Mosky Plexi-m  [schematic]

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howmuch
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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
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If you don't know how HEAVY this band was in the 70s, check Breadfan out.

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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.

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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.

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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 ?

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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.

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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.

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howmuch
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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
R.I.P. 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.

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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.

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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.
R.I.P. 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.

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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

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Post by Manfred »

Finally I recorded the frequency response depending on the position of the tone potentiometer.
Plexi-m Frequency response.jpg

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Post by howmuch »

Hey Manfred,
My Plexi-m is giving a fizzy, crackly-ness to the notes, which is especially noticeable on note decay or when you play softly or reduce the guitar's volume.
Best way to reproduce it is Tone=Max, Gain=Min, into a clean amp.
Does yours have this fizz also?
R.I.P. 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.

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Post by Manfred »

howmuch wrote: 30 Jul 2021, 13:07 Hey Manfred,
My Plexi-m is giving a fizzy, crackly-ness to the notes, which is especially noticeable on note decay or when you play softly or reduce the guitar's volume.
Best way to reproduce it is Tone=Max, Gain=Min, into a clean amp.
Does yours have this fizz also?
Yes, I was able to reproduce that with the settings you gave.
I fed the effect output into the input of the Mooer Hornet in clean mode.
I will look at the output signal with the oscilloscope the next few days.

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Post by howmuch »

I looked at a few YT demos of the Plexi-m and the comments were interesting.
A few people say that they have had the fizz problem, one person with three bad units.
They all seem to be from 2021. Perhaps they were OK before this.

Anyway, I finally got around to de-bugging this fizz problem.
BTW, in the process, I removed C4 and measured it - it was 100pF.

I got the sig. gen. and 'scope out.
Looking at the output of the "clipping stage" op amp, I could see crossover distortion.
So now I know what crossover distortion sounds like.
Crossover Distortion
Crossover Distortion
Crossover Distortion
Crossover Distortion
I figured the only way that could happen in this circuit is a bad op amp.
So I replaced the LF347 with a new one, and that fixed it. :)
With a guitar, note decay is now clear and natural with no fizz. :D
Mosky must either be using bad chips or damaging them in their process.
New Op amp - Good Signal
New Op amp - Good Signal
R.I.P. 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.

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Post by temol »

I figured the only way that could happen in this circuit is a bad op amp.
So I replaced the LF347 with a new one, and that fixed it. :)
I have couple dozens of cheap TL072 opamps (from Aliexpress). All with a similar issue.

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Post by Manfred »

temol wrote: 10 Aug 2021, 10:02
I figured the only way that could happen in this circuit is a bad op amp.
So I replaced the LF347 with a new one, and that fixed it. :)
I have couple dozens of cheap TL072 opamps (from Aliexpress). All with a similar issue.
There arises neither the question fakes or bought out rejects what is sold there in certain countries of East Asia.
As mentioned elsewhere, I am cured after some negative experiences and no longer buy in these countries.
Also, you have to be careful, there are, I call them container companies,
that pretend to buy goods from your own country.

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Post by Manfred »

Anyway, I finally got around to de-bugging this fizz problem.
BTW, in the process, I removed C4 and measured it - it was 100pF.
Yes, C4 with 100pF is the correct value. I made the mistake and did not take into account the capacitance of the LEDs connected in parallel.
I measured the capacitances of red LEDs of different sizes and different current ratings, unfortunately I don't have SMD LEDs available.
The measured capacitances were between 5pF up to 45pF.

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Post by howmuch »

I can be stupid sometimes. I forgot to make a recording of this fizz problem before I fixed it. :slap:
So could someone who has the fizz make a simple recording and add it here for posterity?

I'd suggest this - Gain=Min, Tone=Max, Level=whatever, into a very clean amp/channel. Strum a single chord and let it die out slowly.
Thanks.

BTW - Looking at that manufacturer logo on the LF347 in the photos above, does anyone know what manufacturer that is?
It looks a bit like Texas Instruments' logo if you squint. Is this chip just a bad/fake rip-off?
R.I.P. 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.

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Post by Manfred »

The logo looks familiar, it looks like a fake Texas logo.
I recently had this on transistors I bought from a dealer in Germany with this logo.
The measurement with my semiconductor analyzer gave values far from the specifications.
This dealer pretended to have purchased them from a well known distributor, that's how far it has come in the meantime,
It is funny that Mosky became a victim of fakers from their own country.

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Post by temol »

If you still have this IC you can make a simple setup and check with the oscilloscope if there's crossover distortion present.
Build a simple gain stage, insert 1kHz sinewave and that's all :)

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