EHX Bad Stone Phase Shifter EH5850 V1 'Mummy Face' Traced  [schematic]

Discussion regarding early stompbox technology: 1960-1975 Please keep discussion focused and contribute what info you have...
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TWSpedals
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Post by TWSpedals »

Crazy this doesn't seem to have been traced before.

I was compelled to trace this unit as the only available schematic (a factory one at that) didn't match what I had infront of me.

Noticed differences between factory schematic and my trace:
•There's numerous resistor value differences
•A whole output buffer added
•No resistor between ground and the + input on the final all-pass stage.
•The colour switch and blend labelled the right way.
•No input 'pulldown' resistor.

Quad op-amps have been sanded.
Jfets have no identifiers other than 'SCOTT'
Attachments
Power section, last all-pass filter, LFO
Power section, last all-pass filter, LFO
95% of signal path
95% of signal path
Have a nice day!

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uncleboko
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Post by uncleboko »

I think from memory the FETs were 2N4302, but I'm sure I made one using 2N3819!

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audiohub
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Post by audiohub »

My roommate had one of these back in the mid 70's, and we traced it back then. Your trace looks pretty much like I remember it from way back then.
We figured out that the opamps were probably LM324s, which had an application note for those 27k resistors tied to -9 volts on the outputs (plus, we were actually able to still read the number on one of the packages).
It was a pretty good sounding phaser, as I remember.
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george giblet
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Post by george giblet »

TWSpedals wrote: 21 Apr 2024, 19:18 Crazy this doesn't seem to have been traced before.

I was compelled to trace this unit as the only available schematic (a factory one at that) didn't match what I had infront of me.
Good work. It's amazing what you find when you do a trace of some pedals. Plenty of cases where schematics having been kicking around for years and have been wrong, or, there's more than one version.

(I thought I had a schematic for a JFET based EHX Phaser. I looked through one of my archives and couldn't find it. It may be in a later archive but those aren't easy to get to ATM.)

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fanheritage
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Post by fanheritage »

uncleboko wrote: 21 Apr 2024, 20:53 I think from memory the FETs were 2N4302, but I'm sure I made one using 2N3819!dinosaur game
Will it give the same results?

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uncleboko
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Post by uncleboko »

fanheritage wrote: 13 May 2024, 02:16
uncleboko wrote: 21 Apr 2024, 20:53 I think from memory the FETs were 2N4302, but I'm sure I made one using 2N3819!dinosaur game
Will it give the same results?
That I can't remember - 40 years or so ago!

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