Here's an alternate EH Octave Multiplexer Schematic, 1975 (if anyone cares!)

[Praying_V]

Hey everyone, I have an Electro Harmonix Octave Multiplexer from 1975.  I'm not sure, but I think this may be the earliest generation of this pedal (but then again, who knows... theres NO information about this pedal anywhere online).  When I got it, it needed some work, and I found a similar schematic over at Topopiccione:
http://topopiccione.atspace.com/pjimages/EHOctaveMultiplexer.sch.gif  .........This original factory schem is similar to my pedal, but not quite.  This schem seems to match the board layout for an old Octave Multiplexer that's currently on ebay.  The seller is calling it a 1976 pedal (the year is printed on the circuit board).  In my pedal (with a 1975 circuit board) there's handful of differences, so I used the factory schem as a reference, and traced mine out.  So here's my revised schematic for the 1975 version:
http://www.geocities.com/kiyomasa_demo/EH_Octave_Multiplexer_1975.GIF
And here's my 1975 circuit board pic:
http://www.geocities.com/kiyomasa_demo/eh-octave-1975.jpg
If you compare the two schematics, you'll see mine disagrees on a few resistors and a cap, and some of the wiring... The most noticeable difference is the 1975 board uses two quad opamps, while the 1976 & later boards use a quad & and two dual opamps.  Here's a quick list of differences:

1975 board is missing C17, R11, and R29 (i think those are the numbers, its hard to read)
1975 board uses 560 ohm for R6, where the later board uses 470 ohm.
1975 board has an additional resistor and slightly different wiring at the "Blend" pot
1975 board uses a quad opamp where the later board uses two dual opamps.
The four opamps near the 4013 are connected differently in both pedals.

I would assume that EH made the changes after the 1975 release to improve the design.  I wish I knew, because the tracking is not so impressive (yeah, I know thats a common complaint for analog octaves).  The tone of the square waves is very mellow compared to some other octave pedals (which I like).  Interestingly, because of that extra 47K resistor, the earlier pedal has no pot setting that will give you straight un-effected sound at the output - even at the lowest settings, you have very prominent square waves. Very lo-fi...  Like many early EH effects, there's a slight volume drop when switched on (versus true-bypass).

Anyway, hopefully someday this schem helps someone out. I realize this is one of the least-popular pedal projects out there, but who knows... I guess if you collect schematics you might like to take a look at it.  Some component values on the factory schem were unreadable, so while I traced out the pedal I fixed them as well.  If anyone has any input on how the later-circuit-changes would affect the sound, then do tell.  Mark Hammer, wanna take a stab at it? ................................................ken

[Ry]

Interesting to see the differences.  Thanks for sharing!

Ry

[ollie]

What can you use a multiplexer for with guitar? I thought they were used to send more than one signal down one wire?

[Praying_V]

Its just the name they gave it.  Its an octave-down generator, like an OC-2, Rocktace Divider, Octabass, etc.

[nelson]

Thanks

[Praying_V]

...
Original link no longer works... so here it is again.

[Rob Strand]

You did a good write-up of that one.

Somehow the new links are broken as well.

Here's the 1975 schematic.
[Praying_V  updated the 1976 schematic to match the 1975 unit.
As such these part designators do not appear on the 1975 version R11 (10k), C17 (22n), R29 (150k).
Also a resistor 47k resistor has been added which has no designator.]



Here's the 1976 schematic which seems to have disappeared from the web.   I've cleaned up a few things.  In particular the 15k's and 18k's which are hard to read.  (The same values are on the 1975 unit.)  I did confirm these values against a PCB.

[zmix]

I just got a "pedal" version of this, it has two footswitches, two filter knobs and a toggle switch.  The second footswitch adds an octave up effect, does the "box" version have this?

[Mark Hammer]

Fascinating!  No, indeed, the box version does NOT have a second switch for an octave up.  This foot-treadle version (which I had no idea existed until just now) seems to have been in competition with the Mutron Octave box, that also has one up and one down.

[FUZZZZzzzz]

looks more like someone rehoused his EHX pedal   i never quite got to like my multiplexer for guitar. we used it more frequently with a mic to sound like barry white.

[Mark Hammer]

If it is, indeed, rehoused, then it used the same wah shell EHX was using at that time.  Those would have been an absolute nightmare to machine.

[StephenGiles]

This appears to be an EH deluxe Octave Multiplexer board, but 2 LM311??

Aha, found it - one is used as a muting circuit and the other as a squarer for the adaptive filter sine wave output!