Noisy Cricket MkII

[Gabriel Nunes]

Hi all! I'm desperately trying to get te Noisy Cricket to work. I posted a few comments in the layout page http://tagboardeffects.blogspot.pt/2012/04/noisy-cricket-mkii.html (see the last ones) but no luck.
I've built it with no changes but the JFET: used a BF245c and also tried a 2N5457 (are these ok?). The result was everything working as it should (volume, tone and gain pots, and grit and bass switches), but the volume through a 8ohm 2watt speaker is very low (like in headphones), and the LM386 heats up like crazy in a few seconds. Tried with 9v battery and also wall wart.
Every thing looked ok, but nevertheless I built the whole thing again from scratch (only using the same 220uf caps), and got the same exact result. My only clue is the caps are defective, but I don’t have any tools to test them. Is this plausible?
I measured the LM386 and got this:
PIN1: 1.23
PIN2: 0.01
PIN3: 0.00
PIN4: 0.00
PIN5: 3.40
PIN6: 8.88
PIN7: 4.43
PIN8: 0.00
At the Drain of 2N5457 I get also 8.88.
One other thing I noiticed is that the IC only heats up if both input and output jacks are being used. If feed power to the amp but without a guitar and a speaker connected, the IC does not heat up.

Does this give you any ideas? Thanks.

[induction]

Sounds like you may be shorting the output to ground.

If you post some hi resolution pictures we might be able to spot something. Include all the wiring, especially the front and back of the board, and the jacks.

[Gabriel Nunes]

Ok, I'll do that latter this evening. Thanks.

[Gabriel Nunes]

Bull's eye, induction!!

I was indeed shorting the output to ground, but in a strange way... the damn output jack was faulty!! I started touching every place with the meter to look for unintended grounding, and found that the tip and the sleeve of the output jack were making contact; this is not visible from the outsider so it must be inside, so jack went to garbage. Put in a new one and that's it: cricket rocking! Boxed up the thing and I'm done. Beautiful little amp.
Never would have thought the jack was faulty!

By the way, just for me to learn a bit more, what made you suspect I was grounding the output? The readings in the IC?

Thanks a lot mate,
Gabriel.

[induction]

It was because the IC was heating up, especially since it only happened when you had something plugged into the input and output.

The thing that heats up ic's, especially power amp chips, is current. Pre-amps are designed to amplify voltage, but power amps are designed to amplify current (the 386 actually does both). The amount of current (I) through the chip depends on the output load it sees. Higher impedance (Z) loads reduce the current at a given voltage (V) accoarding to I=V/Z, so they produce less heat. If you short the output, the impedance goes way down, current goes way up, and the chip will get really hot. (Higher power chip-amps run much hotter and almost always need to be heat-sinked.)

I suspect your defective jack only shorted when a plug was inserted, otherwise the chip would get hot with nothing plugged into the output. Sometimes the connectors can rotate until the lugs or connection tabs touch. The same thing can happen if you use a stereo jack and wire the signal to the ring instead of the tip. Likewise, if you wire the signal to the tip, and ground the ring, the chip will overheat when nothing is plugged into the output.

Glad you got it working.

[Gabriel Nunes]

Thanks a lot for all this information.