Hello! As I mentioned in another post, I’ve been keeping a low profile while I’ve been dealing with some minor medical stuff.
I have started to do some technical things again and this story is about one of the pedals in a cheap boxful that I bought a few months ago.
It is a Danelectro Fab Tone distortion pedal. Not a tone I like, but it wasn’t working properly and needed to be fixed.
It turned out that a leaking battery had damaged some components on the main circuit board. A few resistors, a couple of capacitors, and the LED – LEDs always seem susceptible to acid fumes (it eats away the legs). Plus the 4013 CMOS flip-flop chip that Danelectro uses for the soft-switch, just like the Boss flip-flop in discrete components, but all in one chip.
The pedal was encrusted with gunk – my guess is that it failed 20 years ago and has been sitting in a junk drawer for that time.
OK, no worries. I cleaned it and replaced the damaged components … and the pedal refused to switch on and off reliably. I checked all the components again – sure enough the chip replacement was a known good 4013 flipflop.
I tried to figure it out for an hour. Then another hour with the scope. Then an hour scratching my head before it dawned on me what was going on.
This is the input blocks of the original chip (a 4013):
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This is the input blocks of the replacement 4013:
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If you look closely you can see a tiny difference on the clock input – that squiggle means that the correct version has a “Schmidt trigger” input, and the standard one I used to replace it did not. For technical reasons, the Schmidt trigger input works well with a pedal switch. The standard input does not.
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I ordered some identical chips to the one that came out and viola, the pedal is restored and works perfectly.
Danelectro Fab Tone repair
- Pepe
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Re: Danelectro Fab Tone repair
Interesting! I have never seen this certain symbol.
Re: Danelectro Fab Tone repair
That’s attention to detail Laurie ! As a complete Luddite when it comes to electronics, I’m interested to learn how these problems are diagnosed and fixed. Thanks, as always, for sharing mate.
- laurie
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Re: Danelectro Fab Tone repair
In case you have insomnia... this is what that little squiggle actually means.
In the incorrect chip, if the pedal switch press isn't exactly the right timing the clocking fails. The correct chip doesn't care.
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This is the wrong one:
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This is the right one:
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In the incorrect chip, if the pedal switch press isn't exactly the right timing the clocking fails. The correct chip doesn't care.
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This is the wrong one:
. .
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This is the right one:
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