Yamaha RX-V450 - failed repair

[TargetAlpha]

Hi there,

i recently purchased a non functional Yamaha RX-V450. After all it's just the C405 that needs to be replaced
But i didn't have the right replacement cap. So i ordered a few. In the meantime i thought i would look around what other cap i had around.
So after a bit of looking around i found a similiar cap. Same type, same voltage rating of 600V but 100nF instaed of 22nF. LCR meter said it had around ~70nF.
Close enough i thought. So i put it in and the amp worked perfectly.

Until today (a day later). The Yamaha doesn't turn on anymore. I opened it up again to put in the proper 22nF cap that arrived today as well.
But when i took a closer look at the board i found out that the power supply kind of exploded :\
R412 (2.2kohm resistor) is pretty much blown away and some traces on the back are blown off as well.
I fixed the traces, replaced the resistor, the cap and the mosfet Q404 for good measure. But it's still dead. The fuses are fine. But it doesn't draw any power.

I'm not done trouble shooting yet but i just wanted to ask: what the hell happened there? ^^

edit:

Btw. this is the service manual: http://www.audio-circuit.dk/downloads/schematics_v-z/Yamaha-RXV550-V450-HTR-5750-5740-DSP-AX450-av-sm.pdf

The affected board is on page 49/82. The traces are damaged from the source of Q404 over D405 up to R412. D405 and R412 were destroyed and i have replaced them.
The amp now draws 5W when plugged in but refuses to turn on.
Maybe because my zener diode has the wrong value. I only have some BZX55C5.1 around. The right one would be a MTZJ10B. I have also replaced Q404 (K3491) with a 2N60LB. The 2N60LB is apparently not quite as high speed as the original K3491.

Still not sure what happened though. Was C405 not completely discharged due to the higher capacity and charged up to a high voltage? Like in a flash charging circuit? I'm confused.

[madires]

The too large value of the C405 replacement caused an increase of the current flowing through C405. And that caused the other trouble.

[David_AVD]

That was pretty silly using a capacitor 4 times the correct value in a mains feed.  Just asking for trouble.

My guess is the 4013 chip blew when D405 went O/C due to the greatly increased current you forced through it.

[HalfSpace]

Hi TargetAlpha,

C405 (22nf) has an impedance of 144.7k at mains frequency 50Hz. This impedance is used to limit current flow through the 10V zener D405 and R412. By changing C405 to 70nF the impedance has changed to 45.5k increasing current flow in R412 and D405. Heat dissipation in these components would now increase beyond their ratings.

D405 needs to be 10V. Using a 5.1V (BZX55C5.1) will just cause problems. Q404 should be replaced with the same type K3491 (2SK3491).

Please use an mains isolation transformer when working on this circuit for your own safety.

If the amp is now drawing 5 watts check the voltage across C411, it should be around 10v.

HalfSpace 

[TargetAlpha]

That sounds reasonable. ^^

Well, i have replaced the zener diode with the correct value and now it's working again.
The 4013 ic was apparently not damaged.
I haven't checked if the original mosfet is still working but my replacement seems to be working ok.

But i wonder if using an isolation transformer is always a good idea. The traces and pins are sometimes so close together that it's quite likely that a person will touch L and N at the same time. And then you will get shocked anyway.
So maybe it's better to trust the RCD (if you have one)? And if you are behind an isolation transformer the RCD won't trip even if you get electrocuted.

Maybe someone can correct me on this if i'm wrong.

[commongrounder]

If you contact the Line and Neutral inside the receiver, you will get a shock whether there is a ground fault device, or isolation transformer for that matter, in use.  Ground fault devices only protect you from shocks between Line/Hot and a ground/earth path that bypasses the GFI device.  There is no replacement for your own knowledge and caution to prevent this kind of shock.  Please read the several recent threads on this board concerning isolation transformers and ground fault/RCD devices.