Help repair Harman Kardon AVR 1710 input buttons

[joystik]

Hi fellow EE enthusiasts.

A friend brought me this HK Receiver because it couldnt power on, only std by amber light. So I went ahead and opened it. Some corrosion and dust. Cleaned everything tested the usual suspects in the SMPS, but everything tested fine.
I downloaded the service manual which is available at electrotanya searching for AVR 1710. After performing a MCU reset it powered up on its own so while powered on I tested all the voltages and they were on spec. So, smps is good (Phew!). However the issue persists, it cant be powered off or on with the front power button. Only way to power off is to pull the power cord. I have tracked the button signal (KEY_INPUT_INT) to the MCU pin 18 and I can see it is sending a signal to the pin (Every button push on front panel sends different voltage), however it is strange because some of the other buttons use the same pin and those work fine. Not sure where to look next.
Any help greatly appreciated.

JoY!

[coromonadalix]

if i recall, the cpu is controlled  by voltage not data signal etc...   unless you have a button key stuck,  you should see a voltage between zero and 5 volts

maybe q906 or q907 have problems, normally they act as a switch, maybe not,   check  key 1 and key 2 pins  on the main connector, you should have some voltage too

[joystik]

Hi. Thanks for your reply.

I found key1 responds with lower voltage when buttons pressed, but key 2 does not. however, i found no continuity from connector pin 20 to pin 27 on MCU.

Cant find the missing trace. Everything looks intact.
Joy.

[coromonadalix]

is the front panel is kinda  snapped into the connectors  Ie  : at 90 degree vs the main board, or its a wired connector ??

[joystik]

Its a wired connector using a copper ´flex´cable. like in the pic.

I traced the missing voltage to pin 20 on the connector at the MCU logic board. I lost it from there. The schematic says it should go straight to pin 27 of the MCU and resistor R1511, however, I was unable to trace continuity there. There is always 3.3V at pin 27, even when pressing the buttons.

[joystik]

Ok, after burning some eyesight I managed to find the broken trace in 2 different parts under the overlay. Weird. No burn marks. Fixed them.

Now buttons show what I believe is the expected change in voltage at the key1 and key2 pins on the MCU. However, its not powering up.

There has been a change though. It is no longer resetting. Its just ignoring the input signals from the buttons, even though its there.



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[joystik]

Sorry for the late update. Been busy on the day job.

Good news: the receiver is now fully operational.

After days of being stuck, I decided to desolder all chips that showed signs of corrosion before cleaning.
Cleaned all solder pads and resoldered the chips.

The device came back to life after that.

Thank you for your time and helpful advice. I can now sleep with something else on my mind.

Joy.


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