Engineer's Thumb Troubleshooting (midwayfair perf)

[seedlings]

Circuit:


Vero:


Pics:





2-knob version Substitutions:
LM13600 (in place of LM13700)
GT108V germanium (in place of BC327)
Some of my connections may be one hole off from the vero, but they are very near

Power supply is 11V, and both opamps pin 8 show 11V, as does pin 11 of the lm13600
Ground verified pin 4 both opamps, and pins 6, 7, 8, 9, 10, 16 of the lm13600
5.5v pins 3, 6, 7 of top opamp as well as pins 3, 6 of bottom opamp

Audio signal on pin 1, 6, 7 of bottom opamp, quieter on pins 3, 1 top opamp, even quieter pin 5 on lm13600

The guitar signal is amplified well, 'sustain' is clean boost, output volume works as it should, but there is no compression.  If you have some pointers, that would be great.  I didn't know if the germanium pnp would be an OK substitue - it's the only kind of pnp I had on hand.

Thanks!
CHAD

[merlinb]

I dunno, I'm kinda suspicious of that germanium. Doesn't halp that the data sheet is in Russian, so I can't even work out if you've connected it the right way. Try measuring the voltage at the junction of the 100R and 1M release resistor. Does it go down as you play louder?

[seedlings]

Left to right in the perf layout, the emitter is 1.422V, base is 1.489V and collector is 1.069V

CHAD

[seedlings]

I dunno, I'm kinda suspicious of that germanium. Doesn't halp that the data sheet is in Russian, so I can't even work out if you've connected it the right way. Try measuring the voltage at the junction of the 100R and 1M resistor. Does it go down as you play louder?

Voltage stays 4.89V while playing or not playing.

CHAD

[merlinb]

Voltage stays 4.89V while playing or not playing.

It should go down, suggesting there is a problem with the peak detector IC1b. Check the connections around there.
I also don't think it's a good idea to power this circuit from 11V, as you could end up dumping too much current into the LM13700's control pin.

[seedlings]

Lowered overall voltage to 7.93V (from 11).

The emitter had a bad connection to pin 2 of U2a.  Transistor voltages 3.98V, 3.96V, 1.08V.

The voltage of 1M 100R node goes down from 2.7V to 2.3V with a big strum and the ratio knob up.

Now it is a squishy overdrivemore than a compressor (Pigtronics philosophers tone).

CHAD

[merlinb]

Suggest you post all your new voltages.

[midwayfair]

Hrm, that looks to be my old layout. The 1K is in the wrong place, even though it shouldn't noticeably affect performance. Double check the PDF that I didn't make any other corrections (no parts should have moved, it might just be clearer):

https://dl.dropboxusercontent.com/u/9878279/Jon%20Patton%27s%20mini%20layouts/Nonmini%20builds/Engineer%27s%20Thumb.pdf

[seedlings]

Got the correction fixed Jon for that jumper.

Swapped power supplies and here are voltages:
Lower opamp
1- 4.67
2-4.72
3-4.19
4-0
5-4.19
6-4.68
7-4.77
8-9.40

Upper opamp
1-4.68
2-4.70
3-4.20
4-0
5-4.44
6-4.71
7-4.71
8-9.39

Transistor
Emitter (left)-4.70
Base (center)-4.68
Collector - 1.109

Lm13600
1-1.109
2-10mV
3-4.71
4-4.71
5-4.71
6-1mV
7-0
8-0
9-0
10-0
11-9.41
12-6mV wandering
13-6mV wandering
14-7mV wandering
15-12V wandering
16-0

Maybe slight compression through the first third of ratio totation, then mild compressed overdrive.

CHAD

Edit: clean sound coming out of pin 1 of U1a, and gated/rectified sound pin 7 U1b.  Sputtery grunts from U2b pin 1, and faint souttery grunts on lm13600 pin 4.

[MrStab]

i don't have much to contribute, but you said "mild overdrive", which makes me wanna point out that I and others have had issues with the clip-softening LEDs, particularly on guitars with a high output.

the germanium makes me uneasy too, but i can't imagine it would degrade the tone in this context.

[seedlings]

i don't have much to contribute, but you said "mild overdrive", which makes me wanna point out that I and others have had issues with the clip-softening LEDs, particularly on guitars with a high output.

the germanium makes me uneasy too, but i can't imagine it would degrade the tone in this context.

Keep thinking, please!  The signal through the LEDs is ok, and clean at full ratio.  This is at the output of pin 1 of Q1a.  My guitar pickups are not high output.  The distortion is introduced by the compression components, but I don't know what the signal should sound like at each node in the signal chain.

CHAD

[seedlings]

Sound doesn't change if the pnp or lm13600 are removed.  I have tried many gt108a and also 3 different gt313v (might all be too high leakage).  I ordered some 2n3906.  Re-checked continuity through components from each pin of each chip without finding an error - except the reverse polarity diode wasn't grounded.

I don't have a spare lm13600.  Will order an lm13700.  Read where deadastronaut rob had some bad chips that were distorting.

If someone with a working circuit could audio probe
Pins 1 and 7 of U1
Pin 1 of U2
Pins 1, 3, 4, 5 of the LM13700

Let me know what each are supposed to sound like.  I use a cap clipped to the tip of a guitar cord, going to an amp.

CHAD

[merlinb]

Your voltages all look reasonable.

Edit: clean sound coming out of pin 1 of U1a, and gated/rectified sound pin 7 U1b. Sputtery grunts from U2b pin 1,

That all makes sense.

and faint souttery grunts on lm13600 pin 4.

Wha? That pin should be connected to the 4.5 Vref...

[seedlings]

Wha? That pin should be connected to the 4.5 Vref...

What does your circuit sound like on pin 4 with a guitar strum?  This may be where a problem lies.  I hear something like a transistor that is biased soo poorly that only a very hard strum can get it to make a blipping sound.

CHAD

[midwayfair]

Wha? That pin should be connected to the 4.5 Vref...

What does your circuit sound like on pin 4 with a guitar strum?  This may be where a problem lies.  I hear something like a transistor that is biased soo poorly that only a very hard strum can get it to make a blipping sound.

CHAD

4.5V is AC ground for the op amp. I'm 99.999% sure you shouldn't hear ANYTHING.

[seedlings]

Gah!!!! The lm13600 was bad!!!

I nabbed one out of the tri-vibe, and the thumb works!!!  Thanks everyone!


CHAD