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DIY Stompboxes => Building your own stompbox => Topic started by: Scruffie on January 24, 2022, 06:40:28 PM

Title: EHX EH-8000 Guitar Synth (Schematic)
Post by: Scruffie on January 24, 2022, 06:40:28 PM
Redrawn schematics for the rack mount EH-8000 guitar synth, mods included to replace the CA3094 with LM13700 and the TL607 with standard CMOS (not sure if this will work in practice, perhaps someone else can advise).

Don't have a schematic for the power supply but it puts out +/-12v from recollection.

(https://i.postimg.cc/Fd53kfCZ/Guitar-Synth-P1.gif) (https://postimg.cc/Fd53kfCZ)

(https://i.postimg.cc/rz9xzw11/Guitar-Synth-P2.gif) (https://postimg.cc/rz9xzw11)

(https://i.postimg.cc/Z0m6TddY/Guitar-Synth-P3.gif) (https://postimg.cc/Z0m6TddY)

(https://i.postimg.cc/dZJCnbYZ/Guitar-Synth-P4.gif) (https://postimg.cc/dZJCnbYZ)

(https://i.postimg.cc/ftcXcw6h/Guitar-Synth-P5.gif) (https://postimg.cc/ftcXcw6h)
Title: Re: EHX EH-8000 Guitar Synth (Schematic)
Post by: Mark Hammer on January 24, 2022, 11:06:17 PM
Holy moly!!!!! :icon_eek:
Title: Re: EHX EH-8000 Guitar Synth (Schematic)
Post by: bluebunny on January 25, 2022, 03:51:15 AM
^^ What he said!  :icon_eek: :icon_eek:
Title: Re: EHX EH-8000 Guitar Synth (Schematic)
Post by: StephenGiles on January 25, 2022, 04:51:15 PM
Holy moly!!!!! :icon_eek:
or to put it another way - "well strap me to a tree and call me Brenda"!! Excellent drawing - a veritable work of art.

Here is the power supply circuit I have:
(https://i.postimg.cc/JsNKqPdM/G-Synth-Power-Supply.jpg) (https://postimg.cc/JsNKqPdM)
Title: Re: EHX EH-8000 Guitar Synth (Schematic)
Post by: StephenGiles on January 25, 2022, 04:55:43 PM
I also have a sample & hold circuit designed by John Williams at EH UK which I'll try to find.
Title: Re: EHX EH-8000 Guitar Synth (Schematic)
Post by: Scruffie on January 25, 2022, 05:07:52 PM
Holy moly!!!!! :icon_eek:
or to put it another way - "well strap me to a tree and call me Brenda"!! Excellent drawing - a veritable work of art.

Here is the power supply circuit I have:
Thank you very much, Brenda! While this design is slightly different to the 2 x TL601 version you have the schematic for, this still seems to match up to the photos I have of the power board in the unit I repaired from a quick peruse.

I also have a sample & hold circuit designed by John Williams at EH UK which I'll try to find.
Colour me extremely intrigued :o
Title: Re: EHX EH-8000 Guitar Synth (Schematic)
Post by: StephenGiles on January 27, 2022, 03:45:58 PM
I also have a sample & hold circuit designed by John Williams at EH UK which I'll try to find.

Here you go!
(https://i.postimg.cc/Lqxb25QC/Guitar-Synth-Mod-Board.jpg) (https://postimg.cc/Lqxb25QC)
Title: Re: EHX EH-8000 Guitar Synth (Schematic)
Post by: Scruffie on January 27, 2022, 05:19:16 PM
I also have a sample & hold circuit designed by John Williams at EH UK which I'll try to find.

Here you go!
(https://i.postimg.cc/Lqxb25QC/Guitar-Synth-Mod-Board.jpg) (https://postimg.cc/Lqxb25QC)
Oh I see! I was thinking you meant a stand-alone effect, that being the case, I better get building a guitar synth.

Thank you very much, Stephen!
Title: Re: EHX EH-8000 Guitar Synth (Schematic)
Post by: bean on January 27, 2022, 07:58:57 PM
Badass Scruffie. Thanks for the share.

