Arduino Duemilanove with broken power switching circuit

Development Tools IDE 1.x
1

Good day!

I've an Arduino Duemilanove board clone with power issues. I want to get my hands dirty and repair it.
Here is a schematics that I use: http://arduino.cc/en/uploads/Main/arduino-duemilanove-schematic.pdf

Issue description: When powered from USB the +5V is about 3,8V, when powered from DC jack -- I have 5V on +5V line.
That's not USB port issue, I've tried a couple of PC and standalone USB chargers.

What I've measured when powered from USB:

  • I've 5V on USB jack
  • I've 5V before and after fuse F1
  • I've 5V on T1's D pad
  • The voltage drop on diode inside T1 (measured from D to S) is about 1V
  • When powered from USB I've 2,56V on G pin of T1 (I suppose it should be low?)
  • When powered from USB I've 3.3V in Vin

Questions:

  1. Is voltage drop on T1's D-S junction in 1V a correct value? If anyone can measure the voltage drop between 2 and 3 pin of T1 on working board and post results that's will be great!

  2. When I look into schematics I assume that when board is powered from USB I should read 0V from Vin. Can anyone check on their board?

  3. What voltage is on your +5V line when powered from USB?

  4. Is there anything else I can check?

Thanks in advance!

2

Measure the voltages on the opamp (IC5, 8 pins near the end of the ATmega socket). pin 2 should be 3.3V, pin 3 should be 1/2 the Vin voltage (which if the USB is connected and the power jack is not, should be 2.4V or less.) pins 1, 5, 6 and 7 should all be low (a fraction of a volt).

The low voltage then goes to the gate of the p-channel MOSFET (which I think you are calling T1) to hold it on.

3

Here are voltage pins of LM358 when powered from USB (no DC jack):

1 - 0
2 - 3.3
3 - 1,66
4 - GND
5 - 0
6 - 2,59
7 - 2,59
8 - 4 (Vcc)

4

Thank you a lot -- I've found that some pins of LM358 was not soldered, so I resolder them and it works.