Electro Harmonix Small Clone

I had one of these come that was pretty much dead. Didn't pass audio; LED didn't come on. Well, the LED was easy; someone had installed that backwards. Restoring audio was easy too - the 4558 was dead and I have plenty of those. The delay was working, but no modulation. The LFO wasn't working, and the 358 tested bad out of circuit. But replacing the 358 with a 4558 did not work.

The LFO is made up of a 2 op amp integrator/comparator pair that generate a nice triangle wave at pin 1 of the LM358 dual op amp.

I recently found out the hard way that you need to use a LM358, or at least something similar spec'd, or else it will not oscillate. I simply didn't have a LM358, and figured why shouldn't any dual op amp work here? Well, the LM358 is guaranteed for Vout to go to the V- rail, and that isn't true for your general purpose op amps, like the 4558 used for the audio path. I dug around in the op amp bin until I came up with some TS462 "rail-to-rail" dual op amps. Subbing the 462 did the trick. So this is a great example of when you can't just throw any old op amp into the circuit and expect it to work.