The Electro-Harmonix 16 Second Digital Delay


02/02/2005 update : I kinda bought a new EH16 Reissue with the mindset of taking it apart, which I've done. Here's my writeup.

Here's what I have found sofar.

1. I've taken scans ( digital camera wasnt high enough resolution ) of the boards from an EH16. Also made some scans of a much cleaner set of schematics. The schematics listed further on have some notes on them which are very helpful so if I was grabbing schematics to repair a unit I'd grab them all.

2. *The* hard to find part is the Reticon RF5609A. This beast is a 7pole Elliptic Switched Capacitor Filter. Doing countless Websearches for it didnt turn up anything so I drove down to Reticon's old HQ and they were extremely helpful and gave me copies of the datasheets which I have scanned in. Page 1 Page 2 Page 3 Page 4. You probably only need the first 2 pages, but I'm kinda anal so even the packaging info is there. I've found a few chips that look like they'll make good replacements in a clone, but nothing thats pin compatible.

3. Here's a schematic you've probably seen around. Its a redrawing of the first page of the scanned handdrawn schematics that you also find around a lot PDF-Analog-Schematic It looks like it might have a few issues around the Compander, but then again so do the next schematics

4. Here are the handdrawn schematics. If you are adept at Ancient Egyptian then they should be *perfect*, otherwise they are like a murky but tantalizing view of the inards Analog Digital These schematics came from Bob Sellon's website ( see link below )

5. The memory in these units looks to be standard 4164 64kb DRAM. No fancy buffered RAM as rumored by some people. The audio ADC is an AM25L04 12bit SAR which hasnt been made since 1992, but some of Bob's notes lead me to believe that a 74C905 should also work ( look at the Digital Board layout ), I am looking for a datasheet and will add to the site when I find it. There is a ROM hiding in the midst of the Digital circuitry. A26 on the schematic if you can read it. No part number is referenced but judging from the number of address lines its a 32byte ROM. This could be a total showstopper if its bad and you're trying to fix one of these units.

6. AMD came through! Datasheets on the AM25L04 are here. Faxed but readable.

7. Well, the ROM shouldnt be a showstopper anymore. Here is a text dump. Should you need one, a 74288 should be a nifty replacement for you if you can find a blank one. You could make your own programmer easily enough, buy an old Data I/O like I did, or ask me to burn you one. I make the smart-ass comment about finding a blank one after buying some from Jameco to find they arnt blank. NTE sells this chip, I'm sure its painfully expensive from them too.


Bob Sellon has a treasure-trove of information based on his work with these units. Definatly check out his EH 16 Second page.

Who Am I?

I'm Tom Arnold ( no relation ). I've been working with electronics for about 30 years sometimes as business and sometimes as hobby. I can be found on all the usual Synth mailing lists ( Analogue Heaven, Synth-DIY... ). I have quite a few commercial synths and I'm always working on new stuff which is in various stages of completion. My favorite effects are reverb/delay/echo effects.

You can email me


updated 02/03/2005