Multiplex Echo Machine - Sounds Dark/Muffled [SOLVED]

[Hexjibber]

Arrgh! Ok I give up! I've tried everything I can think of! Really hoping someone here can help!

My 2nd Multiplex build is not working as it should, the delay section is working perfectly but the overall sound is dark and muffled and slightly distorted, including the dry signal.

I've triple checked my values on resistors and caps, IC orientation, cap orientation. I've measured the voltage throughout the circuit and checked against my working Multiplex and all reads the same or very similar.

It seems like the answer is staring me in the face but I despite coming back to it several times I just can't see what I've done wrong!

I cracked out the audio probe but it didn't make things any clearer, I got as far as IC1 and the signal was already showing the fault which seemed to only indicate a handful of components, all of which are the correct value.

Just at my wits end now, does anyone have any suggestions I can try?

Some readings:
IC1
1 - 4.08
2 - 4.08
3 - 4.06
4 - 0
5 - 4.06
6 - 4.08
7 - 4.08
8 - 8.17

IC2
1 - 5.01    9 - 2.5
2 - 2.3     10 - 2.5
3 - 0        11 - 2.5
4 - 0        12 - 2.5
5 - 2.78   13 - 2.5
6 - 2.3     14 - 2.5
7 - 0.1      15 - 2.5
8 - 0.93   16 - 2.5

IC3
1 - 5.01    9 - 2.5
2 - 2.5     10 - 2.5
3 - 0        11 - 2.5
4 - 0        12 - 2.5
5 - 2.72   13 - 2.5
6 - 2.3     14 - 2.5
7 - 0.09   15 - 2.5
8 - 0.64   16 - 2.5

Reg
1 - 8.17
2 - 0
3 - 5

H11F1
1 - 0
2 - 0.01
3 - 0
4 - 0
5 - 1.70
6 - 2.25

+9v - 9.35

[gtr2]

You might have a bad solder joint in your signal path...have you tried reflowing the joints?

Also, try another TL072.  It's unlikely but maybe its bad.

Josh

[mgwhit]

What Josh said...plus I'd consider swapping out C7 and C6.  Those ceramics can be total crap.

Are you really getting dark, muffled signal at IC1's input (pin2)?  Like you said, there are only so many parts, joints, etc.  Good luck!

[gtr2]

Clarifying the muffled spot will help us

[Hexjibber]

Hi guys,

Thanks very much for the replies, in answer to your questions the 'dark' sound is present at pin 2 of IC1, I checked this against how it sounds on pin 2 of my working version and it's certainly different. It sounds ok at R5, I thought I'd identified the point as being R10 as it sounded dark there but after checking the value I found it to be correct, C7 is also the correct value. In comparison the signal sounds fine at R10 on my working build.

I swapped out the TL072 with the one from my working build and it made no difference.

I guess reflowing my solder joints is the next best thing to try, never had a solder problem before but there is always a first time for everything!

Cheers chaps,
Graham

[mgwhit]

[T]he 'dark' sound is present at pin 2 of IC1, I checked this against how it sounds on pin 2 of my working version and it's certainly different. It sounds ok at R5, I thought I'd identified the point as being R10 as it sounded dark there but after checking the value I found it to be correct, C7 is also the correct value. In comparison the signal sounds fine at R10 on my working build.

I swapped out the TL072 with the one from my working build and it made no difference.

I guess reflowing my solder joints is the next best thing to try, never had a solder problem before but there is always a first time for everything!

If the reflows don't fix anything (and fingers crossed, they do) I'd seriously consider swapping out C7 no matter what it says it is.  Unless, of course, you've tested it.  (My multimeter won't test capacitance values that low.)  I honestly think it's the most likely culprit, and, even if it's not, it's a 10-cent part.

[Hexjibber]

Hi chaps,

So it turns out my 47pf cap was in fact a 4700pf cap, the answer was there in little tiny numbers staring me in the face the whole time! Feeling a bit embarrassed but to be honest had I not posted I don't think I would have focused my attention in that area so wanted to say thanks very much for the assist!

Mental note: When ordering parts from Banzai, despite the value being written on the little bag, check that they have put the right one in there!!

So glad I've sorted it though as this one is for a mate who is going on tour this weekend so the pressure was on!

[mgwhit]

So it turns out my 47pf cap was in fact a 4700pf cap, the answer was there in little tiny numbers staring me in the face the whole time!

That will do it every time!  Glad you got it working.

Feeling a bit embarrassed but to be honest had I not posted I don't think I would have focused my attention in that area so wanted to say thanks very much for the assist!

You're quite welcome.  Your first post had good photos, IC voltages and information from your audio probing, all of which contributed to isolating the problem very quickly.  It really doesn't get better than that!

I hope your friend loves the pedal.

[gtr2]

Glad you got it sorted!

Josh

[Hexjibber]

Thanks again to the both of you, I really appreciate it as this was stressing me out big time!

Another notch on the experience belt eh!