The errors of my ways... Can you help me find my mistake? (EQ woes!)

[jfrabat]

Hey, guys.

As some of you know, I am making my own home studio gear.  I got a bunch of projects on the bench, but the one I am tackling now is the EQ.  It is a 4-band semi-parametric EQ.  I have build 2 of these already, but I decided to make one for the studio with the knobs facing forward (the other two had them facing up).  You can read a lot more about the project here if you are interested.

But to peek your interest, here is the original pedal (1590Q):



And here is the studio version (1402D):



Now, the boards are identical (minimum production of 3 at OSH) except for mounting holes drilled into the studio version in areas with no vias (I tripple-checked before drilling, and was the first thing I verified when troubleshooting) and the pots (the 10K ones specifically) mounted off board.

When I was building it, I built another just like the original for my daughter, and the studio version one.  Rather than explain the issue, let me show you!

It should work like this:

Please no comments on the LED brightness; that was a request!  Anyway, as for the other one... 

It is exactly the same board (print 3 out of 3), using the same components, except:

• Violet LED's instead of white
• CLRs increased resistance dim the LEDs a bit
• Dual gang pots are A100K in my daughter's and B100K in the studio version
• 12K resistors are from a different batch (Mouser 5% in the studio version and cheap Chinese Amazon 10% in my daughter's)

Since I had both built, I also took readings of voltages and resistance to compare:



Normaly, I would not bother measuring resistance when everything is in the board, but since I have 2 identical boards, I figured it would make a good comparison.  For this, I turned the pots all the way to the left (all 8 of them) to have a consistent reading in both EQs:



As for pics of the board, here they are:





And here is the schematic:



I also used the audio probe to locate the problem.  The signal is making out past the 3rd band with no issues (no reduction in volume) and everything (EQ) working as it should.  But right after R3, it all goes to hell.  I first thought it could be IC1, so I replaced it, but nothing.  It was a bit loose on the socket, so I made sure it had a better contact.  But same issue.  Then I thought it could be the cut/boost pot; after all, it was behaving erratically.  So I replaced it, but no change.  I then started checking resistors to see if there was anything off.  R20 was the only one not close to the value, so I unsoldered one leg and tested.  It is exactly where it has to be.  Double checked cap value for the 4th band.  Also check out fine.  Checked C9 capacitance; also fine.  What am I missing?

[11-90-an]

I may be seeing some cold solder joints...

I'm not sure if it would help but... can you check if the joints that AREN'T supposed to be grounded have any continuity to ground? Just to make sure nothing was accidentaly grounded...

[Rob Strand]

Try pulling IC1 out.  The signal should pass to the output.

Then try adjusting the boost-cut pot of the 4th band.

At the *cut* end the signal should pass with a small amount of loss.   Then ,as you turn the boost-cut pot to full boost you should hear a little more signal loss but not much.

If you don't get that behaviour there might be a short somewhere.

[jfrabat]

Try pulling IC1 out.  The signal should pass to the output.

Then try adjusting the boost-cut pot of the 4th band.

At the *cut* end the signal should pass with a small amount of loss.   Then ,as you turn the boost-cut pot to full boost you should hear a little more signal loss but not much.

If you don't get that behaviour there might be a short somewhere.

That narrows it a bit.  Without IC1, signal goes out fine.  But the cut/boost pot of band 4 does nothing as far as sound goes when I turn it without IC1 in there.

I may be seeing some cold solder joints...

I'm not sure if it would help but... can you check if the joints that AREN'T supposed to be grounded have any continuity to ground? Just to make sure nothing was accidentaly grounded...

Figure it is worth a shot...  and now it got weird!

R3 has continuity to GND.  According to the schematic, it should not.  But checking the other bands, the equivalent resistor also has continuity.  BUT, in case of R3, the beeping is continuous, while in the other bands, it beeps for about a second and then dies until I touch it again.

