was caps, they looked sound, but were cheap crappy caps. I re-capped it anyway, with no result. There was also a suspect SOT-23 transistor which drives the power LED, when touched, the distortion changed. I deleted the LED circuit and connected the LED with a resistor direct to the power rail. No love. Then I noticed that touching part of the board near the power switch caused the volume to increase and the distortion improve. I shorted across the power switch and the unit came to life. I didn't even suspect the power switch, it is a sealed self-contained unit which is built into the back of the volume control. So I shorted the switch with a blob of solder (no chance getting a replacement dual gang + switch unit). Now the unit works great. Just with no off =)