DMB Lexi pedal, gated fuzz!?!?!

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Erok
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Post by Erok »

I have here a 3 knob DMB Lexi pedal that sounds like a gated fuzz pedal.
It is supposed to sound like a glorius, cranked PLexi sound...get it?
The company is out of business and I can't find a schematic.
Uses 3 x J201 and 2 x MPF102's and 1 x 5099.
It's gotta be a close relative to a BSIAB, yes?

All the knobs work, Gibson SG into JCM800 low(clean) input.

Level knob makes it louder
Bite knob makes it brighter
Gain knob...all the way off does nothing-pedal is off unless you SLAM the guitar strings, then a short fuzz/sputter comes through the amp, very short decay. Turn Gain to 9 oclock and it is easier to fuzz/sputter, still a shortish decay. Noon is really easy to fuzz/sputter and full up sounds like a fuzz pedal. Sounds nothing like the video below.

Supposed to sound like:
https://www.youtube.com/watch?v=tBa3kp_xks4
Last edited by Erok on 03 Jul 2020, 02:04, edited 1 time in total.

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soulsonic
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Post by soulsonic »

Does it have trim pots? If it does, maybe one got knocked out of whack and now one of the JFETs is misbiased. In any case, it sounds like a JFET is misbiased for some reason. Measuring the voltage at the Drain of each one should tell you easily enough if something is way off.
Should post up some pics. Would be helpful.
"Analog electronics in music is dead. Analog effects pedal design is a dead art." - Fran

Erok
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Post by Erok »

Thanks for the help
Voltages:
MPF102
7.78
6.04
3.71

MPF102
5.96
1.9
0.00

J201
7.57
4.75
3.6

J201
5.94
1.98
0.00

J201
3.5
.83
0.00

2N5099
7.59
-320mv
0.00

Pics!
Attachments
Lexi_top_web.jpg
Lexi_under board.jpg
Lexi_pot guts.jpg

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soulsonic
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Post by soulsonic »

Seeing over 7v on the Drains of a couple of the JFETs makes me think the bias is off on those. But that wouldn't be the case if they were used as buffers in the circuit. It's hard for me to tell what does what from the photos. Someone needs to trace it out.
Just based on guessing, I would try adjusting the trimmer that's next to the J201 that you measured 7.57 volts on and see if adjusting it can get it to read around 4.5v, and if so, does that improve the sound. Maybe that's the problem spot.
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Erok
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Post by Erok »

Thanks, I have already adjusted the mini pot, there is only one spot where it allows the pedal to pass signal and that spot is where it is now at = horrible.
Yeah, I will try to trace the circuit, messy little bugger for $200 pedal.

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Intripped
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Post by Intripped »

trimmers can be very touchy, even a micro-adjustment could be significant

Erok
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Post by Erok »

Tried the pot again today : )
There truly is only a minute position that allows the ugly sound to come through, however...

I was really close to the speaker. I could actually hear a cranked "plexi" sound coming through the amp, only at microscopic hearing levels.
The adjustment pot needed to be at a lower setting though, if I turned the pot higher the background signal disappeared(I'm thinking over-biased/cutoff at that point).

So there is a trace signal getting through somewhere, does that point to a bad part?
Maybe a bad ground?

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Post by Erok »

FIXED!...? Amperage starved?
I am going to remove the 9v battery hookup from the pedal.
Found out my Pigtronix Micro Disnortion pedal needs 500mA to work correctly, so, I took that information and applied it to the DMB Lexi.
My isolated multi-power supply only has two max a 9V 300mA taps.
I tried it on a 100mA tap and it sounded a little muddy on the low notes and more fuzz-like.
Switched to one of the 300mA taps and it came alive, sounded cleaner/brighter/tighter like a cranked plexi, bass notes much less muddy.
Kind of like an OCD 9v vs 18v.
I'm going to hit the Salvation army and see if I can find a 1-3 Amp wallwart.

This is how stupid manufacturers are these days(or pedal owners : ), they don't even have information on current draw for their own products(not labeled on the products).
I could have fixed this simply, two years ago.

Also, thanks for the replies(Soulsonic, Intripped), without that extra interaction I probably would have gutted the pedal and built a mini Klon clone, whew!

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Intripped
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Post by Intripped »

i'm happy you have the sound back!
it's strange anyway that this pedal is so current-hungry. There can be exceptions, but usually analog pedals don't need many milli-amperes, and this circuit seems to be quite simple.
....maybe something else is happening?
with some other pics of the top, & bottom sides as well (disturbing green foam and light reflections), someone here could for sure trace the circuit and make things clearer.

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Post by Erok »

I'll get clearer pics.
Green foam is remnants from glued on sheet of foam.

Better pics:
Attachments
Lexi Green pad.jpg
Lexi Top CU.jpg
Lexi Bottom.jpg

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Post by Intripped »

Thanks, let's see if someone will help in the tracing job. I'll do my best but for the next days I'm with my phone only, and I'll be travelling. be patient

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Post by Manfred »

I intend to create a PCB clone, but I should know what is behind the green mass.
Is it possible to remove this mass?

Erok
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Post by Erok »

I'll trade you a lump of green mass for correct voltages and possible better upgrades and maybe the addition of "clean" blend knob.
https://www.youtube.com/watch?v=esnQaZ2ldz4

Clean pic
Attachments
Lexi cleaned.jpg

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Intripped
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Post by Intripped »

Hey,
I'm working on the schematic and I'm 90% done.
I can't read many components' values.
Erok, could you please write them down? That would be great

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Post by Manfred »

Hi Erok,

what are the dimensions of the PCB?

Erok
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Post by Erok »

Cap values
Attachments
Lexi-Caps.jpg

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Post by Manfred »

Manfred wrote:
06 Jul 2020, 19:20
Hi Erok,

what are the dimensions of the PCB?
My estimate is 66 x 44mm.
I am planning to make a PCB clone, please tell me the correct dimensions.

Erok
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Post by Erok »

39 x 58mm

More detail shots and high contrast back of the board.
Attachments
Deet7.jpg
Deet1.jpg
Deet2.jpg
Deet3.jpg
Deet4.jpg
Deet5.jpg

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Post by Manfred »

Hi Erok
I made this PCB clone.
The colors and order of the rings on the resistors are hard to recognize.
Please check the values I entered and correct them if they do not match.
I have numbered the wire connections, please fill in the destination to which they lead.
What is the value of the trim import potentiometer?
DMB LEXI Layout.JPG
DMB LEXI PCB Componet side.jpg
DMB LEXI PCB Wire Connectios.jpg
DMB LEXI PCB Wire Connectios.jpg (5.42 KiB) Viewed 24 times

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Post by Intripped »

I'm in a very isolated area and i have problem with internet connection, so i don't know if i can upload a picture of the schematic. I'll try later anyway.

In the meanwhile i can write the wires designations:
1 - red - +9V
2 - blue - fx IN
3 - blue - gain3
4 - green - gain2
Not on PCB - black - gain1 - ground
5 - black - ground
6 - blue - volume3
Not on PCB - green - volume2 - fx OUT
Not on PCB - Black - volume1 - ground
7 - orange - tone1
8 - green - tone2
(9) - blue - tone3

PS: wire #9 is close to wire #7 on the PCB

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