Noisy Cricket debugging help

[bigandtall]

Hey all,

I'm trying to complete my first from scratch build. I'm putting together a Noisy Cricket amp from Beavis Audio with a headphone jack.

I worked from a PCB and just finished putting it all together.

Of course, it didn't work at all on first fire up.

I went over the wiring diagrams and pcb layout and found that I did make a mistake or two, which I fixed. Still, no startup.

I did a very simple Multimeter test and here's my question:

THe battery read at a little over 9V. I followed it through the whole circuit board and the 9v seems relatively consistent till I get to the 220uf cap in C8 (http://beavisaudio.com/Projects/NoisyCricket/DIY/NoisyCricket_Mark2_BuildGuide.pdf).

On the + side of the terminal, it reads 9V. On the other side it read 0. So, I resoldered the connection and then the - side read 9v as well.

Then, I hooked up headphones and a bass and heard a small pop in the headphones when I turned everything on... but no other bass sound.

ANd then when I remeasured the cap to see if I had broken anything loose in the testing, I found that the - side was reading 0 again.

Any idea what's wrong here? Any direction to point me in. Seems that the signal is not flowing all the way through to the output and that is my main problem.  I'm not sure how to fix it, though.

[bigandtall]

Also, the LED does not come on when I fire up the amp.

I'm sorting my way through the troubleshooting sticky links, but just thought I'd ask in case it's an obvious problem.

[BaLaClavaAa]

I just built one and am still debugging it, and ofcoarse it didn't work. What I did wrong though was I put the LED in backwards which fried my transistor and IC. What did you originally mess up? I'd say double check your wiring and check the joints ect, and if not then check those two things. Break easy

[bigandtall]

I forgot to put in a jumper at one part and then didn't ground something properly.

So, should I go ahead and just reverse my LED? How do I know if the transistor is fried?

[BaLaClavaAa]

No, just double check you put it the right way round, it's easy to miss. Well I knew because mine started smoking :p

[bigandtall]

I just spent an hour debugging... I did in fact put the LED in backwards. I fixed that and found that I had mixed up my volume and gain pots and fixed that as well. I can still trace my 9V going through the PCB, so I guess tomorrow I'll look at it carefully with a magnifying glass and a razor blade and make sure that there aren't any fine solder hairs messing things up.

I also did test the LED and it works.

Any other advice?

[anchovie]

THe battery read at a little over 9V. I followed it through the whole circuit board and the 9v seems relatively consistent till I get to the 220uf cap in C8 (http://beavisaudio.com/Projects/NoisyCricket/DIY/NoisyCricket_Mark2_BuildGuide.pdf).

On the + side of the terminal, it reads 9V. On the other side it read 0. So, I resoldered the connection and then the - side read 9v as well.

Then, I hooked up headphones and a bass and heard a small pop in the headphones when I turned everything on... but no other bass sound.

ANd then when I remeasured the cap to see if I had broken anything loose in the testing, I found that the - side was reading 0 again.

Any idea what's wrong here? Any direction to point me in. Seems that the signal is not flowing all the way through to the output and that is my main problem.  I'm not sure how to fix it, though.

You're confusing signal and power. The 9v is to power the chip. Capacitors block DC voltages (your guitar signal is AC) so the purpose of C8 is to ensure that AC only hits the speaker, hence a reading of 0V on the - terminal of the cap is a correct reading.

If you're seeing 9V all through the audio path then that's a major problem. There should be 9V on pin 6 of the IC and the drain of Q1 (double-check that you've got the orientation of your JFET right), some other voltage (not 0 or 9) on the source of Q1, 0V on IC pins 3 & 4 and 0V on the gain and tone pot terminals (they're surrounded by caps so this is another case where the DC has been blocked).

[bigandtall]

Thanks for the thorough response.

I rechecked things last night and found that the board was not properly grounded. I fixed that this morning and now the LED will come on (small victories!).

It still doesn't work, but now I'm not getting a 9v reading everywhere on the board anymore.

It took some more readings and it's looking closer to the numbers in your response:

U1
p1: 0
p2: 0
p3: 0
p4: 0
p5: 8.72
p6: 9.35
p7: 4.1
p8: .13

Q1
D: 9.36
S:  1.82
G:  0

Any other ideas?

[anchovie]

I think you need to make an audio probe (or use an oscilloscope if you have one) and start testing from the input until you find the point where the signal disappears.

[bigandtall]

Thanks for all the help and ideas and info. that has helped me to learn more about this process.

I went through the circuit board with a razor blade and a magnifying glass and made sure that all of the sloppy solder had been cleared out. Then did another check of things and eureka... it works!

Now to just tie up some loose ends, neaten some things, add some shrink tubing, finish boxing it all up-- and I'm done.

[anchovie]

Glad to hear of your success!