Why doesn't this work?

[FingerBlisters]

Here's a schematic. Apologies for the lack of indicators, Eagle can only show names on the PCB not the value as far as I know. Q1/Q2 are MPSA 18 (just says 3904 as that's the part in the eagle library. There's a 2n3906, and a NPN at the end (2n3904).



Imgur link to same:

https://i.imgur.com/LC25hD9.png

It's a devi Hyperion, into a passive Baxendall/James, into a LBP1 to recover volume lost in the tone stack.

Looks fine to me. The Hyperion schematic was lifted from PedalPCB's version except amalgamating the 2 x 100n into a single 220n, and removing the 100n to GND. Also a 1m pull down before the input gain pot to lessen switch pops.

The Baxendall schematic is pretty common and is from the Duncan TC and this guy:

http://pedalparts.co.uk/docs/BaxMuff.pdf

The LBP1 is something we all know and love.

The bit on the end 4.7U/1k/39n/VOL is a low pass filter before the pot to tame off any residual squeal/hiss.

What am I missing here? The LED comes on but no action.

[Vivek]

Would an Audio probe be of help here ?

You could find out an which stage the Audio signal is not as expected.

[FingerBlisters]

It probably would be, I was just wondering if there's any obvious schematic issues anyone could see and I've never used an audio probe before. I've been looking at this thing so long everything looks the same.

[Vivek]

There can be problems of 2 types

Problems with the circuit schematic

Problems with a particular build.

Audio probe can help as first level of diagnostics


You can make an audio probe very easily.

I think RG Keen and Craig posted diagrams on how to make it with an alligator clip and a cap

You can go to the output of each stage of your circuit and see if you get the expected output.

[GibsonGM]

Yes, audio probe. I like to build things like this in complete sections, and listen to the output of each to be sure they actually work.   I do this on breadboard, and again when building on a PCB (I tend to use perf, not real PCBs).  That way any errors are isolated to a particular block.   

And also, at the end, you know the whole thing works - so if you make a mistake when installing the switching and power jacks etc, it's clear there is a problem in them and not the board if it doesn't work!

[antonis]

except amalgamating the 2 x 100n into a single 220n,

Replacement of 2 X 100nF in series need a 50nF single cap..
(47nF should also be OK..)

[ElectricDruid]

The only thing I can see that looks weird on the schematic is that 1M on the input. It's not doing anything because it's in parallel with the input gain pot (which is of a lower value so will tend to dominate the eventual value) and when the pot is turned down, in parallel with 1K (and 1K in parallel with 1M is basically 1K).

I agree with what others have said. Get an audio probe and check the output at the end of each stage and find out which one isn't working. Probably one has an error and doesn't look like your schematic in reality.

Also you said you've been looking at it so long it all looks the same. That's fatal for debugging. Go and do something else. Bake a cake. Take the dog for a walk. Go camping for the weekend. Whatever. Then come back and take a fresh look.

[FingerBlisters]

posted in error. Sorry.

[FingerBlisters]

The only thing I can see that looks weird on the schematic is that 1M on the input. It's not doing anything because it's in parallel with the input gain pot (which is of a lower value so will tend to dominate the eventual value) and when the pot is turned down, in parallel with 1K (and 1K in parallel with 1M is basically 1K).

I agree with what others have said. Get an audio probe and check the output at the end of each stage and find out which one isn't working. Probably one has an error and doesn't look like your schematic in reality.

Also you said you've been looking at it so long it all looks the same. That's fatal for debugging. Go and do something else. Bake a cake. Take the dog for a walk. Go camping for the weekend. Whatever. Then come back and take a fresh look.

Here's the PCB iit's made on if that's a help



I did fashion an audio probe from a guitar cable and ran a looper into the problem pedal.

I get a signal from the input on the bottom, through the 1m, into lug 2 of the bottom left gain pot, then rotating the knob sends signal to lugs 3 or 1. Putting the signal to lug 3 then gets me a signal to the left side of the 100n below, but that signal DOES NOT come out the right side of the 100n.

So, I removed the cap and installed some sockets. I've tried a few caps (including new ones) and I cannot get signal to come across to the right hand side where it would then go into the 2m2 above.

However, what I can do is pull the right leg of the 100n out of the socket, bend it across to the 2m2 and... no signal. However, I can get signal if I touch the cap leg to the left side of the 2m2 but it's a lot quieter.

Does this mean the red trace going up from 100n to 2m2 is bad? The fact I can't get a signal across the cap when it's in circuit is puzzling me.