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Sony STR-AV970X reciever, no audio output.

This Sony STR-AV970X powers up ok, relay clicks on, but there's no audio at the speaker outputs. I don't have a schematic so the only information I've been able to figure out is that the output transistors have no bias voltage on them and consequently there is no bias current across the emitter resistors. There are just the supply voltages coming into the collectors which are +/- 51 volts. The low voltage supply is putting out +/-12 volts and +5 volts at the regulator sources. Sitting in the middle of all this is a dual voltage amplifier, an STK3102III, which I suspect may be faulty but I was wondering if somebody could tell me where the low voltage supply enters the mix in the power amplifier section.

Thanks for your reply.

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David Farber
David Farber's Service Center
L.A., CA
Reply to
David Farber
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I think I may have worked on one or two STR-AV*'s in a former life...

Seems like 100% of the cases where the outputs were NOT bad, it was the STK* that was bad.

Not sure what you mean by "enters the mix" but the STK's typically use op amps, and I've seen cases where they go bad, and output a DC voltage which drives the outputs hard off.

Reply to
Mr. Land

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I wish there were some kind of DC voltage. The offset is zero. The driver and output transistors have no base-emitter voltage on either channel. It would seem to me that the STK-3102III would not want to output any DC to the amplifier input. I recently repaired a newer Sony receiver that had some bad digital muting transistors. That prevented the low voltage from reaching the driver stage. I was thinking something similar was going on here but without a schematic, it's tough to figure out.

Thanks for your reply.

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David Farber
David Farber's Service Center
L.A., CA
Reply to
David Farber

Does the headphone jack work?

Get the data sheet for the STK*. Test to make sure it is being powered properly. Test for output.

Low voltage PS generally powers things like logic circuits such as the microprocessor controls, LED's, displays, etc in an amp. Surround sound receivers have one or more decoder/mixer IC's which require logic-level power.

The STK3102 uses internal transistors and is rated to V(cc) +/-75VDC with a recommended operating voltage of +/-50V so you would be right on the money with +/-51VDC on your supply rails for this amp IC.

If you've got another power amp available, try taking the L & R channel outptus from the STK3102 to the power amp inputs. That will tell you at least if the STK is okay and point you towards your output transistors or associated bias resistor networks.

In my experience it's unlikely that BOTH outputs die at the same time unless someone has, say, dumped a drink into the amp or something similarly catastrophic. The STK is a part common to both channels, I'd say there's a decent likelihood it's gone bad.

Good luck

Dave

Reply to
Dave

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I replaced the STK-3102III and that corrected the problem. Pins 5,6 and

10,11 provide the needed +/- 1.1 bias voltage to the output circuitry. Previously there were 0 volts on these pins. 
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David Farber
David Farber's Service Center
L.A., CA
Reply to
David Farber

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