[kraal]

Hi,

Maybe it's not related, but I would check this out:
It seems that pin 2 of three out of four of the two gang pots have not been soldered. According to the schematics, it is ok not to solder pin1, but pin2 must be sodered to have a variable resistor.
(I assume that the pin order of your footprints is 1 2 3)

Regards,

M.

[11-90-an]

Reflow also, while you're at it...   

[kraal]

I cannot find R8 on your schematics. Are you sure of the references ? Found it... 
(side note, ordering references based on position in schematics, and keeping same reference order for similar building blocks makes things ways easier to read)

[Rob Strand]

What about put IC1 back in and short C1 with a wire.
When you do this confirm pin 5 IC1 is connected to ground.

In this case you should expect:
- the signal to pass without a drop
- boost/cut pot does nothing
- frequency pot does nothing.

If not something is wrong around IC1B, R3, R7.

[antonis]

<off-topic>

Any particular reason for R27/R35 different values..??
(LPFs on split dual supply..)

[Rob Strand]

<off-topic>

Any particular reason for R27/R35 different values..??
(LPFs on split dual supply..)

jfrbat tweaked the resistors to get equal supply rail voltages.  It was in the old thread.
There's slightly different currents on + and - rails.

[antonis]

to get equal supply rail voltages.

Ahaaaa...!!!
(another one of those perfectionists..)

[Rob Strand]

Ahaaaa...!!!
(another one of those perfectionists..)

It didn't start that way. 

Initially one rail was quite a bit off so he had to tweak the resistors.  Later we worked out one of the chips was fried and replacing the chip the currents went back to normal and the rail current were nearly equal - so we had to swap he resistors back.  Since he had already measured the voltages and had to the change the resistor anyway he put in a more finely tuned value.

[jfrabat]

Something I found out after replying and before going to bed is that R3 only has continuity to ground when the pedal is plugged in ans turned on.  Same for the other bands.  Not sure why...

Maybe it's not related, but I would check this out:
It seems that pin 2 of three out of four of the two gang pots have not been soldered. According to the schematics, it is ok not to solder pin1, but pin2 must be sodered to have a variable resistor.
(I assume that the pin order of your footprints is 1 2 3)

Regards,

M.

Pin 1 and 2 are connected at the pot.  One less wire, same effect.  The other 2 versions are the same.

What about put IC1 back in and short C1 with a wire.
When you do this confirm pin 5 IC1 is connected to ground.

In this case you should expect:
- the signal to pass without a drop
- boost/cut pot does nothing
- frequency pot does nothing.

If not something is wrong around IC1B, R3, R7.

OK, will try this.

[kraal]

It seems that pin 2 of three out of four of the two gang pots have not been soldered. According to the schematics, it is ok not to solder pin1, but pin2 must be sodered to have a variable resistor.
(I assume that the pin order of your footprints is 1 2 3)

Pin 1 and 2 are connected at the pot.  One less wire, same effect.  The other 2 versions are the same.

While using the same number of wires, "soldering pins 1-2 of the pot to pins 1-3 of the footprint" is less readable than "soldering pins 2-3 of the pot to pins 2-3 of the footprint" (as shown on your schematics). Don't you think ? 
Anyway... it's not related to your issue, so forget about it.

[Mark Hammer]

First, let me compliment you on beautiful professional-level builds.  Just gorgeous.  Both the enclosures and the boards, and the overall look.  I wish I was even half as methodical.

But on to the problem, and a potential source.  There are a LOT of twisted wires in there. Nothing intrinsically wrong with that, but I've had more than one occasion where a wire I thought was providing electrical continuity was not doing so, because it was fractured internally.  As well, in recent years I have taken to slipping a piece of heatshrink over the connection between wires and pot solder lugs because I can't always guarantee that the uninsulated lugs aren't touching anything they shouldn't be touching.

Always just one more thing to check off the troubleshooting list....sigh. 

[jfrabat]

It seems that pin 2 of three out of four of the two gang pots have not been soldered. According to the schematics, it is ok not to solder pin1, but pin2 must be sodered to have a variable resistor.
(I assume that the pin order of your footprints is 1 2 3)

Pin 1 and 2 are connected at the pot.  One less wire, same effect.  The other 2 versions are the same.

