DEBUG: Helicopter in Zendrive!

[Germanium_Boy]

A few days ago, I finished a Zendrive, and it gets me really crazy because is not working properly, and I don't find the source...

In a few words, the only sound that I get is a horrible helicopter-type sound, a pop-pop noise, squealing and sometimes motorboating. The gain, volume and tone controls shape the sound, so it seems that they works as it's intendeed to. Maybe the voice control doesn't change the sound so much.

The layout I'm using is the following, done and verified by other people:

http://tagboardeffects.blogspot.com.es/2012/01/hermida-zendrive.html

The only change was D3. I used a 1N34 diode, instead another BAT41. About the voltages, some are a bit strange, so it's a good place to an analysis:

IC (AD712)

1: 4.60-5.20
2: 8.68
3: 4.60
4: 0
5: 10.00
6: 4.90
7: 4.90
8: 1.55

And the Jfets of the clipping stage:

Q1
S: 8.70
G: 5.04
D: 5.00

Q2
S: 5.75
G: 8.70
D: 8.70

I checked most of the typical mistakes... unwanted bridges, placements and cuts... the resistors, and that the electrolytic capacitors work properly... this thing is a simple stompbox, so it's so frustrating!

Thanks in advance for your help!

[JerS]

The tricky thing about that particular layout is the "Double Links" as noted at the bottom of the page. Check to make sure you have made those additional linkages, and see if that cures it.

Also - I think maybe you have the pins listed incorrectly here - pin 8 should see 9V+ as it is directly connected to the 9V+ lead. it does not make sense that you would be seeing only 1.55V on that pin, but be seeing the full voltage elsewhere.

These are some ideas to get you started anyway!

[Germanium_Boy]

Thanks, JerS! Yes, I checked the double links, it's so easy to miss those kind of details!

And I committed that stupid and usual mistake of numbering incorrectly the IC's pins... so...

1: 4.60-5.20
2: 8.68
3: 4.60
4: 0
5: 1.55
6: 4.90
7: 4.90
8: 10.00

By the way... It's the first time that I use the ONEspot powersuplly of Visual Sounds, and... it gives me around 10 volts!

[mth5044]

It seems odd to me that you'd have that high a voltage on pin 2. Check to make sure that you are using 10k's for the voltage divider, and then make sure that the voltage there is ~1/2 the supply voltage (10V in your case, so perhaps look for 5V). For future threads, always post a schematic when asking for help. Vero layouts are very hard to debug from. The schematic can be viewed here:

http://revolutiondeux.blogspot.com/2008/05/zendrive-project-at-fsborg.html

[Germanium_Boy]

Mr mth5044, you're right... I should post the schematic... Sorry about it, and thanks for your advice!

I followed the power supply path to pin 2. The resistors are correct in value. But it happens something that I don't understand.

Following the schematic of Revolutiondeux blog, in V2 I read around 5v. But after the 100n capacitor, and voice control, I get 8.75v. Maybe a faulty capacitor here?

[aron]

Thank goodness a schematic.
OK, yes, check voltages at V2. So if V2 is around 5v (I guess you running on a power supply not battery right?), then somewhere around R7 or the feedback loop, you have a problem where something is touching your V+ (10 volts). Check carefully around the feedback loop/ gain pot etc...

[aron]

If I had to guess, you might have forgotten the trace cut by pin 8.

[Germanium_Boy]

Thanks, Aron! The trace cut between pins 1-8 is done, and the multimeter doesn't find continuity there... So it's time for me to look after another issues...

The increase in voltage after C5 is what I have to study now, is something that I barely understand...

[Germanium_Boy]

I change C5 capacitor, and the result is the same. Between 10K resistors in the power supply, and the entry of C5, I get 5V. But between C5 and voice pot, I read 8. V. I suppose that in this point is the fault, but I'm not able of finding it... Sooo frustrating, but with any troubleshooting building I always learn a lot

[aron]

I'm not seeing it. Triple check all of your trace cuts. It's not C5. The problem is occurring somewhere by pin 2. There are 5 parts that are connected to pin 2 - so the problem is there OR the problem is at the end of the 1K (R7) that connects to the voice pot. If you used an IC socket (I hope you did). Pull the IC and measure the pins again. Again, the problem is at the either end of R7 IMO.
Look carefully at the components connected to the 9V traces. Post a picture if you can of the top and bottom of the board.

[Germanium_Boy]

Triple checking, and triple thanks to you, Aron! I decided to score between the tracks with my small hacksaw, even with the multimeter said me that there was no problem. Anyway, using a magnifying glass, I think I found a bridge in a cut, next to pin 2, as you bet!

