Another Tube Sound Fuzz question!

[hookus]

When the effect is engaged, all I get is a little static scratchy sound that increases in volume as the volume pot is turned.

I etched a board with the layout from Tonepad, using the Red Llama parts list. Checked voltage at all the pins, and had about 1vdc Seems a bit low. Could I have damaged the chip with ESD?

[hookus]

Just realized, I used the 1K resistor on the 9V power run, like a Red Llama, if my voltage reading was correct, I may not have enough current for the 4049. The 4049 spec sheet says it needs about 10 mA, and if the 1K resistor was dropping 8 volts, that would only give me about 8 mA. Sound correct?

[Mark Hammer]

Sounds like plausible reasoning to me. The 4049 actually uses a fair amount of current. The original EHX Hot Tubes that used a 4049 in similar mode actually came with an onboard power transformer. I've been able to run mine off a 9v battery, but they do not last all that long.

[hookus]

Well, the next step will be to go back to a 100 Ohm resistor, we will see if that works.

[Mark Hammer]

If it doesn't, then your problem is somewhere else.

[hookus]

Well, I have come to the conclusion that this is rediculous. I cannot beleive a 10 component circuit has defeated me. I go from repairing expensive military electronics, to being beaten by a beginner circuit, drives me nuts.

I replaced the resistor, measure about 7 volts at the 4049, much higher, should be around 15 mA. Now, when I first turn the pedal on, I get a very low output fuzz, like 50% of bypassed volume. After perhaps thirty seconds, the output just goes to very loud static, then nothing. Driving me absolutely nuts. I figure maybe I put one of the two whole radial caps in backwards, nope. The volume pot is the scratchy one, I usually ground the back of my pots, so think perhaps I cooked it. Nope, 10K one way, ground the other, and the resistance sweeps correctly. Remember that it is unbuffered, so put my bass in passive mode. Still no go. The only deviation from the schematic was a .22 uf input cap, rather than .1 uf, and .51 pf caps in each feedback loop, but I don't think any of those should make a difference.

Is this where I get to throw this one out the window, and start from scratch?

[frank.clarke]

All the inverter pins should be at about half the supply voltage. See if there is any sound coming out of the first inverter stage, divide and conquer.

[hookus]

I don't have an o scope, what should I look for sound with?

[frank.clarke]

A wire to the volume control, see "audio probe":
http://www.diystompboxes.com/pedals/debug.html
You can listen to the signal as it goes thru each stage, perhaps it dies at some specific point. The chip could be dead, but I haven't managed to kill one yet.

[hookus]

Thanks! That helps a lot. I now have two different circuits with the exact same symptom. The second circuit worked when I tested it outside the enclosure, but when I mounted it, it worked right away, then slowly died away to nothing but a scratchy static. Wonder if my active bass is killing them.

[Mark Hammer]

"It worked fine before I boxed it" is an all too common occurrence.

Why does it happen? Occasionally it happens because:

a) ground connections that were made on the breadboard are not all replicated during the boxing

b) wires get fractured during the transfer

c) pot or other lugs or bits of metal sticking out short against the chassis or other parts of the circuit; pots whose outside lugs manage to touch against a nearby pot chassis or even the lugs of a nearby pot are so common an occurrence when tightening up the outside hex nuts to secure the pot, that I routinely stick a piece of shrink tubing over the end of the connecting wire and pot lug, just to insulate it

[hookus]

Good point, it can be a very tight fit inside a box. I need to pay more attention to that. Perhaps I need to be more judicious in cleaning flux off the board when I am finished as well.

[Mark Hammer]

The flux does not NEED to be cleaned off but it certainly helps to have a clean board when inspecting for bridges, cracks, etc.