help repair & modification to sony str k780

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hi, to all.
i have bought an sony k780 amp 5.1 5*85W +passive sub.
the problem is that the C_ch is not working.
i look the wiring diagram and i finde that the power transistor are mn2488+mp1620. this transistor are drived by a stk 350-530.
i try to touch the c_ch input in the stk input and i hear sone noise.
to be sure for the pow transistore i change the c_ch transistor with the L-ch transistor and the L-ch work and the C-ch still dosnt work..
can i connect the stk output of the rear speaker to the C-ch to drive the final?
can i connect in breadge this amp and how without damage?
can i use 4ohm speaker?
and what you think can i build the subwofer section in the board?
best regards to all..
my idea

in the picture i show my idea. please replay.:wchair:


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What type of signal source are you feeding to this 5.1 channel amplifier?
In order for all channels to be driven, the mode needs to switch to "Dolby Pro Logic" as indicated on the display panel. The receiver then synthesizes C, Rear and subwoofer signals where the source is compatible - see instructions.

Otherwise, stereo sources (2 ch) only drive front (L + R) channels so it should not be surprising if there was no centre channel output with only stereo input. The fact that there is some output when you touch the C channel input should tell you that the problem (if there is one) is elsewhere.

Digital players have outputs that drive all power amplifiers in DTS mode and provide subwoofer signal. Other audio sources ordinarily do not.

You will not be able to bridge outputs effectively. The outputs are rated for min. 6 ohms loads. In bridge mode, this means a min. rating of 12 ohms loads, so I hope you get the picture that you will not get more useful audio power, likely less before the protection circuits activate on 4 ohm impedance speakers.

Subwoofer power requirements are greater than other channels and will require extra cooling arrangements according to the class of amplifier used. It would be more effective to use a powered subwoofer based on a ready-made plate amplifier of >100W or compromise by using the rear channel amplifiers to power twin subwoofer drivers or simply use only one channel, if necessary.
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hi, thanks for replay.
i switch to all mode and i let activated the 5 ch mode with cd/dvd in source and still the C-ch dosnt work,I reset all setting but nothing.
I dont know if you have a look to the schematic but in the amp board is a place for the sub-woofer section that is empty just for the sub (model k680).
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Is the source really Dolby Pro Logic compatible? If you try a DVD etc and still have no centre channel, I guess you may have a processor or board fault as Sakis suggests, but without applying a test signal to the centre channel i.e. doing proper diagnostic tests, you are wasting your time guessing about the power amplifier or other possibilities.

Any added amplifier still needs power and cooling. You already have rear channels with power and cooling that could be used for a subwoofer. If you believe you are competent to modify the receiver and squeeze more inside, then that is your choice.

Frankly, I don't think it makes sense to mess with receivers when the flexible plate amplifiers just screw to any subwoofer box, contain appropriate crossover and level controls, auto power-up, have a wide choice of power suiting 4 ohm loads and are very simple to fit and get operational. I cannot see your location but I'll post a link to a US vendor to illustrate the choices. There are world-wide agencies for this type of product.
Subwoofer Plate Amplifiers in the Speaker Components Department at Parts Express | 332
hi, the pin soldering are all ok.can be a audio processor problem?what you think?
I want to build 2 case with 2 speaker each so i need 4 ch and 1 sub.
this is my idea and i need the c-ch or sub-ch
but without spending more money.

ps. did you look the picture for the connection that i have in mind???
regards from Albania
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Honestly, I can't follow what you are planning there. The schematic is too small to follow, even on my large screen.

As I understand your last post, you want to use front and rear channels to bi-amp your 2-way speakers. That occupies 4 channels but you think your centre channel is dead so you have no spare channel to drive a straight subwoofer? If you think you can fit another amplifier within the case as in your opening post, you still have to buy that or the parts at least, so I can't see much savings there.

I think you should test the centre channel with a signal direct to the input you heard noise from previously and test, not guess or presume something is wrong with the centre channel. Let us know the status when you have determined if it works or not

This is easy to do by simply feeding the centre pin of your signal source to that point where you heard sound and ground the input outer ring where the RCA audio inputs plug in. It will be at full volume so don't fit speakers until you add a couple of resistors to attenuate the signal.
Place a 10-50k Resistor in series to your amplifier and add 1-5k from the signal lead end of this to ground. Keep the ratio of the resistors the same i.e. 10:1 or even more. This cuts the volume level significantly. If you still hear no signal even though you hear sound by touching the centre channel's amplifier input, you have a problem with the source, not the amplifier channel.

We cannot repair your amplifier if it is broken. That is down to your abilities and understanding of how it operates and how logical tests can identify faults, if they exist. If you don't have the parts or basic equipment to test even, there is not much we can suggest to help. I do add a caution that you should not hack into the circuit just because you think it will work again after you do this. Keep it functional and sort any problem first.
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