[el_mejor]

The receiver didn't work after changing the amplifier IC in almost completely dismounted state. After measuring some voltages, i decided to assemble the receiver again (w/o the defective part soldered in) to store it in the cellar. After that I tried to switch it on a last time. And - it worked. I suppose there was some groundconnection missing. So I soldered in the spare part again which was i bit tricky because the wires were a bit to short (because it wasn*t the Yamaha spare part with the "Y"-appendix) and i had to lenghten them.

But in the end all the work was worth it and it was only the power stage IC defective. Thanks for all the help!

[el_mejor]

Yesterday, I received the spare-part. I changed the defective part, but there must be another defect. I resetted the error memory, but nothing. The error code means that ther is something wrong with the power supply, but all voltages are in spec. I think that the fault is in the diagnostic curcuit. We will see...

[el_mejor]

Ah im such a dumbass. Of course is the type I ordered having 15 pins, but the 3rd is just missing. I just counted the existing ones. Have checked the datasheet again...

Now I´m looking forward to the delivery.

[el_mejor]

I soldered the bad stk out of the curcuit and measured and compared the resistance of the output stages and one was short-curcuited.

I have another problem. Yamaha uses the stk402-120Y (14pins) and I suppose, that that one i ordered is only a STK402-120 (15pins). I noticed this difference to late. Unfortunatly I couldn´t found a shop which is selling the "Y"-Type. I´m waiting for the delivery. If it´s the wrong type, I had bad luck and I´m not able to get the receiver repaired. Then I better buy a brand new one.

[vtech]

No doubt it is all stk's. The reading on the test mode can help but you'd need to have the service manual and more than likely, knowing yamaha's protection circuitry is definately detecting the presence .

Assuming that you tried monitoring the voltages on the pin based on the datasheets to determine the bad STk... right?

As a general rule with audio stk's utilizing the datasheets;
you should be able to measure the plus& minus rails(must be balanced) along with close to zero at the output pins. When/if there is an internal failure within the stk, it almost always causes a dc level to appear at the output pin(s) which will trip the protection ckt.

[el_mejor]

There are two STk402-120. I guess one for the front channels and one for the rear. There is a STK404-120 also, I guess this one is for the center speaker, because it s an single-channel amp.

I soldered the STK404 out, just to make sure that I didn´t measure something wrong. Now the receiver shuts down immidiately after turning it off. This is telling me, that there MUST be some control mechanism inside of it and I hope, that after changing that defective STK402 the unit will work again.

In the meanwhile, I found out, that there is some diagnosis mode, which can be entered by holding down the "stereo/effect" and the "6CH input" button while turning the receiver on by pressing the standby-button. But without explaination, I don´t understand what the messages have to say. One of them could be some check for the supply. "DC:100% PS:10%". But I don´t know what this means.

[vtech]

Ok, I see . Nothing wrong with your english... & yes the STk is infact an IC which yamaha is apparently started using.
But I am willing to bet that you are looking at rear channels ONLY. The STK in question is only a 2 channel audio amp while that suppose to be a 5 channel receiver.
I know for a fact that some other brands have used STK's as rear channels combined with seprate transistors for the mains. --sounds like yamaha is doing the same? On the other hand if they are going to use STK's only, there has got to be more than one?

[Guest]

My problem is, that my english is not that good, to be sure, that I´m saying everything correct.
The defective part is an stk402-120, and I think this is an IC, an integrated curcuit. I thought this is meaning the same as discrete amp.
If you study the datasheet, you can see, that every channel of the ampflifier consists of to stages, one for every polarity. And one of the two stages is short-curcuited.

I tryied to switch the unit on, with all of the submodules connected to it. Because I assumed, that the receiver does shut down, because one of them is missing. Result was, that the reciver shuts down even quicker as whitout them. So I guess, that there could be another fault. But that doesn´t make so much sense, because I think the defective amplifier stage is the result of overload and not of another defect in the unit.

I ordered the STK402-120 already, so I will try to change this part and hope that the receiver is a smart one and just detected the missing part.

[vtech]

Hmm...exactly what is the IC? I was not aware Yamaha has started using Ic's for outputs! I'm used to seeing discrete outputs & thought that you were able to find a defective output transistor which is rather easy to spot w/ a meter. If infact an Ic, how did you determine it being bad w/out any reference points or perhaps you have a diagram?

and you are correct: If it was a bad output IC, removing it , SHOULD allow the unit to at least stay powered On. ..at this point just not sure if you are in the right track & definately do not recommend any shotgun approach.

[Guest]

I checked all semiconductors and the emitter-resistor, I´m pretty sure that the final stage IC is the only part I have to change.
The only thing I am wondering about is, that the receiver even shuts down if the defective amp IC is soldered out. I don´t think that the control curcuit is detecting, that this part is missing. Because of this, I´m affraid that I didn´t find all the faults. Or is there any reset functionality to tell the receiver, everything is alright now?

ANyway! I ordered the spare part and hope I´d be successful. I´m reporting if the receiver is running or smoking after the repairment.