Help! APC B1M pot catching fire!!

[npx]

Well I certainly didn't think my first post here would be about the following situation!

I've been lurking here for a couple of months... a couple weeks ago I built an Atari Punk Console from this schematic:



The bold dots are body contacts, plus I also added a normally closed momentary switch, and a switch to toggle between normal (constantly on) and momentary modes.

I had the circuit working fine and I put it away. Today I etched the enclosure, and tonight I've been assembling the little beast. I got everything in the box, and plugged in the battery to make sure I had the on/off switch pointing the correct way. Great, it works! I unplugged it and took it into the next room. Then I looked down and the 3rd lug of one of the 1M pots (the one to the right of the 9v+ on the schematic) spontaneously caught fire! Needless to say, I was a little surprised. It subsided, and I figured the pot must now be burned out... so I plugged it in again to see if it still worked, and to my surprise, it did... I turned the pot and it sonically did what it was supposed to... and then ignited again! 

I'm confused. Can anyone tell me what is going on??

Thanks for any help!!

[DougH]

You need a fixed resistor between your 1M pots and ground to limit the current when the pot is set to minimum resistance. When the pot is set to minimum, there is a dead short across the battery. That's what is burning up the pot.

[npx]

Wow, thanks for the quick reply! You're right, that pot was set at minimum when it happened...

So what value resistor/s should I use? And where exactly should I put it/them? (I'm still new at all this!) 

[DougH]

A 1k 1/4W should be more than enough for 9v. For longer battery life, use as large of resistance value as you can get away with without changng the control in a way you don't like. (Try 10k-20k for example as that limits the current draw to 1ma or less.)  Break the connection between the pot and ground and insert the resistor there.

[npx]

Thanks Doug, I'll try that in the morning.