Tubes go dark after reset calibration

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3 months 3 weeks ago #10247 by w8an
I have a Rev 5 kit that I built a few months ago. I am now ready to implement the electronics into a cabinet that I have been working on. The clock had been working ok the last time I fired it up.

Today I applied power and it was struggling to light the Nixies. I could see some activity on 3 tubes, the others flickered occasionally. I measured the HV and it was 158v which is a bit low for the National NL-841 tubes I'm using. After reviewing the schematic and builders manuals, there doesn't seem to be any manual adjustment to the high voltage, but a reset should launch a high-voltage calibration. So I did that.

Upon reset, five of the tubes go into a "glow mode" then a "digit mode" switches in and all 6 of the tubes show a number that changes. It seems to alternate between the glow mode and digit mode which I suspect is the calibration method running. Eventually it stops that and all the tubes go dark. At this point the HV reads 18.6V.

There is no device attached to the LDR input, it is floating. I had no problems with this thing a while back but am now wondering if the missing LDR device is a problem. Can I connect a variable resistor here? I'm not really interested in auto-dimming.

Also, I do have the Wifi module and see no setting that would affect any of this behavior.

Any insight is appreciated,
Steve

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3 months 3 weeks ago #10249 by Ty_Eeberfest
LDR should have no effect on the HV calibration since the program explicitly disables it during calibration. Thus, LDR or no LDR the tubes should be lighting up at high brightness while the HV calibration is doing its counting routines. After calibration the LDR mode may come on in which case you'll want to turn it off via the WiFi web interface,

The very first thing I would check is your external power supply. If supply voltage is sagging below 9V the HV generator won't work. If the power supply checks out (under load!) then the HV generator circuit needs looking at, probably starting with the MOSFET.

Look into it later when the dust is clearing off the crater.
The following user(s) said Thank You: Ian

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3 months 3 weeks ago #10260 by Ian
Also, please note that in test mode (all digits "00:00:00" then "11:11:11", "22:22:22" etcetera, the HV turns off after 60 seconds. This is a protection measure.

I don't think that is what you are saying, but just to mention it,

Also 18.6V is a weird reading. Are you measuring with with a meter or an oscilloscope? What is the input voltage you are using?

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3 months 3 weeks ago #10265 by w8an
My power supply is a 12 volt 1 amp wall wart. Measuring the input at the clock board I read 16.4 volts. The Vcc on the board is 5.015 V.

I ran the calibration again and now the tubes do light, poorly, and there is unstable high voltage. It seems to fluctuate from about 165 to 200 volts at about 1.5 Hz.

I have taken a video of the high voltage waveform on the scope and also of the tubes.

The videos can be seen here...

Oscilloscope trace of high voltage: https://youtu.be/c-7G93qsF7I
Nixie display tubes: https://youtu.be/dKPX9li37qY

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3 months 3 weeks ago #10266 by Ty_Eeberfest
Just noticed I didn't answer your original question about dimming completely. So: Yes you can put a variable resistor in place of the LDR and use it as a brightness control. The exact value isn't critical. I'd try a 10K pot and see if the response looks good.

Anyway... thanks for the clear and useful videos. I can see what it's doing but so far I'm not sure why it's doing it. You didn't modify the firmware or any of the HV related config values did you? Because it looks like the firmware may be "hunting" while trying to bring the voltage exactly to the setpoint. The waveform looks a lot like an under-damped control loop. I've only ever seen this when I was fooling around in the code trying to improve the tuning of the HV control loop. I'm now trying to think of any way a hardware issue could cause this but so far no ideas. Maybe Ian can see something here that I'm missing.

Meanwhile, could you put your scope on the MOSFET gate and see what you get? I'm expecting to see PWM with duty cycle fluctuating at the same rate as the HV voltage fluctuates, but let's make sure.

Look into it later when the dust is clearing off the crater.

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3 months 3 weeks ago #10267 by Ty_Eeberfest
And one other thing... you say your allegedly 12V wall wart power supply is drifting up to 16 volts, presumably under no/low load. Sounds to me like it is not regulated. See if the input voltage to the clock board (VIn, not 5V Vcc) is swinging around when the HV is swinging up and down. Or try powering the board from good solid regulated 12V supply (got a bench power supply?) and see if the clock acts better.

Look into it later when the dust is clearing off the crater.

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