MXR Distortion II build

[dthurstan]

Hi

Im having some problems with a vero build of the Distortion II. Copy of the original schematic below;


I removed the switching network, the output buffer and the Vref buffer. Something is pulling down the Vref and I'm not sure what it could be. Here is a stripped down clearer version of the schematic;


Heres the vero layout;


And photos of the vero board;



The voltages are
IC1
1: 0.71
2: 0.71
3: 0.83
4: 0
5: 0.9
6: 0.71
7: 0.71
8:9.02

IC2
1: 0.76
2: 0.74
3: 0.9
4: 0
5: 0.9
6: 0.72
7: 0.76
8: 9.02

IC3
1: 0.69
2: 0.69
3: 0.75
4:0
5: 0.68
6: 0.79
7: 0.67
8: 9.02

When I remove all ICs I get 3.9 Vref. I can't see anything shorting the Vref line so I'm at a loss as to what could be up. Can anyone tell me if I'm missing anything?

Thanks

Dave

[Tony Forestiere]

...voltages on most dual op-amps in MOST circuits want to be:
pin1 - 1/2 V+
pin2 - 1/2 V+
pin3 - 1/2 V+
Pin4 - 0V - (continuity to GND) A lot of folks figure if they are reading 0 volts, they are good without checking true continuity to GND.
pin5 - 1/2 V+
pin6 - 1/2 V+
pin7 - 1/2 V+
pin8 V+ - (supply voltage)

This should be pinned to a Debugging thread.

[Govmnt_Lacky]

Remove the jumper directly below R5 

Nevermind!

[dthurstan]

...voltages on most dual op-amps in MOST circuits want to be:
pin1 - 1/2 V+
pin2 - 1/2 V+
pin3 - 1/2 V+
Pin4 - 0V - (continuity to GND) A lot of folks figure if they are reading 0 volts, they are good without checking true continuity to GND.
pin5 - 1/2 V+
pin6 - 1/2 V+
pin7 - 1/2 V+
pin8 V+ - (supply voltage)

This should be pinned to a Debugging thread.

Yep learned that when I had problems building an easyvibe. But I think there an issue with the Vref as it doesnt sit at 4.5. I think Ive drawn the circuit correctly.

[Tony Forestiere]

I am by no means a Vero guy. My little mind cannot grasp the logic behind conceiving a layout on medium constrained by one way traces. Perf or PCB for me.  I digress as to (one of many) shortcomings.

I did notice some areas under the board that looked a bit suspect to me as being possible bridges. Could be lighting, shadows, or my older vision.



Please realize that I am lazy and attempt the easiest fix for lost/missing/low readings first. My soldering skills(?) are not the greatest, and are usually my downfall. Best of luck finding your gremlin.

[dthurstan]

I am by no means a Vero guy. My little mind cannot grasp the logic behind conceiving a layout on medium constrained by one way traces. Perf or PCB for me.  I digress as to (one of many) shortcomings.

I did notice some areas under the board that looked a bit suspect to me as being possible bridges. Could be lighting, shadows, or my older vision.
...
Please realize that I am lazy and attempt the easiest fix for lost/missing/low readings first. My soldering skills(?) are not the greatest, and are usually my downfall. Best of luck finding your gremlin.

Thanks for your help. It's often the most obvious things that are cause. But unfortunately not in this case. I checked before I posted and checked again with a meter just now. As you can see my soldering skills aren't the best either. Sorry for the poor quality photo too, it makes it look like there is lots of possible bridges.

As I mentioned before when I remove all the IC's the Vref jumps up to 3.9V. I don't understand why this isn't 4.5V, I've checked the resistors are the same colour value and the caps are the right way round. I just wonder if I have redrawn it correctly.

[Govmnt_Lacky]

Back again but this time I actually have something logical to add/ask...

Do you happen to know what the "A1" is in the original schematic? Is that just a flip/flop used for bypass switching?

Also, I would believe that MXR used that half of U4 as the Vref buffer for a reason. Maybe eliminating that is contributing to your issues.

Good Luck 

[dthurstan]

Back again but this time I actually have something logical to add/ask...

Do you happen to know what the "A1" is in the original schematic? Is that just a flip/flop used for bypass switching?

