DMB Lexi pedal, gated fuzz!?!?!

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Erok
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Joined: 18 Mar 2013, 04:25

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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Joined: 18 Mar 2013, 04:25

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.
"Analog electronics in music is dead. Analog effects pedal design is a dead art." - Fran

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?

Erok
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Joined: 18 Mar 2013, 04:25

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.

Erok
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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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Intripped
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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

Manfred
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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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Joined: 18 Mar 2013, 04:25

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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