Author Topic: EH Holy Grail Nano Type Switch Problem  (Read 2088 times)


EH Holy Grail Nano Type Switch Problem
« on: November 16, 2016, 01:12:02 PM »
Hi all, I picked up a "broken" EH Holy Grail Nano for cheap, and the issue is that the DPDT (on-on-on) Reverb Type switch was broken and stuck on 'spring'. I pulled it apart and tried to replace the switch with an off-board DPDT, and nothing is changing with the switch. I started trying to connect each lug to ground separately, and all combinations of them, and I couldn't seem to get it to change then either. The DPDT switch I pulled out was definitely broken, the sliding metal contacts inside were cracked along with part of the plastic mechanism.

Does anyone know logically how the switching works here? I've found a couple of other Holy Grail derivative schematics (here:, and they seem to all be slightly different variations on pulling some logic pins to ground. The Nano has a different main DSP chip, with datasheet here: When doing a continuity test though, none of the pins of the switch connect directly to any of the pins on the chip, except the one that goes to ground obviously.

Is there anything I'm missing? Is the RESET function important while switching, or just power-on? Should I be connecting some voltage here? I'm hesitant to try because of the delicate specs of the DSP chip.

(Posted in the DSP forum because I'm trying to understand the seemingly digital logic behind the switching, if that's wrong, let me know)
(End game for this is that it's going in a combo pedal with a fuzz, so if all else fails and no one has any clue what black magic EHX does, I'm fine with just using the hole I drilled for the switch for a filter or something)
breaking more things than I'm building


Re: EH Holy Grail Nano Type Switch Problem
« Reply #1 on: November 17, 2016, 07:54:59 PM »
Ok, going on what's in the schematic Mark Hammer posted in the link you sent, I've got the following:

Flerb is PIO0 pulled high, PIO1 low.
Hall is PIO0 and PIO1 pulled high.
Spring is PIO1 pulled high, PIO0 low.

If the switch you took out was a DPDT, I'd assume that one section does PIO0 and the other section does PIO1. Both pins have pull-ups to +5V close to the Cirrus chip, and the switch is centre-off. Each section has one of its pins tied to ground to ground the required input. So switched up, one section grounds PIO0 to give you Spring. Switched down, the other section grounds PIO1 to give you Flerb. In the centre off, the pull-ups take both lines high, giving you Hall. Does that seem reasonable?

Interestingly, if the firmware is the same, you ought to be able to get a Room reverb by having both pins low, and you can switch between short and long by adding a switch from PIO3 to ground (I'm assuming it's got the pull-up fitted, which it would if the firmware *is* the same).


Edit: This thread suggests the switch *is* a On-off-on:
« Last Edit: November 17, 2016, 07:57:09 PM by ElectricDruid »


Re: EH Holy Grail Nano Type Switch Problem
« Reply #2 on: December 30, 2016, 12:31:30 PM »
Any luck with this? If it was accomplished, could you post pics and a quick how-to?