Digit Brightness, Classic Rev6, IN-8 Tubes

Hmmm. Not much else to check in the anode circuitry. Just to make sure, are the optos definitely EL817C (the "C" may matter - the trailing letter is an indication of current transfer ratio)?

And then on the cathode side the only potential culprit would be the 74141. I've never heard of a "weak" one but that doesn't mean it couldn't happen I suppose. Worth a try swapping it out if you have another one on hand. Also verify the chip's supply pins are well soldered.

Oh that's a really good point. We're using EL817C which has a transfer function (effectively a "gain") of 200%-400%. That means at best it can give out 4 times the current in at the base.

If we're putting in 5V and the forward drop is 1.5V with a 1k resistor that means we could source at best (5-1.5)/1000 * 4 = 14mA (400% TR) and at worst 7mA (200% TR)

IF the resistors were 10K instead of 1k, we'd get just a tenth of that, i.e. 1.5mA at best, 0.7mA at worst.

Jedidiah wrote: Just checked them all (r1, r2, r3, r12, r13, r14) and they are all indeed 1k.

Well there goes that theory...

Ian wrote:

Jedidiah wrote: Just checked them all (r1, r2, r3, r12, r13, r14) and they are all indeed 1k.

Well there goes that theory...

But it was a nice theory while it lasted...

Looking at the latest scope shot...

Counting squares on the screen I'll estimate the voltage drop at 23V. (Aside - not sure why the scope says Maximum is 33.6V, which seems to conflict with what the trace shows, unless that huge spike on leading edge of first pulse is throwing it off). 23V drop seems fairly reasonable given the data sheet voltage drop across the tube is 150V, so 177V - 150V = 27V. (177V supply taken from your post on Page 2 of thread)

So if it's dropping 23V across 3K my math says the tube is getting 7.66mA peak. Divide by 6 (multiplexing) gives 1.28 "effective mA" to the tube. If I use the Mean voltage from the scope instead I get 1.06mA. Either way it's low for a tube that wants 2.5mA typically. BUT dividing by 6 is an assumption on my part - MUX math may not be as simple as I'm making it out to be!

Anyhow... I don't have a Classic R6 but I have a few running Modulars which use identical drive circuitry. When I get home in a little while, if Ian hasn't already posted some results, I'll do the scope tests on mine and report back.