help with 1708 problems, 2 MCU board's

[spitaleet]

Hello.
Firstly, thanks for the scans, they help me a lot in maintenance!.
I have two boards 1708 here with similar problems, it is important to note that I am sure that both the problem is in the MCU board, because already replaces the power board of both ...

The first one doesn't have 1V on Lt \ RT, would U9 be a voltage regulator? It is not short, and the transistors sensors are ok.

The second does not have 3V3 on any power board button, only the "B" of the MCU has signal.

Any help?

[RDC]

C7 on the MCU board seems to fail and short, it's on the 3.3v rail, if it were bad you'd have nothing at all work on the controller.

U9 is a 3.2v Regulator, TLV70732 You could try to reflow it and see if anything changes.

Checking it with a DMM it should measure roughly..

1 - ~1v
2 - GND
3 - ~0.25v
4 - 3.3v


That other controller is odd, because the 3.3v you would measure on the button lines means there is an issue with U4, it's a TCA6416RTWR, but you said you've tried different boards, so you may have more than one board with issues there. The 3.3v that U4 uses is the 3.3v rail, and if it were dead nothing at all would work on the controller. U4 should have 3.3v (power rail) on pins 18, 21 and 23. If it does, but still has no 3.3v on the button lines, I'd suspect U4 is bad.

[spitaleet]

Thanks for the help!

In the first, C7 not short, you know the value of C7?, on TLV70732  i have:

1 - 0v
2 - GND
3 - 0v
4 - 3.3v

Reflow not work, probably i need replace the U9? i can use a u9 from a 1697 board?


In the second,
I have another identical board tested, with the functional u4, when connecting the defective pcb MCU, 3V3 does not appear ...

I realized that if I just turn on only the pcb power, u4 doesn't release 3v3 for the buttons, would a signal from "up" be needed for u4?

[RDC]

C7 is 33uF, 6.3v or more.

If U9 has no voltage on pin 3 (Enable) then it will have no voltage on pin 1 (Output) so U9 could be bad, or there could be a problem with R38 (0ohm) or R40 (100k) If both Resistors are fine you can try removing U9 first and check voltages again, the ~0.25v should be there. If it is not, then the MCU is not working correctly. U9 is the same on all boards so far.


U4 is an IO Expander, so it needs to communicate with the MCU board. You will need a scope or Logic Analyzer to see if the I2C bus is working. About all you can check there without a scope or LA is R41 (0ohm) that is in the SDA line. Also check R4 (0hm) If either of those were open or missing, or the traces damaged, U4 would not work.

[spitaleet]

R38 and r40 ok.
Without U9, same results...,without enable signal, is no use change u9 correct?


R4 and R41 ok too, changed another pcb for check U4, same problem, probably a dead end   

[RDC]

No enable signal is probably a bad trace or a bad MCU.

You'll need a scope or LA to see what is or is not working with U4.

[spitaleet]

I don't have an oscilloscope right now ...
Probably the customer will not wait much longer, I will have to return them (incredibly both are from the same customer!).
Even so, I appreciate the knowledge transmitted and i hope can contribute  with something in the future!