Smoking DEFX Laika Fuzz v2

[Itchy Scratcherson]

for anyone interested, here's a slightly more legible reference layout of a defx Laika Fuzz V1, as well as the Millennium Bypass portion of it's schematic along with the power section that feeds it... (n.b. any mistakes on this layout or schematic are my own) HTH!
(note: updated the Ferrite Bead labels like the 7th post below)



updated the symbol for the BS250 in the power section:

[Itchy Scratcherson]

Looking for some Millennium Bypass Help! Pic shows relevant parts of the schematic, my voltages & notes.

Still can't get the LED to behave. Now it's ON all the time.   
When NOT bypassed, should the LED cathode drop to 0V?

"The BS170 has a very low gate threshold voltage of around 2.1V... "
...If I understand correctly, when the circuit is turned on, the 4148 diode leaks just enough voltage to "pull up" the BS170's gate, thereby allowing voltage to flow from source to drain (toward the LED).

I'm only familiar with a LED lighting on a pedal by receiving voltage at its anode while its cathode is grounded. But here's a situation where, and I may be wrong, the LED lights if anode voltage > cathode voltage by at least ...however much voltage is required to light the LED.

In any case, in theory, lifting the gate should get the LED to light ...unless you're like me.

In his Millennium Bypass article, Mr. Keen states that the circuit is triggered when a threshold amount of voltage is reached, which is determined by the difference between 2 leaking diodes: the 4148 and the 3904's C-B junction.

Among other things, I'm not understanding how the "difference between voltages" leaked by the two diodes is managed by the circuit. Maybe I'm being too literal?

Thank You very much!

[Itchy Scratcherson]

At a quick glance, it looks like four of my defx builds use the Millennium bypass scheme (I can check their voltages, etc. against my Laika's voltages).
1.) Anticarus,
2.) Calypso,
3.) Fuzzimile, and
4.) Inchindown

more learnin' !

[duck_arse]

I use the millenium in everything I build. hard to go wrong with it.

you can pull the Q5 (diode) and the mill will still work, as long as you have the led the right way around - Anode to R6 and supply, and Kathode to the Source Drain. it will remain ON as long as nothing is connected to switch-end of R4. when that resistor connects to the SIGNAL pot thru the bypass switch, the led turns OFF. if yours doesn't do this, then either -

bad led, bad led orientation - inspect, test, verify, correct
the mosfet is bad or incorrect orientation - the 2N7000 for ex is reverse pinout to the BS170
the mosfet gate can't see the pot to pull it down - bad wiring, bad switch, bad pot - test and correct.

the only voltages we care about in the millenium circuit is 0V on the gate = led off, otherwise DON'T CARE don't measure, and the voltage at the mosfet Drain - will be near 0V when led is lit [functioning correctly], and something higher than that [up to the supply voltage] when led is off. your 4V8 suggest a fault/error.

you might pull Q7 from its socket for the testing phase, it might [might] be a contributing factor.

as for the supply protection mosfet Q1, if you think it a problem, pull it out of the board, and link across the drain and source pins on the socket. this will render the board unprotected from reverse polarity, but you'd be being careful about that.

[edit :] noted correction. red face noted.

[Itchy Scratcherson]

I appreciate that very much! I can't thank you enough for such a thorough reply! Now I understand that «the only voltages we care about in the millenium circuit is 0V on the gate = led off»

However, not sure how you meant it, but according to the schematics, the LED's Kathode connects to the MOSFET's Drain — as in: («Anode to R6 and supply, and Kathode to the Drain,») right?

[Itchy Scratcherson]

Got the LED working! But my 3PDT bypass switch wiring needs some re-working (again), b/c only unprocessed audio is passing through the pcb at the moment regardless of stomp switch's state. So the LED problem turned out to be a trace I forgot to bridge... a solder pad I burned off was connected 2 other points, and I'd only bridged one of 'em. 

[duck_arse]

erm, sorry for the drain mixup. I knew if I typed that enough times I'd get it arsed-about sooner or later. and, shows you are learning.

your dmm should show you where your signal wiring is going or not going.

[Itchy Scratcherson]

for anyone interested, here's a slightly more legible reference layout of a defx Laika Fuzz V1, <<...>> (n.b. any mistakes on this layout are my own) HTH!

Ferrite Bead numbers & R14 label updated in this v1 Laika layout reference:

[Itchy Scratcherson]

I replaced the SK30A-Y (Q8) with a J201 (à la the original trace schematic), and finally the Scan pot is at least  trying to respond by crackling at a couple points in the rotation. So, I'm still wrestling with this final leg of the schematic, including the diode-reverse switch. In the meantime, I built a Fugly-Face with 100% success, and started a Sparsh OD pcb today. I really like this Laika, even in the less-than-100% functional state. The Fugly Face is, um, an acquired taste.

[duck_arse]

and you bent the legs around on the J201 to match the K30-A pinout, right?

[Itchy Scratcherson]

Yes, thanks for the reminder. I used a tiny MMBFJ201 chip already pre-mounted and then creatively pinned it to the Laika PCB. Finished the defx Sparsh today w/100% success. Debugging the Laika Fuzz in earnest again tomorrow!

[Itchy Scratcherson]

Well, I'm going a little insane doing the same things over and over expecting a different result. I got to that point a week ago and thought about just replacing the pcb & starting over. Then I made 4 other projects... trouble-free. Taking this on again as a learning experience, I've now burned off so many pads I have maybe 9 bridges of my own wired on. But I'm determined!

