Digit Brightness, Classic Rev6, IN-8 Tubes

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6 months 3 weeks ago #12766 by Jedidiah
No worries, BS_Jim. Thanks for reengaging! Below is a trace of the digit 9 cathode when energized.

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6 months 3 weeks ago #12767 by Ty_Eeberfest
Here's a random idea: tube healing mode. It lights up the digit of your choice with NO multiplexing involved. This takes a lot of variables out of the equation. The anode for the selected tube is held on (+HV) constantly. The cathode for the selected number is also held on (ground) constantly. Thus the display is in a truly static state.

Activate healing mode per instructions on page 13 of the User Manual . Run through a few digits. How do your tubes look now? What voltage is being dropped across the 3K resistor of the active digit?

You can stop reading here if you want. Following is just me rambling on about why reading cathode voltages is a hot mess.

Also... I poked around in my clock some more and concluded that trying to get meaningful readings of cathodes is an exercise in futility, Consider that for each cathode all 6 tubes are tied together, There are interactions that are not well defined. When HV is applied to a tube's anode, the grounded cathode obviously lights but less obviously all unlit cathodes float up to some voltage where 0 < V < HV. What that voltage will be depends on stuff like the surface area of each cathode and probably some other stuff I'm not thinking of. In short, as the MUX scans thru the 6 anodes the voltage on any cathode driver pin jumps all over the place. To see this, pull all optos except the 1 second tube. Read from Pin 1 of the 74141 to ground. When the tube displays 9 you can see it pull down to ground but when other numbers are displayed observe the voltage bouncing around.

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

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6 months 3 weeks ago #12768 by Ty_Eeberfest
Okay, one more and then I'm out for the night.

Having re-read Jim's post about cathode readings and thought about it for a while I better understand what he was getting at: looking for a cathode to ground at the right time and not worrying about where the voltage goes at other times, With all but one tube pulled this would show if cathodes are getting solidly pulled down to ground. Judging by the scope trace you posted I'd say the cathode side of the circuit looks fine.

I'm also linking a couple Burroughs app notes that I haven't seen before. While they don't solve the present problem they do make for interesting reading about tube characteristics. Don't get too hung up on the "pre-bias" stuff. I find it confusingly written and it's not relevant to the situation at hand. All they are really saying is that your cathode drive transistors must be able to handle the voltage that unlit cathodes drift up to without experiencing reverse breakdown, and the 74141 (and K155ID1 clones) are fine in that regard. Pre-bias was probably more relevant when the app notes were written, before the 7441 was invented and they were trying to drive tubes with discrete (germanium? omg!) transistors!

Nixie App Note 1
Nixie App Note 2

No idea why Firefox is giving me security alerts when I try to download from those links. There is nothing wrong with the files.

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

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6 months 3 weeks ago #12769 by BS_Jim
Yeah Ty, I was just interested in seeing that the cathodes got pulled to ground and that some phantom resistance wasn't involved that we hadn't accounted for. The scope traces appear to confirm that so it's just another idea that didn't pan out.

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6 months 3 weeks ago #12770 by Ty_Eeberfest
Jim, yep I figured that out after re-reading a few times and playing around with a probe on the 9 cathode (right at the 74141 chip). What I'm really anxious to see now is what happens when he puts it into healing mode, That will put an end to blaming nonexistent compensating features in software since that mode takes software out of the picture as much as it possibly can be.

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

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6 months 3 weeks ago #12771 by BS_Jim
The other thing I'd like to see is a photo of the operating clock to see just how dim it is. I don't know if it's exceptionally dim for a nixie clock or it's just not living up to expectations.

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