Goodday, I had just completed assembling the K8020 control valve amplifier. What I am writing here is my troubleshooting history on the K8020 when it failed to switch on. I need your advice.
Here goes: I had doubled checked all component placement and solder joints before powering on. All components values, placement and connection are correct and electrically sound.
However, the amplifer switches on and then off when power is first applied. The buttons were not depressed at any point when power is applied. I can see the relay pull on for a brief moment and then off when the power is applied. The Power/ Mute LED then stays lit and for a brief moment turns off and then comes on again. The LED would repeat this behaviour and the amplifier remain unresponsive to any button depress.
I suspected faulty T06 (BC557) that controls the mute/ output reed relay, since pin 10 of the ucontroller connects to the base of this transistor and the LED. A faulty T06 might not allow the ucontroller to function properly causing the LED to blink. However all active components were tested OK.
My attention turn to the PIC ucontroller. Pin 10 is low when powered on then will pull to high and then low (this explains why the LED blinks). The PIC does not respond to any depress of buttons. All output pins (pin 2, 6, 7, 8) are high. All input pins (pin 11, 12, 13, 17) are high (which is normal to my understanding, only when buttons are pressed should the pins be pulled low). All logic high values are 5.07V which indicate the 5V supply is good. Next, I check the RC OSC of the PIC. The values are right as depicted in the manual, so the PIC clock should be OK.
When I pull any of the output pins to low with a wire at pin 2, 6, 7, 8, the relay comes on with LD8 on (this was done without the PIC in its socket). Thus the amp circuit is good, just that the PIC is not responding to inputs. Next I tested to see if the pins are pulled low at pin 11, 12, 13, 17 when the push buttons are pressed. Yes I have 0V when pressed, so the PIC has good connections.
I thought the PIC is faulty and ordered a replacement part through SINTA ELECTRONICS in Singapore. However the new PIC gave me the same problem.
I am stumped. Please advice!