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sony STR-D965 blown R channel

Ok the front panel was dead, had 2 fuses blown, and the MN2488 and MP1620 was shorted in the right channel. the rest was fine.

Well, i replaced both fuses, replaced both outputs and the 2 10ohm base resistors. I checked every other resistors and found things to be normal. i replaced a high ESR 47uf cap also.

there was a small PNP transistor near the 2 outputs that was shorted also, replaced that.

The set now turns on and works, except as soon as I hook a speaker up to the right channel output, it goes into protector. i checked and double checked the resistors in the output circuit. If i leave the speaker unhooked, its fine. there is NO DC offset according to me meter on the right channel. if i set the meter on AC, I can see the AC signal on the meter, and on my scope just normal. if i hook the speaker up to the right channel, it fires protector mode. the other channels work fine.

any ideas?

Reply to
Mike
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I removed the protector current detector from the right channel, and if i scope it, the top half the wave is clipped off. meaning no drive to NPN?

and all resistor paths to the NPN is fine, so im suspecting the STK3102 driver is fuxified.

Reply to
Mike

as soon as i unhook the speaker, the wave is fine. any ideas???

Reply to
Mike

well, the output of that channel is normal with no load, HOWEVER the base drive signal to both transistors is clipped on the bottom half. results in a top half clip with a load. :(

Reply to
Mike

With no load on the output stage, the output transistors will not be required to pass any current, other than the few mA idle current. This in turn, means that the output transistor bases are demanding no current from the driver stage, again other than the very small current required to drive the idle current through them. However, once there is a load, and signal, the outputs will demand current from the drivers, and if they are unable to supply this in a symmetrical fashion, as your scope checks would suggest, then the output stage will shift its midpoint towards one side, resulting in effectively, a DC offset that the protection circuit will detect as such, causing the shutdown that you are experiencing.

The driver stages within the STK will likely be very similar in topology to the discrete output stages, employing an NPN / PNP transistor pair. If you are seeing one side of your drive waveform clipped, then it is likely that either the NPN or the PNP transistor is failing to supply the current being demanded by the output transistor that it's driving. This will be because either the transistor has failed in some way, or some internal resistor has failed, either of which has likely been caused by the original output transistor failure. I would suspect the STK first, closely followed by any external bias resistors.

Arfa

Reply to
Arfa Daily

it has to be the STK.

because the output looks normal on no-load condition, however, the base drive to both transistors is clipped on the bottom of the waveform.

load and no load. its clipped. the other channel is fine signal.

So, the chip most likely. because all the resistors and capacitors test fine. same with the 2 current limiting/bias transistors. it also tests fine.

STK is last.

Reply to
Mike

One per channel? Swap to verify....

jak

Reply to
jakdedert

I suspect you've missed a bad resistor...

Mark Z.

Reply to
Mark D. Zacharias

well according to how its wired, the right front, and center, is run by the STK3102.

the left channel which works fine is run by an STK350-230.

Well I decided to test the center channel, and is also distorted, and that channel wasnt touched. so the STK has to be the problem.

Reply to
Mike

replacing the STK fixed the issue. :)

Reply to
Mike

Glad to hear it. Thanks for posting your end result. It's always nice when someone actually bothers, and helps to make the time spent by everyone venturing help and advice, seem worthwhile.

Arfa

Reply to
Arfa Daily

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