EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: davew_nz on January 28, 2019, 06:32:28 am
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Hi All
Now below is example of Duck Debugging, ( https://en.wikipedia.org/wiki/Rubber_duck_debugging (https://en.wikipedia.org/wiki/Rubber_duck_debugging) ), the very act of writing this post to the forum gave me the answer, you will see the tone change as I typed, I thought it was worth sending anyway
As below;
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I have a Philips DCM230 that is pushing 10 years old that has given up the ghost a few weeks ago. I can throw it away but instead I thought I would give repairing it a crack.
The fault started as the display starting to wash out over a period of a few weeks before it got to the point where it would not power on.
I have located the Service Manual ( https://www.manualslib.com/manual/1184542/Philips-Dcm230.html (https://www.manualslib.com/manual/1184542/Philips-Dcm230.html) ).
My first thought was that the issue relates to the supply voltage. The external PSU provides 14.97v DC as required.
There are a couple of Caps for power smoothing (I think) so I pulled those and the 6600uf one was showing low so I ordered new ones and replaced them, no change.
I tested the 7806 power regulator and the "Q3" P channel MOSFET (SPD18P06P) and both appear ok. The MOSFET is showing 14.8v on the Gate & Source so the trigger to turn off the is still powered up.
Now I was running out of ideas, I have had this on my desk for weeks looking back at me.
On the MCU there is a +5v and +7v line and plus an ECO & POWER-CTL lines. There doesn't appear to be power to the MCU so I traced back the MCU 5V line to Q11 voltage regulator and saw that was only getting 0.5v, the next component up the line was a 1/2watt resistor (R61) and this is where the penny dropped. R61 connected directly back to the Mosfet source, so it should have been showing 14.8v, it was showing 0.5v.
There are no other components only a direct trace on the PCB between them, could that be the problem? The trace was covered in old glue that had turned a yukky brown colour and I remember seeing a YouTube video (Mr Carson's Lab) about this glue breaking down over time.
I ran a wire from R61 directly back to the 14.8v source, connected it up and powered it on.... IT LIVES ! That was the problem.
Thanks for providing a location where I can ask questions, sometimes the act of just thinking through the problem as you write it is all that is needed.
Regards
Dave