Yamaha RX-V496RDS problem

[HellasTechn]

Good day,

Theese days i am working on a Yamaha RX-V496RDS. Here is a description of the problem.

The device blew a fuse (2.5A) on the PSU board. I replaced it and it blew again. When i pushed the power button i could hear the standby relay click on and on second power button press click off (with fuse blown) but the rest of the device was dead.
I checked the psu board and looks ok. I found one A1695 and one C4468 transistor dead at the output section. I replaced them and now when i press the power button the fuse is ok, i can hear the standby relay click and a few seconds later a second relay click (i think it is the relay that enables the output to speakers) but the display is dead.

3rd picture shows a board with some regulators( 79M12A, 7812A, 7805A) mounted on the heatsinc that i replaced with new but still the display is off.

I dont know what else to check. any ideas ?

Thank you.



Thank you .

 

[shrtrnd]

What I would do, is run a continuity check of the semidconductors, and look for an open in one of them.
The fuse not blowing any more tells me you found the shorted devices, now it' a matter of finding the ones that burned 'open' circuit.
Good luck.
If you can trace the power path beyond the fuses, locating where you lose the power in the circuit should help isolate the 'open'.

 

[quantumtangles]

What I would do, is run a continuity check of the semidconductors, and look for an open in one of them.
The fuse not blowing any more tells me you found the shorted devices, now it' a matter of finding the ones that burned 'open' circuit.
Good luck.
If you can trace the power path beyond the fuses, locating where you lose the power in the circuit should help isolate the 'open'.

Good advice to which I add my fascination with such devices led me to what appears to be the service manual for this attractive machine. I make no warranties as I did not attempt to download the manual but here is the link in the hope it may be of help.

http://elektrotanya.com/yamaha_rx-v496_rx-v496rds_htr-5240_5240rds.pdf/download.html

 

[HellasTechn]

What I would do, is run a continuity check of the semidconductors, and look for an open in one of them.
The fuse not blowing any more tells me you found the shorted devices, now it' a matter of finding the ones that burned 'open' circuit.
Good luck.
If you can trace the power path beyond the fuses, locating where you lose the power in the circuit should help isolate the 'open'.

The problem is that after the fuse power goes to the main board and separates to a large number of IC's that i cant really check.

 

[HellasTechn]

Thank you all.

I will look for the service manual and try to locate test points and see what happens !

 

[quantumtangles]

Thank you all.

I will look for the service manual and try to locate test points and see what happens !

A further idea. There is an excellent video on Youtube by Dave Jones from Australia (EEV Blog) about tracking down defunct ICs (in this case Op amps) that failed due to over voltage arising from a defective power supply rail. He iterates that 'under voltage' issues can never cause problems with ICs. They always arise from 'over voltage' situations as you would expect. Check out his forensic approach and methodology. The problem you face, though not identical, is rather similar when viewed at a higher level of abstraction.

This is the link to Part 3 of his attempts to fix an HP35670A DSA

His thermal approach in this video just may offer a solution. Check out the power dissipation from the data sheets, and the thermal resistance. You can calculate the temperature above ambient for normal operation and if they are running hot, you may have your culprit.

Do let us know how you get on. Interesting project.