quick debug question - commonsound tremulous

[ethrbunny]

I built the commonsound tremulous using a board from tonepad. Its a quick build and went together pretty nicely. My LED is firing like its supposed to but the LDR is not happy. The value stays at 0 ohms even though the LED is turning on and off. I have it light shielded so I don't think its a leakage problem.

One wierd thing - I tried taking out the TL072. This fixed the 0 ohms problem between pins 2 and 6. Does this mean I have a bad opamp?

[flibbergibbin]

no.

[Paul Perry (Frostwave)]

Actually, if the TL072 had fried (power supply polarity wrong) and turned to a puddle of silicon, it might very well have very low resistance between pins.
MIND YOU, you can't measure the resistance of the LDR while the circuit is active!! because, circuit voltages will probably interfere with the meter!

[ethrbunny]

Hmm.. well then Im wondering why I don't get *any* signal from in to out. That half of the circuit is v straightforward. The opamp is getting 9V and ground at the appropriate places. Its on a tonepad PCB so the wiring is all correct.

Ill go back and check, check and recheck. Solder bridge somewhere?

But just for curiousity - is there any way to know if the TL072 is dead?

[Paul Perry (Frostwave)]

The only reliable way to test a chip, is to make a very simple circuit with a socket, put the chip in, and see if it goes. (for TL072, I'd use one of those LFO circuits that drive a led).
Usually not worth doing.. but places doing mass production, sometimes have a "test" PCB with all sockets, so you can test any chip that you might be suspicious of. (an incredible timesaver for complex boards).

The problem with just putting another TL072 in, is that if there is a fault in the circuit that is cooking the TL072, you might get through a few of them Most of us have been there.

A quick test to see if a chip is royally roasted, if you have a known good chip of the same kind, is to run around the chips with a multimeter on ohms & see the resiatance between the + power pin & all the others. Readigs forthe two chips should be much the same for matching pins. If not, something is ......fishy.

[ethrbunny]

Wow - great points. I would have cooked several before I wised up. 

Its a pretty simply circuit. Ill tear into it and see what Ive mucked up.