[SOLVED] Amplifier Yamaha RX-V657 not turning on

[Veketti]

Dear All,

I've got an amplifier Yamaha RX-V657 which just doesn't turn on from the standby mode anymore. I found out that it is common that Yamaha amplifiers fail the Capacitor C405 22nF 600V from standby circuit and will not turn on anymore at some point. I removed that cap and it was only 8nF and esr ~400ohm. Bought two types of caps:
https://www.reichelt.de/wima-film-capacitor-impulse-resistant-mkp10-630-22n-p31974.html?r=1
https://www.reichelt.de/film-capacitor-22nf-630v-105-c-phe450-22n-630-p206683.html?&trstct=pos_0
Original capacitor looked like this:

And tried both of those film capacitors, still that amplifier is not turning on. Did some measurements and I'm only getting fraction of voltage to the transformer. Please see attached picture of the measurements. Any idea what else might be wrong?

 

[betwixt]

Possibly that value is different for models running from 115V and 230V, can you read the value on the original one to check the new one is the same.
If it is, the next thing to check is the supply to the IC. The voltage across D405 should be about 9.4V and across the IC pins 7 and 14 you should measure about 9V. If the IC supply is low it will not be able to fully drive Q404 which in turn will limit the current to the power transformer.

Be careful, some of those voltages are not isolated from the AC power lines so keep fingers well out of the way and never use grounded test equipment.

Brian.

 

[Veketti]

Thank you for reply. Value of the capacitor is the same, I can read 223K 630 from that. I measured the voltage across D405 and it was 4.3VAC. Across IC pins 7 and 14 415mVAC. So it is lacking voltage. What component might be the culprint?

 

[betwixt]

The voltage across pins 7 and 14 should be DC, please check again using a DC range on the meter.
If the voltage is still low, my first guess at the problem would be that C405 has gone open circuit but it could be many things. Check that voltage first, measured directly across the pins and let me know what it is.

Brian.

 

[Veketti]

Oh, it is DC. I thought that it is AC as there is tilde on the drawing. Measured it again with DC and it was 7.48V. Just as a side question is the D406 responsible of making it DC?

 

[betwixt]

Partly. Some AC coupling is made from the incoming power through C405 and R419. Zener diode D405 conducts at ~0.6V on negative half cycles and at about 10V on positive half cycles so there are 10V bursts across it. D406 conducts those through to C405 to produce a steady supply to the IC. The return current to the line is via the bridge rectifier D408.

You can try a dirty trick to override the power switch if you are sure there are no problems further down the line, just short out the source and drain pins of Q404. Do it at your own risk though, if there is a fault elsewhere in the amplifier the protection won't operate.

Brian.

 

[Veketti]

Shorted the drain and source and the amplifier turned on from the button. Any idea what might be the failed component, or do I need to solder those two together to make it work again... :sad:
And btw. no magic smoke left the device when shorting those...

Edit: I found out a thread where someone had that IC (flipflop) faulty. Could that be the the same in my case. However there the puredirect led was blinking and mine doesn't.
https://www.electronicspoint.com/forums/threads/yamaha-rx-v750-doesnt-switch-on.272858/

Or the Q404? Wouldn't want to start ordering new components blindly so anything else to measure to be sure what's the issue...

 

[betwixt]

If shorting the MOSFET made it work normally the fault has to be in the area of the IC. Any of the parts around there could be faulty, it's difficult to make a diagnosis remotely.

If the MOSFET is working, there is a test you could try but it isn't conclusive: measure the voltage between source and gate pins of the MOSFET, it should be >7V when the power is on. if it isn't, either the MOSFET has an internal gate to source short (unlikely) or the IC could be at fault. 4013 flip flops are very inexpensive and easy to find.

Brian.

 

[Veketti]

I removed the mosfet and did some measurements. First of all it was draining 10mA even if gate was 0V. Then when rising the gate voltage it conducted max ~40mA. I probably at some point managed to destroy that mosfet as now it is shorted between gate and source.. Most likely it was already faulty. Did the measurements according to this video
When I tried to charge the gate with the multimeter positive probe like in the video it didn't became conductive.

Just ordered new mosfet, IC and diodes around the IC. Just noticed also that Yamaha updated that mosfet from 2SK3850 to 2SK3491 and I had that earlier one. Let's see once I get those new components whether it will come back alive.

Btw. That subtrans didn't have D413, R402 and D412 populated. I wonder what's the purpose of those..

 

[betwixt]

Btw. That subtrans didn't have D413, R402 and D412 populated. I wonder what's the purpose of those..

I'm not sure but it seems to be something to do with holding the MOSFET in the off state if the supply voltage is too low. The link bypasses it so I guess Yamaha decided it wasn't useful. Perhaps they were protecting against unpredictable operation if it was run from a marginal AC supply, for example if it was run from a generator running low on fuel.

Brian.

 

[Veketti]

Wohoo, it's fixed! Just received my digikey order and changed Q404, IC, D404, D405 and D406 (previously C405). I'm pretty sure that Q404 as well as the C405 were broken. Q404 is now STD2HNK60Z-1 which can handle 2A instead of the old 0.7A so hopefully it will now work for years..

Btw. I'm not sure why but now the voltage around the transformer is >200VAC and 12VAC. I wonder were those voltages in that schematic correct, because 7.5 sounds too low for 12V relay coil.

Thanks betwixt for helping to diagnose. Another device saved from going to garbage bin and bringing music to my ears..

- - - Updated - - -

Btw. I'm not sure why but now the voltage around the transformer is >200VAC and 12VAC. I wonder were those voltages in that schematic correct, because 7.5 sounds too low for 12V relay coil.

Wasn't able to edit my previous post. This was not correct. When it was off no voltage on primary coil and when turned on around 152VAC like in that schematic..

 

[betwixt]

Well done - saved the amplifier and help save the planet in one move. :-D
It grieves me to see the piles of recent electronic devices at the local recycling facility, knowing that almost all of it could be restored to perfect working order for very little cost instead of minimal metal salvage or even worse, going to landfill.

Brian.