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William Buchholz
 
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Default Sony STR-D865 Receiver "Protection" mode

I am working on a Sony receiver model STR-D865. When I turn it on the
relay will click and after about 10 seconds it displays "Protection"
and then stops responding except for the power button. This happens
with or without speakers connected. I checked all the output
transistors and they test OK. I also have tested all of the tiny
transistors on the main amp board. The 5V line tests OK. The output
transistors have either +38V or -38V volts applied to them. I looked
up the problem in the Google archives and found that the driver IC may
be bad. I removed IC671 and IC601 both marked 3875. However when I
powered it on again the protection message still comes up. Before
replacing the driver ICs I want to know of the absence of the driver
ICs would also cause a protection error or am I missing something
else? Any help is appreciated. Thanks
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Mark
 
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Default Sony STR-D865 Receiver "Protection" mode

William:

Yes, if you remove the driver IC the feedback loop around the output
amplifier is broken, and there is no guarantee that the output will go to 0
VDC like it should. However, there are a couple of more things that you can
check before concluding that you have a bad driver IC.

Usually, if a driver IC fails it will cause an output transistor to also
fail, in my experience. So chances are that the drivers are good, or you
would have noticed a failed output device.

In most Sony amplifiers, if an output transistor ever fails it will also
take out the 180-ohm resistor connected to its base terminal. So I would
check all of the base resistors on the output transistors. If one of them is
open then the output stage can go to one of the two supply rails and cause
the protection circuit to activate. Sometimes the 0.22-ohm emitter resistors
can open, so check those also.

An easy way to narrow in on the problem is to check the voltages on the
three pins of the large white 0.22-ohm emitter resistors while the amplifier
is powered on. Of course, you will have to reinstall the driver ICs to do
this. They should all be close to zero volts. If you see any that are
greater than about 0.1 V, then turn off power and check the resistor, the
output devices and the base resistor in that area.

Good luck,

Mark


"William Buchholz" wrote in message
om...
I am working on a Sony receiver model STR-D865. When I turn it on the
relay will click and after about 10 seconds it displays "Protection"
and then stops responding except for the power button. This happens
with or without speakers connected. I checked all the output
transistors and they test OK. I also have tested all of the tiny
transistors on the main amp board. The 5V line tests OK. The output
transistors have either +38V or -38V volts applied to them. I looked
up the problem in the Google archives and found that the driver IC may
be bad. I removed IC671 and IC601 both marked 3875. However when I
powered it on again the protection message still comes up. Before
replacing the driver ICs I want to know of the absence of the driver
ICs would also cause a protection error or am I missing something
else? Any help is appreciated. Thanks



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