Mosky Plexi-m  [schematic]

All about modern commercial stompbox circuits from Electro Harmonix over MXR, Boss and Ibanez into the nineties.
Post Reply
User avatar
howmuch
Information
Posts: 46
Joined: 09 Nov 2014, 18:31
Has thanked: 12 times
Been thanked: 36 times

Post by howmuch »

This is obviously based on the Single Channel Carl Martin Plexitone.
I really like the sound of this pedal. It's got that Bright-Channel Plexi character and a lot of gain.
If you're after a pedal that will help you get an EVH brown sound, you could do a lot worse than trying this one.
At less than £20 new, you can't go wrong.

I can't find a schematic of the Single Channel Plexitone - I wonder how close the Plexi-m is to it.
It would be interesting to see a schematic of the CM Panama to compare as well (do you have one bugg?).
Guessing it's very similar, with an extra variable-HPF "Damping" control.
The circuit is slightly different to the (discontinued?) Caline English Man, going by the schematic that was posted here.

I've sketched a schematic based on the visible traces on the component side and referring to the "PlexiTuna" and "Boneyard" schematics.
So some of it will be correct and some of it is guess-work.
Some of the SMD capacitors can't be measured in-circuit, so I've included values in parentheses from the other schematics.
Since the Single Channel Plexitone was tweaked from the original, those capacitor values could be wrong.
The 7660S is configured to generate -Vcc from the supplied Vcc.

Nearly all of the components are on the bottom side of the full-size PCB.
There's a smaller PCB for the pots, LED, DC Jack and a 1N5819 (could be 1N5817) diode.
The two ICs are LF347 quad op amp and 7660SCBA voltage converter.
The asymmetric clipping is done with 3 red LEDs.
The only thing I don't like is that the In/Out jacks are only hanging on by their nuts.

Here's some photos and that rough schematic.
Let me know if you spot any mistakes.
P1070717.png
P1070780.png
P1070731.png
P1070751.png
P1070754.png
P1070762.png
Mosky_Plexi-m.PNG
Sad news concerning Burke Shelley, from one of my favourite (and much underrated) bands - Budgie
Their songs have been covered by Van Halen, Iron Maiden, Soundgarden, Metallica and more.
If you don't know how HEAVY this band was in the 70s, check Breadfan out.

User avatar
FuzzMonkey
Breadboard Brother
Information
Posts: 102
Joined: 28 Dec 2009, 22:10
Has thanked: 29 times
Been thanked: 23 times

Post by FuzzMonkey »

I've been tempted to pick up one of these as I also heard it was based on the single-channel PlexiTone.

User avatar
Manfred
Opamp Operator
Information
Posts: 1305
Joined: 04 Apr 2009, 23:42
Has thanked: 1182 times
Been thanked: 799 times

Post by Manfred »

I ordered a Plexi-m today.
When I receive it I can find out the missing capacitance values in the signal path.
With my PEAK LCR45 LCR and impedance meter I can measure the impedance of the RC circuits and determine the capacitor by calculation.

User avatar
Ice-9
Transistor Tuner
Information
Posts: 1074
Joined: 31 Mar 2009, 15:29
my favorite amplifier: Marshall
Completed builds: Far to many to afford the enclosures for them all. :)
Has thanked: 132 times
Been thanked: 354 times
Contact:

Post by Ice-9 »

Manfred wrote: 17 Mar 2021, 00:33 I ordered a Plexi-m today.
When I receive it I can find out the missing capacitance values in the signal path.
With my PEAK LCR45 LCR and impedance meter I can measure the impedance of the RC circuits and determine the capacitor by calculation.
I think I would be tempted to just remove and measure the caps, far quicker ;)
It looks like\a nice little pedal especially for £20 ish
It's fairly straight forward, if you want to start it , press start. You can work out the rest of the controls for yourself !

No silicon heaven ? preposterous ! Where would all the calculators go ?

