Bat detector kit, odd behaviour

[snes]

Hi everyone,

First post, and a plea for help.

I have just been helping my daughter put together the bat detector kit currently on sale in the Elektor shop. We followed the debugging procedure in the manual, but it is exhibiting erratic behaviour. Symptoms are 9 time out of ten after switching on, it just emits a lot of white noise and the sensitivity of the microphone is almost nil. It appears that repeatedly switching it off and on again will occasionally get it into a correctly functioning state where there is very little noise and ultrasonic noises can be effectively tuned in.

What could be causing it to get into the bad state at power up? Once switched on, it doesn't ever appear to get out of this state. What should I be checking?

It's this one: http://www.elektor.com/bat-detector-kit-140259-91

The design looks like a superhet, using a 555 timer for a LO and a CD2003GB AM radio receiver chip for its mixer and AF filter. It has a one transistor preamp between an ultrasonic mic and the radio chip. I am assuming it is the radio chip that gets into a bad state, but am at a loss to know why this might be.

Many thanks!

[ag]

Hi snes,

Based on the symtoms you described, my best guess would be a cold solder connection. You appear to have enough electronics background to know what that is, so maybe you could check or maybe just re-solder all the connections made when building the kit. A second possibility which comes to mind is perhaps an ill-mounted microphone which is causing a feedback loop. Maybe check that as well.

Regards,
Ag

[snes]

Thanks for the tips Ag.

No obviously dry joints, but the pre-assembled SM components was done with lead free solder so they all look dry

So far, I reflowed the radio IC and all the passives immediately adjacent to it, but it made no difference at all. I also loosely twisted the pair of leads to the mic to try and cancel any interference, as they are quite long and come close to the PCB when the lid is closed. Again, no perceivable difference.

It's not as though it doesn't work. Just flicking the power switch a few times until there's no loud hiss gets it working every time. It would be nice if we didn't have to do that though.

I am wondering if a power rail is coming up too slowly or something like that. There are two in the design, 9V direct from the battery and 5V from a low power 7805 derivative. (The battery we're using is a brand new alkaline by the way.)

[ag]

Hi snes,

Although you mentioned going through the debugging procedure in the manual, I don't know what that manual contains, and therefore I offer this link http://www.elektronik-labor.de/Lernpakete/Fledermaus.html which has some detailed instructions and pictures, including schematics as well as construction and debugging hints. Unfortunately, it is only in German, but it might still be of some help. In particular, I am guessing that the loud hiss is feedback from the microphone. Is the mic glued/sealed properly (it should be mounted with glue and be solid and air-tight against the enclosure)? Also insure that the polarity of the mic is correct. If you have an oscilloscope, you might try probing the circuit when it is in the hissing mode and perhaps be able to localize the problem. Also, you might try touching/tapping your way through the circuit, looking for areas of sensitivity in regard to the symptom. Start with a dry fingertip, and if necessary, progress with a very slightly moistened fingertip. I assume you have already double-checked the polarity of the electrolytic capacitors and verified that R5 and the potentiometers were delivered with the correct values.

Regards,
Ag