EHX Microsynth schematic doubts

[DVB_master]

The 2017 Revised Schematic is the most correct schematic.   

The identified errors on this schematic are:
- There should be an additional 10k resistor wired between the circled (1)'s  on the two schematic pages.
   (based on STM's fixes to the 1996 schematic)
- The value of C5 has not been confirm on a PCB. (However STM's fixes to the 1996 schematic didn't say this needed fixing so it is assumed to be correct.)
- R12 should be 7k5 not 1k5.  (As it is a standard rectifier circuit.)
- R14, SQUELCH, is a trimpot.   I suspect it is set so the output of ICA3A pin 1 is zero volts. (However there may be a production setting for this.)

You are definitely correct. Thanks for summarizing.

One thing I did note is the numbered wire connections on the actual PCB are *not* the circled numbers on the schematic.  The numbered wire connections are not shown on the schematic.

Correct. However, in the 2002 revision they match the PCB silkscreen.

[Rob Strand]

You are definitely correct. Thanks for summarizing.

Ahhh, it's still not correct!

I found some bugs.   
- stm's swt's list has not been fully covered
- I think I stuffed up on the 10k resistor between the circled (1)'s.
- When I trace the PCB from pin 1 IC A2A  I see another 10k resistor a 27k resistor (R57) .

Give me a bit of time and I'll clean them up.
If you want to check as well and we can compare notes.

[bean]

The other end of #1 connects to R57...so that could just be 36k instead of 27k and skip the 10k. Or, am I reading your note wrong?

[Rob Strand]

The other end of #1 connects to R57...so that could just be 36k instead of 27k and skip the 10k. Or, am I reading your note wrong?

Thanks.  Yes, I've got that one sorted.   There's two circled 1's on sheet two so I've re-phrased the 10k + 1k fix. The fix/added resistor applies locally around R105 not between the two sheets.  I also noted there's 10k + 1k not 10k + 10k.

Had to trace the PCB pics again what a pain.
It was worth it because I found other details.

I also said stm did the fixes when it was actually swt.

[Rob Strand]

Electro Harmonix Guitar Microsynth - Fixes and Notes
Rob S, V1.0, 26 April 2018
-----------------------------------------------
Notes:
- Circled numbers on the schematic are not the numbered wire connections on the PCB.
  They just link points on the schematic.
  The numbered wire connections are not shown on the schematic. *** see below
- The schematic does not show which pot terminals are CW an CCW.
- The unit is constructed with two PCBs.
-----------------------------------------------
Schematic Errors

The 2017 Revised Schematic is the most correct schematic.

   https://www.docdroid.net/u3cx2Xj/electro-harmonix-micro-synthesizer-revised-2017.pdf

The identified errors on this schematic are:
- There should be a 10k resistor between the circled (1) near R105 and R105 on sheet 2.
  (Note: R57 (27k) connects to IC A2A. 
   So the circle (1) near R57 connects to the (1) on sheet 1 near IC A2A.)
- On the schematic, R105 should be 1k, not 10k.
  The junction of the two resistors wires to the footswitch S1 (bypass signal).
- R12 should be 7k5 not 1k5. 
  (As it is a standard rectifier circuit.)
- R3, PRE-AMP GAIN, is a trimpot.
- R14, SQUELCH, is a trimpot.
- The value of C5 has not been confirm on a PCB.
  (However swt's fixes to the 1996 schematic didn't say this needed fixing so it is assumed to be correct.)
-----------------------------------------------
Parts:
JFET    Q2 bf245a
Diodes   1n4148
-----------------------------------------------
Supplies:   
   +8V to +9V
   -10V
   0V
-----------------------------------------------
The unit has 10 pots and 2 trimpots.
- trimpots 100k
- all pots 100K linear

Trimpots:
R3    PRE-AMP GAIN    (sheet 1)
   Presumably set for unity gain when Footswitch S1 in bypass position.
R14    SQUELCH      (sheet 1)
   Presumably set for 0V at output of IC A3A pin 1 with no input signal.

