2 70s Electro-Harmonic Small Stone phasers

2 vintage Electro-Harmonix Small stone phasers. The artwork for both of these is almost identical to the 90s reissue (which uses LM13700 OTAs) but the lack of any LED indicators gives away that these are older - probably late 70s or 80s.

Small Stone #1

I think this is the pedal I have owned the longest with repairing. I started buying broken stuff  to repair from eBay in 2011, and picked up a couple of vintage Big Muffs. One was a lot that came with this pedal, a Small Stone, pretty much as pictured - no knob, no pot, no switch and a lot of broken wiring.

The Small Stone has 4 phase shifting stages, using 1 OTA as an LFO and 4 more as variable RC filters that give varying phase shifts. Early versions of the Small Stone used CA3094 OTAs branded as "EH1048", a house-marking for Electro-Harmonix. This version is an "Issue J" and has the 5 EH1048 chips, dated to 1977.

Issue J

EH1048 - 1977, week 32.

I bought a new 24mm reverse log pot, installed a 3PDT switch and re-wired the pedal (true bypass, why not). It didn't pass an effected signal. I think I put it aside at this point, I suspected that the OTAs might be bad and didn't have any replacements.


I came back to this recently. The oscilloscope showed that there was no LFO signal anywhere on the board, even though most of the voltages on oscillator OTA looked reasonable. I bought an RCA metal can CA3094 and replaced it - now I had an LFO, but still no wet signal. Looking at the input and output pins of each OTA, I could see that the first stage was phase shifting, but the second had no output. I shorted together the input and outputs of the second stage, and now I had a phase-shifted signal. It wasn't quite as deep as it should be, with only 3 or 4 stages active, but it verified that the other two ICs were good. I ordered one more CA3094 to replace the dead EH1048, and complete things.

Before re-housing

I sourced a new hockey put knob that fit the new 24mm pot. Originals are hard to come by, but it fits the right aesthetic.

This was inspected by... Elsa? Cheers, Elsa.

There is usually some foam behind the PCB on the back panel of the housing. The PCBs just hang off the back of the rate pot, the foam is to prevent it from shorting out on the back panel. This foam had perished, so I taped some card down to insulate the PCB instead.

Small Stone #2

This second unit is a similar vintage, I picked it up hoping it might help repair the first one.

This is a slightly different PCB (with a phenolic substrate instead of fibreglass?), but looks to be the same circuit more-or-less. It also has 5 EH1048s, dated to 1979.

This one actually worked despite being sold as faulty (this is not that unusual). It just had a couple of quirks. It was a big help in verifying the switch wiring on the other unit, and for taking reference voltage readings off the OTA chips.

Phenolic PCB, instead of fibreglass?

The first was that the rate pot had some odd damage, the casing was partially open. I guess this could have been caused by dropping the pedal onto the knob, or by pulling on the PCB while it was still attached to the enclosure. Or during factory assembly, this was EHX after all. This was easy to close and re-crimp with pliers.

Opened pot housing...

...closed again

The second was that it would start to oscillate with the "color" switch in the up position and no input. It works fine with a guitar connected, or a buffered pedal in front of it, but with no cable or dangling unconnected cable it will start to ring at the top of the phaser sweep. As far as I can tell this is just something that this revision does, to fix it I would have to switch to a shorting input jack or modify the pedal to reduce the positive feedback when the color switch is in the up position, neither of which I really want to do.

These turned out really well. Feels good to have them done. They sound almost identical, the only change that jumps out is that the two different brands of pots don't quite match up - the plastic shaft CTS pot physically rotates further than the new Alpha pot, so the rates are slightly different when the pointers are matched by eye.
I don't have a modern Small Stone, or a Sovtek, to compare with. They definitely sound smoother than JFET phasers I'm used to.

One or both of these will probably hit Reverb in the next couple of days, get in touch if interested.

Eurotec Black Box Faze Module

Eurotec was a brand run by Sola/Colorsound in the late 70s. They released a line of pedals called the "Black Box" series which seemed to be re-issues of their earlier products as well as some clones of contemporary designs. The pedals could be used individually with battery power, or plugged into a "Black Box Module Energiser" base station that could power and route signals to four modules. I don't know if this modular system was successful but it's interesting to see an earlier commercial stab at a pedalboard concept.

Image from https://www.tonehome.de/eurotec/

I have the phaser of the series, and it doesn't pass an effected signal. There are no schematics for this series that I could find online, but 4 JFETs and 4 opamps nearby give the game away - this is most likely a Phase 90 clone. The "React" knob seems to replace the mixing resistors for the clean and wet signals on the original so the phasing effect can be adjusted in intensity.

Build quality is a little cheap, but not unusual for the period - a single sided board that looks like a phenolic substrate. Traces will probably lift if overheated. The footswitch is unlike any I've seen before, it has a threaded black plastic cover to anchor itself to the enclosure. The enclosure is more like a sheet metal can than the usual diecast aluminium.

I went through this expecting to find some dead capacitors, but everything measured OK out of circuit. I did find one capacitor lead that had either broken loose from a cold solder joint or was never soldered in at the factory.

Feeding a sine wave into the pedal and probing the opamps showed that all had phase-shifted signals on the outputs except one - replacing this opamp restored the effected signal.

It really sounds great - the "react" control will dial the phasing all the way to a very resonant "thumping" sound - it can get much more extreme than the MXR originals.

Adding external power:

Despite having an LED, there is no indication that the pedal is active when pushing the footswitch.

I found it interesting that the output jack on these boards are stereo, and the ring contact is tied to a diode that switches out the battery clip. Pictures of the original base station seem to show stereo TRS jacks, so it's possible that they supplied power through the additional conductor.

Full modular system - image from https://www.tonehome.de/eurotec/

I wired up a a mono 1/8" jack to a stereo 1/8" connector and added a 9V DC connector to the ring. When plugged into the output jack, the output signal is passed through and 9V power is used instead of the battery. As a bonus, the LED works as well.

This seems like a worthwhile cable if you don't want to modify the enclosure.