Here's a vid I found that does a bit of a deep dive on the compressor/adaptive filtering input section:

Title: Re: EHX EH-8000 Guitar Synth (Schematic)
Post by: StephenGiles on January 28, 2022, 08:01:36 AM
Fantastic, last time I saw those waveforms was at EH Uk 40 years ago!!! Where did you find the video?
Title: Re: EHX EH-8000 Guitar Synth (Schematic)
Post by: bean on January 28, 2022, 08:19:27 AM
Fantastic, last time I saw those waveforms was at EH Uk 40 years ago!!! Where did you find the video?

It was the only thing I could find on youTube searching for the EH-8000. I don't even know what this thing sounds like.  :)

There are some other interesting guitar synth related videos on that channel, as well.
https://www.youtube.com/c/AirgunShootingAndRelatedActivities-ASRA/videos
Title: Re: EHX EH-8000 Guitar Synth (Schematic)
Post by: bean on January 28, 2022, 08:25:25 AM
Here's the calibration instructions for the EH-8000. This came from a Reverb listing:



(https://i.postimg.cc/F16C8t5s/EH8000-calib.jpg) (https://postimg.cc/F16C8t5s)
Title: Re: EHX EH-8000 Guitar Synth (Schematic)
Post by: Scruffie on January 28, 2022, 08:33:55 AM
I'll have to check the video out in a little bit.
Fantastic, last time I saw those waveforms was at EH Uk 40 years ago!!! Where did you find the video?
I don't even know what this thing sounds like.  :)

It's been about a year since I played with it, but it really does sound like a proper synth and is insanely fun to play with. This is the only demo I know of it online which is far more out of tune than it's capable of and doesn't really show off many of the voices, modulation effects or the portamento http://jeffcreamermusic.com/eh-8000-guitar-synthesizer/

Here's the calibration instructions for the EH-8000. This came from a Reverb listing:



(https://i.postimg.cc/F16C8t5s/EH8000-calib.jpg) (https://postimg.cc/F16C8t5s)
Ah, I was going to have to type up the copy I had, thanks for that :)
Title: Re: EHX EH-8000 Guitar Synth (Schematic)
Post by: StephenGiles on January 28, 2022, 04:12:33 PM
 I did record a few things on reel to reel back in the 1980s using my diy EH Guitar synth in pink BASF tape I remember, which I doubt that I still have yet alone a working reel to reel tape recorder.
Title: Re: EHX EH-8000 Guitar Synth (Schematic)
Post by: StephenGiles on January 29, 2022, 09:12:47 AM
Badass Scruffie. Thanks for the share.

Here's a vid I found that does a bit of a deep dive on the compressor/adaptive filtering input section:



fuzz box fanatics might look at this guy's facebook page, linked in the youtube video!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Title: Re: EHX EH-8000 Guitar Synth (Schematic)
Post by: StephenGiles on January 30, 2022, 07:20:09 AM
Here are my drawings from the 1980 circuits I was given by the designer.
(https://i.postimg.cc/z36x1R02/EH-Synth-input-conditioning.jpg) (https://postimg.cc/z36x1R02)

(https://i.postimg.cc/ZWhjWzkr/EH-Synth-p-to-v-tracking-VCOs.jpg) (https://postimg.cc/ZWhjWzkr)

(https://i.postimg.cc/3dGnrJq8/EH-sine.gif) (https://postimg.cc/3dGnrJq8)
Title: Re: EHX EH-8000 Guitar Synth (Schematic)
Post by: StephenGiles on February 03, 2022, 08:11:12 AM
If anyone is interested, I drew those in MS Excel 1997!!!!!
Title: Re: EHX EH-8000 Guitar Synth (Schematic)
Post by: Scruffie on February 03, 2022, 08:23:35 AM
I should point out so it's clear, Stephen's drawings are for an earlier version (schematic dated August '78), identified by the use of 2 x TL601 chips instead of the 1 x TL607 in my drawings (from a schematic dated June '79).