Pots, from left to right on the front panel:
R31   TRIGGER      (sheet 1)
R61    SUB OCTAVE   (sheet 2)
R6    GUITAR      (sheet 1)
R28    OCTAVE      (sheet 1)
R36    SQUARE WAVE   (sheet 1)
R101    ATTACK DELAY   (sheet 2)
R86   RESONANCE   (sheet 2)
R92   START FREQ   (sheet 2)
R65   STOP FREQ   (sheet 2)
R74   (sweep) RATE   (sheet 2)
-----------------------------------------------
Wires

The input/output PCB has 13 numbered wire connections and 4 un-numbered wire connections.

Un-numbered wires:
Yellow wire:   R1 (68k) to input socket  ; input signal
Violet wire:   R1 (470R) ground side to input socket ground
Green wire:   R105 (between 1k and 10k) to footwitch S1 (bypass signal)
Brown wire:   R104 (470R) to footswitch S1 (effect signal)

Numbered wires 1 through 13
TBD *** see below
-----------------------------------------------
***

The 2017 Revised Schematic is the most correct schematic.   

The identified errors on this schematic are:
- There should be an additional 10k resistor wired between the circled (1)'s  on the two schematic pages.
   (based on STM's fixes to the 1996 schematic)
- The value of C5 has not been confirm on a PCB. (However STM's fixes to the 1996 schematic didn't say this needed fixing so it is assumed to be correct.)
- R12 should be 7k5 not 1k5.  (As it is a standard rectifier circuit.)
- R14, SQUELCH, is a trimpot.   I suspect it is set so the output of ICA3A pin 1 is zero volts. (However there may be a production setting for this.)

You are definitely correct. Thanks for summarizing.

One thing I did note is the numbered wire connections on the actual PCB are *not* the circled numbers on the schematic.  The numbered wire connections are not shown on the schematic.

Correct. However, in the 2002 revision they match the PCB silkscreen.

So the circled numbers on the 2002 schematic match the PCB silkscreen.

[DVB_master]

Just to summarize the outcome:  (I don't know how I got through that yesterday I was pretty darn tired.)

The 2017 Revised Schematic is the most correct schematic.   (BTW: Thanks for putting that up.)
https://www.docdroid.net/u3cx2Xj/electro-harmonix-micro-synthesizer-revised-2017.pdf

The identified errors on this schematic are:
- There should be an additional 10k resistor wired between the circled (1)'s  on the two schematic pages.
   (based on swt's fixes to the 1996 schematic)
- The value of C5 has not been confirm on a PCB. (However swt's fixes to the 1996 schematic didn't say this needed fixing so it is assumed to be correct.)
- R12 should be 7k5 not 1k5.  (As it is a standard rectifier circuit.)
- R14, SQUELCH, is a trimpot.   I suspect it is set so the output of ICA3A pin 1 is zero volts. (However there may be a production setting for this.)

One thing I did note is the numbered wire connections on the actual PCB are *not* the circled numbers on the schematic.  The numbered wire connections are not shown on the schematic.

Sorry, I missed that 10k/1k divider at the output... Your list is correct . If I find some time, maybe I can review the schematic and upload somehow.
Thanks for your support and the discussion. It helped a lot to clarify some aspects of this effect!

[DVB_master]

Hi Rob, I am reading your notes. In the preamp section:

Input impedance: 68k,  this will load the guitar pickup.
Variable gain:   x1 to x22

The gain of x22 is reached when the trimpot is 10k (as in the old schematic) and the center taper is in the CW position. With a 100k trimpot, the maximum gain of the preamp is around 200...
This let me think that the 10k pot was OK... What do you think?

I have another question about the output signal. When the effect is bypassed, I assume that the output should be (roughly) at the same amplitude as the guitar input. Since we have a 10k/1k divider at the "bypass fx out", the output signal is equal to the output of the preamp divided by 11. This let me think that the preamp trimmer should be set to achieve a gain of 11 in order to compensate for the output divider in "bypass condition". Do you or anyone else have some comments?

[Rob Strand]

Sorry I missed your reply the other day.

This let me think that the 10k pot was OK... What do you think?

Probably the case.  From what I could work out I wrote that before the fixes arrived.  I know I didn't get back to finish analyzing that device.

I have another question about the output signal. When the effect is bypassed, I assume that the output should be (roughly) at the same amplitude as the guitar input. Since we have a 10k/1k divider at the "bypass fx out", the output signal is equal to the output of the preamp divided by 11. This let me think that the preamp trimmer should be set to achieve a gain of 11 in order to compensate for the output divider in "bypass condition". Do you or anyone else have some comments?

