Important: CE2-type chorus problem

[Fuzz]

Hi guys!

I recently took an old Tokai TCH1 chorus that was not functioning, due to a problem in the LFO zone.  After checking it a while, I found that the schematic is identical to the Boss CE2 one, with the exeption of the chorus/vibrato selector that is integrated in the Tokai (and is a popular mod for the CE2). First try I made, after working on the bias trimmer to maximize the chorus sound, was to substitute the LFO original OpAmp (BA728) with an "always-functioning" TL072 (socketing the PCB)...and *YES*, the chorus started working, but quite thin and very asymmetrical, with the first half of the rate pot absolutely unuseful.

Than, trying different OpAmp thanks to the socket, I gave the BA728 a second chance, and it has been working for a bunch of seconds. From that moment, even the TL072 doesn't seem to be enough. I am sure that the chorus is working because if I move the depht pot knob up an down quickly with my hand, I can hear the chorus sounding, and if I short on the PCB the point that goes to the central lug of the pot with one of the points that go to the external ones, I can hear the maximum rating chorus. I also tried to substitute the rate pot, but nothing.

Are in this forum some chorus gurus that can explain to me what is happening?

Many thanx in advance!
Fuzz

[Mark Hammer]

Look at the LFO of just about any flanger or chorus or phaser, and you'll see a few things in common.

First they just about all use a standard 2-opamp circuit made up of a comparator and an integrator.  The comparator will be one op-amp with a Vref feeding one input pin.  The integrator will be the other op-amp with a cap as the feedback path.  Look over several such LFOs and you'll see that:

a) there is generally a wide range of resistance values in the parts that make up the Vref, indicating different assumptions about current (e.g., the Ibanez Flanger uses a pair of 5k1 resistors for the Vref divider, the Morley Crystal Chorus uses an asymmetrical pair of 2.2M and 1.8M to do the same thing)

b) there is generally a wide range of fixed input resistors to the integrator stage

What all of this suggests to me is that the LFOs are designed around certain assumptions about what that particular op-amp wants/needs as input current to do what it has to do.  It may be possible to switch chips *within* classes (e.g., swap a TL072 for a TL082 or LF412 or CA3240) but much less feasible to swap chips between classes (e.g., NE5532 to a TL082) and still have it produce the same output.

You will note that the majority of LFOs in modulation pedals tend to use relative cheap chips, generally not BiFet.

[Fuzz]

Hi Mark!

Thanx and yes, you are completely right. The problem is that I DO turn back to the original OpAmp, after checking that it was not the problem. I try to explain it better:

1- The pedal was not functioning, but I could hear the chorusing sound just by moving the depht control = LFO problem (I thought)

2- Substituted the original LFO OpAmp for first with a socketed TL072: got the chorus, a bad one (due to the unappropriate OpAmp features), but it seemed to be a LFO OpAmp problem...

3- ...but NOT! Put again the original one in the socket, and I had the chorus for a while, than stop. But if I short two of the three lugs of the rate pot (only on pcb solderings, not directly on the pot's lugs; THAT'S STRANGE!) I can hear the chorus at maximum rate = LFO OpAmp DO work!

Could it be a problem of old and bad solderings in the PCB LFO zone? (it doesn't seem, but...)

I really would have that chorus back, because it sounds very nice (warm and liquid, but even transparent), at least for what I was able to hear in that few times of "half-working" conditions.

Thanx again Mark, I'm pleased to talk with you! Bye,
Fuzz