Well it's taken almost all evening (not quite, I've been to my youngest lads school performance, plus cooked tea for me and my eldest) but I finally discovered why I couldn't get the HV circuit to work...
After testing, and replacing anyway, all the transistors, testing the diodes, trying various open/short circuit connections, trying different bias resistor values, and even testing the timing capacitor(!), it was while trying the different bias values I spotted my mistake! The 100k resistor from the flyback diodes anode, was connected wrong - it was connected to the oscillator transistors emitter! Literally, on the wrong end of the emitter 1k resistor. It should have connected at the 'supply' end. You can easily see in the photo above of the board, where I have had to change it for a fresh one with long enough leads to reach the right connection point.
I'm very pleased to have got it working, as this board has been hand milled using my Dremel, so a lot of physical effort has gone into its making. I was starting to think I would have to abandon it.
With the neon lamp in the regulator chain glowing, and the ammeter now showing 1.2mA rather than 35mA (itself an improvement on the 1.8mA I was getting on the breadboard, probably from changing the BC327 for a BC212L with more gain), I'm now going to leave it until tomorrow to add the indication circuitry.
This circuit now has a random mish-mash of transistors in it, rather than the nice complementary pair it was designed with - a BC212L as oscillator, MPSA42 as switch, and BC337 as feedback. The feedback transistor may yet be changed again, the current design uses collector control feedback. When my supply of high-gain MPSA18's arrives, I may change this for base control feedback, to see if I can lower the current drawn even more.
Regardless, its late now, and I'm quite tired. Since towards the end of fault finding I noticed myself rather too casually holding the board by the G-M tube anode clip (in other words - the place that should have had 400V on it!) it would probably have been a little too risky to work on it anymore tonight!
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| Working HV circuit |
I'm very pleased to have got it working, as this board has been hand milled using my Dremel, so a lot of physical effort has gone into its making. I was starting to think I would have to abandon it.
With the neon lamp in the regulator chain glowing, and the ammeter now showing 1.2mA rather than 35mA (itself an improvement on the 1.8mA I was getting on the breadboard, probably from changing the BC327 for a BC212L with more gain), I'm now going to leave it until tomorrow to add the indication circuitry.
This circuit now has a random mish-mash of transistors in it, rather than the nice complementary pair it was designed with - a BC212L as oscillator, MPSA42 as switch, and BC337 as feedback. The feedback transistor may yet be changed again, the current design uses collector control feedback. When my supply of high-gain MPSA18's arrives, I may change this for base control feedback, to see if I can lower the current drawn even more.
Regardless, its late now, and I'm quite tired. Since towards the end of fault finding I noticed myself rather too casually holding the board by the G-M tube anode clip (in other words - the place that should have had 400V on it!) it would probably have been a little too risky to work on it anymore tonight!
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