Digit Brightness, Classic Rev6, IN-8 Tubes

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6 months 3 weeks ago - 6 months 3 weeks ago #12760 by Ty_Eeberfest
Here is a scope shot. Modular clock, IN-14 tubes, LDR dimming and other effects OFF, subjectively the display looks "good and bright". Flashing neon separators (colons) disabled to stabilize bus voltage.

Sorry, no color or cool cursors - this is a photo off the screen of one of the earliest "service man" portable scopes Fluke ever made. Ugly but still accurate.



Digits on the screen:
Big one = peak voltage
Small one = average voltage

Other relevant info:
HV ("+170") bus = 180V
Mux frequency = 96.9Hz
Pulse width = 1.55mS
Duty Cycle = 15.3%

I'm just gonna to leave this info here without comment because it's not helping me think of reasons why your tubes are dim. I'm just not seeing significant difference between your readings and mine.

EDIT: Probes were on the 1-minute digit, not that it should matter. I wasn't paying attention to what number was being displayed. I left it going for half an hour and looked at it periodically between distractions. Scope readings did not vary so I think it's safe to say it makes no difference what number is being displayed.

Look into it later when the dust is clearing off the crater.
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Last edit: 6 months 3 weeks ago by Ty_Eeberfest.
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6 months 3 weeks ago #12761 by Ty_Eeberfest
I need to correct myself on one point. It DOES matter what number the tube is displaying, Don't know if this is particularly meaningful or not but here are the numbers...

Displaying a "1"
Peak = 20.9V
Avg = 3.24V

Displaying an "8"
Peak = 26.5V
Avg = 4.02V

Other numbers seem to vary as you might expect: numbers with larger glow area pull a bit more current.

Also I've noticed now that the readings do wander a bit when the number displayed is not changing, The HV bus voltage is wandering more or less proportionally. I attribute this to the software based voltage regulation hunting for the "perfect" voltage, which it probably never quite finds because the load is varying every second. I'm tempted to disable the regulation in code as a test to see if the bus is more stable or less without it. Not that this has much to do with your issue,,,

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

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6 months 3 weeks ago #12762 by Ty_Eeberfest
And finally, for now... every way I crunch the numbers I get 1.8K as the ideal anode resistor and yet I'm looking at a perfectly bright clock that has 3K anode resistors. Also, a few pages back I see you tried a 1.5K resistor and it made no difference in brightness. This is all very weird to me and I'm writing it out here partially to help myself in trying to understand what's going on.

Is there some "X Factor" we are all overlooking??

I don't think it's in the software. There's nothing in there alters MUX properties to compensate for anything. The only compensation-like code is the voltage regulation that monitors the HV bus and makes small changes to the HV generator PWM duty cycle (HV generator, NOT MUX duty cycle!) to continuously "trim" the HV.

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

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6 months 3 weeks ago #12763 by Jedidiah
IMHO, it has to be in the software. Otherwise v=ir is no longer valid 0:>) .....

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6 months 3 weeks ago #12764 by Ty_Eeberfest

Jedidiah wrote: IMHO, it has to be in the software. Otherwise v=ir is no longer valid 0:>) .....


With neon lamps/tubes/etc. v=ir may actually not work as expected. I remember learning that they exhibit a negative resistance phenomenon under certain conditions but that's all I remember. I'll try to do some reading on the subject tonight.

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

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6 months 3 weeks ago #12765 by BS_Jim
I didn't just hit and run with a reply, Jedidiah. I am at a loss to explain what's going on here and am thankful that Ty picked up the ball when he did. I can't explain that 1.5k anode resistor causing no difference in brightness either without asking what was the voltage at the cathode. This would need to be measured with your scope because of the short duration of the multiplexing. It would be easiest to measure on the minutes or 10 minutes position because it will stay the same long enough to take a measurement. We've all been focusing on the anode voltage and just assuming the cathode was being grounded through the 74141. There could be some resistance to ground that we didn't account for. I'd like to see a trace of the cathode pin of any digit of whichever tube is easiest to work with when that digit is active. If it's not very close to 0v, I'd suspect it would be even higher with a 1.5k anode resistor on the tube.

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