Yamaha DRX-730 won't start

[frankie31]

Hello everybody,

I'm back with a new problem... on my audio-video system Yamaha DRX-730 that doesn't want to power on.

http://elektrotanya.com/yamaha_drx-7.../download.html

As I push the power button, the main relay that cliks (sticks) and 2 seconds later unsticks...

The relay is powered independently of the main system by a little transformer (T502) and its voltages are ok so I don't think there is a problem here with the relay command stage.

On the other side, the Q501 (main mosfet that powers the main transformer T501 that powers the system) doesn't chop the voltage (but the 350V on its drain is ok).

I suspect the mosfet driver (IC502, L6566BTR). But even its Vcc voltage isn't good (very fluctuant from 9V to 12V measured with my DC voltmeter).

About it : I don't understand on the schematic how it is powered ? It is at the secondary from T501 but if Q501 doesn't work... no secondary voltages isn't it ?

So... now I'm stuck... any idea ?

Thank you very much, regards, François.

[budm]

Re: Yamaha DRX-730 won't start

https://cdn.badcaps-static.com/pdfs/...613cc2ca91.pdf

"But even its Vcc voltage isn't good (very fluctuant from 9V to 12V measured with my DC voltmeter)."
Vcc turn on threshold is 14V, right now it stuck in the loop, it can be bad startup cap on the VCC pin or the load is too much so it goes into start-up-shut down loop.
Read the spec sheet and learn how the IC function.

[frankie31]

Re: Yamaha DRX-730 won't start

Thank you budm for helping me

I think you are right I need to go further in the understanding of this IC.

So let's measure the voltages on the L6566 chip :

Pin 1 : The voltage for HVS (the pin whose aim is to charge an external cap to create VCC) is about 350V : it's ok.

Pin 5 : Vcc changes between 5V and 12V as previously said ; the electrolytic cap C512 is ok (tested with my ESR-meter)

Pin 4 : At the gate driver output (GD) which pilots the main power transistor Q501 (probe plugged on D502 cathode) I see nothing with my scope when I push the power button (flat curve the same as unplugged probe)

All the other pins : the voltages = 0V !

My IC has got no Vref.

Could it be the IC ? Or do you think the problem is elsewhere ?

Thank you very much, François.

[frankie31]

Re: Yamaha DRX-730 won't start

I would like to add more informations : the measurements are made with only the 4 pin connector (W503) connected between the main board and the sub board (the power supply board) just so I can push on the power button to make the relay stick.

Additional measures : on this 4 pin connector W503

No power/Push on :

CGND (ground) : 0V/0V
PWR_RY (relay control) : 0V/4.8V
+5SUB : 13.55V/11.46V
+12_RES : 15.10V/12.67V


Thank you

[budm]

Re: Yamaha DRX-730 won't start

Thank you budm for helping me

I think you are right I need to go further in the understanding of this IC.

So let's measure the voltages on the L6566 chip :

Pin 1 : The voltage for HVS (the pin whose aim is to charge an external cap to create VCC) is about 350V : it's ok.

Pin 5 : Vcc changes between 5V and 12V as previously said ; the electrolytic cap C512 is ok (tested with my ESR-meter)

Pin 4 : At the gate driver output (GD) which pilots the main power transistor Q501 (probe plugged on D502 cathode) I see nothing with my scope when I push the power button (flat curve the same as unplugged probe)

All the other pins : the voltages = 0V !

My IC has got no Vref.

Could it be the IC ? Or do you think the problem is elsewhere ?

Thank you very much, François.

What do you use for ground ref point for the meter?

[budm]

Re: Yamaha DRX-730 won't start

Lets take the resistance reading of L6566 IC between pin 3 (Ground) against all other pins.
There may be problem with the internal current source that charges the cap on the VCC pin.

[budm]

Re: Yamaha DRX-730 won't start

BTW, I look at the diagram and see that there is Zener diode D504 connected to the VCC pin, if there is leakage on the that Zener diode or the Cap has leakage resistance on that VCC pin, then the voltage on the VCC may not be able to get to 14V for the IC to start up.
You also need to verify the voltage readings on the IC pins again using the ground pin as the ref gnd. I expect the AC OK pin 16 will have 3V per diagram and 1V on Vref unless those two 470K resistors are bad.

[frankie31]

Re: Yamaha DRX-730 won't start

What do you use for ground ref point for the meter?

For the ground ref point I use an alligator clip put on Q501 heatsink

[frankie31]

Re: Yamaha DRX-730 won't start

I confirm the voltage readings for all my pins.

I add some measurements : for AC_OK I agree something is going wrong -> I have my voltage after R558 (320V) but nothing after R559... hum hum...

So the resistance for R558 is ok (469K) but... not for R559 : it seems to be bad !

Nothing is parallel-connected so this resistor is dead...

I have to find another one to test.

Do you recommand me to change for a bigger one (higher wattage) ?


Thank you, again, François.

[budm]

Re: Yamaha DRX-730 won't start

It has less than 0.1W dissipation on those two 470K, 1/4W will be than enough. They had to use two resistors due to each resistor has breakdown voltage of 250V, for it to handle 400V they put two in series.
Worst case at 400V/(470K * 2) = 0.42mA of current flowing through those two resistors if pin 16 shorted out.

[frankie31]

Re: Yamaha DRX-730 won't start

Yes !

It works !

It was definitely the 470K resistor

One thing that I don't understand :

The voltage divider made of R556,557,558,559 gives me for the pin 16 :

V_pin_16=V_capacitor*(R556+R557)/[(R556+R557)+R558+R559]

V_pin_16=332*(4.7+15)/[(4.7+15)+470+470]=6.81V

And I finally found 3.1V after changing the R558 and R559... what's the difference between the theory and the pratice ?


Thank you very much, regards

[budm]

Re: Yamaha DRX-730 won't start

Glad to hear that it works.

[Chungalin]

Re: Yamaha DRX-730 won't start

Suprising coincidence!

Today I've received a DRX-730 with exactly the same symptom. After pressing power button, main relay switches on and two seconds later switches off. Nothing on display.

I've checked R558 and R559 without hesitation and in this case it's been R558 (open). Replaced by same value (bigger one, I don't have that small) and voila!

This coincidence reveals that this could be a common fault in this model.

Thanks a lot budm and frankie. You have saved me a lot of time.

[frankie31]

Re: Yamaha DRX-730 won't start

Hi !

Good work Chungalin !

[PAULO SOZA]

Re: Yamaha DRX-730 won't start

Hi! I have the same problem, but I'm not an expert, so, can you upload some photos?? Thanks!

[budm]

Re: Yamaha DRX-730 won't start

Hi! I have the same problem, but I'm not an expert, so, can you upload some photos?? Thanks!

Did you download the service manual?

[PAULO SOZA]

Re: Yamaha DRX-730 won't start

hello budm, thanks for responding so quickly!
I have attached photos of the measurements that I made, I hope you can guide me ..
Thanks again

[budm]

Re: Yamaha DRX-730 won't start

I would take both of them off the board and test them or at least unsolder one leg off the board and test them. R558 looks bad.
If I were you I would replace them since they are known to go bad.

[PAULO SOZA]

Re: Yamaha DRX-730 won't start

Hi again, I removed the resistors.
I have attached photos of the measurements.
It seems a success!
I will answer when finished

Thanks a lot!

[budm]

Re: Yamaha DRX-730 won't start

Same proble, R558 is bad, so you are lucky that it is the same common failure as other posters have found.
Put in two new resistors and then you will be on your way.