For so many hours I haven't made any progress at all. I'd fix the LED not lighting, but then discover it lights all the time. I cannot get the MOSFET gate to bleed it's voltage & get LED to work properly. So, lots of checking of the Mil Bypass again today. OK. Looks good...

Silly question: can a LED light with 9V going to it's anode AND with a few volts to it's kathode? or, MUST the Kathode be grounded?

Just re-installed the Ge transistors in their sockets & So now I'm ready to fire it up b/c everything looks good & checks out, and then... when I gave the pcb 9V, I fried *another* BS250! I noticed right away that the bypass LED worked for a few seconds, then got dimmer & then faded to nothing... like my attention span, for the moment.

[Itchy Scratcherson]

Bypassing the BS250 for the moment. Out comes the probe.

[idy]

The difference in voltage between anode and cathode is called forward voltage. It varies. mostly by color. Let's quote as 1.8v for red to 3.3 for blue.

If you just put a 2v LED between + and - of a 9v battery, it will burn out before you can measure it. The little Current Limiting Resistor we always add drops all the other voltage. So with a 2v LED you would measure 2v across the LED and the other 7 across the R.

[Itchy Scratcherson]

I've been having trouble knowing what is the designed behavior so I can't confirm whether the kathode is supposed to be grounded in order to light up, or does it hold some voltage. Unlike other pedal circuits I've worked with, Laika's Mil Bypass connects to other parts of the circuit. It's usually more "black box", isolated, modular, so Laika's really been challenging to troubleshoot.
I'm not sure I understand the behavior, I think, of fwdV in a diode...

• Depending on the direction of the applied voltage, a diode either conducts or resists/limits the amount of voltage that can pass through it, correct?
• If 9V is fed to a forward-biased diode which has a fwdV spec of 1.8V, will that diode ONLY allow 1.8V to pass through in this direction?
• And/But, is forward voltage also related to the amount of voltage required to actually light up the LED?

THANKS!!

[Itchy Scratcherson]

Simple Millenium Bypass question re: MOSFET behavior: Do I understand correctly that when the gate threshold is reached. signal is then allowed to pass from source to drain? But... what when the source = GROUND? In that case does the flow actually go drain-to-source?

This is a crucial fundamental I'm not grasping in this complex circuit. Laika's Mil Bypass also connects to the Drift portion of the circuit. The four transistors are sort of daisy-chained such that one's drain feeds the next's gate... hence the question above.
Thank you so much!

[duck_arse]

I've been having trouble knowing what is the designed behavior so I can't confirm whether the kathode is supposed to be grounded in order to light up, or does it hold some voltage. Unlike other pedal circuits I've worked with, Laika's Mil Bypass connects to other parts of the circuit. It's usually more "black box", isolated, modular, so Laika's really been challenging to troubleshoot.
I'm not sure I understand the behavior, I think, of fwdV in a diode...

• Depending on the direction of the applied voltage, a diode either conducts or resists/limits the amount of voltage that can pass through it, correct?
• If 9V is fed to a forward-biased diode which has a fwdV spec of 1.8V, will that diode ONLY allow 1.8V to pass through in this direction?
• And/But, is forward voltage also related to the amount of voltage required to actually light up the LED?

THANKS!!

a diode conducts, or doesn't conduct. it only conducts when the Kathode is closer to ground than the Anode. closer to ground includes ground, 0V, earth, even a negative supply. it doesn't matter where the current limit resistor is, above or below the led, only whether the K points to the lower voltage. think of the led symbol as an arrow, might help.

the diode/led doesn't limit the voltage, it eats some of the applied voltage. and the CLR eats the rest - or the diode expires.

the forward voltage is the voltage the led needs to light. how much current you allow it [via the current limit resistor] determines how bright the light.


re mill mosfet. if the source is connected to ground, and the gate is then turned on, whatever is at the drain then sees ground, is taken to ground, dissappears into ground. if a led K is above the drain and the gate is off, no light, no current path thru the led or the mosfet. if the gate then turns on, the K does see ground, current does flow through the mosfet to ground, the led does light.

[antonis]

MOSFET behavior: Do I understand correctly that when the gate threshold is reached. signal is then allowed to pass from source to drain?

The other way round..
(from Drain to Source..)

https://www.electronics-tutorials.ws/transistor/tran_7.html

[Itchy Scratcherson]

Thank you so much, guys. I'm still on it. Is it possible my Q6 (& Q7) BS170 mosfets have been DSG pinout all along (I inserted them assuming DGS?) Before I desolder & resolder these for the sixth time, it'd be convenient to know what the behavior/ symptoms might be if in fact I simply hadn't checked the pinouts carefully enough. (Bottom line is I think it would prevent the LED from being able to light, and Q8 would never see action. Hmmm.)
My little finger tells me this is a long shot b/c I did have 2n7000's in there, properly aligned, for a few days. ...then again, the Drift section wasn't working at that time either...

[antonis]

Is it possible my Q6 (& Q7) BS170 mosfets have been DSG pinout all along (I inserted them assuming DGS?) Before I desolder & resolder these for the sixth time,

It depends on your particular version..
https://in.pinterest.com/pin/bs170-mosfet-pinout-datasheet-equivalent-specs--802344489876788219/
https://www.onsemi.com/pdf/datasheet/mmbf170-d.pdf

So, a testing verification should be quite useful..
(bearing in mind that MosFets dislike repeated solderings..)