Sony STR-AV970X reciever, no audio output.

[David Farber]

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.

 

[Mr. Land]

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.

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.

 

[David Farber]

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.

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.

 

[Dave]

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.

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

 

[David Farber]

audio this

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.

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.