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Old 18th September 2010, 12:44 PM   #1
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Default No sound from Sony STR-K850P, HELP!

Hello All,

I've tried to no avail with other forums, and I found this forum looks to be the bes for real information regarding problems, so here's mine.

I purchased a Sony STR-K850P AV receiver on E-Bay, appears it "may" have been damaged in shipping. Seller stated it was working before shipping, but I cannot get any sound out of it, from any speaker connection or input. Below is a list of items looked at, and reviewed so far:

No Sound from any input, through any speaker connection (tested w/ 8 ohm speakers). Tuner will not “Auto Tune”, even with antenna attached. Will direct tune to a station number, but no audio.
· Unit powers on, display lights up, react to button inputs
· Unit does NOT “heat up” when left on
· Unit responds to remote control commands
· Checked main fuse (125V, 6A), fuse functional
· Relay on standby power circuit board “clicks” once when powered on
· Both power circuit board connections seem to be functional (receiver did not power up when disconnected connectors)
· Unit does detect input signal (i.e. optical vs analog, 5.1 vs stereo)
· Unit does NOT show “PROTECT” message
· Have NOT tried headphone jack for sound output (no headphones w/ Ľ” plug)
· “Sleep” function works: unit will power off
· Have tried “reset & Memory clear” per Sony manual-No effect
· Very slight hum from power transformer when powered on.
· Nothing internal visibly broken, not connected.
· Was told that unit was fully operational before shipping. Received with dent above volume control. Internally looks like nothing is damaged w/volume control.
I've read that a likely cause is the thermal fuse may be blown, but wouldn't that prevent the unit from powering on at all?

Any help is appreciated, and please excuse me that I am really a novice. I am merely trying to check everything, and decide if a trip to the repair shop is worth it.

Thanks so much for any help or direction.
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Old 18th September 2010, 09:30 PM   #2
BrianVG is offline BrianVG  Belgium
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It won't be easy to find the problem and even worse solve it. Repairing such commercial devices yourself is not easy because it uses probably SMD. Do you have warranty? First of all try to use that if you can.
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Old 19th September 2010, 09:19 AM   #3
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Try this: connect an amp (+ speakers) to the tape rec out, connect any source device to let's say CD input. Now you should be able to select the source with the Sony.

If you have no sound there could be something wrong with the input selector or it's power supply. If you have sound I would definately check the volume pot, it might have been internnally broken, you can check it with a multimeter.
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Old 19th September 2010, 11:46 AM   #4
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Originally Posted by BrianVG View Post
It won't be easy to find the problem and even worse solve it. Repairing such commercial devices yourself is not easy because it uses probably SMD. Do you have warranty? First of all try to use that if you can.
Thank you,

I do not have a warranty on this unit, and yes I am trying to avoid expensive repairs.

Thanks again.
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Old 19th September 2010, 11:49 AM   #5
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Quote:
Originally Posted by boyofwax View Post
Try this: connect an amp (+ speakers) to the tape rec out, connect any source device to let's say CD input. Now you should be able to select the source with the Sony.

If you have no sound there could be something wrong with the input selector or it's power supply. If you have sound I would definately check the volume pot, it might have been internnally broken, you can check it with a multimeter.
Thanks boyof wax,

I did not try any pass through for either audio or video, and I was wondering about that.

I have equipment that I can set up to try your suggestion, so I'll give it a try. I'll post my results.

Thanks!
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Old 19th September 2010, 02:39 PM   #6
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Quote:
Originally Posted by boyofwax View Post
Try this: connect an amp (+ speakers) to the tape rec out, connect any source device to let's say CD input. Now you should be able to select the source with the Sony.

If you have no sound there could be something wrong with the input selector or it's power supply. If you have sound I would definately check the volume pot, it might have been internnally broken, you can check it with a multimeter.
Well, it looks like we're narrowing it down to the input selector or the power supply.

I tried your suggestion of connecting an amp/speakers, no sound at all. I tried it across various inputs og both MD/Tape amd video in/out. I did verify all of the components were functional.

I'm thinking there is something in the power supply; the unit does not even "warm" when left on. My other Sony receivers tend to get very warm even after 5-10 minutes. The heat sink is ice cold, and the power transformer isbasically at room temperature.

One last question; do you think a power supply repair would be a costly one? I pretty much have nothing into this receiver now, so I hope to avoid "good money after bad".

Thanks again!
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Old 19th September 2010, 03:09 PM   #7
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Hey bizwiz, if you suspect the power supply('s) here are some ideas to check em out!

For the amplifier's power stage you can put a voltmeter across the big power supply caps. For input circuitry specifically I would recommend you'd google the partnumbers of the IC's and measure the voltage across the supply pins. Voltage and pinout are listed in the datasheet's.

Is this amp discrete or does it have power opamps?? Power opamps usually have a mute or standby pin, there might be a fault controlling it. If it uses power opamps you can also check the pin-status from the datasheet.

Any repair you can DIY is very cheap, except for the transformer if it was broken. If you'd bring it to a repair shop you'd be charged for the hours of labour. Not to mention that most repair's involve PCB swap.
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Old 19th September 2010, 09:46 PM   #8
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Quote:
Originally Posted by boyofwax View Post
Hey bizwiz, if you suspect the power supply('s) here are some ideas to check em out!

For the amplifier's power stage you can put a voltmeter across the big power supply caps. For input circuitry specifically I would recommend you'd google the partnumbers of the IC's and measure the voltage across the supply pins. Voltage and pinout are listed in the datasheet's.

Is this amp discrete or does it have power opamps?? Power opamps usually have a mute or standby pin, there might be a fault controlling it. If it uses power opamps you can also check the pin-status from the datasheet.

Any repair you can DIY is very cheap, except for the transformer if it was broken. If you'd bring it to a repair shop you'd be charged for the hours of labour. Not to mention that most repair's involve PCB swap.
Okay,

This is where I admit my complete ignorance!

Exactly what are the "big power supply caps"? Where would they be located?

Are the "I/C"s the black rectangular items which have part numbers on them?

I am not sure if this amp uses discrete or power opamps. I think I know what you mean by these.

Thanks again for your help, you're the best I've found so far. At this point it is more of the challenge of finding the failure cause, and perhaps the adventure of a DIY repair. I must add this is a long way from the crystal radio I built as a kid!
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Old 20th September 2010, 05:33 AM   #9
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The caps look like two big cans on the board and say something like 4700µF or 10000µF on them.

A transistor look like a small black blob with 3 legs or like the large ones on the heatsink. If the heatsinked items have more than 3 legs, they can be power opamps.

An ic look like a black rectangular box with 8 or more legs.
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Old 20th September 2010, 08:28 PM   #10
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Originally Posted by Tekko View Post
The caps look like two big cans on the board and say something like 4700µF or 10000µF on them.

A transistor look like a small black blob with 3 legs or like the large ones on the heatsink. If the heatsinked items have more than 3 legs, they can be power opamps.

An ic look like a black rectangular box with 8 or more legs.
Thanks,

Though now I really feel dumb, I looked at "caps" again and realized what that meant! Wow, what a 'duh' moment.

I do have another question re: power supply root cause; during some more review, I disconnected the connector to the speaker selector switch, and of course the display went out. But since the power comes from the amplifier board, does this mean the amp is getting power?

Thanks again for your patience and help.
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