Hi all,

as my recent time-waster electronix project I'm troubleshooting a Mitsubishi HS-M55 VCR which actually works fine expect for a seemingly dead vacuum flourescent display.

I checked all the voltages on the VFD, and they are within spec: the cathode filament is driven by 4VAC straight from the PSU and biased to -30VDC, while the anode is grounded. There's activity on the segment pins between -30V and GND, so the drive voltages look ok.

For reasons beyond me, there's no voltage across the filament; when I unplug the display, I measure the 4VAC on the connector coming from the PSU, but when I hook it up, the 4VAC is nowhere to be seen on the filament -- only the -30VDC bias. Not suprisingly, the filament doesn't glow.

But when I apply about 6-8VDC across the filament from an external PSU, the VFD lights up!

Since there appears to be nothing wrong with the drive voltages, I'm guessing I'll simply have to tweak this thing. Is there any simple way to apply the extra 6-8V from the PSU without blowing it? (Simply tapping the 5VDC supply, for instance, would surely short the PSU, since the cathode is at -30VDC).

Thanks for any hints,

--Roland