Yamaha RX-V750 doesn't switch ON.

[Raghavendra K]

Hi Kris,
Found your responses to FUM3Z regarding Yamaha receiver and thought you could help me with some tips.
I have Yamaha Receiver RX-V750, which isn't switching ON. The relay (MCU signal PRY) powering the main transformer isn't triggering when I plugin power or push the standby button. I have checked the voltages for/to be PDET = 4.0v, 5M=5V & Reset=4.3v and PRY=0.6v
The receiver doesn't even enter diag mode and absolutely no display. I have tried shorting the relay points and found that the main transformer is working fine. I have even tried connecting only Subtrans and MCU board, but still the relay doesn't get enough voltage. The PRY remains 0.6v even after pressing the standby button. The PSW signal returns 4.3v to MCU when I press standby button.

Upon diagnosing the cause, I found 3 items being wrong.
1. The Diode (after regulator) which feeds the MCU with 5V was conducting on both bias.
2. The MCU was burnt near the Power input side. (VCC) There was sure sign of physical bulge.
3. The regulator transistor to convert 10.5V into 5V (for 5M, VSS for CPU) was burnt.

I have replaced all the 3 parts but the problem isn't solved still.
Concerns :
1. When i plugin the receiver, the regulator transistor gets over heated. I have already burnt two transistors.
2. The diode seem to be performing correctly.
3. Since I replaced the MCU purchased from DIGI-Key online, do I need to flash it (upload firmware)? If so, how can I upload the firmware? There is a provision (slot) for the flash connection. I am not sure how this works.
The MCU I replaced was M30626FHPFP. This MCU is Flash memory version and the burnt CPU was also same. However, the service manual has "M30622MHP-161FP" as the MCU, which is Mask-ROM version.

I even found that the resistance between VCC (pin 62) & VSS (pin 64) of MCU is only 18-Ohms. Is it normal?
btw, the model is L series (230-240v). And the capacitor 0.33F / 5.5v doesnt retain any voltage when I power-off the AC mains.
Further I noticed that the 10.5V from subtrans slowly gets dropped to 4-6v. and the sub-tansformer is getting overheated too.



Please help.

 

[KrisBlueNZ]

Hi there. Sorry I haven't got around to replying to this question yet. I'll reply later today. I've deleted your repeat thread.

 

[Raghavendra K]

Thank you Kris. Delighted to see your response.
Sure, I shall wait. I have nightmares fixing this problem. Any help is appreciated.

 

[Raghavendra K]

Hi,

Could you please put some light on this issue. Been waiting for some response.

 

[shabazxz]

Hello. i have one Yamaha Receiver RX-V650, and do not power up. when i press power ....nothing. I have changed the 22nf/630v, others 2 capacitors and one c1815. I have by-passed the relay and still nothing. On the IC501 i have 10v and 5.7. After the D501 i have 5v. But still no power. If one channel it is broken, the receiver power up and show something or ....do nothing. What can i do? Thanks

 

[Raghavendra K]

Hey Shabazxz, what are the voltages at PRY, +5M, PDET & RES of sub-trans?
Dont bypass the relay. Just troubleshoot the initial things first to help identify the problem in SubTrans & regulator section.

 

[shabazxz]

I mesured, but it was a litlle difficult. i messured on the function 2 pcb. and i found PRY - 0, 5M - 4.8v, PDET 1.8, and RES 2.6v. i dont know how accurate were the mesure, i did not press to much on the bord.

 

[shabazxz]

Hy, i saw the button Pure Direct flicker very dim blue light. Today i will change the C407. On this it's write F42 and below this 472j. On the schematic it is write 4700P. This mean 4.7uF, but no voltage. I have 2 10uF /630v and i will put those 2 in series obtainig 5uf. Hope to be good. Do you have another ideea? Thanks and sorry for my english.

 

[Raghavendra K]

You can use 16V or 25V safely. However, don't try the series connection. It doesn't work as you are expecting.
I feel there is some issue at SubTrans section. Check those TLP 421, Zener Diodes & Capacitors next to the IC402.
Unless you get proper voltage @PDET & RES, your receiver won't power ON. And instead of checking the voltages on Function 2 Board, check on SubTrans section. Refer to the page 86 of the service manual for required voltages.

 

[shabazxz]

ok, i will buy 4.7uF/16-25v. Can i remove the SubTrans from reciver and put 220 and make the mesurement like this?

 

[shabazxz]

Sorry, it is 4,7nf, no uf. it is C407 and it is no voltage. i found on 100v .

 

[Raghavendra K]

The service manual says it is MYLAR Capacitor of 4.7nf (4700pf) 50v. You can safely use 100v replacement. Meantime, if you remove the Sub Trans board out of cabinet, you have to connect the terminals using jumper cables. Otherwise the +5M & PDET/RES will show 0v.
+5M is output of regulator on function-2 PCB and that is sent to SubTrans for deriving PDET & RES.

 

[Raghavendra K]

Have you tried "Debugging mode" of the system?

 

[shabazxz]

Hello. how to do that?Thanks

 

[shabazxz]

I have changed the TLP421, but with TLP521 because i did not found TLP421. I changed and C407 and still nothing. I will solder wires on the board and i will verify the voltage.

 

[Raghavendra K]

Hey, TLP421 is for triggering the flow of voltage at receiving side of RES & PDET.
Anyway, did you check the capacitors & zener diodes? Replacing the capacitors will give proven-boost in voltage. Be sure to check all the capacitors in SubTrans & regulator section of the function-2 PCB.
Do you have access to service manual? If not, search for it on google and you should get it freely.
The manual has step by step instructions on how to enter De-Bugging mode. It helps you bypass all the protection modes, except over current.
Be assured that that you are very close to the success.

 

[shabazxz]

Hello again. I have try the debug...but nothing. I will replace all capacitors on subtrans and i will look after voltages. I have the manuals and i look there after the right value. I will tell u what i will found

 

[shabazxz]

Hello. I have mesured the voltages at the little transformer from the Subtrans and i have 86-90v. On the schematic it is 158

 

[shabazxz]

I have found strange things. on the SubTrans Q404 is k3491 but on schematics is k3850. Between pin 7 and 14 of the TC4013 the voltage is 0. D408 - between alternative pins, i have mesured with the gouge set to diode, shows nothing. What to do ? I suspect the D408 - S1NB60. What do u say? Thanks

 

[shabazxz]

On the output of the transformer from SubTrans i have 5V, but after the rectifier and the condensator i have 9,9-10v. On the function pcb 1/2 , i have on S10 - 9,9-10V and at 5M - 5,55V. Still no good PDET and RES.