OK, all of the obvious culprits have been ruled out so now we have to consider weird things.
Wish you had a schematic, but it's not mandatory. 1. With your scope, do you see similar on/off times at the base of each NPN 'predriver' ? It should be in the milisecond range. Trying to rule-out the possibility of the control logic 'skipping-over' the tubes that aren't lit. 2. This might be risky, but you could try momentarily connecting the anode of the bad tubes to the anode of a good tube. You may want to play it safe and use a resistor of a few K-ohms instead of a short. If all 4 tubes are off, there's a low-impedance path somewhere. 3. Connect a 10K resistor to +170V supply. This will not damage the PNP, and assuming it takes 150V to illuminate the tube, the 10K resistor will limit the current to a safe value of 2mA (1 mA per tube). It wont be a strong glow, but you will see it if the tubes are alive. Connect a voltmeter to the other side of the resistor, and GND. Next, momentarily touch the resistor (voltmeter side, not +170V side!) to the dead tube's anode. You should see the 2 dead tubes glow, and see around +150V. If you see significantly less voltage, or zero, there is a short or low-impedance path somewhere. It could be the voltage divider, or a bad tube. You may need to unsolder one or both anodes to confirm If the voltmeter still reads ~170 when you touch the tube, and there's no glow, there could be an open somewhere. You'll need to buzz-out the connections between the tube's anode pin, anode resistor, and PNP. If that still fails, then I see no other culprit besides the tubes. -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To unsubscribe from this group and stop receiving emails from it, send an email to neonixie-l+unsubscr...@googlegroups.com. To post to this group, send an email to neonixie-l@googlegroups.com. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/12435ea0-c2ee-4293-9533-ec01403e4729%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
