Debugging Tim Escobedo PWM

[lightcycle]

Hello!

I tried building Tim Escobedo's PWM circuit, but can't get it to work as expected, and can't find what I did wrong. Does anyone have any suggestions what I could try?

Here's some info  

1.What does it do, not do, and sound like?
There is sound from the pedal, both engaged and bypassed. The pulse width pot does however not act as expected. With pot fully CCW there is no sound. If the pot is turned ever so slightly CW there is a thin and weak sound, if turned slightly more CW the sound gets louder and fuller, kind of the square sound that is expected, but from there and through the remaining 95% of the pots rotation the sound doesn't change at all. I would the sound would change all through the pots rotation. This is all with the LFO not engaged, I thought I'd get it to work without LFO before looking into how the LFO works.

2.Name of the circuit = Tim Escobedo's PWM

3.Source of the circuit (URL of schematic or project) = http://www.jiggawoo.eclipse.co.uk/guitarhq/Circuitsnippets/snippets.html

4.Any modifications to the circuit?
The Ez LFO shown in the schematic is incorporated. I put a switch to connect/disconnect the collector of the transistor to pin 3.

5.Any parts substitutions? If yes, list them.
The 386 I used is an LM386N-1.
I used a 1M pot for pulse width, paralleled with an 1M resistor and a switchable jack which disconnects the pot when connected. The idea of this was to be able to connect a 1M footpedal to control pulsewidth. All tests so far has been with footpedal disconnected. Measuring from pin 3 of the 40106 to ground gives 450k when pot is fully CW, 0 when fully CCW with a smooth rise of resistance when pot is turned CW.

6.Positive ground to negative ground conversion?  N

7.  Voltages

What is the out of circuit battery voltage? => I have not installed a battery clip in the pedal, I therefore used a regulated supply. Measured 8.9 V.

Voltage at the circuit board end of the red battery lead = 8.9
Voltage at the circuit board end of the black battery lead = 0

LM 386:
1 = 1.31
2 = 0
3 = 0
4 = 0
5 = 4.36
6 = 8.9
7 = 4.5
8 = 1.3

40106
1 = 0.6
2 = 8.08
3 = 4.99
4 = 4.99
5 = 0
6 = 8.9
7 = 0
8 = 8.9
9 = 0
10 = 8.9
11 = 0
12 = 8.72
13 = 0
14 = 8.9

3904
E = 0
B = 0.62
C = 0.02

I also tried substituting the 40106 with a 40014, with the same pot behaviour and similar voltage measurements.

[GibsonGM]

Check your wiring!  Do you have the .005u cap there?     Not sure what the proper voltages on the logic chips should be...but, seems like you need to do the usual look, look again, look some more and all that. 

Check ALL your pot connections, make sure none are incorrect.  Try to take 'bites' out of the problem.  For example: any way you can tell if you have a signal from the LFO?  Maybe via hooking a speaker to it (audio probe is ideal, maybe you have one?).  There should be a signal at pin 3 of the 40106, which is the output of the entire LFO section.    If it works, you can omit it from your scrutiny {for now}.

Does the 386 work?  Try connecting a sound source to pin 3 of it, and if you have sound...same deal.

I suspect something not wired right around that logic chip...

[pinkjimiphoton]

to me, it seems like that 3904 isn't right. collector should be higher than base, no?

fwiw, this pedal really doesn't sound all that great, and the lfo is kind of a waste of effort to add, as the pedal just isnt very good. sweeping the pulse width with an lfo doesn't really work well imho...

i built this a long time ago, and revisited just a couple days ago... and a/b'd it with insonicbloom's Pong War Machine (with lfo) and they both sound pretty much identical...

like poo.  it's cool, cuzza what it is, but ....

i think you may like the ugly face better.

is there any possibility you wired the pot wrong?

just my opinion fwiw..... i was really disappointed by this circuit, especially once i added the lfo.

it's only good for like super mario sounds imo

[GibsonGM]

to me, it seems like that 3904 isn't right. collector should be higher than base, no?

Depends on what pin 3 of the 40106 is sitting at, right?  Collector isn't going to +V, it's off to the chip.  Tim's circuits are fun, weird, and different, but can be a little difficult to understand!

Like I said, you find out if the LFO works, the 386 audio section, and then you know you did something wrong to the schmitt trigger (I totally suspect an error there).   The troubleshooting process is the same no matter what you're doing, altho other things may complicate it, like how sensitive an IC may be (could fry it in 1 second, or not at all...).   Without know circuits to sub the IC into to test that, you are stuck with going over the supporting components over and over and over....which is why I want to see him eliminate the other blocks first...

[lightcycle]

Thanks for your replies! You were right GibsonGM, going over the connections around the schmitt trigger i found a short I somehow missed earlier between pin 3 and 4. Now the pedal sounds as it should, very dramatic when the pulse width pot is swept.

Contrary to you pinkjimiphoton I actually really like the sound of this pedal. I already have an Uglyface, and while I do think it's a very good pedal also I do not view it as being very similar to the PWM soundwise. This is probably not a pedal to be used every day, but for anyone liking the sound of it (synthy pulse wave modulation) I think it's totally worth building.

Regarding the LFO I however agree. I really just built it in because it was on the schematic and didn't require many additional parts. I have built in a switch to disengage the LFO and envision using this pedal mostly with manual sweep of the pulsewidth. Anyone building this pedal mainly wanting to have it swept by LFO should probably look into adding a better LFO circuit.

[pinkjimiphoton]

hi marcus, if you diggit, it's cool!!

i was just really dissappointed with the build, and the pong war machine from insonicbloom, too. just nothing i could or would ever really use, but i'm glad you like it and it's working!!

to me, it seems like it needs way more modulation to make it useful... it just doesn't really seem to DO anything other than bitcrushed distortion to my ear..

but whaddahell do i know? there's an old saying in this biz... that that sounds good, is!! and that's all that's important.

to me, the ugly face without lfo has a lot of the same or similar sounds, maybe i was disappointed at just how much more i could get out of the UF than the pwm.