Klone build trouble

[jonwhitear]

Hello all,

I'm building a Klon Centaur overdrive pedal clone kit, acquired from AliExpress.

The kit and circuit are as described in this TDPRI thread and circuit diagram.

Unfortunately, it's not working very well, and I'd appreciate guidance from someone who understands how the circuit works.

The pedal works in "bypass" i.e. buffered mode, but is very quiet in effect mode. I have checked component values, and measured supply voltages OK, e.g. at the opamp pins.

Long story short, when I feed a 500Hz signal from my scope into the input, with all the controls at 12 o'clock and the effect engaged, I see a nice clean signal at the output of the first opamp (pin 1 of U1a), a nicely distorted signal at the junction of the clipping diodes, and a significantly amplified signal at the output of U2a (pin 1.) However, I don't see any signal at the input of U2b (pin 6.) I seem to lose the signal across the 100K resistor between pin1 and pin 6.

Before I go desoldering stuff and generally making a mess of it, what would your next steps be? I have continuity from pin 1 to one leg of the 100K resistor, and from the other leg to pin 6. The resistance across that resistor measures about 14k in situ.

Cheers,

Jon

[Simon Barden]

Can you desolder one leg of the 100k resistor and measure its resistance? Have you double checked all the associated resistor ring values, just in case there's one wrong value that's sneaked in.

Any of the resistors around U2-b with a short to ground at one end?

There's a very good description of the original Klon circuit here https://www.electrosmash.com/klon-centaur-analysis (web certificate seems to now need renewing but it's been a good site before with no issues). Also a lot of other pedals described in detail.

[DrNomis_44]

My guess is that it could be caused by a dry solder joint on the 100k resistor between pin 1 of U2 A, and pin 6 of U2 B, or, that 100k resistor might have gone high in value, or gone open-circuit, there could also be a partial-short to ground from pin 6 of U2 B, as Simon said, first thing I would do is de-solder one leg of that 100k resistor and measure it with a digital multimeter, if it checks out to be within tolerance then that will eliminate the resistor as being the cause, hope that helps.


That 10k B output pot could also be the cause of the low output when un-bypassed, sometimes the solder-lugs on pots can go a bit loose if too much heat is put on them when soldering to the lugs, this will cause an intermittent connection to the carbon track.

[jonwhitear]

Thanks guys.

I removed the 100k resistor, but that tested OK, so I put it back. While I was testing all the components in that part of the circuit to make sure I had continuity where I should, and no ground connection where I shouldn't, I found that the silk screen labels for the 100k resistor in the feedback loop of U2b and the 560R resistor before the output pot where swapped, so I had a 560R resistor in the feedback loop (which presumably explains why there appeared to be no signal, as it was pretty mach all being fed back into the negative loop) and 100k at the output pot.

I have swapped those round and now it's working nicely. Initial testing with my p90 equipped ES335alike sounds promising.

Cheers, Jon.

[McCreed]

I know SFA about building pedals, but I just noticed your latest avatar... Noice!!!

Oh, and glad you got it sorted too!