MKC acting strange

[Anonymous]

Hi all,

This is my first guitarpcb kit, but I’ve made many pedals (and a 5E3) before.

I finished the build last night. I put no bass boost switch, but did put the switchable higher gain. I tried it out through my interface which is an 18i20 into Helix Native and it sounded pretty good, though it was different from other klon based pedals I’ve tried and I had to keep the tone/treble up almost all the way to match the dry signal but it was not bad at all. The ‘normal’ gain setting in particular had almost no gain at all compared to other klons I’ve tried, but the extra gain had loads, as well as having extra treble.

Today I plugged it into my Princeton Reverb and it sounds much different and so much worse. Almost zero extra gain on the normal channel, and very very little treble. The higher gain channel again had more treble but the overdrive/distortion was very “ripping paper” and weak sounding.

Plugged it into the interface again and it’s back to ‘normal’.

At first I thought it was an impedance issue, but the impedance of the 18i20 is 1M5 and that of the Princeton input 1 is 1M. The difference seems negligible, but what do I know…

The voltages on the IC pins all read ok.

Any ideas?

[Barry]

Strange indeed.

Regardless of build experience (we all make mistakes) we still need clear photos of on-board, off-board, component side and solder side views to be able to get a glimpse of what might be going on. This is not normal at all one way or the other so we should start by having a look.

[Anonymous]

Here are a few pics, hopefully it’s just some sloppy mistake I made

The soldering is a bit dodgy in some spots because I’ve soldered/desoldered some spots numerous times trying to figure out what’s going on.

thanks!

[Cybercow]

Double-check the voltages on the opamps. Make there is +9v on pin #8 of both and that pins #3 & #5 are  at ~ +4.5v on IC2.

If those voltages are correct, then I’d wager a resistor or cap somewhere is an incorrect value.

[Anonymous]

Yeah it looks like the voltages are correct.

I took a look through the entire circuit the other night and didn’t find any problems but I’ll take a third look today or tomorrow

Thanks for the help

[Anonymous]

My apologies I didn’t get the off board pics.  I did notice I have pins 1 and 3 flipped on the gain potentiometer.

I also used two 1N34As from different sources, if you notice in the pic. One of them is substantially bigger than the other, so I’m assuming a different manufacturer and all

Could either of these things be the culprit? While I’ve done a lot of pedals I’ve always been painting-by-numbers, I’m very weak on the circuit analysis side

[Anonymous]

My apologies I didn’t get the off board pics.  I did take another look through the resistors and caps and didn’t notice any problems. I did notice I have pins 1 and 3 flipped on the gain potentiometer though

I also used two 1N34As from different sources, if you notice in the pic. One of them is substantially bigger than the other, so I’m assuming a different manufacturer and all

Could either of these things be the culprit? While I’ve done a lot of pedals I’ve always been painting-by-numbers, I’m very weak on the circuit analysis side

Thanks again

[Cybercow]

Justin – the diodes will not impact the build as described in the initial post. The flipped pins on the gain pot will have an impact on the performance of that pot – it will work backwards. Are you using a dual-gang pot for the “Gain” control – or separate pots?

[Barry]

I would also be really sure your other pots are not wired backwards.

It is impossible to tell from photos.

[Anonymous]

Thanks Barry, Cybercow for your help and input

Tonight I took out the trim pot for the other gain and wired the dualgang pot as “standard” and jumpered R26 as well instead of the resistor and now it’s working as it should. I did both at the same time so I’m not sure which it was but it sounds great now.

Thanks again!

[Author]

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