EHX - Octave Multiplexer  [schematic]

All about modern commercial stompbox circuits from Electro Harmonix over MXR, Boss and Ibanez into the nineties.
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JimSalabim
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Post by JimSalabim »

Thanks! To make it easier to understand I will speak about two "modes" here that my Octave Multiplexer has currently:
"Working mode" and "non-working" mode.

"Working mode":
Sometimes the pedal will power on in working mode and everything is fine. Tracking works as expected here and all knobs work as expected, no crackling, no strange behaviour, no strange sounds, everything just fine. But after some time the pedal will often suddenly jump into "non-working" mode.

"Non-working" mode:
Very often the pedal will power in non-working mode. This is when all the strange stuff happens. It already starts with 1 or 2 seconds of noise after switching it on. In non-working mode only the low E string is tracked very badly, nothing else is tracked, there are weird noises and the controls crackle in extreme positions and have strange behaviour there.

In the last video that I posted the pedal was in "non-working" mode. In "working-mode" the Blend knob at 0 has just the dry signal, just as it should be.

When the pedal is in "working-mode" every one of the four electric guitars that I have works just fine, so there’s no issue here.
When the pedal is in "non-working" mode then no signal will really get tracked, no matter what kind of signal and how clean or loud the signal is.

Yes, the other 14 pin chip is an LM324 quad opamp (IC1 in the schematic). The 4558s would be IC2 in the schematic. There seem to be versions of the Octave Multiplexer with two LM324s and versions with one LM324 and two 4558s.

The 4558s are soldered directly to the board with no socket, so unfortunately I can’t take them out easily to check it without IC2.

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Andy2No
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Post by Andy2No »

Fair enough. That really does sound like either a faulty component, or a dry joint. Both can be very mysterious.

Do you have another LM324 to try? Or any other quad op amp with the same pinout... they mostly correspond well enough to just swap them out. I think the only really significant differences are supply voltage requirements (any should be okay) and input impedances (probably not a big deal, in this case). They have different noise levels, clipping behaviour and power drain too, but probably none of those things matter too much, in a circuit like this - they'd make a qualitative difference, but nothing fundamental.

Here's a thought - connect a test meter into the circuit to monitor voltage levels, e.g. at the supply pins of IC1. If there's something dragging the supply voltage down, like a bad smoothing capacitor, that would make it sound distorted. The burst of almost white noise, in that video, was odd, but I guess if the supply voltage drops low enough you can get some strange behaviour. Op amps can sound really nasty when they clip, and a normal input level would suddenly start clipping if the supply voltage drops, plus below a certain voltage, the op amps would just behave in ways they wouldn't normally.

If the voltage stays stable while it's happening, and swapping the LM324 makes no real difference, I'd guess it was something in that first bit of signal path, from the input socket to the + input of IC1d, including just a dry joint, which basically gets to do anything it likes, at any time, with no rules that the likes of us could fathom... I bet there's an idea for a really expensive boutique pedal or synth module, in that :)

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Andy2No
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Post by Andy2No »

... monitoring the Vbias voltage might be worthwhile too. It's easier for a bad component / solder joint to affect that, than the supply voltage.

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JimSalabim
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Post by JimSalabim »

Thanks. With a 9V battery the supply voltage is stable at 8.1V and the Vbias voltage is stable at between 3.3 and 3.4V.
I don’t have another quad opamp left, I will get a few and try with these.

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JimSalabim
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Post by JimSalabim »

Good news! Finally I bought a new quad opamp (LM324). Well, it seems the old one was damaged. Now with the new one everything works just perfectly. :-) No more strange behaviour at all!

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Andy2No
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Post by Andy2No »

Excellent. I'm glad to hear it. :applause:

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JimSalabim
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Post by JimSalabim »

Thanks so much for your help and for making me understand the circuit a lot better!

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Andy2No
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Post by Andy2No »

You're welcome, Jim.

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DoomchildXXL
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Post by DoomchildXXL »

Hey, guys!

Can anyone post some working voltages of a modern one with all the smd stuff like Andy2No posted? I've got one on the bench for a buddy and can't make head or tails of why it isn't working. :scratch:

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Post by DoomchildXXL »

Different approach :D

Has anybody got an etchable layout for me? Pretty please?

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bamslam69
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Post by bamslam69 »

Hi all, I’ve got the modern one all soldered up but it’s a bit problematic. Does anyone have a copy of the schematic that refers to the Modern layout on the tag board fix page?
Thanks in advance.

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