Auto Wah

[col]

I built an auto wah on stripboard out one of one of the Babani books of Music Projects and on the test rig it worked fine. It is based on an LM13700N chip. When I put it into the box it seems to oscillate like mad and produces a ptewping ptewping ping type noise at the end of every note. The pots don't have much effect on this and at full settings the strange effect is constant. As soon as I open the box up leaving enverything connected it works correctly! Nothing is touching, there are no shorts and no wires are bent into any others. The back of the stripboard is covered in insulating tape and there is velcro on the back of this, the sides are not touching the box so there are no shorts there.
The only thing I can think of is to replace the box with a bigger one or put it in a plastic box as the close proximity of the metal box may have something to do with it. Has anyone else had a problem like this? I'd like to get it sorted as on the test rig it sounds really good and I've spent quite a lot of time trying to solve this.

Thanks for reading,

Col

[Mark Hammer]

Oh that's a familiar one:  "It works fine when I have the lid off"

Not all the time, but often enough to mention here, I find the little rivets on my pots may be loose.  I suppose if I used thinner stranded wire for conencting them it might behave differently, but sometimes the leads from the board are short enough that wire that gauge (#22) is stiff enough to move the lugs and disturb internal contact in the pot.  Sometimes a nice stiff pinch with some needlenose can fix that, and sometimes you just have to change pots.

Other times, the bunching up of leads that occurs when a lid is closed can bend things like vertically mounted resistors, and they short out against things.  Still other times, you can find yourself with boards shorting out against the backs of pots (e.g., with vertically mounted resistors or those poly-film caps that look like stuffed goalposts).

Planning out he location and length of one's leads is obviously very important, not only for oscillation-defeating and neatness but for allowing comfortable and problem-free chassis closure.  Planning out the location and mounting of the board is also important, as is planning for the height/clearance of components.  Too often, I whip up a perfed project without thinking over how high certain components stand off the board and where the pots are going to be, and find myself needing to unsolder most of the board because that blasted 100uf cap or trimpot is sitting right over where I installed that toggle switch and I can't close the cover.

Hate it, hate it, hate it.

[col]

Thanks for the reply. I will check again but I'm sure nothing is touching when I close the box. I will check the pots as I have had a couple of these with problems such as the carbon contact breaking off. I might try putting it in a different box and shortening the wires if I can. This is one of those really frustrating things that keeps being put down and picked up again. I'm sure that one day I'll pick it up and think "Oh no-that's it".

[petemoore]

Bad when its V+ to V- shoritng out.
 I find myself testing for continuity between V_ and V+ alot.
 After I do that, I start wiggling, then loosening it from the box, often times it'll pop to life as the board moves slightly, ahh I'm getting 'Warm!, then it's just a dogmatic search to figure out exactly where the short is, and usually a quick fix like put a piece of hard plastic packaging material or layers of electrical tape.

[SolderBoy]

Col,
I know this stuff can be frustrating, but if its any consolation I built that circuit a few years ago, and it works great.  I ended up modifying it too, from memory.  I think I ended up using the LP output of the 2nd opamp, and tweaking one or two other things...

I love those little babani books.  I have half a dozen of 'em.  Try an amazon search on "Babani" some time.  Tonnes of interesting titles...

[col]

Solderboy,

I have several of the Babani books as my local electronics supply shop is selling them all off at half price. Of all the circuits I have built from them this is the only one that has caused me problems. I'll just have to persevere with it but at the moment I'm a bit strapped for time.

Thanks for the relpies,

Col