Blue Box is motorboating - starting to puzzle me (fixed)

[Coriolis]

Built a Blue Box from the Tonepad layout a while ago. It worked fine, until I put in an enclosure. Then all I got was very weak distortion at low volume settings, and weird oscillations at higher settings. Figured I had damaged the 4013 with static, but then I started audioprobing around the dual opamp (4558).
I have some input on the noninverting pin (pin 3), but get what I believe is called motorboating on the output (pin 1). Then on pin 7, I get more a sort of high frequency warbling, and not as loud.
Now, I searched this forum, and read R.G's explanation of this phenomenon, and I see 2 places in this circuit where it could be an issue:

-Power supply - when I had boxed it, and found it suddenly wasn't working, I was using a regulated psu (that has never let me down).
This doesn't fit with sag in the power causing motorboating. Right now though as I'm debugging, I'm using a 9v batt which yields 7v under  load, so it's perhaps not too fresh. The sound coming out of the circuit is the same as with the regulated psu. So probably not the psu.

-There's a cap in the feedback loop of IC1a, which I believe is supposed to suppress oscillation. Now I don't know for sure whether that cap is good or not (haven't tried replacing it). It's supposed to be 300pf, but only had 330pf. But that is more likely to improve suppression, yes?


There seems to plenty of psu filtering, 100uf for the opamp, and an additional 100uf for the 4013, so it seems unlikely that the 4013 is contaminating the supply, especially since I tried taking it out, and the opamp still does what it does. I read that someone had oscillations with a 4558 in the circuit, but that a 5532 or TL072 cured that. Tried that, didn't work, except it seems I get a bit more output from pin 1 with the 072. Other than that, I've gone over the schem and checked for misplaced components, reflowed solder joints, etc.

It's probably something silly, but that motorboating is bugging me, mostly because I don't see how it could be there at all (barring a bad component).

Anyone care to comment?

C

[Mark Hammer]

It worked fine, until I put in an enclosure. Then .....

If I had a nickel for every time........

Make absolutely sure nothing is getting shorted out or crammed together.  Pot lugs can end up touching things.  Boards placed over top (underneath, actually) of pots end up havingresistor kleads shorting out against pot covers.  Leads that were soldered just fine fracture unseen at the solder lug when you stat twisting things around to fit it in.  Phone jack solder lugs or contacts short out against the chassis because you didn't leave enough clearance.  The list goes on and on.

There are times when stuffing a circuit into a box and NOT having it misbehave is a bit like putting 3 children in the back seat of a car for a long drive and expecting them NOT to fight or start shouting.

[Coriolis]

 
I should have mentioned, that the board is out of the enclosure for the debugging.

Seems I also forgot to provide some voltages on IC1 (the opamp):

Pin 1: 1,02
Pin 2: 0,70
Pin 3: 0,69
Pin 4: 0,00
Pin 5: 0,90
Pin 6: 0,91
Pin 7: 1,08
Pin 8: 1,82

This is with a battery that now reads 5,95v under load (measured at the battery poles) - not much.
Pin 8 is supposed to get the full supply voltage, since there are no dividers that I can see.
Btw, the 4013 seems to be getting the same voltage on it's power pin (14), so I guess the power is messed up somehow.
The only thing in the way of anything, is R24, a 330ohm resistor between the 4013 and power. Voltage reads the same on either side of that. Can't see any shorts along the power line to the opamp, not even with a magnifying glass.

Wait a minute! I just took both plugs out of the jacks, and now pin 8 reads full supply voltage! Same on pin 14 of the 4013.
So basically, when I plug in, I lose most of the power, biasing the opamp too low to do anything useful?
Gotta look closer at my connections then.

C

[Coriolis]

These are the ramblings of a moron!

Right; I plugged the thing back in again, and took some more readings. I found this: When I measure voltages with the - probe of my dmm on the negative pole of the battery, I get the correct supply voltages on the supply pins of the chips - effect plugged or unplugged.
When I put the negative probe on the in/out jacks though, I got 1.82 volts on my supply pins.

Then I saw it: I had ground coming from the battery to the board, and ground between the two jacks, BUT NO GROUND FROM JACKS TO THE BOARD! If there is an opening for village idiot on this forum, I'm so there...

Anyway, it works now (though we'll see about that when I put it back in the box, won't we), and sounds fine.
Sometimes it seems, all I have to do, to to find the problem, is post here. And not because of any answers I might get, but because it somehow makes me look harder...or...something.

Allright people, nothing to see here! 

C(lown)

[Mark Hammer]

Ah yes, the famous "Every ground connection present and accounted for except the one that matters".

One of the deepest traps in DIY, I find, are those connections you plan to make "later on".  And then what happens?  Well, somebody strolls in and is having difficulty with their homework.  Somebody else strolls in and HAS to tell you about their frustrating day at work.  Then a telemarketer calls.  Then a child's friend calls and wants to know if the kid can come over to play.  Then you have to get supper ready.  And before you know it, you've become amnesic for all those last minute touches you needed to do.  When you come back to the pedal the next day, you have absolutely no recollection of what was done and what remained to be done.

People think your greatest ally in DIY is a meter...or maybe a decent iron....or maybe transistor manuals or datasheets....or ready access to schematics....or people to ask.  Wrong on all counts.  Your greatest ally is always.....drum roll please.....continuity and lack of interruptions.

[TheBishop]

deleted since the subject was changed to fixed.
starting my own thread.

[Coriolis]

Thanks for understanding, Mark.

I think was exactly a case of "I'll do that later". I think I had that specific ground connection set up with an alligator clip, and then, when it was time to box it, I forgot to solder a wire in. That simple - and that stupid!

Well, whenever I feel bad about my accomplishments in diy fx, I just take a look at the photo you call "My shame", the one of your unfinished- circuits-pile, if you recall. Then I feel a lot better...
It's good to remember that it happens to everyone, and I suppose I should be glad that this one was easy to fix...

C

[Paul Perry (Frostwave)]

Your greatest ally is always.....drum roll please.....continuity and lack of interruptions.

Mark, I read about a study done of various workplaces, to determine why some places were much more efficient at computer programming. The major factor: if programmers had their own rooms & someone to screen their incoming phone calls, they did MUCH better. After I read that, I stopped ringing my friends at their work.

[sta63bmx]

There are times when stuffing a circuit into a box and NOT having it misbehave is a bit like putting 3 children in the back seat of a car for a long drive and expecting them NOT to fight or start shouting.

LMAO!  I built that two-channel tubescreamer for another guy, got it all debugged and working perfectly, opened the box to take pictures, and then carefully packed it and sent it.  You guesed it!  No sound when it arrived.  If he jiggled the box it would work.  He opened it, checked it, and then was able to put it all in there and make it work. 

I like covering the bottom of the board with electrical tape or putting it in a little paper envelope or something, or taping paper around it.  Looks stupid, but...

[j.frad]

Yup, I had the same grounding forgotten on my woody! I really had to hit my head on the floor when I realised what was wrong!
I like to put scotch tape on the copper side of my circuits to prevent shorting on the box, might not be the best material because of static electricity but I've never had a problem with it!