Yamaha RX-A380 DC PRT 0L / Circuit Protection Problem (yet another...)

[slater0013]

Hello from France 

I have a Yamaha RX A830 audio/video amplifier ! Worked like a charm for years.

And suddenly, impossible to turn it on, circuit protection turns it off immediatly.

As mentionned in service manual, I went through the Diagnostic menu to see : DC PRT 0L (sometimes 2L 3L 4L)

Using a Voltmeter, I can measure that the FRONT RIGHT speaker output is around 5V when all the other speaker outputs are measured around a few miliVolts.

I guess I found the faulty channel, but what to do now ?

https://cloud.univ-grenoble-alpes.fr/s/Xxgqozd3ZmFMppA

I guess the 5V measured at the speaker output is what I would measure at R1150 ? I need to find the adequates probes to avoid shorting any other components.

I do not really understand why I should cut the resistor and what It could help me with, I suppose the problem is somewhere else.

https://cloud.univ-grenoble-alpes.fr/s/RaccmgyxSfZW2Pg
and
https://cloud.univ-grenoble-alpes.fr/s/gTejszGEGK4N2Qt

shows the components related to FRONT LEFT and FRONT RIGHT channels.

There are quite a few to try ! And if I can manage to test resistors and capacitors, how to test transistors properly ? What would you try first ?

https://cloud.univ-grenoble-alpes.fr/s/bGRz59NanAd3Moa
Just looking at the board, nothing seems to be explicitely dead !

Any ideas ?
Thanks,
GS

[slater0013]

Also uploaded physical implantations of the components on the mainboard:

https://cloud.univ-grenoble-alpes.fr/s/fPSd7jQkSM6AqBJ

[David_AVD]

Was that 5V on the speaker output measured on the centre pin of the emitter resistor, or on the rear panel terminal?

The reason I as is that if the speaker relays are not engaged due to a DC fault, you'll need to measure at the emitter resistors.

[coromonadalix]

who said you had to cut a part    your amplifier at this level is not adjustable, you have electronic problem(s)  you have to differenciate ..


first you check supply lines and all the related voltages,  for sure something is affecting the outputs


are you equipped to do some tests, voltages tests, component tests        normally when possible    at zero volume  you take CAREFULL MEASUREMENTS on the good channel at key points (voltages, in and out of each tansistor chain)   around the biais transistor  etc .... 


AND CROSSCHECK  on the bad channel to see where it begun to drift, could be a simple as :  a cold solder / bad connection  ......... 

[wasedadoc]

I do not really understand why I should cut the resistor and what It could help me with, I suppose the problem is somewhere else.

Yes the problem is elsewhere.  Cutting the resistor makes only a small adjustment.  Cutting it will not cure the fault you have.

[slater0013]

Hello,

Yes I measured directly on the speaker output !

GS

[slater0013]

I just brang the amplifier at home, easier to make measurements.
I managed to find probes !
Will make more tests

[slater0013]

Hello,

A friend of mine took the amplifier for a while, cut, removed and tested the output transistors marked in green Q1052C and Q1052A. Working correctly, PNP and NPN. We tried to start the amplifier again without the transistors and still -5V at the speaker output, and circuit protection still triggers.

https://cloud.univ-grenoble-alpes.fr/s/Q5iqYdjpmEASZqq

My guess is that the first stage of amplification might be faulty (labelled Q1038 and Q1045 if I get It right) ? Would you identify other usual suspects ?

Thanks,
GS