Digit Brightness, Classic Rev6, IN-8 Tubes

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8 months 1 week ago #12748 by Jedidiah
At 9 minutes and 15 seconds after the hour, the ACP routine did occur. No perceived change in brightness, and voltage across the minutes digit fluctuated between 2.9 and 3.5.

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8 months 1 week ago #12749 by Jedidiah
across the minutes digit 3k resistor, that is!

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8 months 1 week ago #12750 by Ian
There's something here that does not make sense at all.

It could be that reading a pulse wave with a voltmeter might not be giving a reasonable result (not all meters can interpret PWM well), I can only think that there is a problem with the pulse width.

I think it is time to break out the scope... Please provide a trace of the input side of the Opto (pin 1, right next to the dot).

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8 months 1 week ago #12751 by Jedidiah
See attached image.
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8 months 1 week ago #12752 by Jedidiah
Here is a better screen capture for you with more info.
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8 months 1 week ago - 8 months 1 week ago #12753 by Ty_Eeberfest
Coming in late here. I just read the whole thread and have a few comments...

First off, I agree with Ian that voltmeter readings across the anode resistors can not be taken literally. Meters do not handle the "odd" wave forms here very well. The only way to know for sure how many volts are dropped is to use the scope and read the peak voltage off the screen.

Your scope trace looks normal. The peak is right about 1.25 volts which matches the forward voltage spec of the EL817 opto. The frequency / duty cycle looks reasonable.

My question is this: has it been absolutely positively verified that the 1K resistors on the inputs of the optos are really 1K?? Remember that optos are not switches, i.e. at low input currents they behave like a transistor that's in its linear region - partially "on". If the input resistor is much more than 1K we'll be operating in that region and the opto's output current will be limited.

EDIT: Saw the new screen capture, does not change what I posted. Okay, 1.38V instead of the 1.25V I wrote but that's still well within the opto's specs.

Look into it later when the dust is clearing off the crater.
Last edit: 8 months 1 week ago by Ty_Eeberfest.

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