TDA2004 amp questions

[Doomguy42]

Hi
I have this audio circuit where the right channel is really quiet. I can here it increasing volume if I adjust it by the digital potentialiometer. But overall it sounds about 15% of what it should be,
I've replaced the
Td42004 amp
X2 digital potentiometers
Most of the larger electrolytic caps.

I can't seem to cure this fault though. Please can anyone explain the amp bootstrap?
It's like the audio is there but it's not getting amplified correctly.

Thanks

 

[Doomguy42]

Tda2004

 

[rjenkinsgb]

The bootstrap cap generally feeds in to near the top of the chain that supplies the output device bases / gates.

It helps keep the current through the drive/bias chain nearer constant, rather than it dropping off when the bases get near the positive rail.

eg. C2 in this example:

 

[rjenkinsgb]

In your amp, verify the resistor values in the feedback dividers that feed the caps to pins 2 & 4.

They control the gain for each channel.

 

[Doomguy42]

Thanks for the lesson. The resistors for the gain were all correct. I couldn't find the fault anywhere... it turned out to be the audio processing on a card and nothing to do with the amp area. Head scratcher and a half

 

[Nigel Goodwin]

Thanks for the lesson. The resistors for the gain were all correct. I couldn't find the fault anywhere... it turned out to be the audio processing on a card and nothing to do with the amp area. Head scratcher and a half

Not really, simple standard fault finding procedures would have easily eliminated a fault around the TDA2004 - a scope would be useful, but even simply shorting POT-L and POT-R together would have shown the fault wasn't there.

Simple procedures, signal injection, signal tracing, and the 'half split' method.

Just randomly changing components for no reason isn't the way to repair anything.

 

[Doomguy42]

Agree totally.

That's my first ever time actually attempting to fault find a stereo audio circuit. Wouldn't say its randomly replacing components as I'm testing them and trying to look for clues.

I've succesfully repaired it myself. The 25year old electrolytic caps are all replaced with new. I've learned about how the Audio amp gains work... its has totally great stereo sound, it has cost me hardly anything other than time.

One lesson I have learned is it must have some gain missing on that channel from the sound processor which I never knew about previously.

Thats good progess for me personally and mission accomplished . When I have done it 20/30 times I will do it better for sure.