The voltages are now correct, but there is some trouble, because I get no sound... but with a little help of my audio probe, I think I'll find the trouble... Oh, man... it's so pleasant to resolve this kind of troubles, and to learn something in the way, with your help!

[duck_arse]

if you used a socket, pull the ic, and test between pins 1 and 5 of the empty socket (power off!).  you want to see from 10k to 60k, depending on the tone setting. your pin 1 volts should be the same as your pin 5 volts, they aren't in your posted voltages.

[Germanium_Boy]

Thanks, duck-arse. I continue with the analysis of the faulty zendrive... I tried what you advise me, but I don't get any reading...

I felt a bit euphoric, reading the good voltage in pin 2... but I realize that the pins 5-7 are a bit wrong. 1-4 seems to be OK:

1: 5.19
2: 5.19
3: 3.50
4: 0
5: 1.31
6: 1.72
7: 1.72
8: 10.30

With the audio probe, I get sound along all the feedback, in IC1, pin2. But from pin 1, 3-7, nothing. Sound too from R4, tone pot and C4. Nothing from C3, until the end of the circuit. Well, from central lug of Volume pot I get sound, but this is very usual, isn't it?

A need a coffee, and a bit of rest... This is a step beyond, but I need a pair of steps more!

[duck_arse]

try lifting the pin 1 end of R4, then audio probe at pin 1. when you say you "don't get any reading", what do you mean? were you measuring for the resistance between those pins?

and can you include the voltage at "V2" (R1//R2) with your measures, please?

[Germanium_Boy]

Sorry for no being very precise in my answer! Anyway, I commited a mistake... The measure between pin 1-pin 5, I read just what you told me: 10k-60k, as I turn up or down the tone pot.

About the voltage in the node R1/R2 (V2), is around 5.10V.

Thanks!

[Germanium_Boy]

With the idea of learning a little bit about debugging voltages in ICs... what would be the reason for a correct values in pins 1-4 and 8, but lower than expected in 5-7? Step by step!

[Germanium_Boy]

Monologue, part. III  After a pair of checks, now I get the correct voltages! But no sound in any point of the circuit... only from the central terminal of Volume pot 

This is the most hard-working pedal of the last months for me! 

[duck_arse]

can we see some photos of your work, both sides, nice and clear? offboard wiring as well, please.

and a new round of IC volts, and V2, and battery V, please.

V2 provides a mid-point bias voltage, and is fed to the non-inverting input of IC1a. this sets the output pin 1 to the same mid-volts point, for good linear swing, you know all that stuff. as the pin 1 is connected via resistors only, to pin 5, THAT pin should also be the same dc volts as pin 1. and pin 5 in another non-inverting input, and that section is wired as a voltage follower, so the same voltage should appear at pin 7 as that at pin 3 (or very very VERY nearly the same).

if your V2 volts and your pin 3 volts are out by more than a small amount, it indicates an excess current being drawn by something being misconnected. is your opamp type a fet-input stage or a bi-polar input stage, what does your datasheet tell you?

seeing as C5 is there, the only DC voltage on pin 2 should be coming from the loop connected to pin 1. if you lift D3 and D2, you will have nothing but the gain pot and C7, have any volts changed? from your last posted volts, those at 5,6 and 7 are correct, following each other, but being fed something wrong. and pin 1 and 2 are right, but not for your voltage at pin 3.

concentrate on the pin 3 parts, test for resistance back to the V2 point from the pin 3, and resistance to ground and to pin 8. and they may be something missing/wrong between pin 1 and pin 5. test for resistances to ground along those parts as well.

[Germanium_Boy]

Mr. Duck Arse, what a great lesson. Serously, as I said at the beggining, it's a lot more instructive a painful, trouble-shooting pedal, that the ones that sounds perfect at the first sight!

The pedal, finally, came to live! And the fail... oh, man... It's one of the most important rules: never give something the OK without checking before. As I didn't get any sound, excepto from central lug of Volume pot, I checked the pot itself. And it's a faulty one. Never, never before I experienced something like these with a pot, even with one cheaper. Luckily, I had one of the same value, so with the change, the pedal got alive.

Thanks, guys, for all of your help... In my next IC-based project, I'll have a pair of good lessons learned!

Now, it's time to return the board to its box!

[aron]

Great!!!! OK well now the only thing is to place it in the box without anything shorting. Good luck with your next projects. I love using IC based circuits because they are general easy to debug for me.