Potentiometer only kicks-in in for the last 10% of the turn (guitar effects pedal)

[Lawrenciumbc]

I am building a guitar effects pedal from a schematic online. It is all working fairly well upon initial tests although one of the potentiometers only appears to kick in (i.e. influences the guitar sound) for the last 10% or so of the turn. During that 10% the effect (sustain on this pot) cuts in and it works. I have tested the range of the pot in question and it does cover 0 to 100k so I don't think it is faulty. I wondered if this is a problem that can pop up in circuits using potentiometers and what might cause it?

Measuring the resistance at the point the sound kicks in, it is 50k (up to the 100k), so it is still utilising 50% of the range (seems to be about the last 10-20% of the the actual pot remembering it is a log pot and not linear). So its as if the lower resistances don't initiate the sustain effect.

The schematic for the pedal is HERE and as mentioned, the pot I am having trouble with is the Sustain pot.

A full run down of the problem including links to images and transistor voltages I have measured is posted HERE on the forum for the site that created the pedal schematic, if you feel that this extra information might help. I am asking the question on this site from a general 'troubleshooting potentiometer problems' perspective and so I am just looking for what the next logical test steps might be.

Also, I am using 100k log potentiometers that came in the kit, but after measuring their resistances, one of them (NOT the one in the above fault) appears to only get to around 60k. Is this the sign of a faulty pot? I read a tolerance of up to 20% should be allowed for which would obviously mean anything between 80k to 120k and therefore this pot is even out of of that range.

 

[duke37]

I see the board and component positions but do not see the schematic.
It appears that the pot is OK but it will be necessary to detgermine the effect of the pot in the circuit and if there is a critical threshold.

 

[Lawrenciumbc]

ok yes, interpreting what it is supposed to do to the circuit electrically makes sense.

Sorry, the schematic link I posted is for the vero board layout, though a version of the schematic from a different site is here:



located from HERE (bigger image)

 

[Lawrenciumbc]

I have just uploaded a demo video of the pedal's performance for additional info located HERE

 

[BobK]

Did you notice the note that says "linear taper potentiometers?"

Bob

 

[Lawrenciumbc]

Good spot! I didn't notice that. The components I have used came in a kit based on the layout on the tag board effects link and aside from checking they worked, I didn't question the potentiometer type.

The schematic is actually from a different site as tag board effects didn't have one but its the same pedal. looking at the kit contents on the site I bought the kit from, it lists them as log pots so I don't think it was a packing error. The pots I have are 'A100K'.

I will get hold of some linear taper pots to replace them with and see how that affects the sound if you think that this could possibly hold the key. Theoretically then, 50% of the sustain pot should 'work' instead of the final 10% as it currently stands.

In addition, I have found this page (on the same site as the schematic) which appears to comprehensively go through the general circuit and so might provide more insight: http://www.kitrae.net/music/big_muff_guts.html

On this page, it also mentions the pots:

"REPLACEMENT POTS - Vintage USA Big Muffs (V1, V2, V3) have single-gang, linear taper, 24mm,100k potentiomers. Electro-Harmonix used pots from various makers, but they were usually the 24mm size, with 1/4" D shaped knob shafts. 250k linear can be substituted for the volume and sustain pots if 100k are not available. 100k Logarithmic taper pots, or "audio" pots, will also work, but they are more expensive than linear, and the useful volume range gets squeezed into a smaller area of the knob rotation. The difference between linear and logarithmic/audio pot types is that linear increases (electrical resistance) evenly as you turn the knob, whereas log has a shorter increase at both ends of the knob rotation, but a longer, increase in the middle. That that wider spectrum in the middle means a more usable range to allow better fine tuning with a log pot, in a tone knob for example, but not the best for a Big Muff volume pot. Both types sound the same, but the knobs will be in different positions for the same volume, tone, or gain level.

Something to keep in mind, some pots are letter coded with A or B to represent linear or logarithmic, but there is no worldwide standard for which is which. In general, the USA standard, which I believe came from Japan, is A=log/audio and B= linear. The European standard is A=linear and B=log/audio. For example, a USA made pot may be marked B100k, which means 100k linear taper. A100k would mean 100k logarithmic/audio taper."

 

[duke37]

Q2, Q3 and Q4 are identical amplifiers which seem to be more complicated than necessary.
Q3 and Q4 clip the signal when it is large enough. The sustain pot alters the gain to push the signal up into limiting.
The gain of Q4 can be raised a little by bypassing R22 with an electrolytic capacitor.
A pot with a different law will only change the threshold position which perhaps is what you want. It will not change the range of action.

Is this the schematic of what you have?

 

[Lawrenciumbc]

The schematic is very close to what I have after cross checking many of the components, it is certainly setup in the Q1, Q2, Q3, Q4 blocks. Essentially, the kit I have is based on the vero board layout for it which is what I have followed. The schematic I have posted is from a different site that shows the traditional schematic for the traditional effects pedal (as there isn't a schematic for the kit I have in existence), but it is all based on the same thing with the kit having slight variations in resistor or capacitor values that have been upgraded over time by the community or the guy who created it I think.

"A pot with a different law will only change the threshold position which perhaps is what you want."
- Yeah I thought that would be the case. If the sustain pot only appears to work from 50k to 100k as 10% of the log pot then it will be the final 50% of a linear pot presumably. It probably won't solve the problem, but what I wondered was whether this could be the 'intended' full range based on the circuit design and therefore stretching it out would be preferable, but i'm not yet experienced enough to be able to tell exactly what is going to happen based on the schematic.

"Q2, Q3 and Q4 are identical amplifiers which seem to be more complicated than necessary."
- I *guess* that this is probably because it is a classic pedal design and maybe the over complicated design helps create the 'classic tone'........but that's simply a guess. From my point of view, I would prefer to stick to the kit intentions. Maybe optimising it could be a future project when I understand it more.

I'm not sure if you watched my video demo of it that I posted above but that demonstrates the strange behaviour with the sustain pot. In fact it actually appears to cut the whole sound throughout part of the turning of the pot.

If the sustain pot looks like it is directly between Q3 and Q4 and they push the gain of the signal up so that it is limiting (by 'limiting' do you mean to make the signal clip?), is it possible that the incoming signal isn't strong enough for some reason and therefore it is not being amplified by enough?

The basis for this is my measured transistor voltages compared to another person (who claims to have a working pedal). My values in all Q's are a tad lower apart from in Q4. Could this be the unbalance that is causing the issue?

MY TRANSISTOR VOLTAGES:
Q1 c = 3.39v b = 0.60v e = 40.00mv
Q2 c = 3.62v b = 0.60v e = 19.70mv
Q3 c = 4.00v b = 0.63v e = 37.90mv
Q4 c = 4.70v b = 1.75v e = 1.1670v

OTHER PERSON'S 'CORRECT' TRANSISTOR VOLTAGES:
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