Having some trouble getting the cicuit to function.

Submitted by montanamax on Wed, 12/14/2011 - 12:55

I'm having trouble getting sound out of my Stella. After lots of testing I can confirm an audio signal coming out of lug 2 on the 10k level potentiometer and going into the TBA820M on pin 3, but no signal comes out of the TBA820M on pin 5. I do have voltage on the positive side of each of the electrolytic caps.

I'm wondering if it's likely I have a bad TBA820M chip, and if there is any more specific testing I can do in the circuit to prove this? Does someone have a list of the positive voltage values for each electrolytic capacitor when the circuit is powered at 9 volts I could use to double check my circuit? Or should I just order in another TBA820M chip and cross my fingers?

Thanks!

Member for

11 years 5 months

John

Sun, 12/18/2011 - 01:27

Is the TBA820M in right side up? The notch faces up towards the pots.

I only ask because earlier this week I soldered in an entire chip exactly backwards after double and then triple checking that I had it in the right orientation. Then I applied power, and Vcc went to Gnd and Gnd went to Vcc and I got to see some magic smoke. :-/ Then my 2 year old got to expand his vocabulary. Then a little later I got to practice my parenting skills. "No honey, we can't say "Shit!" around the neighbors. I know it's funny but it's not polite." "Shit! Shit! Shit!"

Anyway. My next question is, are you sure that no signal is coming out of the TBA820M? Are you checking for signal or did you just hook up the speaker and you aren't hearing anything out of the speaker? Check for a bad speaker. And remember that the Stella amp can put out over 1 watt of power so if you have a little 0.25 watt speaker it will fry it, fast. I fried a little .25w speaker in about two seconds once and it made barely a whisper of noise while it was doing so.

So, voltage readings. I fired up my test Stella with a 9V battery and took a reading at each pin of the TBA820M. This is with no signal connected and all pots turned all the way down:

Vcc: 8.72v

Pin 1: 0.63v
Pin 2: 0.49v
Pin 3: 0v (this is the input and caused my stella to hiss quite a bit)
Pin 4: 0v
Pin 5: 4.00v
Pin 6: 8.72v
Pin 7: 8.65v
Pin 8: 5.51v

Not sure what you mean when you say a list of positive voltage values for each electrolytic capacitor. Do you mean a list of voltage readings for the + leg of each electrolytic capacitor? If that is what you mean, then here you go:

C1: 0.42
C2: 8.65
C5: 3.993
C6: 5.47
C7: 8.57
C12: 4.23

If you are still having problems, can you take a picture of the front and back of your circuit? Make sure it is well lit and in focus. Put your camera in macro mode if possible. Thanks!

Member for

11 years 1 month

montanamax

Fri, 12/23/2011 - 08:12

Thanks for your help!! I have the chip in the right way, and have my power connected right. I did make that mistake on another project. It's amazing how fast the jacket from an exploding electrolytic cap can shoot into the air from reversed voltage. Glad I had my glasses on.

But I still think the magic smoke might have escaped at some point. Almost all the voltage readings look good, but I do have a 0 voltage reading on Pin 8 of the TBA820M. So I ordered a couple new chips off an eBay listing and will pick up the debugging again in a week or two when they arrive.

Thanks again for helping me figure this out.

- Jon