Focal Alpha 80 repair (no sound)

[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.

[rodrigopires]

Redid the probing and found that the 79l12 reg is ok, only the 78l12 was nok so went ro replace it when i found under the black silicone a burn between the resistor pad (Vin) and ground on the reg. Removed the reg, cleaned as much as possible the area and soldered a new 78l12 reg but still it doesnt work... Now i have 20v on Vin and -1V on Vout, i was able to get 12v out wich gave the TL074 chips 23v across the midle pins but it was just for a breaf moment and it went to -1 V output... Removed the new reg and tested it outside the board and it is ok, resoldered on the board and again -1v output...

[Audiorepair]

You might have lost the Gnd connection to the 7812 since you say it's burnt around there.
Hard wire the Gnd pin to the 7912 Gnd to check, and try and bypass any burnt bits of PCB in case they are conductive.

Otherwise, there may be a short downstream from the 7812, a blown op-amp or cap etc.

[rodrigopires]

connected the +12v rail to a external 12v PS and i got sound! I suspected it could be a problem with the Vin resistor shown in the image and after desoldering it was broken in half... The number is not readable i only know it ends with a 0. The 79l12 resistor is not very well marked but e get around 10 ohm with a multimeter and the number looks to be 130 so 13 ohm looks about right. Should it be safe to apply the same value to the 78l12 a 10 ohm resisitor?

[Audiorepair]

It is far more likely the fuse resistors are 10 ohms than 13, the value is not really critical, it just has to blow the resistor if something bad happens, which it did.

From your description, it appears there was a conductive path between Vin and Gnd on the 7812 regulator, which blew the fuse resistor.

Since the 7812 in your photo is covered in that black glue, I would suspect that the black glue, which is known to go conductive and corrosive, is your problem here, and that is what shorted Vin and Gnd.

I've lost count of the number of times I have seen this problem now in powered monitors, and it beggars belief that manufacturers are STILL DOING THIS.   

[rodrigopires]

Substituted the resistor and repair was completed  oddly enough that problem with conductive black glue only apers after warranty expired... no wonder the manufacturers are not woried... 

Thanks eveyone for all the help!

[Audiorepair]

Check the other speaker, it will be in a similar state.