DMB Lexi pedal, gated fuzz!?!?!

Manfred · Post by **Manfred** » 16 Jul 2020, 23:22

For comparison I have ran simulations with the AC127 and the 2N5089.
With the AC127 the output voltage was always about 80% of the input voltage, with the 2N5089 between about 0.015% and 25%.

Manfred · Post by **Manfred** » 16 Jul 2020, 23:58

Insert a 2.7 megaohm resistor between V+ and the emitter of the 2N5089, the stage will work properly with it.

Erok · Post by **Erok** » 17 Jul 2020, 00:02

Made a 2.7M resistor(series 2.2M + 200k)patched into circuit.
Would this be a positive feedback loop?

2N5089 readings 9V battery reads 8.2V.
E = 3.1V
B = 596mv
C = 114.3mv

Anyway, the pedal is louder, but just barely reaches unity gain+. The sound is not satisfying, mushy/splatty fuzz(does not come close to the video link in the first post). The gain knob is really only effective at the end of the range between 9-10.

So, DMB manufactured a non-working pedal, no wonder they went out of business.

Manfred · Post by **Manfred** » 17 Jul 2020, 06:05

C = 114.3mv

?
UC should be 8.2V.

Replace the 47k resistor between the first two stages with a 470k resistor.
In similar circuits from which the LEXI circuit was obviously derived, the parallel circuit of a 470k resistor and a 470p capacitor was used at this point.

Erok · Post by **Erok** » 17 Jul 2020, 06:51

That should just be the regular plexi treble peaking circuit?
The pedal sounds more plexi with the 2N5089 bypassed.

I'll try it though.

Intripped · Post by **Intripped** » 17 Jul 2020, 12:23

... So the last transistor is a 2N5089, reverse mounted?
Have you already tried to substitute it with a new one?
i'll update the schematic soon

Manfred · Post by **Manfred** » 17 Jul 2020, 13:58

I see, I overlooked that 2N5089 is reverse mounted.
I have now done the simulation without the bias resistor, there is also the same very small and distorted signal at the ouput.
The interesting thing is that the amplitude is decreasing with time and disappears more and more.
With the bias resistor of 2Megohms the output amplitude is only about 20% of the input amplitude.
With a bias resistor of 2 Megohms the negative signal amplitude starts to be rounded at about Vpeak = 180 millivolts and to be limited at about Vpeak = 240 millivolts.
The curve shape is then also similar to the clipping behavior of the cathode follower.
During the simulations I always fed the signal via the series circuit consisting of a resistor of 20 Kiloohms and a capacitor of 1 microfarad to the base terminal.
The behaviour of the circuit depends on the value of the bias resistor, so you have to find the right value in the real circuit.

Erok · Post by **Erok** » 17 Jul 2020, 16:15

I have not tried a new 2N5089(don't have one available). I'll check online and see what I can get.
I still think there is a bigger fault here...somewhere...

Ill also try flipping the 2N5089 around, just for noob yucks.

What about the output section on this schematic.
Why the difference between the two circuits?

One more hypothetical...
How about a bigger bias resistor on the 2N5089?
Maybe a 470k or a 220k like in this schematic?
100k like JCM800 cathode follower?
Wouldn't that output a higher signal?

And...another thing. The battery was drained from 8.51V to 7.8v in about 2 hours?
Seems to be drawing too much amperage from somewhere.

Erok · Post by **Erok** » 17 Jul 2020, 17:51

LMK what you guys think?
I swapped the last 4.7k "cathode" resistor for a 200k.
The pedal came ALIVE!
Sounds like a great cranked plexi, open full chords and plenty of growl, no sputtering, plenty of gain and more importantly Volume above unity.

Is it safe to run the circuit like that?
I'll try 220k and 470k as well.
I think 470k will be too much based on other schematics.
I wish I knew how to do electronic math better(would save me from "shooting in the dark"). Basic Trigonometry you say?

The 220k resistor sounded best to me.

Still troubling how much amperage this pedal wants.
I am testing battery drain now, crappy 7.11v at start, we'll see what the voltage is in an hour.
Results:
7.11V initial battery voltage...
1 hour later battery reads 5.98V!

deltafred · Post by **deltafred** » 17 Jul 2020, 18:12

Erok wrote: ↑17 Jul 2020, 17:51 Is it safe to run the circuit like that?

Perfectly safe.

You will not damage anything and it will draw less current.

Erok · Post by **Erok** » 17 Jul 2020, 18:31

Looks like all the transistors have a high maximum voltage rating(30V).
Will running at 18v hurt the pedal?

I'll also install a .1uf decoupling cap and 220k to the input of the Volume pot, i'll figure something out.

Somebody needs to get fired.

Erok · Post by **Erok** » 17 Jul 2020, 20:26

Found out!
Looks like a Wampler Pinnacle clone minus the cool stuff on the output section, almost verbatim.

Manfred · Post by **Manfred** » 17 Jul 2020, 20:52

I swapped the last 4.7k "cathode" resistor for a 200k....

Did you keep the transistor in inverted mode?
With or without bias resistor?

In inverse operation the current gain and therefore the collector current is very much lower.
Of course this means that a higher resistance between collector and ground is needed to achieve a higher voltage at the collector.
It's all confusingly different modifications contribute to the output stage working, only the original circuitry doesn't work.
The video from the link shows the newer version of the LEXI, maybe all bugs were fixed.
Maybe it is a better solution to change the LEXI to the Wampler Pinnacle circuit.

Erok · Post by **Erok** » 17 Jul 2020, 21:10

I did not flip the 2N5089, kept everything stock except for swapping the 4.7k to a 220k between Collector and ground.
It's like DMB were in the Dare to be Different camp of circuit design.

Any idea why the pedal needs so much amperage?
I'm going to search for "pedal drains battery fast".

Again, thanks to the community for inspiring me to try and fix this thing.
I think I am ready for a testing breadboard and thousands of dollars of parts/equipment.

Manfred · Post by **Manfred** » 19 Jul 2020, 11:46

Even if the pedal does not work properly here is the corrected PCB layout for the sake of completeness.

LexiLayoutComponentSideV2.pdf: (17.97 KiB) Downloaded 94 times

LexiLayoutSolderSideV2.pdf: (5.05 KiB) Downloaded 87 times

SprintLayout 6.0 File:

LexiV2.zip: (22.34 KiB) Downloaded 87 times

Manfred · Post by **Manfred** » 24 Jul 2020, 17:14

Erok tested the 1 Megohm resistors soldered out again and confirmed the values, thanks for that.
Here the confirmed original PCB layout:

LexiLayoutComponentSideV3.pdf: (17.85 KiB) Downloaded 67 times

SprintLayout 6.0 file:

DMB LexiV3.zip: (22.24 KiB) Downloaded 92 times

Manfred · Post by **Manfred** » 26 Jul 2020, 21:00

Finally, the redrawing of the traced schematic of the original board.

Nocentelli · Post by **Nocentelli** » 27 Jul 2020, 00:18

Manfred wrote: ↑26 Jul 2020, 21:00 redrawing of the traced schematic

Gain pot connections look odd - output should be from lug 2 rather than 3?

Manfred · Post by **Manfred** » 27 Jul 2020, 10:10

Gain pot connections look odd - output should be from lug 2 rather than 3?

Thanks, you are right I made a drawing mistake.

Ichabod_Crane · Post by **Ichabod_Crane** » 04 Aug 2020, 09:03

So, in the end that is really that the correct schematic?
I tried to emulate it with the software, but I got low signal and an important cut of bass and very large treble roll off.
The trimmer doesn't help much. Should it carry 4.5v at the J201 drain?