Electronics > Repair

Focal Alpha 80 repair (no sound)

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rodrigopires:
Hello Everyone! I have focal alpha 80 speaker that outputs no sound, yet it detects input audio and exits standby mode.

It has two amplifiers (one for each speaker) one TDA7296 and one TDA7294: on pins 15 and 13 i get 37v on one amp and 75v on the other, on pins 8 and 7 e get the same voltages on the corresondent amp. I probe the input (pin 1 and 3/2) with an asciloscope and got nothing, same on the outputs (pin 1 and 14).

So the amplifier chips are geting power but no signal, i looked before and there are some more amplifier chips 074C (searched for GZNS534 and found nothing) these chips are all getting aound 10v VCC but the outputs some have 1v some have close to 0. Probing with the osciloscope gave no concrete result.

I presume the main amplifiers are ok but the signal is not gething there somehow lost betwin the standby detection and the amp chip.

Bellow a provide a picture of the board. I have not been able to find a shcemtic and going in blind is more dificult  :(

How should i proceed? I'm a bit new in audio equipment repair so i appreciate every help! :-+

Audiorepair:
The TL074 chips should be getting something like +15v and - 15v or more on the middle pins either side.

If you are measuring 10v to them, then that is a problem.


Ah, in fact the photo shows the two regulators that will be providing the power to the TL074's, marked 78L12 and 79L12 (U9), so they should be producing + and - 12v, measured on the outer right pins.


Lets hope all that black glue hasn't gone conductive, or you have an even bigger problem.

rodrigopires:
You sir are correct! i mesured the inputs and outputs and for this:

for the 79L12:
Pin 1 to GRD -> 18.7V
Pin 3 to GRD -> 6.88V

for the 78L12:
Pin 1 to GRD -> -1.48V
Pin 3 to GRD -> -80 to -100 mV (oscilating)

None of the values are right and none make sence not even the regulator Vin... Could a bad regulator afect the input votage like this or it must be a problem before the reg's?

Alex_Baker:
Are those fusible resistors just to the left of the regs in the picture?

Audiorepair:
The 7812 Gnd pin is pin 2, Vin pin 1.
The 7912 Gnd pin is pin 1, Vin pin 2.

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