SMD Scream Machine can make a decent sound, but the controls are not operating correctly.

[jcpst]

Hey guys. Baby’s first SMD build. Even with a clumsy tip, I managed to get it assembled. Here’s the behavior.

• Volume: seems to work fine.
• Gain: knob does nothing at all.
• Tone full CW: lower than unity and the high end is cut off.
• Tone full CCW: all the mid and high end is there, it’s a bit louder than unity, and it has a nice crunch.

It took me a long time to decide the orientation of the IC. I wasn’t able to see a dot. But I did notice one side had a longer slope. But it not having the dot made me suspicious…

The other thing I did was a substitute for 1uf caps. I didn’t find any, so I stacked 470+470+68uf and made a little parallel tower with them.

Since there are more tube screamers on the planet than people, I thought this might be a scenario that people may have come across. Apologies if this pops up in a forum search and I missed it.

It’s pretty small stuff, so let me know if I need to grab a detail shot.

 

[Nostradoomus]

First I'd do a thorough inspection of the parts, because I'm not gonna decipher that for you lol. Then I'd solder the breakout board input jack and board out pads a bit more, looks like they're barely hanging on. Next I'd scrub the board. Then I'd take some DMM measurements of the transistors.

EDIT: The cap towers are sus too, you can change the coupling caps in a TS to 470n without many repercussions (if any)

 

[Robert]

The resistor values look correct. There's no way to visually inspect the SMD capacitors.

Make sure there are no solder bridges between any of the pins of the IC.

Got a DMM? Let's check something...

Turn gain all the way down, measure resistance between pins 1 and 2 of the opamp.
Turn gain all the way up, measure again.

 

[jcpst]

Turn gain all the way down, measure resistance between pins 1 and 2 of the opamp.
Turn gain all the way up, measure again.

Gain down, 40.5k
Gain up, 54k

Then I'd solder the breakout board input jack and board out pads a bit more, looks like they're barely hanging on.

They’re on there pretty good- I see what you mean, I think it’s bad lighting on my part.

 

[jcpst]

Also didn’t see any visible bridges on the IC pins under magnification.

 

[Nostradoomus]

maybe try with a continuity tester just to be sure, something is definitely funky with the gain pot/that area. I have working tubescreamer circuits in front of me and the resistance goes from 50k min to 500k max between pin 1-2

 

[jcpst]

Ok so mine is definitely off. I have to leave the bench for the night, but I’ll start going through that area tomorrow. Thanks, I appreciate it.

 

[jwin615]

Did you use uF or nF caps?
To make 1uF you needed nF.

 

[Nostradoomus]

Did you use uF or nF caps?
To make 1uF you needed nF.

Guessing that was a typo because those caps would be way bigger if it was uf

 

[jwin615]

Guessing that was a typo because those caps would be way bigger if it was uf

Figured. But I'm not all that familiar with tiny tants.

 

[jcpst]

Sorry, I was just being a mF-er

 

[jcpst]

Found a short between r1 and c1 to ground, fixed that.

Noticed there was no continuity between IC1.1, R9, and IC1.5. Pulled a pad off r9 and ran a lead to pin1.

No continuity between q1 base and r2/r3. Pulled a pad. Ran another lead. No continuity between q2 base and c8. Ended up running a lead between q2 base and r12.

Now I have no sound when engaged. Wondering if I overheated those transistors…

I feel kind of like a caveman that’s using a rock on this thing. I might end up putting it away for another day.