Also, I would believe that MXR used that half of U4 as the Vref buffer for a reason. Maybe eliminating that is contributing to your issues.

Good Luck 

A1 is a CD4066 chip used for switching. May the Vref does need a buffer, but Made by Mike has made this pedal and Im sure he hasn't used the Vref buffer looking at the build pics. And surely with no ICs in the circuit it should goto 4.5V.

[dthurstan]

Looking at the original schematic again I think there is an issue around the opamp just before the output pot. If I lift the ground from lug 3 and remove IC3 I get 4.5V for Vref. I think I need to include C17 from lug 2 and R25 which goes to Vref. I thought this was part of the switching network but I guess the output needs to reference Vref? Can anyone back me up on this one?

Thanks
Dave

[EATyourGuitar]

MadebyMike posted this on another forum and I don't think it has been posted anywhere else yet. this one works and it is without the switching stuff. just what you want.

[dthurstan]

Hey thats great thanks. Saves me losing my mind trying to see off a picture of one of his boards. I'll trace the signal through and see where I've gone wrong.

[dthurstan]

I've had a look through Mike's vero and apart from his amendments to the circuit (changing the gain and bass responses) I look to have built it correctly. The only thing that is different is the original had the res pot connected to Vref (Vs) where as Mike's is connected to ground. So I wired mine the same. I now get a weird sub octave oscillation when the res is all the way up. I can't see any reason for this. If anyone has some helpful pointers to where the issue maybe that would be great. I guess it's somewhere in the res section with the 2nd gain stage??

Thanks

[dthurstan]

Hi

So I'm still having issues with this pedal.

I've been over the vero again and again and I can't find any errors. I've checked all the solder joints and made sure there was no solder bridges. I don't think its the layout as it's kinda similar to Mike's.

There is a strange problem in that when I first plug it in I get around 1.2 volts at Vref, but when I pull IC2 out a bit and push it back in Vref jumps up to 4.4 volts. The pedal sounds ok but it does have a gated octave sound with res all the way up.

The voltages are
IC1
1: 4.57
2: 4.57
3: 4.10
4: 0
5: 4.49
6: 4.57
7: 4.57
8:9.02

IC2
1: 4.55
2: 4.59
3: 4.50
4: 0
5: 4.50
6: 8.07
7: 4.57
8: 9.02

IC3
1: 4.55
2: 4.57
3: 3.87
4:0
5: 4.45
6: 4.53
7: 4.55
8: 9.02

So there seems to be something up with the second half of IC2 (res control) and the first half of IC3 (filter). I've also replaced all the ICs with TL074 just in case, I know an IC is usually the last thing to look at but what the hell.

I just can't figure out why it doesn't work. Can anyone help?  

[MartyMart]

Hi David,

Your vero layout is quite tight - can you check that your long runs at C6 & R7 are NOT touching the pot connections R8a and R18a ?

I would normally cover those long leads in some plastic wire cover removed from a few cms of hook up wire.

I am also struggling to follow your "nodes" of connections from IC2 to IC3, can you verify that the whole
R13/14/15/16 / C9 etc is correct .... ?   something doesn't look quite right there.... I don't mean totally "wrong"
but sometimes you need a more direct "node" to ground or V-ref that makes the circuit work a little better - if that makes sense?

Layout can mean EVERYtHING even if the "path" is correct, doesn't mean it will always work correctly !!

Cheers,
Marty.

[samhay]

Your bias voltage (Vref) is not stable as you probably do not have enough current through your biasing resistors (R26,R27). They were 470k in the original design as they were only intended to feed into 1 buffer - not 3 dual op-amps.
As usual, geofex provides a usesful read on bias networks: http://www.geofex.com/circuits/biasnet.htm
For reference, my stock op-amp bias network usually uses two 10k resistors and a 47-100u cap from Vref to ground (your C21), or 1M bias resistors if I I am buffering Vref with an op-amp like in the original MXR schem. These vales are far from optimal, but haven't failed me yet. If you are using TL062s, which are low-current op-amps, you will probably get away with significanly higher values, but it look like 470k is too high.

[hex_void]

Clearly a bias resistor problem. Buffer the Vref bias voltage with another opamp and reduce the bias resistors to around 100k just to be sure.