User avatar
Manfred
Opamp Operator
Information
Posts: 1305
Joined: 04 Apr 2009, 23:42
Has thanked: 1182 times
Been thanked: 799 times

Post by Manfred »

Ice-9 wrote: 17 Mar 2021, 21:51
Manfred wrote: 17 Mar 2021, 00:33 I ordered a Plexi-m today.
When I receive it I can find out the missing capacitance values in the signal path.
With my PEAK LCR45 LCR and impedance meter I can measure the impedance of the RC circuits and determine the capacitor by calculation.
I think I would be tempted to just remove and measure the caps, far quicker ;)
It looks like\a nice little pedal especially for £20 ish
Thanks for the hint but measuring and calculating is faster for me, I know how to calculate AC circuits with the help of complex numbers.
I won't touch the SMD components.

User avatar
Manfred
Opamp Operator
Information
Posts: 1305
Joined: 04 Apr 2009, 23:42
Has thanked: 1182 times
Been thanked: 799 times

Post by Manfred »

Yesterday the Mosky Plexi-m I ordered arrived.
I found the following capacitance values:
C4 = 150pF, C7 = 100nF, C9 = 47p.
C14 is shorted by BF1, so I could not measure its value, the capacitor makes no sense at this point.
The measurement of the leds showed that they are all of the same type:
Vf=1,89V at 5,00mA
Here is the measured P/N junction curve:
PN-junction Clipping Single LED Mosky Plexi-m .jpg
Last edited by Manfred on 03 Apr 2021, 00:21, edited 1 time in total.

User avatar
howmuch
Information
Posts: 46
Joined: 09 Nov 2014, 18:31
Has thanked: 12 times
Been thanked: 36 times

Post by howmuch »

Manfred wrote: 31 Mar 2021, 00:46 Yesterday the Mosky Plexi-m I ordered arrived.
I found the following capacitance values:
C4 = 150pF, C7 = 100nF, C9 = 47p.
Thanks Manfred.
I've added them to the schematic. Here's v02
Mosky_PlexiM_v02_Schematic.PNG
Sad news concerning Burke Shelley, from one of my favourite (and much underrated) bands - Budgie
Their songs have been covered by Van Halen, Iron Maiden, Soundgarden, Metallica and more.
If you don't know how HEAVY this band was in the 70s, check Breadfan out.

User avatar
Manfred
Opamp Operator
Information
Posts: 1305
Joined: 04 Apr 2009, 23:42
Has thanked: 1182 times
Been thanked: 799 times

Post by Manfred »

i have searched in some applications with ICs 7660S and LT1044. In most of them there is no film capacitor connected in parallel to the electrolytic capacitor on Vout.
Only one circuit had a 100nF connected in parallel, so I think that also here C16 = 100n is correct.
Also you can remove BF1 and C14 from the schematic.

User avatar
howmuch
Information
Posts: 46
Joined: 09 Nov 2014, 18:31
Has thanked: 12 times
Been thanked: 36 times

Post by howmuch »

Manfred wrote: 31 Mar 2021, 15:08 ...so I think that also here C16 = 100n is correct.
Cheers Manfred. I won't bother re-doing the schematic again for that; next time round.
Manfred wrote: 31 Mar 2021, 15:08 Also you can remove BF1 and C14 from the schematic.
Yes, technically I could remove BF1, BF2, C14 and D6, but I drew the schematic to represent the Mosky Plexi-m, so since those components are there, I'll leave them.
If anyone wanted to build their own from this (although even if you already had all the components, it's far easier to just spend the £20), hopefully they'd have the sense to optimise those parts.
Sad news concerning Burke Shelley, from one of my favourite (and much underrated) bands - Budgie
Their songs have been covered by Van Halen, Iron Maiden, Soundgarden, Metallica and more.
If you don't know how HEAVY this band was in the 70s, check Breadfan out.

User avatar
Manfred
Opamp Operator
Information
Posts: 1305
Joined: 04 Apr 2009, 23:42
Has thanked: 1182 times
Been thanked: 799 times

Post by Manfred »

The PN-junktion V/I curve I measured seemed strange to me, so I repeated the measurement.
It is difficult to contact the SMD components in the circuit.
I edit the curve and the value in the above post.
Here are the curves of D3 and D1&D2 together in one picture.
PN-junction Clipping LED Mosky Plexi-m.jpg

Post Reply