Controller's displays failing to turn on - Page 1
Forum section: electronics / repair

author: alps
author_rank: Contributor
timestamp: May 28, 2016, 11:36:58 pm
content: 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' issuealready 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 - 850D10: 650 - 1D11: 647 - 1The 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 - 1D12: 602 - 1D21: 604 - 1At 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, 1.82D9: -1.84, -2.03D21: -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 apossible D21-Q21 issue I replaced both of them. Apropos of replacingcomponents, 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.

author: Ian.M
author_rank: Super Contributor
timestamp: May 29, 2016, 12:55:39 am
content: The MCU U1 is aCOBpackaged device. Its not replaceable so if its bad the whole board is FUBARed.

author: jitter
author_rank: Frequent Contributor
timestamp: May 29, 2016, 06:37:13 am
content: Just by looking at the photos I can see your board must be sick.The partially visible electrolytic capacitor in the first photo, top left, looks bulged. Problem 1. The hissing noise might have been coming from a venting cap. But you wrote you replaced them, so I guess that one is solved.Then the heat damage around the diodes is probably caused by R5 becoming very hot. The discolouration of the board around R5 indicates that, and the heat seems to have soaked to the other side where those diodes are. Problem 2.Problem 3: your C2ZD coded component only findsthisChinese site of an ALT81729C2TR with the right footprint. I can't access the pdf but a translation of the page indicates that this might be an LED-driver chip.To be honest, it doesn't look good. The failed caps and burned spot on the board might mean that overvoltage has been the cause of the board failing.Edit: hmm, found anotherthreadsomewhere on the internet. It also had that burned spot, so that is probably the result of the design.Your best bet: get the power supply up and running. And also make sure that all solder joints are okay and have not cracked with age and heat cycling. I would also check continuity of the tracks in the discoloured area, perhaps those are cracked, especially around vias and through holes.

author: alps
author_rank: Contributor
timestamp: May 29, 2016, 03:57:29 pm
content: Thanks for your replies!As I have ruled out issues regarding capacitors (just replaced), R5 and R51 resistors (replaced as well), I now would like to investigate the SOT23-5 efficiency (translation of a chinese webpage says that it is a power chip).Using the diode test mode, I have found that putting the leads on the first and third pin (both directions) makes my tester beep. I haven't desoldered this component yet even because I'm afraid to break it: I don't use any hot-air gun or hot air soldering station.According to you, could that beep be normal? If not, could I replace this component with a non SMD 5-pin transistor?A few time ago I remember to have successfully replaced some npn SOT-23 transistors (marked 1p) with some BBC 337-40. Would it be possible to do something like that in this situation?