Big Muff 73 Rams Head Sustain Pot only approx. final 10% of turn works

[lawrenciumbc]

Pedal Name: Big Muff 73 Rams Head (Bitsbox kit)

Layout: using the Ivlark layout (http://3.bp.blogspot.com/-SZzinuajzrY/UR_Lu4KAtiI/AAAAAAAAE6E/rPAQ4ox_Wyc/s1600/EHX+73+Rams+Head+Big+Muff.png)
and
the Madbean standard wiring guide for the rest of it (http://www.madbeanpedals.com/tutorials/downloads/StandardWiring_MBP.pdf). The only addition i’ve made (which was the difference between the pedal working and not) is grounding the output socket.

The pedal is not yet boxed, but it sounds pretty decent through testing apart from the sustain pot which only appears to kick in properly about 80 or 90% round when turned. After that point it sounds great, prior to that, there is no significant affect on the sound. I don’t appear to suffer from any unwanted hum or anything which is good.

These are my transistor voltages:
q1
e = 40mv
b = 0.6v
c = 3.398v

q2
e = 19.7mv
b = 0.6v
c = 3.62v

q3
e = 37.9mv
b = 0.633v
c = 4v

q4
e = 1.167v
b = 1.749v
c = 4.7v

Comparing them to JaviCap’s (on the tagboardeffects page for the pedal in the comments), my values aren’t too far off, just a little low in some places, but as I say the pedal works generally.

(JaviCaps values)
JaviCAP (30th Dec 2014)
Q1: C: 4.98V B: 0.65V E: 0.06V
Q2: C: 4.42V B: 0.67V E: 0.06V
Q3: C: 4.32V B: 0.62V E: 0.06V
Q4: C: 4.10V B: 1.54V E: 1.02V

Could it be the slightly lower voltages causing this or is it more likely to be to do with the pot connections (or a faulty pot).

I wondered if this was a fairly common problem and what the best steps to solve it might be.

I have stored some images in my dropbox if you feel these might help. Apologies for any messy soldering, I am fairly inexperienced and this is my first pedal, all tracks and cuts have been carefully continuity checked. I was also going to carefully cut the overboard to size after too:

circuit above view

circuit above showing all components

bottom of vero board shot

sustain pot closeup

footswitch closeup 1

footswitch closeup 2 

I have generally tried to use the same colours for the wires off the board as Ivlark’s schematic for easier viewing. Also, my ground lead from my 1/4” Jack output is temporarily a croc lead.

[lawrenciumbc]

I have uploaded a video demonstrating the fault onto youtube.

[lawrenciumbc]

Still struggling with this, it's the only thing that's holding me up and it would be a shame to finish off the pedal with it not quite working correctly.

I noticed my link to the video before wasn't quite right, it should have been: https://www.youtube.com/watch?v=zHyA_xqA1cE&feature=youtu.be

I am now trying to build the circuit in NI Multisim to see if that might reveal something else I could try.

I wondered if when the resistance is lower on the sustain pot (i.e. <50k), all of the current is bypassing the Q4 transistor for some reason and going to ground, after looking at the schematic on kitrae (http://www.bigmuffpage.com/Big_Muff_Pi_versions_schematics_part1.html)

I will try and increase R23 to create more 'fixed resistance' on this route and hopefully allow the sustain pot to kick in much sooner.

I'll report back with my findings for future users.

[Addy Bart]

Have you checked the pot is ok with a multimeter?
Also, looking at your pictures, you might want to tin your wires in future before soldering them to the board. I had some problems on my first builds with little strands of wire making connections where they shouldn't.

[lawrenciumbc]

Hi Addy,

Yeah I've checked the pot and it certainly goes from 0 to 100K in terms of resistance so I think that it is functioning correctly.

In terms of the wires, I did tin them but maybe I didn't use enough solder if it doesn't look like I did haha! I might de-solder the wires and re-do them on the board end in case that is causing the issues.

I also wanted to confirm if my transistor voltages look high enough. They look close but compared to others but the loss seems to be around Q1 and Q2. The problem is, i'm not sure what the best way of checking exactly which point the loss is occurring. Is it simply a case of checking voltages and manually doing the maths on the circuit to workout what the voltage should be at each point or is there a more efficient way?

[Ciaran Haslett]

I'm on my phone (comp still not fixed !!!) so may be off base here.  

Most if not all of your cuts have stray copper dangerously close to causing a short.  
There appears to be a component lead shorting the right hand side on your copper pic
Where was the sustain pot when you took dc measurements?  
Measure Sustain 3 for DC.  May explain your noise when turning it.  The row above it is your V+.  A short between these rows can can introduce DC or a dodgy/shorted cap on the same row.
Reflow all your 12K and 100r resistors.

I'd try cleaning up all your cuts.  Examine them with a magnifying glass and test with a meter.  Knife all the gaps and ensure there are no shorts.  And maybe reflow some iffy looking joints.

