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Old 05-10-2004, 08:56 PM   #11
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The mic is like one you'd see in a karoke bar, It had a 1/4" plug.
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Old 05-10-2004, 09:21 PM   #12
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_________7.46V

_________.698V

_________.548V
.697V
____2.5mV

That'* what I get out of the circuit at those points... Does it look like I did a stupid?
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Old 05-11-2004, 12:57 AM   #13
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Quote:
Originally Posted by Damemorder
That'* what I get out of the circuit at those points... Does it look like I did a stupid?
The topmost voltage is approximately correct -- the others are *way* too low. You're drawing a lot of current through R4, and it'* dropping most of that original 7.4 volts across it. (You might notice that R4 gets kinda hot when the power'* on.) With a base voltage of 2.5mV, the transistor'* off, so that current has to be going somewhere else.

It looks almost like you've got a resistive short to ground in parallel with C5 -- that could happen if C5 is (approximately) shorted or in backwards. Check C5'* polarity and make sure the stripe, notch, or + marker is facing away from ground. If that'* correct, pull it from the circuit and touch its leads together (to equalize any charge it may have built up while it was in the circuit). If you've got a capacitance setting on your meter, use it and check its capacitance and make sure it'* within 20% of its rated value. Otherwise, take your multimeter, set it to about the 20 or 200 ohm scale, and put the red lead on the positive lead of the cap and the black lead on the negative lead. The resistance reading should start off low and climb steadily. If it stays low, it'* shorted out.

I am a little worried about the fact that the base voltage is so low, though -- at 2.5mV it looks almost like it'* shorted to ground, too, but it should be well isolated from ground with the microphone disconnected. I'd suspect that the transistor was toasted if you hadn't tested it and got a reasonable hfe Did you solder all this together? Or did you build it on a breadboard or protoboard?

-b
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Old 05-12-2004, 01:14 AM   #14
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I put this all on a breadboard, And I went ahead and turned C5 the right way....

I played around with it a bit checking the voltage at odd points, Get this, No one had a 120 for R4, So I used a 100 and two 10'* in series, Makes sense to me (prolly the screw up), But after the 100 is where it drops to these low voltages...
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Old 05-12-2004, 03:32 AM   #15
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Quote:
Originally Posted by Damemorder
I put this all on a breadboard, And I went ahead and turned C5 the right way....
So C5 *was* in backwards? If so, that'* where the current was going. If you didn't destroy it by running current through it backwards (electrolytic capacitors are kinda fragile), the circuit should work now. There'* a chance it *is* wasted now, though, and if so, it'* probably shorted in both directions. If you're still getting a huge voltage drop across the 120 ohm resistor with the cap in the right way, try the resistance test I posted above on C5 to determine if it'* still capacitating

Quote:
Originally Posted by Damemorder
I played around with it a bit checking the voltage at odd points, Get this, No one had a 120 for R4, So I used a 100 and two 10'* in series, Makes sense to me (prolly the screw up), But after the 100 is where it drops to these low voltages...
That combination works fine But the resistor isn't the cause of the problem, at least as far as I can tell -- the real problem is that you've got a current sink somewhere in the circuit that'* causing R4 to drop more voltage than it'* supposed to (Ohm'* Law: voltage dropped = current * resistance). Eliminate the current sink, and you'll eliminate that voltage drop.

Which leads me to yet another test you can do on this circuit: pull those capacitors out and chuck them across the room. Without a mic signal, those capacitors really do nothing once they're charged up -- so you could check those test voltages I posted earlier with the caps removed from the circuit. If they look right, you know the caps are the culprits. If not, the caps could still be jacked up, but there'* something else going wrong too -- and at that point, all that'* left is a couple of resistors, some wire, and a transistor. And only one of those three things regularly fails spectacularly -- here'* a hint: it'* not the wire or the resistors

-b
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Old 05-13-2004, 11:29 PM   #16
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RadioShack blows. I was screwing around and checked the resistirs. That bastard up there handed me a 100K for the 100. Granted, I should have noticed it, But still... I swapped that out to a 100, I get close to the right voltages, It just doesn't work. Which leads me to beleive that the transistor is dead eh?
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Old 05-14-2004, 02:38 PM   #17
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Quote:
Originally Posted by Damemorder
RadioShack blows. I was screwing around and checked the resistirs. That bastard up there handed me a 100K for the 100. Granted, I should have noticed it, But still... I swapped that out to a 100, I get close to the right voltages, It just doesn't work. Which leads me to beleive that the transistor is dead eh?
Radio Shack blows.

The voltages at those points are partially determined by the transistor; it'* called a "quiescent bias point," it'* where the transistor likes to hang out when there'* no input signal. If the Q-point is correct, the transistor'* probably working, at least a little bit.

What voltages did you get in the circuit when you fixed R4? And if they're all good, try the scope test I detailed above -- if you can inject a known signal into the microphone, you can use the oscilloscope to check if that signal is making it through the circuit to the output. If it'* not, you'll be able to see where it disappears.

-b
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Old 05-14-2004, 03:21 PM   #18
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Quote:
Originally Posted by enmityst
So C5 *was* in backwards? If so, that'* where the current was going. If you didn't destroy it by running current through it backwards (electrolytic capacitors are kinda fragile), the circuit should work now. There'* a chance it *is* wasted now, though, and if so, it'* probably shorted in both directions.
Something fun to try with a wasted electrolytic capacitor: try running higher-than-rated voltage through with the polarity reversed: BANG! suggest putting it in a metal bucket while you do this, to shield from shrapnel!
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Old 05-16-2004, 12:52 AM   #19
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Ah Ha! Got it working, hooked up the output ground and changed the battery, works like a charm, except for the horendous feedback... You gotta wait like a second after the other guy talks or he gets nothing but garbled feedback... Oh yeah, I'm using this to put sound into my cellphone for a speaker phone in the camper, out in the woods. I got annoyed trying to start a fire and holding the phone....
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Old 05-16-2004, 01:52 AM   #20
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Quote:
Originally Posted by Damemorder
Ah Ha! Got it working, hooked up the output ground and changed the battery, works like a charm, except for the horendous feedback... You gotta wait like a second after the other guy talks or he gets nothing but garbled feedback... Oh yeah, I'm using this to put sound into my cellphone for a speaker phone in the camper, out in the woods. I got annoyed trying to start a fire and holding the phone....
Heh, congrats. You know, they make external speakerphone kits for cell phones...

-b
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