Controller's displays failing to turn on

Discussion in 'Embedded Systems and Microcontrollers' started by Leicester, May 28, 2016.

  1. Leicester

    Thread Starter New Member

    May 13, 2016
    Hi folks!

    I was trying to solve a problem with power supply of my controller (TORO DDC-4) when suddenly the displays turned off. Then I made some simple measurements with my digital multimeter, starting from an area which earlier then this last displays' issue already seemed a bit burnt (between the orange SMD diodes ZD1 and D10).
    In the diode test mode I read these values (leads in both directions):
    ZD1: 1 - 850
    D10: 650 - 1
    D11: 647 - 1
    The other orange diodes (to their left), some of which hidden by the upper display, have similar values then those of D10 and D11. The only with a higher value is the zener (ZD1). I don't know if this can be considered an anomaly.
    The other bigger diodes (D9, D12 and D21) showed these values:
    D9: 623 - 1
    D12: 602 - 1
    D21: 604 - 1
    At a glance all would seem regular.

    So I measured the voltage at diodes' poles:
    ZD1: -2.24 (both poles)
    D10: -2.10, 1.83 (cathode close to D11)
    D11: 2.40, 1.82
    D9: -1.84, -2.03
    D21: -2.19 (anode, successive to the zener in the circuit), -1.75 (cathode)

    It could sound a bit strange but when I measured the voltage at D21 cathode I heard a hissing noise. The same thing happened to the successive poles of the components after D21 cathode (Q21 base and C21 upper pole). To rule out a possible D21-Q21 issue I replaced both of them. Apropos of replacing components, I even replaced all the other transistors. After this action, I haven't heard any hissing noise anymore (maybe the new components are more efficient). Within the replaced transistors there was Q4, at whose collector there was no voltage. With the new one there is no voltage as well (perhaps this could depend on the malfunction of the central unit).

    Probably, a short circuit or an overvoltage has damaged this PCB. I'm quite sure that this problem comes from the relay: I measured 12V arriving there after AC entry and a short circuit in the relay or its malfunction could have sent these 12V throughout the circuit, thus damaging the components which cannot resist to that voltage.
    And I'm afraid that within these components there's the central unit.

    Now, to rule out various PCB components, I decided to replace the relay, the electrolytic capacitors, the internal lithium battery, 2 resistors (over than the above mentioned transistors).
    Original components are still the varistor (already checked), the keys, the SMD capacitors and resistors, the central unit (U1), the crystal (Y1), the inductors (L1 and L2), all the diodes (except for D21), the displays and a SOT23-5 (at least I guess: its code is C2ZD).

    My attention is now focused on that SOT23-5 (U2 in the circuit, 2nd image, top center, focus on the last image). It should be a voltage regulator, am I wrong?
    How can I measure its efficiency with a multimeter possibly without desoldering it?
    Would it be possible to replace it with a non SMD transistor?
    Measuring voltage I've read these values (from the left to the right): -2.11; 1.70; 0.85.

    And finally, would it be possible to check the central unit (U1)? Should it be desoldered? It's a kind of little black dome outside with no readable code. Do you think it could be easily replaced?
    I forgot to say that there are no electronic schematics available at the moment.