Dod 250 clone that isn't clipping and gain knob has no effect

[jdoughty]

Hey guys, so I have made a dod 250 clone from https://www.pedalpcb.com/product/dist250/

The gain knob doesn't alter the sound at all, and there's no sign of a clipped signal.

I made a couple alterations. I made the 47 uf cap foot switchable to a 220 uf (I'd read it would boost the clipping... Yet to be seen), and each diode is on an on off on switch with a 4148 silicon diode and a 1n34a germanium.

I've taken an audio probe to the entire circuit and verified I can hear signal all through out. I've checked at the base of where the diodes are and there's signal, though nothing massive, which I don't know if it shouldn't sound bigger before the diodes, and theres some quiet clipped signal that can be heard between the diode and the switch.

Also I tested the pot for the gain and I can hear it go from no signal in the middle lug to the full signal from the chip.

Pins of 741
       1. 2.3
   2. 4.4
   3. 3.6
   4. 0
   5. 2.4
   6. 4.4
   7. 8.9
   8. 0

Diodes: 0

Pos:9.1

[Vivek]

For easy reference:

[Vivek]

First Observation:

The Inverting and non inverting pins of an Opamp are usually at the same voltage

Pin 2 and 3 should have been at the same voltage as pin 6

Unless the Opamp is actually oscillating and we cannot see that on a DMM

Then I would put a 100pf cap across R2 to try and kill RF oscillations.


Or Pin 2 is actually floating, ie some problem with soldering of R2



Observation 2 :

C7 47uF cap is for power supply smoothing. Changing it wont change tone/gain/sound


Answer 1:

At max gain, the Opamp should be multiplying signals at 200x ie the Opamp will clip

So even if there are no diodes at all, at max gain, you should get very loud, very clipped signal at Pin 6

If you do not get very loud, very clipped signal at Pin 6 when at max gain, it means there is something wrong with R2, C3, R3, Gain control.


So I would check value and soldering of R2 as my first step.
and check C3, R3, Gain control continuity as second step
(try connecting junction of R3 and Lug 1 of Gain control directly to ground, and see if you get very loud, very distorted sound from pin 6. That might get an answer to your comment "The gain knob doesn't alter the sound at all")

and put 100pf across R2 as 3rd step.

Hope that helps.

[antonis]

The Inverting and non inverting pins of an Opamp are usually at the same voltage
Pin 2 and 3 should have been at the same voltage as pin 6

Actually, they are..
(DMM's impedance shunts non-inverting input to GND hence forms a voltage divider with 1M bias resistor..)

For 800mV "offset" (4.4 - 3.6) it acts as 5M5 resistor..

P.S.
R6 & R7 values are unecessarily high..

[willienillie]

That schematic is more MXR Dist+ than DOD 250.  Nearly the same thing though.

OP, those wires are HUGE.  I see some possible shorting bits, but it's too blurry to say for sure.

[Vivek]

The Inverting and non inverting pins of an Opamp are usually at the same voltage
Pin 2 and 3 should have been at the same voltage as pin 6

Actually, they are..
(DMM's impedance shunts non-inverting input to GND hence forms a voltage divider with 1M bias resistor..)

For 800mV "offset" (4.4 - 3.6) it acts as 5M5 resistor..

P.S.
R6 & R7 values are unecessarily high..

I agree that you are right on both points !

[duck_arse]

P.S.
R6 & R7 values are unecessarily high..

.... but are nearly always so.

[jdoughty]

Ok, I'll check the resistors and caps in the gain circuit and look for possible shorts. Hopefully my smaller wire will be coming in so I can avoid some of the wire related madness. Hopefully I'll have something this afternoon

[iainpunk]

2 things:
1) get yourself a huge amount of thin wire, stranded or solid core is up to personal preference, i like solid core.
2) make an audio probe! you can test at any node if there is singal there, if not, you know where is was last. it is my favourite debugging tool


cheers, Iain

[antonis]

R6 & R7 values are unecessarily high..

.... but are nearly always so.

.... sad but true.

[jdoughty]

Found it... I'm a dope...

There was one other alteration... I didn't have a 1M pot so I used a 500k and put a 500k resistor after it... Removed that resistor and go figure, we have loads of clipping!

[antonis]

So, you managed to obtain max gain only X3 ...
(pas mal..)

With 500k pot, max gain will be the same as with 1M but min gain will be 3 instead of 2..
(and, of course, different gain "responce" for shaft rotation..)

[jdoughty]

So, you managed to obtain max gain only X3 ...
(pas mal..)

With 500k pot, max gain will be the same as with 1M but min gain will be 3 instead of 2..
(and, of course, different gain "responce" for shaft rotation..)

Thanks for the explanation, I'll probably leave it then

Thanks everyone for the help! Wish me luck stuffing these over sized wires into this enclosure...

[willienillie]

The MXR used a 500K gain pot, so nothing "wrong" with that.

[jdoughty]

Oh my God this thing came out awesome!




So final alterations:

47nf cap foot switchable to a 220nf cap, this is like a fat mid push that is awesome for filling space, though stock has that classic clarity. Kinda like a fender vs marshall amp sound difference. There's two leds now that alternate blue and green base on which I have active.

Diodes are on off on switched between 1n4148, none, and then a 1n34a diode

Gain pot is a B500k that I'd recommend replacing with a A500k as there's a sweep of distortion levels held up in the last quarter turn.

The 1u cap had to be replaced with a 470n because it was the biggest I had. I'd intially tried putting 3 caps together, but it didn't fit and I kinda just wanted to move on.

It sounds fantastic! I was supposed to be fixing 3 pedals tonight, but I've been stuck flipping switches and trying different amp and guitar combos!

The

[willienillie]

Gain pot is a B500k that I'd recommend replacing with a A500k as there's a sweep of distortion levels held up in the last quarter turn.

C500K is what you seek.  Same as the MXR.

[Vivek]

Found it... I'm a dope...

There was one other alteration... I didn't have a 1M pot so I used a 500k and put a 500k resistor after it... Removed that resistor and go figure, we have loads of clipping!

Great that its working !!!

However it should have worked even with 500K pot + 500K resistor in series if done properly. Something else was the issue.

But all's well that end's well !!

[iainpunk]

However it should have worked even with 500K pot + 500K resistor in series if done properly. Something else was the issue.

no.
remember, having a minimum of 500k is like having only the first half of travel on a potentiometer, and in a dod 250/dist+ that's 2 or 3 times gain, which would never clip the signal of single coil guitars, and maybe a tiny bit on humbuckers

[duck_arse]

nice indicator holes. awful knobs [I have some of those too].

[Vivek]

However it should have worked even with 500K pot + 500K resistor in series if done properly. Something else was the issue.

no.
remember, having a minimum of 500k is like having only the first half of travel on a potentiometer, and in a dod 250/dist+ that's 2 or 3 times gain, which would never clip the signal of single coil guitars, and maybe a tiny bit on humbuckers

I made some assumptions on how the OP would have built up 1M by adding 500k to 500k pot

And then connecting top of that contraption to wiper, as shown in the schematic.

In retrospect, I shouldn't have spoken without asking exactly how he made his connections.

Earlier comment struck out.