Yamaha RX-497 No main power?

[redrooster]

I've been given this Yammmy to repair, there are no relay clicks or display lights when I press the power button. There is however S10v,5MV, /RES (Reset) 4.7v. PDET (power detect) is 0.110mv and PRY (power relay) is around 100mv. That's the only power there is except for 7 or 8vac on the small transformer. Any help is appreciated.

 

[patrick101]

The S10 is the 10V standby voltage for the amp, if it is low the amp will not operate.

Please disconnect the CB106 cable in the amp board first and check again the S10 voltage in the M3 small board to see if we can get 10V output.

 

[redrooster]

The S10 is the 10V standby voltage for the amp, if it is low the amp will not operate.

Please disconnect the CB106 cable in the amp board first and check again the S10 voltage in the M3 small board to see if we can get 10V output.

Is there some other way I can do this because that connection is a major pain to get at and get off! I will do it if I have to though?

 

[patrick101]

In order to fix the amp, I think you have to loose some PCB to test it.

 

[redrooster]

I finally got it off, the small board measures nothing on the S10v. That board is easy to get out, I've had it out a few times to check everything and replace a 2.2uf 50v electro capacitor that I thought just maybe a bit low?

 

[patrick101]

I finally got it off, the small board measures nothing on the S10v. That board is easy to get out, I've had it out a few times to check everything and replace a 2.2uf 50v electro capacitor that I thought just maybe a bit low?

Do you disconnected the cable to the power amp board?

You said there is 8V AC from the small transformer therefore you should get the 10V standby voltage.

 

[redrooster]

OK, I think the problem may be D261 the small bridge diode S1NB20. At least 1 diode out of the 4 seems to be shorted? I will desolder Electro capacitor C-260 to find out for sure?

 

[redrooster]

Do you disconnected the cable to the power amp board?

You said there is 8V AC from the small transformer therefore you should get the 10V standby voltage.

Yes, I disconnected the cable to the power amp board. I've found that D261, S1NB20 the small bridge diode has two blown diodes. So I hope that's all that is wrong with it?

 

[patrick101]

ok, seems a good sign

 

[redrooster]

ok, seems a good sign

Thanks for putting me onto the right track Patrick. Now I understand what's happening with the power-up circuit. Yay! Thanks again Mate!

 

[patrick101]

Do not mention that, you did the good testing first.

 

[redrooster]

Do not mention that, you did the good testing first.

Yes, but if you hadn't told me about S10V being the power-up voltage I would still be trying to figure things out? That's when it all clicked and I understood how it worked!

 

[patrick101]

Forget to remind you that please also check the circuit resistance after the S10 output, it maybe a short circuit there to cause the small bridge fail.

To do that, plug back the cable and measure the resistance between S10 pin to ground.

 

[redrooster]

I had measured the Diode bridge wrong, I don't think there is anything wrong with it because it reads the same as the other small diode bridge on the board it's connected to? D259, S1NB60 next to the Mosfet Q254. Then I looked at IC251 and IC253 I don't get any diode reading from either one in circuit?

 

[patrick101]

I'm not talking test the AC board with the small transformer, I mean test the power amp board side S10 pin resistance to the ground where S10 voltage supply to the whole system.

 

[redrooster]

S10 pin resistance from the main power board is 410.5 ohms that cycles down to 0 ohms then back again to 410.5 ohms and repeats.

 

[patrick101]

S10 pin resistance from the main power board is 410.5 ohms that cycles down to 0 ohms then back again to 410.5 ohms and repeats.

Why this reading?

 

[redrooster]

Why this reading?

I don't know? I'm using a Fluke 87V Multimeter. What should the reading be?

 

[patrick101]

ok, maybe some caps charge and discharge in the circuit, a reading of 410.5 ohms seems ok.

Just wait you replace the small bridge rectifier and pit back the whole amp to power test.

 

[redrooster]

I put it back together and measured 17.77 vac across the small transformer secondary, the little bridge diode is not putting out any voltage. IC251 and IC253 are not showing any diode voltage with a diode test? I'll wait for the little diode bridges to get here and see how it goes then? Thanks for your help, Patrick. It's appreciated.