Electro-Harmonix Graphic Equalizer

Another EHX big box pedal, a mains powered 10 band graphic eq. I'm guessing this one is a late 70s/early 80s vintage. Google finds at least two versions, this one and a more modern looking black-and-yellow version with a power switch and LED/lamp. Oddly enough, none have a knob in the lower left corner instead of a stomp switch, so this may be a less common variant or someone's science project. It doesn't work - moving the sliders just brings in some noise, the knob in the corners seems to attenuate this.

A few things are apparent once the lid is off. The "master volume" is not stock, it's someone's mod in place of a bypass switch. The jacks are probably replacements, as EHX didn't use those isolated plastic jacks. There is a third jack that has been added, probably for a remote footswitch for bypass. The PCB is also broken! It's held in place to stand-offs with screws and washers and one corner has broken off. Some of the plastic sliders of the potentiometers are sheared off, it's clear that someone stamped over the sliders hard and managed to break the PCB as well. There is a pair of washers and a screw holding the board in that corner, and it actually seems stable mechanically.

A schematic was easy enough to find, the model name is the same as on the PCB art and the circuit seems to match. It's 9 notch filters (using single opamp gyrators) in parallel with different centre frequencies plus a passive 16 kHz filter. This seems to be a common design for guitar EQ. Power supplies are approximately +-13.5V using a centre tapped transformer and zener regulators.

Schematic found online

I took the board out of the box to take a lot at the pots. Date codes are from 1979. Some black foam was attached at some point, probably to keep dirt out, and over the decades it has perished and turned to sticky fluffs of black shite that are now all over the PCB and inside the pots. I cleaned these with contact cleaner and cotton buds until the pots slid smoothly. Also washed the enclosure while I had it gutted, removed the plastic jacks, installed some new metal open-frame 1/4" jacks and pulled out the volume knob and wiring.

Some of the pots have broken sliders, so it's possible that they have broken tracks or the wipers have disconnected from the tracks. I tried to measure the resistance of the left-most pot from the backside and it didn't seem to reading correctly. All the pots extremes should be wired together in parallel, and it turned out that left hand one had no continuity to it's neighbour. I tried to desolder it to measure out of circuit, and the pad and track lifted right away. The pad on the next pot also started to lift as soon as it was heated. Usually I don't see any lifted traces or pads when desoldering. It turned out that these tracks were physically broken, when the PCB corner was broken the pots must have been pulled away from the board. The left-side sliders were completely disconnected, the ones on the far right were fine and the middle were a bit flaky. I ran a piece of bus wire across the top pads to connect them, much like how the pot cases are grounded together.

Cracked pads

Pots jumpered with bus bar

At this point I clipped a test signal to the board and fired it up. Power supplies were OK. The signal didn't get any further the first opamp stage, so after checking the feedback network looked good I replaced it with a new 4558 opamp. Everything works afterward, all bands boost and cut and none have any noise or crackling.

I added a new switch and wired for true-bypass, and then managed to confuse myself when I saw a huge 50 Hz hum on the output. I thought the power supplies had gone bad and replaced the filter caps with new Nichicons. It turned out that although the enclosure is grounded, and the PCB is bolted to the enclosure, the PCB is not grounded through these bolts. Running a wire from the input jack fixed this up. At least I know this won't need new filter caps any time soon.

The extra hole in the enclosure is a decent fit for a large LED bezel and some spare hardware, I've added this as a bypass indicator.

Repaired and cleaned up.

It sounds exactly like a 10-band EQ sounds. The +-13.5V supplies mean that this has huge headroom, and it can be used to filter high amplitude signals without distorting as soon as a 9V or 18V unit.