Tube Screamer debug help!

noby · 03-09-2013 12:33PM #1

I know there are a few pedal builders on here.

I built a TS clone ages ago, worked perfectly. Then I decided to relocate it into a different/smaller enclosure. That's when the trouble started.

At first I was getting complete overloaded distortion, no matter what the gain setting. Last night I took it all out of the enclosure, gutted all the wires and started all the wiring from scratch.

So now I have no output, and measuring a few voltages I see that my Vref is only 3.6v as opposed to 4.5V. I'm guessing something is pulling it down. I've checked the board for shorts and CSJs. So my plan is to isolate the voltage divider stage to ensure I have 4.5v, then reconnect different stages checking each time that my Vref stays at 4.5V Is this my best plan of action (before I start lifting components/cutting tracks)?
Obviously I'm not expecting someone to pinpoint my issue just from that sketchy bit of info, but is there anything obvious that I might be missing?

KeithTS · 03-09-2013 01:01PM

Have you got the schematics and stuff to hand?
And the actual component values you used. did you do the maths yourself and stuff to know exactly what your Vref should be or was before the problems started, do you get anything when the circuit is bypassed and so on?

Any info on the actual circuit would help. I would suggest looking back over every part to ensure there's no misplaced component, no grounding issues and so on. Any electrolytic caps with incorrect polarity or a BJT the wrong way round?

The fact that it did work suggests that your wiring or a component is mixed up somewhere or that you have a dodgey solder joint.

zafo · 03-09-2013 04:19PM

If you're powering off battery and it's down to 7.2V then Vref is gonna be 3.6V. the completely overloaded distortion sounds like a short on the gain pot so make sure nothing is touching the case. Now the no output sounds like a short or ground where it shouldn't be. Have you a signal probe? If not make one and probe along the circuit, from the input, til you hit a dead end.

noby · 04-09-2013 06:12PM

Thanks for the replies guys. Sorry but I've been up the walls with work.

The pedal is Der Screamer from Musikding. Details here: http://diy.musikding.de/?p=59&lang=en

Yes, I presumed the distortion was from a short (I'm trying to get it into a smaller enclosure so the pots are a bit cosy). That's why I pulled it out to start again. Second time round it was dead. Noticed one of the wires starting to come loose from all the pulling, so decided to completely re-wire it. I'll double check my wiring when I get a chance to make sure it wasn't that.

As for the power, I'm using a power supply, so my input voltage is spot on 9v. The voltage divider uses two 10k resistors (I double checked the values) but I'm getting a voltage drop of 5.6 volts across R17, and end up with Vref of 3.5 or so. I had no need to check this before, but I'm assuming with equal resistors I should have 4.5V

I checked the voltage on the IC and I seem to be only getting 1.4v on pin1, 3.4(Vref) on the others, and 0v and 9v on pins 4 and 8. This pin1 voltage seems odd to me. Perhaps I need to double check my drive pot.

I need to make myself a signal probe I think, although it'll probably be next week before I get to look at it again.

zafo · 05-09-2013 02:05PM

Yeah your Vref should 4.5V if your supply is 9V, just whipped open a Screamer here and I have 4.5 or thereabouts on Pins 1,2,3,5,6 and 7 then 0 on pin 4 ground and 9 on Pin8. Sounds like something wrong with your voltage divider network but could be something else pulling down your Vref afterwards. I'd be looking at the drive pot too but other than that I'm not sure unless I have it in front of me.

Tube Screamer debug help!

Comments