Sorry I can't be any more help than this.  Trying to compare your pics to the layout when on a phone is hell!

[lawrenciumbc]

Hi Ciaron,

No problem, I will double check my cuts and clean them up a bit and reflow some of the connections, in particular the resistor values you have suggested. Good spot on the stray component lead but I think it was just the angle of the photo as it wasn't actually shorting but I have removed that stray wire now.

All of the pots were at maximum (highest resistance) I believe, when I originally took my measurements. This actually links to something I have just found which is likely to help with finding the source of the fault; The base of Q2 (affected by the sustain pot), ranges from around 196mV to 0.638V at either end of the sustain pot (min to max respectively). This means that it only actually gets to 0.6V towards the end of the range and so only just reaches the bias point of the transistor to switch it on. Therefore, I guess the range should actually start at just below 0.6V (not 196mV!) in order for the pot to have any affect for the majority of its turn?

Adding to this, between the 7.5k resistor and the 150nf cap, the voltage range is 0.5V to 3.5V and so this highlights the strange drop around this area of the circuit (given the above fact that the other side of the 150nf cap is 196mV to 0.638V).

The voltage range at sustain 3 is also 0.5V to 3.5V.

I've also realised now that the image links I have posted up seem to add ';cid=1475327281669-286' to the end of each of them in the address bar, so these are the direct links:

https://dl.dropboxusercontent.com/u/36766365/sustain%20pot%20issue/2016-09-18%2019.23.01.jpg

https://dl.dropboxusercontent.com/u/36766365/sustain%20pot%20issue/2016-09-18%2019.23.44.jpg

https://dl.dropboxusercontent.com/u/36766365/sustain%20pot%20issue/2016-09-18%2019.24.59.jpg

https://dl.dropboxusercontent.com/u/36766365/sustain%20pot%20issue/2016-09-18%2019.25.41.jpg

https://dl.dropboxusercontent.com/u/36766365/sustain%20pot%20issue/2016-09-18%2019.24.14.jpg

https://dl.dropboxusercontent.com/u/36766365/sustain%20pot%20issue/2016-09-18%2019.24.25.jpg

[Ciaran Haslett]

One more stupid question....what transistors are you using and have you tried changing them?

[lawrenciumbc]

The transistors from the kit are BC550C's and I tested them before I put them into the circuit but changing them might have been worth a try.

I have just reflowed some of the joints and cleaned the holes up as you suggested and the voltages of the transistors are looking a little higher for both Q1 and Q2 which is great! These are my new values:

Q1 C=4.00V B=0.630V E=40.6mV
Q2 C=4.06V B=0.640V E=39.9mV
Q3 C=4.03V B=0.637V E=37.6mV
Q4 C=4.72V B=1.748V E=1.164V

The practical result is that the sustain pot appears to work correctly now and the pedal sounds great!

Thanks for your help and pointers! Now to finish off the enclosure!

 

[Ciaran Haslett]

Excellent!!!  Glad to hear it.  Remember...just because it's vero and not PCB, doesn't mean it has to be rough and ready.  The more care you take with every step, cuts, soldering, placement etc, the better chance you have of building a successful circuit first try.

I've been doing this for years now and I still measure every component before it goes in.  Still check continuity on every cut and link.....hell I even file down each side of the vero after I cut it just because it looks better (and removes any stray copper from where the vero was cut).  You're not in a race.  Patience is more than a virtue in this game....it's a prerequisite!

Looking forward to seeing your completed build!

Ciaran

[Silver Blues]

Ciaran Haslett wrote

Excellent!!!  Glad to hear it.  Remember...just because it's vero and not PCB, doesn't mean it has to be rough and ready.  The more care you take with every step, cuts, soldering, placement etc, the better chance you have of building a successful circuit first try.

I've been doing this for years now and I still measure every component before it goes in.  Still check continuity on every cut and link.....hell I even file down each side of the vero after I cut it just because it looks better (and removes any stray copper from where the vero was cut).  You're not in a race.  Patience is more than a virtue in this game....it's a prerequisite!

Looking forward to seeing your completed build!

Ciaran

Just want to echo all of this, being meticulous pays massive dividends in the long run. It's way better to spend twice as long building it the first time than go back to debug it for six times as long. Have a meter handy and check resistance on every joint, after examining it for quality (you can tell when a joint is not good just by looking at it usually). And the classic advice from this forum, knife the gap!

[lawrenciumbc]

yes I can't agree more, I tried to be very careful, especially with it being my first pedal. I read the guides, checked every component with my meter and checked continuity throughout but I think its just inexperience that got me on this one! I'd say i've certainly learnt the power of the reflow and having good joints!

I'll be sure to post up my final project when its done and condense my experiences for others to follow/learn from!

Then onto building that bit crusher I always wanted but could never justify the price to buy a new one!

[Silver Blues]