Yes agree 100%.  In my summaries above I put some notes about the trimpot probably being set for unity gain in bypass.

[Cybercow]

Rob Strand - I just wanted to say thank you for the analysis on the EHX Micro Synth and the corrected schematics. They are being a big assist in helping me trouble shoot the bass version of this pedal now on my bench. Tip-o-the-hat to ya!

Moo

[Rob Strand]

Rob Strand - I just wanted to say thank you for the analysis on the EHX Micro Synth and the corrected schematics. They are being a big assist in helping me trouble shoot the bass version of this pedal now on my bench. Tip-o-the-hat to ya!

Thanks for the feedback.

(I've got stuff on my computer like that I can't remember doing at all.)

[carlsoti]

My sincerest thanks to Mr Strand for the analysis. Much like Cybercow, this thread helped me de-bug a first gen vintage Bass Micro Synth. I suspect that a previous owner inadvertently turned the squelch trim pot, as the hole in the back doesn't quite line up with the pre-gain trimmer, requiring removal of the panel to make the appropriate adjustment.

[Rob Strand]

My sincerest thanks to Mr Strand for the analysis. Much like Cybercow, this thread helped me de-bug a first gen vintage Bass Micro Synth.

Thanks for the feedback.  I'm glad I put this up instead of rotting away on my harddrive.   (there's probably several thousand text files with notes on different subjects.)

[Andon]

Apologies for bringing up an older topic (last post was July of 2022). Rob's notes have been extremely helpful in better understanding this circuit, but if I were wanting to just extract the octave up and attack delay portions of the schematic what would I be looking at? On the first page of the schematic it seems like I'd need everything except for all of the square wave blocks (mostly on the left hand side) and likely the peak follower (could be wrong on that last one), and on the second page it feels like all I really need is the bottom left hand portion with the VCA and the networks containing the 2n5088s, though I suspect I might also need Q3 and A13 (which would be fine since I'd already be using half of an LM13700 for the VCA anyway). Also the way the 100K Octave pot connects (via point 4 on both pages) it seems like I could just take that straight to the output if I were wanting to separate the octave from the attack delay, but I'm happy to be shown otherwise.

EDIT: Here's what I'm thinking I need. Green seems likely, yellow is maybe to me (though there may be some stuff in green I don't need).

Page 1:



Page 1 full size: https://i.imgur.com/qZjwhZj.png

Page 2:



Page 2 full size: https://i.imgur.com/Cq7nxLK.png

[Andon]

Bumping for the new week. Starting to breadboard this after a long weekend at work.

[Kevin Mitchell]

Bumping for the new week. Starting to breadboard this after a long weekend at work.

Hey!
I had successfully breadboarded the EH Microsynth a few years back. It was a hell of an undertaking!
I was able to hash out all errors in the available schematics (both the original you're referring to and the Mircosynth one from the other site) after reverse engineering a couple of em'.
-Just thought I'd finally share my notes on it.

The errors in the original schematic;
R12 is 7.5K, not 1.5K (this is fixed in your schematic but not the original)
R105 is 1K, not 10K (same as above, fixed in your schematic but not the original)
And what I see missing;
Where (1) connects to R105 there's actually a 10K resistor in between R105 and (1). This resistor is missing entirely from the schematic.

So you're borrowing from the design for an octave effect and variable delayed volume swell? That's a neat idea though playing Dr. Frankenstein for such a thing is likely more trouble than starting a design from scratch! You're not asking for much after all.

Edit:
Got bored & wanted to draw it out. Here's a simulation of the octave section. - don't need a breadboard to tinker!
https://tinyurl.com/2n6emnax

The additional circuits for the attack delay could very well be revised to suite your needs. Otherwise you'd have to rip the entire envelope design for such a simple task.

[Andon]

Thanks for the input! I ended up adjusting the feedback resistors on the input and output (instead of using a pot), dropping the 3-pole 700Hz filter and that opened up the whole neck, and then I did the back-to-back diode pair + small resistor (470R) to ground with a 10nF in parallel (also to ground) at the output, per Mark Hammer's suggestions on the "cleaner" Green Ringer, and it sounds great!

I think the attack delay in this circuit appeals to me because it seems "easier" or st least more foolproof than others I've seen - any suggestions on that?