You can also increase R20 & R21 to 100k without affecting the tone (assuming you're using TL072 or any high input impedance opamp) and not use a buffer for Vref but change R26 & R27 to 10k.

The Vref buffer is recommended though, as, from what I noticed while experimenting with my high gain distortion, it reduces noise. 100nF ceramic caps across the opamp power supply pins also help but they need to be as close as possible.

[dthurstan]

@ Marty

Cheers Marty

Those pics of the circuit board are old I have put some covers over the longer leads now.

I've had another quick look and I think I've got everything connected correctly around R13/14/15/16 / C9 etc. I also checked it against Mike's layout.

I was concerned that the layout might be an issue but Mike's is also pretty tight (it fits in a 1590B enclosure).

Thanks for the tips. I'm also building a Bassman emulation based on your bassboy schematic. 

@ samhay & hex_void

Yep bias voltage is the issue. Thanks for you advice. I originally thought it must be to do with biasing but because Mike's vero used 470k bias resistors also I assumed it couldn't be that.

I've used 10k resistors and 100uF caps and it's stable now.

I've read through RG's article on biasing before, but I must admit I don't follow it. 

In this circuit most of the opamps are connected straight to Vref, there are 1M and 10k resistors connecting to Vref on the first and last opamps. So that would suggest using 10k or 1k resistors.

Looking at the supply current (http://www.ti.com/lit/ds/symlink/tl062.pdf) 250uA.
Multiply that by 10 and divide into 9volts and half, you get 18k.

I know I've got it working now but I want to understand why (or how to avoid this in the future ).

Now that I have it working, I'm thinking of doubling the supply voltage with a charge pump. The original unit runs of 15v so 18v will give me some more headroom.

I'll need to have the charge pump on a separate veroboard, so I was thinking should I include a Vref buffer with 470k/1M bias resistors?

Thanks again for everyone for putting up with my noob understanding.

[MartyMart]

Hi David,

I don't remember having such a V-ref issue, though I normally follow the guidelines of a 10k/10k node with at least a 10uf or 47uf to ground also.
Almost all my builds have 100r from 9v with a 100uf and 100n cap to ground plus reverse protection diode.
Any instability can also be aided by 100n to ground caps right next to the IC's power connections too.

Cheers for the "Bass Boy" ad, that's a very cool sounding circuit and really shines with a TS-9 in front of it !!

Regards,
Marty.

[dthurstan]

Hey

Thanks for all the help in getting this working. I finished the enclosure last weekend, I'm very happy with it. 





As you can see I've added a charge pump circuit so I get ~17V going into the Distortion II. I'll have to post some clips with it running on 9 and 17 volts.

I think with 17V I'm not getting much gain range compared with 9V. It seems its too distorted with the gain all the way down. I'm sure that with 9V there was more range but it's been a while. I may try and reduce R5 (240k) see if that helps.

Dave

[tommycataus]

Hey guys,

Sorry to piggy-back onto this thread, but it seemed like the most logical place to post my question. By the way that pedal looks awesome Dave, amazing job.

I'm a real noob and I'm hoping that by attempting this and asking questions I will learn a lot about tweaking the circuit and troubleshooting... I've decided to embark on the vero layout of this as I was struggling to read the schematic, I've used the verified one posted further up, MadebyMike's version since it seems like that should work. Well, mine does, and it doesn't.

When I plug it in it gives me a good distortion sound, the drive and the volume both work well although as either of these two knobs are turned I seem to get the wild oscillating sounds that remind me of very old fashioned radio transmissions, or tuning an AM radio. The filter adds to this when turned, although with a guitar plugged in the adjustment it makes is very subtle, too subtle for an ordinary control. The resonance doesn't seem to do anything.

IC1:
1 - 4.61
2 - 4.61
3 - 4.02
4 - 9.08
5 - 4.47
6 - 4.58
7 - 4.58
8 - 4.58
9 - 4.58
10 - 4.47
11 - 0
12 - 4.47
13 - 4.58
14 - 4.58

IC2:
1 - 4.58
2 - 4.58
3 - 4.49
4 - 0
5 - 4.58
6 - 4.65
7 - 4.58
8 - 9.08





I'd really appreciate any feedback you guys are willing to offer! Thanks

Tom