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Author Topic: boss sd-2 effect won't turn off  (Read 6802 times)

Enzo

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Re: boss sd-2 effect won't turn off
« Reply #15 on: August 06, 2015, 10:25:48 PM »
OK, so the pins 1,2,3 gate is working, and the reason it does not toggle its output is that its input is not shifting enough.

Yes, ECB.  I can't read the schematic very well.  But plain old common 2SC945 would work, in fact the exact same circuit uses 2SC945 in some other models.  It really doesn't matter, most any common NPN transistor would work.  The center leg is the collector.  On Q2, the emitter is the leg nearest the board edge, where the ground trace is.  If that collector was really stuck at zero, then the flip flop would not change voltages.

Look, the toggle line was going from 5v to 2.6v, but when you selected a switch position where it was not connected to the IC, then it started going from 5v to close to zero.  That sounds to me like the IC is loading it down.  When I had you ground pins 1,2 and watch pin 3 toggle, that ONLY tested that one gate in the IC, not all four.  I suspect one of those other three gates has a bad input pin. 

Tyr this, leave the selector in that position where nothing is connected to the IC.  Wires 10 and 11 carry the switch feeds down to the ICs.  Try measuring resistance to ground from each of those, power off of course.  They ought to measure the same, does one read lower than the other.  I mean substantially, not a small difference.

In my shop I stock all the CMOS chips, so I would be slapping a new 4011 in there at this point.

nosaj

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Re: boss sd-2 effect won't turn off
« Reply #16 on: August 12, 2015, 08:12:16 PM »
You are the man Enzo!!

Got a new chip in and voila!.

Thanks so much.

nosaj

Enzo

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Re: boss sd-2 effect won't turn off
« Reply #17 on: August 13, 2015, 12:31:40 AM »
Cool.  I hope more than just a working pedal, that you got from all that a systematic approach to tracking down the problem.

nosaj

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Re: boss sd-2 effect won't turn off
« Reply #18 on: August 13, 2015, 06:02:13 PM »
Just to be sure I understand some  On that IC  out of Circuit I could breadboard it and then by grounding the inputs find the switch that is not switching IE no voltage change correct?

Yes I am starting to enjoy the learning. I really thank you for your Time.

nosaj

Enzo

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Re: boss sd-2 effect won't turn off
« Reply #19 on: August 13, 2015, 08:58:10 PM »
Not quite that simple, but exercising logic chips like that is a valid technique.  You can look up "CMOS logic" and find the basics.  TTL logic has the same rules, but voltages are a little different and would not apply to your pedal other than conceptually.  Yes, a breadboard could easily be set up to check the "truth table" for the IC.

Actually my first test is to simply measure resistance to ground (and possibly to V+) from each input pin to see if any show a low resistance - lower than the others anyway.

But considering they are cheap, and my shop stocks them, once I suspect it, I usually just go ahead and change it out.  Parts are cheaper than time here.

nosaj

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Re: boss sd-2 effect won't turn off
« Reply #20 on: August 13, 2015, 09:05:12 PM »
I was thinking I'd learn more by applying it to a breadboard, maybe in principle.

nosaj

Enzo

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Re: boss sd-2 effect won't turn off
« Reply #21 on: August 14, 2015, 12:08:44 AM »
You can breadboard the one IC, put power to it, and make the gates do their thing.  But it is a logic chip, not an amplifying chip, so the output pin of each gate will either be close to zero or close to the power supply voltage, depending on what the input pins are doing.  So you would need to ground certain input pins and connect others to +9v with a resistor, and watch what the output pin does.  The truth table for an IC like that is simply the combinations of input pin states, and what the output pin is expected to do as a result.  For a simple NAND gate like yours, the table only has four possibilities.  Of course the four gates in the IC are independent of each other unless wired together as in your pedal.

In fact, I recently sold a little Heathkit powered breadboard, which would have been ideal for such experimentation.

 

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