MXR Blue Box Clone

[Gatocaster]

Hi.
I recently attempted to build a relatively straightforward clone of a MXR Blue Box. I keep nearly getting it right but I cannot seem to eliminate the crackling noise that it creates...I own a real MXR BB so I know how it ought to sound.The tone is right but the crackle is horrid.

I socketed up all the caps, diodes, transistors and both chips and have changed them all but get the same result.
I have razored the stripboard tracks to make sure there are no solder bridges and triple-checked the entire layout - I have even tried to simulate it on SPICE - but the dreaded noise remains, a sort of crackling, fizzing sound, that comes with the decay of the note.

My audio probe suggests the noise begins after the b-side of the 4558, between it and the Vb track, so I thought it could be a faulty Op amp, but swapping it for another 4558 or even a TL072 makes zero difference.

The voltages at the 4558 are as follows:
Battery 9.08v
Pin 1: 4.42v
Pin 2: 4.42v
Pin 3: 2.19v
Pin 4: 0.11v
Pin 5: 4.30v
Pin 6: 4.10v
Pin 7: 5.08v
Pin 8: 8.84v

The voltages at the 4013 are as follows:
Pin 1: 4.42v
Pin 2: 4.47v
Pin 3: 0.07v
Pin4: 0.07v
Pin5: 4.45v
Pin 6: 0.04v
Pin 7: 0.05v
Pin 8: 0.05v
Pin 9: 8.82v
Pin 10: 0.02v
Pin 11: 3.94v
Pin 12: 8.82v
Pin 13: 0.01v
Pin 14: 8.82v
 
The layout I have used is at http://johnkvintageguitars.homestead.com/Effects/Fuzz-ODs/BlueBox/MXR_BlueBox-Final-Verified.png
It seems many people have built it successfully but not me!

Any help would be greatly appreciated. I am sure that it is something relatively straightforward (it usually is) but 3 days of probing, swapping components, scraping, cleaning and endless inspection has yielded nothing but insomnia.

[Mark Hammer]

"Crackle" is a bit of an ambiguous term, so I may be wayyyyy off here.

But it bears noting that the Blue Box uses that pair of transistors as gates, the idea being that the sound is gated out/off before the note starts to sputter.  You can potentially address that by changing the parameters of the gate, perhaps by the decay time.

[Gatocaster]

Thanks for the prompt reply, Mark.
I presume the decay time could be altered by changing the transistor type? However, I have already tried the circuit with both 5088s and 3904s with the same spluttering result. Any suggestions?

[Mark Hammer]

Nope.  The decay time is set by the resistor and cap to ground after the two diodes.

The transistor type might change the response to that envelope voltage (I assume the 56k resistors on the collectors are selected, based on what they allow the envelope voltage to make the transistors do), but it is no solution itself.

What you're aiming for is to have those transistors turn off just before the triggering of the 4013 flip-flop starts to become unreliable and inconsistent.  Part of doing that may be to turn the transistors off a little sooner.

BTW, it is not uncommon for people using noise gates to complain about what seems like fizz or trill when they set the decay time a little too short or long.

[Gatocaster]

Many thanks for the advice. I am in the learning process here so really useful - I shall play around with the circuit tomorrow when I have caught up on the lost sleep!

[Gatocaster]

Good morning (UK time).
Well, I have tried various combinations of resistors and capacitors and the noise is not affected. If anything, it gets worse with any other combination.

Here is another clue however. When I remove the 4013 the fuzz circuit still works but the noise remains, indicating I guess that the problem is not in the flip-flop part of the circuit?

I am running the circuit in my test box in which other circuits that I have built successfully such as a Boss CE2 run perfectly well with no noise so that eliminates the box as being the source. I also swapped the 4558 with my CE2's and it works fine, so we can rule out a faulty opamp.

This is definitely doing my head in! The only other pedal I ever had this much trouble with was an RM Octavia so I guess octave shifters and me just  aren't  meant to have a harmonious relationship, ha ha.

[duck_arse]

sockets have been known to cause problems with circuits when they aren't contacting to the best of their abillities. the more stuff you shove into sockets, the more likely you'll end up with a socketing problem.

get a resistor lead cut-off, and go around all the sockets on yr build, poking the wire in between component and socket wiper contact connection. see if you get any results.

[Gatocaster]

So...a couple of days on and I have a working Blue Box circuit. The answer was simple - build another one!
This time I used the layout by Harald Sabro at...

http://www.sabrotone.com/wp-content/uploads/2011/09/BlueBox.gif

...as mentioned in a previous thread, "MXR Blue Box Vero Layout?" at...

http://www.diystompboxes.com/smfforum/index.php?topic=42055.msg303740#msg303740at 

I should have used this one in the first place as I have built other boards by HS and they always work, so many thanks Harald.
As for the faulty one, I now have this one AND a real one to compare it to so I ought to be able to get it working soon.
Many thanks everyone.