MidiDac pitch cv problem.

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eggpie
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MidiDac pitch cv problem.

Post by eggpie » Mon Sep 10, 2018 3:00 pm

I have built several modules now and every one till now has worked flawlessly, thanks to Tony's superb documentation and advice.
I have a strange issue with my last built module, the mididac.
It worked fine when I finished the build yesterday, but today at switch on, everything worked except the pitch cv output.
All the midi in is working. I have gate led, 5v at the gate jack.
Pitch bend, mod, aftertouch are all giving the correct voltages at their jacks.
Pin 1 of IC1 is giving me -14.58v regardless of pressing a key.
The pitch cv jack is giving me +14v regardless of pressing a key.
I'm not sure what could have changed in just one day! As I said, yesterday it worked 100%
Hope someone can help as I'm a little baffled!
Thanks.
Tom.

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Synthbuilder
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Post by Synthbuilder » Tue Sep 11, 2018 2:12 am

Try removing U5. Power it up and see if it works - the slide won't but everything else should. If it does then U5 was damaged. That could possibly mean that U5 was a 74HC4066 and not a 4066. The 74HC series aren't designed to work on +15V and will work only for a short time.

Tony

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eggpie
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Post by eggpie » Tue Sep 11, 2018 4:16 am

Thanks Tony.
Will try that later tonight.

Tom.

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eggpie
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Post by eggpie » Tue Sep 11, 2018 3:24 pm

Hi Tony. Tried your suggestion of removing U5, you were 100% spot on. It was a CD4066BE though, and not a 74HC4066. Luckily, Farnell have a MOQ of 5 for these, so I had spares.
So, the question is, what fried it? I am generally quite careful around cmos ics and all other ics for that matter. i use an earthed wrist strap and a proper ic insertion tool/ extractor. Are cmos chips still vulnerable once in their socket of a completed module? Especially when it's not powered up? The last and most likely option; I may have powered up the module by plugging it straight into the PSU which is already switched on (sorry).
Whad'ya reckon?

Thanks.

Tom.

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Post by Synthbuilder » Wed Sep 12, 2018 2:09 am

eggpie wrote:I may have powered up the module by plugging it straight into the PSU which is already switched on.
That could have well done it. I normally check to see if modules don't die with just +15V or -15V, but maybe if +15V and -15V are applied with no 0V that could kill it? When modules are powered up by plugging them in to an already powered up supply you can get a variety of combinations of the three power rails being applied very quickly. Most modern ICs are tough but CMOS doesn't like having too much negative current - which could happen to U5's pin 1 via pin 3 of U4 if the power is applied oddly.

Tony

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