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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- TC4013.jpg (34.45 KiB) Viewed 1443 times
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This is the input blocks of the replacement 4013:
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- CD4013.jpg (28.22 KiB) Viewed 1443 times
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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- TC4013sml.jpg (10.63 KiB) Viewed 1443 times
I ordered some identical chips to the one that came out and viola, the pedal is restored and works perfectly.