I've seen inside a few Guitar Synth's but have only ever seen the TL607 variant.
Title: Re: EHX EH-8000 Guitar Synth (Schematic)
Post by: StephenGiles on February 04, 2022, 08:12:46 AM
Perhaps they had supply problems back then, we shall never know. I wonder if there ever was an official factory schematic. The synth I played with, which was bound for Steve Howe of Yes, I never saw inside.
Title: Re: EHX EH-8000 Guitar Synth (Schematic)
Post by: Scruffie on February 05, 2022, 03:52:53 PM
Perhaps they had supply problems back then, we shall never know. I wonder if there ever was an official factory schematic. The synth I played with, which was bound for Steve Howe of Yes, I never saw inside.
I did have a small correspondence with Julia a little over a year ago and if my memory is correct, I think she was unaware of the redesign but suggested it was probably just done as a cost saving measure/circuit refinement.
If you look at the original drawings, the change to TL607 managed to save a 311 & 3140 too which was probably a fair saving back then.

There was a couple of other small tweaks throughout the circuit I seem to remember.

Oh and this was redrawn from an official factory schematic.
Title: Re: EHX EH-8000 Guitar Synth (Schematic)
Post by: StephenGiles on March 25, 2022, 01:18:16 PM
Here's the calibration instructions for the EH-8000. This came from a Reverb listing:



(https://i.postimg.cc/F16C8t5s/EH8000-calib.jpg) (https://postimg.cc/F16C8t5s)


What do you mean by a Reverb Listing?
Title: Re: EHX EH-8000 Guitar Synth (Schematic)
Post by: PRR on March 25, 2022, 08:01:44 PM
What do you mean by a Reverb Listing?

reverb.com (https://reverb.com) is a used-gear site. Sellers post pictures of gear they want to sell, and sometimes the paperwork also.

https://reverb.com/item/42350571-electro-harmonix-eh-8000-guitar-synthesizer-black

(https://i.postimg.cc/Yv2nygLZ/EH-8000-GSynth-Align.gif) (https://postimg.cc/Yv2nygLZ)
Title: Re: EHX EH-8000 Guitar Synth (Schematic)
Post by: StephenGiles on March 26, 2022, 03:14:07 AM
What do you mean by a Reverb Listing?

reverb.com (https://reverb.com) is a used-gear site. Sellers post pictures of gear they want to sell, and sometimes the paperwork also.

https://reverb.com/item/42350571-electro-harmonix-eh-8000-guitar-synthesizer-black

(https://i.postimg.cc/Yv2nygLZ/EH-8000-GSynth-Align.gif) (https://postimg.cc/Yv2nygLZ)

Many thanks
Title: Re: EHX EH-8000 Guitar Synth (Schematic)
Post by: StephenGiles on March 26, 2022, 09:50:55 AM
EH-8000 Guitar Synthesizer Alignment Procedure:

1) Short IC205 Pin 6 to IC206 Pin 3

2) Adjust R212 for minimum drift at IC206 Pin 6 (the voltage at pin 6 should be close to ground, but the important thing is minimum drift.

3) Remove short installed in step I.

4) Set VCO1 & VCO2 volume to off, adjust VCO3 and direct levels so that tuning comparison may be made. Tuning is most easily done with a signal generator or keyboard instrument so that continuous test, signal may be applied while tuning. Turning “Fuzz" on the direct input will help if your input signal is harmonically pure.

5) Set signal generator to pitch close to top of guitar range (I kHz Is good)

6) Adjust R206 so that VC03 is In tune I octave above Input signal.

7) Set signal generator to pitch at low end of guitar range (80 - 100 Hz).

8) Adjust R208 so that VCO3 is in tune I octave above Input signal.

9) Repeat steps 5 - 8 as necessary until both high and low ends are in tune. Note that if you have access to an oscilloscope the tuning process is much easier if you look at the signal "DIF" at IC204 Pin 6 while you do the tuning. When you see no “beating" on the scope VCO3 is in tune.

10) Set VCO3 volume to off, turn up VCO1 volume.

11) Set signal input to about I kHz.

12) Tune VCOI to unison with the Input signal.

13) Set signal input to 1OOHz.

14) Adjust R223 to bring VCO1 in tune.

15) Repeat steps 11 - 14 as necessary.

16) Repeat steps 11 -15 with VCO2, adjusting R238 instead of R223.

17) R207 can be adjusted for best compromise between tracking speed and overshoot. This adjustment is best made with actual guitar input Alternate between notes an octave or so apart and adjust R207 for the best sound.