Wednesday, January 8, 2014

Tuning up the DC rx

Direct Conersion receiver based on the TA7358AP and xtal 14318kc. This xtal generally oscillates around 14313kc in this ckt.

Original post is here

Tuning this receiver is a very straight forward affairs. You will need xtal oscillator as given in the sept2013 post. Make that and you will have an RF signal source. If possible use the different power supply to power this xtal osc and receiver. Please use the xtal of the same freq value.

In the rx check the power supply rail wether you are getting proper 12vdc or not. If you have put and led in the power rail instandtly you will know this. check also that ta7358 pin 9 is getting 5 volts and not more. Warning more voltages will damage this mixer ic. Also note the orientation of this ic. dont make mistake here. It will damage ic. On pin2 there is 0.1 (104) capacitor. Also chekc the freq atk pin 8 ,

Audio should be quite, there will be white noice hiss . (broad band shhhh sound is called white noice) now touch the base of first pre-amp the noice will increase. increase/decrease the volume and you will know that af chain bc547 and lm386 is working ok. IF motor boating sound comes on putting the volume in maximum position. then increase the feed back of preamp from 33 pf to 100pf. Now, there should be no motorboating and noice level will be quiter. also see that you use bc547b variety not the bc57c. suffix c has higest gain which can create problem here.

Now connect the one feet antenna or any small legth wire to rx input.

Power on the xtal osc. If you have general coverage radio (AM will suffice) you can know that your osc is indeed working. or measure freq with xtal. Now this xtal osc is working put it some distance and also connect one to two feet antenna for radiating the signal. ( both rx and tx antenna are different ones, there will be two antenna wire.)

at this point when your rx is switched on and xtal osc is also switched on you should hear the an audio note. swithc off the xtal osc and it should vanish. Tune the front end bandpass filter for more signal. If need arise keep your xtal osc farther from the rx and retune.

Connect the big antenna / dipole and sound should increase and if lucky there will be little or no broadcast station breakthrough.

If you dont hear the note now there is some problem.
Switch off everything.
Visually inspect the ckt diagrams, your ckt, ic pin orientations, also check power supply.

then test the speakers. LM386, bc547 stage and then rf amp and ta7358stage.
on connecting big antenna without any bandpass filter, you should be able to hear the am/sw stations this is sure sign of working of your dc receiver. by putting the bandpass filter (generally two tanks will be ok sometime you need to put thee tanks in front end.) ............... to be continued....

Tomorrow will be updating/editing this text and putting more fotos and sketches. also first time putting space for comments/your feedback , any dobts, comments etc.

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