DC leak in TC chorus

[stinkfoot]

I just got a TC chorus (80's SCF) home, and decided to add it to my chain. As it happens, whenever I turned any pedals on/off later in the chain I got a severe pop. Turning on a simple booster (which happened to right sit after the chorus) stopped the popping from the other pedals, so I suspected a DC leak in the TC. With my DMM, I measured about 2 volts (!) across the output, which quickly dropped to a steady 0.5v. I replaced the output caps on both outputs (4.7uF electros) but it didn't help. Any ideas?

Also, ideally I'd like to fix it without having to disassemble/assemble the pedal too many times (there are connectors between the pcb's inside that I'd prefer to not damage). Because of this, I have actually thought about simply replacing all the electros, while I'm at it. But would that be going overboard, and is there something else I can try?

/Andreas

[Ronsonic]

Is there some sort of pull down resistor at the output of the TC? Measure from tip to ground on the output jack, if there isn't maybe tack a 1M across there and see if that fixes.

[Auke Haarsma]

Isn't the SCF always on? In 'bypass' it buffers but can still add gain, and in 'active' it adds the chorus?.

[stinkfoot]

Ronsonic - on the left/mono output, right after the 4.7uF output cap, there's a 470ohm resistor connected to ground, through a 0.047uF cap. I'll try tacking on a pulldown resistor across the output and see what happens. On the right output, right before the output resistor, there's a connection to the "ring" terminal on the left/mono jack, through a 2.2K resistor. I suppose it will act as a pulldown to ground.

Auke - yes, it is a fully buffered pedal. The DC leak causes switch pops in the other pedals downstream.

[Zben3129]

Is the output cap electro?

If so maybe a film cap would be worth a try or maybe a nonpolar electro. Not sure if this will help but its not too tough to check.

I'd imagine the pulldown might help though.

Zach

[stinkfoot]

Time for an update...

I actually measured as much as 6 volts DC on the output jacks...  A 1meg pulldown resistor helped, but not enough. Finally, using a 220K resistor I got the DC leak down to 15mV, without any detrimental effects on the pedal̈́'s sound. So now it is useable, at least.

Still, something's not right with the pedal - it should not put 6 volts across the output... I can't help feeling that this was only a temporary fix or a band-aid solution - eventually I'd want to get to the bottom of why the voltage was there to begin with. I did find an FAQ entry at tcelectronic.com which mentions that the SCF may pop when engaged/disengaged from a loop box. This is attributed to a "small DC leak" in the output caps, and they recommend soldering 1M resistors across the output jacks to solve this. However, at 6 volts DC offset, I'd still say my pedal is slightly outside the norm... 

I'll drop TC an e-mail to see if they can shed some light on this. In the mean time, thanks for the pointers!

/Andreas

[Ronsonic]

It has got to be the output cap. Or we are misunderstanding something.

I know you mentioned replacing it - check the polarity and replace it again.

This is pretty simple electronics. You've got a blocking cap and a pulldown resistor - if there's a voltage on the wrong side of the cap it's a bad cap. It's easily possible for the output to float up a little when unterminated, but the pulldown fixes that. Maybe shoehorn a film cap in there, since this is low volts you can get the smallest one possible and you can cheat the value down without hearing a loss of lows you can surely go down to something like a 2.2u without fear. Nuke from orbit.

[Zben3129]

Yep, I still recommend changing the electro to a new one or even better a film 


A cap is (for our purposes) an open circuit to DC. Open circuits don't care what is going on on the other side of the circuit, as it is not connected. DC voltage across a cap is bad.

Zach

[stinkfoot]

Yup, I too thought replacing the output cap (caps, actually - there's two outputs) would block any DC, no matter what's going on before it. But in this case it didn't make any difference... I could replace it again (in case the one I used was a dud) but fitting a 2.2uF film cap in there - not to mention a 4.7uF - will take some doing... As it stands, I got the DC leak down to 15mV, which is low enough for me to be able to use the pedal. I've e-mailed TC tech support, though, and it will be interesting to see if they've got any ideas.

/Andreas

[stinkfoot]

So... TC replied, but no dice.

Here's what I got:

Dear Andreas,

Unfortunately we are not able to help you:

1. Our service technicians do not give repair tips simply because they have no time for that : they must repair what customers send in for repair.
2. The old SCF has been discontinued for 15 years and we are not supporting it any more. None of our tech actually know it.

Sorry !!!

Best regards

/Andreas

[slideman82]

This whole topic gave an idea... I have a SCF too, and there's some kind of trouble when it's working: a noise like a heartbeat appears! Also, there's another problem: dc output of the power transformer is abut 45V, and 15V regulator only stands for less than 35! So, probably a zener will solve the problem (I already did this). 15V regulator could be bad, too, like it was on the one I have.
I'm gonna check the Dc at the output...

PS: all electrolitics could be in not such a good shape, they are more than 20 years!

[Zben3129]

Heart-throb sound is the LFO bleeding into the signal. Do a search and you will find tons of posts on this, more efficient than me typing them out 

Zach

[slideman82]

Thanks a lot! I'm gonna search with those key words... I wish I could fix that pedal tomorrow...

[slideman82]

Changing all electros (not tantalums) didn't make it! Maybe the LM339, I wish I could replace it with a TL064 without doing the dead bug trick! and they're not the same type IC, I think!

[DanishRock]

I knowthis is an old post, i have a scf with the lfo bleed problem, cant find any solutions. Did you fix yours slideman