While using the same number of wires, "soldering pins 1-2 of the pot to pins 1-3 of the footprint" is less readable than "soldering pins 2-3 of the pot to pins 2-3 of the footprint" (as shown on your schematics). Don't you think ? 
Anyway... it's not related to your issue, so forget about it.

Sorry, that's what I meant...  I answered before breakfast (no coffee!).

First, let me compliment you on beautiful professional-level builds.  Just gorgeous.  Both the enclosures and the boards, and the overall look.  I wish I was even half as methodical.

Thanks!  That really means a lot!

But on to the problem, and a potential source.  There are a LOT of twisted wires in there. Nothing intrinsically wrong with that, but I've had more than one occasion where a wire I thought was providing electrical continuity was not doing so, because it was fractured internally.  As well, in recent years I have taken to slipping a piece of heatshrink over the connection between wires and pot solder lugs because I can't always guarantee that the uninsulated lugs aren't touching anything they shouldn't be touching.

Always just one more thing to check off the troubleshooting list....sigh. 

I will check continuity of the wires today, to be sure.  I did test for continuity between the pot lugs, and except where there should be (dual gang pots), it is OK.

Ahaaaa...!!!
(another one of those perfectionists..)

It didn't start that way. 

Initially one rail was quite a bit off so he had to tweak the resistors.  Later we worked out one of the chips was fried and replacing the chip the currents went back to normal and the rail current were nearly equal - so we had to swap he resistors back.  Since he had already measured the voltages and had to the change the resistor anyway he put in a more finely tuned value.

Now, now... Rob, play nice.  Are you trying to imply that my pedal did not work the first time and I had to trouble shoot it?  I have NEVER EVER had to trouble shoot my pedals.  They always work the first time around!!!  You should know that by now!

Antonis, I did it because I AM a perfectionist.  It had nothing to do with troubleshooting!!!   

By the way, I really want to get this working today.  Tomorrow I am taking the family on a well deserved semi-vacation.  We have spent the last 6 months locked up at home.  Tomorrow we are all going to the beach house for a week to change scenery at least!  (well, tomorrow for me, today for them!).  I mean, we will be locked up in that house as well (we cannot actually go down to the beach), but at least there is a lot more land there for the kids to play (and a pool, which will be good for kids and adults alike!).

But Rob, Kraal, Anotnis, 11-90, you guys are not allowed to take vacations.  Remember I got 2 pre-amps in progress, and the compressor board should be arriving any time now, so I am most certainly going to be needing help troubleshooting those as well!  LOL!  Once I finish and actually get these 4 working (EQ, Compressor, and 2 preamps), I decided I will give a De-Esser a shot.

Another idea I have (I may be REALLY be getting ahead of myself here) is to have an Arduino controlled patch-bay type of deal where I can select which pre-amp to use.  My intention is to provide provisions for 3 or 4 preamps, and when you select one on the screen, the mic xlr is connected to that preamp (via relays), and that preamp is connected to the output (which will go to the compressor).  Later on I could eventually build something similar (maybe more akin to the looper) to select which compressors/de-essers/EQ to use.  That would be really cool, but that is at least a year or more away (too much already on my plate!)

[jfrabat]

What about put IC1 back in and short C1 with a wire.
When you do this confirm pin 5 IC1 is connected to ground.

In this case you should expect:
- the signal to pass without a drop
- boost/cut pot does nothing
- frequency pot does nothing.

If not something is wrong around IC1B, R3, R7.

Tried this.  Pin 5 is in effect, grounded.  Signal dies right after R3 (nothing comes out of Pin 7 IC1).

[antonis]

Short left leg of r to right leg of R7,..
(actually, bypass both resistors..)

[jfrabat]

Short left leg of r to right leg of R7,..
(actually, bypass both resistors..)

R7 and which other one?  You left the number out...

[antonis]

 

Right..!!
Short left leg of R3 and right leg of R7..