Author Topic: Engineers Thumb distorting  (Read 410 times)

guidoilieff

Engineers Thumb distorting
« on: August 31, 2019, 07:08:58 PM »
Hi. I've made this pedal using this layout https://www.dropbox.com/s/purgnc68qed3n7t/Engineer’s%20Thumb%20build%20doc.pdf?dl=0

Voltages seem ok. Pin 1 of IC1 is at 8.5v, drops to 4.5 when I pick a string on my bass and slowly rises again to 8.5.



Here is the problem: Using a sound probe signal reaches pin 3 ok, but in pin 1 and 2 the sound comes distorted/ gated, kinda like a bad biased transistor if you know what I mean.


I tried swapping the IC and thats as far as my brain goes. Should I lower the value of R2 or just the opposite? Maybe the problem is on C2 and forward? Idk. Any suggestion is welcome.



Sorry I cant upload a pic, my phones dead.


Thanks!
« Last Edit: August 31, 2019, 07:56:58 PM by guidoilieff »

guidoilieff

Re: Engineers Thumb distorting
« Reply #1 on: August 31, 2019, 10:29:22 PM »
Doesn't the Ratio pot controls gain on IC1?


Maybe I should make it a 2 knob version. What are the best resistor values for bass?
« Last Edit: August 31, 2019, 10:41:19 PM by guidoilieff »

merlinb

Re: Engineers Thumb distorting
« Reply #2 on: September 02, 2019, 05:52:48 AM »
Doesn't the Ratio pot controls gain on IC1?
Yes. Turn the ratio control to minimum and you should have a unity gain buffer, totally clean. If you don't then you've make a build error somewhere.

FYI, one thing that schematic needs is a 220k resistor in series with the release pot, otherwise you'll get funky behaviour when the release is turned down to minimum.

Quote
What are the best resistor values for bass?
Same as for guitar, but you can increase C3 to 2.2uF if you like.

guidoilieff

Re: Engineers Thumb distorting
« Reply #3 on: September 02, 2019, 08:57:56 AM »
Doesn't the Ratio pot controls gain on IC1?
Yes. Turn the ratio control to minimum and you should have a unity gain buffer, totally clean. If you don't then you've make a build error somewhere.

FYI, one thing that schematic needs is a 220k resistor in series with the release pot, otherwise you'll get funky behaviour when the release is turned down to minimum.

Quote
What are the best resistor values for bass?
Same as for guitar, but you can increase C3 to 2.2uF if you like.

Thanks for the reply, such an honor. Sorry for my English.


So its normal that 7/8ths of my ratio pot distorts with this design?

« Last Edit: September 02, 2019, 09:00:25 AM by guidoilieff »

merlinb

Re: Engineers Thumb distorting
« Reply #4 on: September 02, 2019, 09:06:39 AM »
So its normal that 7/8ths of my ratio pot distorts with this design?
Yes that might explain the problem.

guidoilieff

Re: Engineers Thumb distorting
« Reply #5 on: September 02, 2019, 11:04:35 AM »
Well.... I tested the pedal with an amp and the clipping leds started flashing and now the pin1 of IC1 doesn't rise to 8.5v anymore, it is stuck at 4.5.

Maybe in 10 years when my country is no longer in crisis I can get another 13700 because I think it might be broken now... IDK how to check that.
« Last Edit: September 02, 2019, 11:24:48 AM by guidoilieff »

merlinb

Re: Engineers Thumb distorting
« Reply #6 on: September 03, 2019, 04:39:48 AM »
Well.... I tested the pedal with an amp and the clipping leds started flashing
Are you using an active bass, or something?

guidoilieff

Re: Engineers Thumb distorting
« Reply #7 on: September 03, 2019, 11:20:48 AM »
Well.... I tested the pedal with an amp and the clipping leds started flashing
Are you using an active bass, or something?

Nope. Just a passive bass and guitar.

The good thing is that I tested the 13700 in a Coron DS7 and it works.



What caught my attention is that in the 3 lugs of the Ratio and Release pots there is 4.4v (release is actually 4v/variable/4.4v)



And here is a voltage list because now voltages are weird I guess


T1
C 1
B 3.7
E 4.45


ic 1

4.42
4.42
4.30
0

4.40
4.40
4.40
9


ic2

0
0
4
0

0
4.4
3.7
9


ic3
1
0
4.4
4.4
4.4
0
0
0

0
0
9
0
0
0
0
0


Thanks again for your time. I know this can be a little bit abstract.
« Last Edit: September 03, 2019, 11:24:56 AM by guidoilieff »

merlinb

Re: Engineers Thumb distorting
« Reply #8 on: September 04, 2019, 04:13:58 AM »
Judging by your voltages it looks like you have a problem around IC2A, like maybe a short to ground?



befey

Re: Engineers Thumb distorting
« Reply #9 on: September 13, 2019, 11:21:19 AM »
This schematic seems slightly different than the one posted on your website. Especially around the LM13700.