Repair of electronic control board for LED disco light. (Motor not working) - Page 1
Forum section: electronics / beginners

author: elliottveares
author_rank: Contributor
timestamp: November 08, 2014, 01:42:12 am
content: Hi all and welcome to my first post, I have posted here and not in the repair section as I am a beginner.Basically my situation is this; I have a old led disco light from the early 2000's (made in china) and a long time ago the motor stopped working although the lights still work. The light is rather simple, the led's (red, green and blue) turn change to the beat music, and the motor driving a mirror used to turn both ways with the audio as well.To day I thought I would try and find the fault due me thinking it would be simple due to the small board size and few components. Bear in mind that I only have access to a multimeter, temperature probe, and a soldering iron.So far I have determined that the motor works continuously spinning one way if I connect one wire to the inside terminal of the motors terminal block and the other wire directly to the AC input from the transformer.I so far have re-seated the socketed IC, checked for dry solder joints, blown caps, etc.... If I measure between the nearest (to the motors terminal block) pnp trasistor's emitter and inside terminal on the motors terminal block and get 13v dc.It is also to note that I am having trouble tracing out the circuit and knowing where to start.The two main IC's on the board are a EM78P153SP (Had sticker on with 3205v1-111-1) and a LM358P what is a dual op amp. The transistors (expect the main transistor on the heat sink) are of the type ST-8550DBelow are some pictures:Hope I make sense, and sorry if I have not provided enough information.Is it worth me trying to fix this?Regards: Elliott Veares

author: Paul Price
author_rank: Super Contributor
timestamp: November 08, 2014, 07:22:33 am
content: Try tracing the wires coming from the motor to the control board. Check the transistors the transistors that the motor wires connect to on the PCB.You can always find a way to make a simple circuit to control the motor yourself. Reversing the voltage to the motor might reverse motor direction.The EM78P153SP is the microcontroller that also controls both the motor and the lights and it is working at least enough to control the lights but it might be trying to send control signals to a dead transistor that connects directly to the motor. If the transistors seem to be of the same type, they all should measure the same when tested by your ohmmeter diode function between their three pins. Remove them and test them one by one. One or more will likely not match the others when measured this way.

author: SL4P
author_rank: Super Contributor
timestamp: November 08, 2014, 07:37:59 am
content: This looks like a really simple unit. I fix quite a few of the micro based lights, and the problems are about 50% power supply, 45% Assembly quality! Occasionally LEDs or other electronics dead.Attached jpg is my guess of the layout. Not sure how the LEDs are connected. ubt a good startinm gpoiunt. I'm sure you can figure out where I've missed something.

author: Vito_R
author_rank: Regular Contributor
timestamp: November 08, 2014, 07:58:09 am
content: If you don't have access to a schematic for this board then what I would do is simply a component level check on everything on that board since there's not too much on it. Definitely check all transistors and diodes for proper forward voltage drop or leakage with your diode check function on your multimeter. The caps could still be bad even though they don't visually show they are blown or leaking. I regularly replace bad caps with high esr that don't show any signs of leaking or venting and they even test properly for capacitance but are still defective. When replaced it solves the issue. You need an esr meter to do that though, or simply replace all caps especially if they are some cheap possibly fake chinese caps like CrapX oops sorry CapX. Every time I see those whether they test bad or not I simply replace them because they have proven to be unreliable from past experience.

author: Paul Price
author_rank: Super Contributor
timestamp: November 08, 2014, 08:13:06 am
content: Vito-R, there are no CapX capacitors on the board where the circuit is driving the motor that I can see, in fact, just three electrolytic caps total, two in the LED lighting circuit, and the big tall one in the power supply section. I would bet the big one is ok too, but you can easily test them with an AC voltmeter function or an oscilloscope and you would see several volts of AC across the capacitor pins if it was bad. Since the LEDS are flashing correctly and the motor is not moving, I would strongly guess the big cap is 100% ok.You don't need an ESR meter to check electrolytic capacitors, just an AC function on a digital voltmeter, and only if you have a strong suspicion that an electrolytic cap is bad by a high level of AC across it, your best troubleshooting strategy would be to use alligator clips (or just temporarily solder it across the suspect bad cap) to connect a new capacitor across the suspected bad one to see if it removes the AC voltage drop across it or else if the circuit starts working properly. Always match the polarity marks and attach the test capacitor with power off.The tell-tale sign that an electrolytic cap is bad is the flat top of the cap becomes dome-shaped or you can see leaking electrolytic fluid from the base. Most usually, a cap is a good cap if it isn't open circuited and the dome is flat, and you can rely on the fact that if there is measured a large AC voltage across it, it might be worth trying to bridge a new one across it to see if this fixes the problem. Visual inspection and a multimeter AC test best tells you that a electrolytic cap may be bad in a power supply or motor drive circuit, and it is smart, time-saving and convenient to do this even without removing it from a circuit to test it.The H-Bridge transistors are definitely the parts to test first and always look carefully for bad solder joints.Some people would advise you to replace every part on the board. If this is done correctly it usually fixes the problem, but there are drawbacks to this approach, very much time and money are uslessly wasted. I've repaired circuits for tens of years and the idea is get the job done quick, proper and to make money, not wast time, effort and money replacing parts needlessly.

author: Vito_R
author_rank: Regular Contributor
timestamp: November 08, 2014, 08:19:53 am
content: Yes I agree test the transistors first but unless they are high quality Japanese caps (which I can't determine from these pics) I still recommend to test or replace all caps because they can cause weird problems affecting the control circuits and power supply filtering. There are a few smaller electrolytic caps I can see in the picture too, a total of 6 including the large can. It's such a small board just test all components. You're sure to find your problem fast.

author: Vito_R
author_rank: Regular Contributor
timestamp: November 08, 2014, 08:52:33 am
content: Quote from: Paul Price on November 08, 2014, 08:13:06 amSorry I have to disagree with that part you wrote. An ESR meter is the best and proven to be the most reliable efficient method to test capacitors. Testing for leakage is also another method. Visual inspection and using a multimeter will not always tell you whether a capacitor is bad or not, especially if it is not only a power supply issue. ESR is the way to go, and another benefit with an ESR meter is most times you can easily check the cap in circuit without desoldering it. Everyone in the electronics servicing industry will tell you this from years of experience. I've been in the electronics repair and installation business for over 20 years.

author: Paul Price
author_rank: Super Contributor
timestamp: November 08, 2014, 09:02:10 am
content: Not everyone.

author: elliottveares
author_rank: Contributor
timestamp: November 08, 2014, 09:08:14 pm
content: I have verified that some of the ST-8550D are bad, well at least I am getting different readings when I measured the resistance of them when de-solderd. Will try again with the diode function.I also since my original post verified there is a middle layer and the outside motor terminal goes to the transistor below the one I originally thought and is part of the h-bridge (missed traced it originally.)Thanks everyone for their help so far.

author: Vito_R
author_rank: Regular Contributor
timestamp: November 08, 2014, 09:20:26 pm
content: Yes simply use the diode function to test forward voltage drop on all transistors and check for any leakage too. Once you determined that you have a bad transistor make sure you test all components biasing the transistor as it can sometimes damage other parts in the circuit too. As I mentioned before, it's such a small board I would check all components just to be on the safe side and to ensure all issues are resolved. You can also be 100% confident of the repair and the general condition of your board.

author: elliottveares
author_rank: Contributor
timestamp: November 08, 2014, 10:09:11 pm
content: Ok now I am confused?From what I can tell the ST-8550D is a PNP transistor with the pins being EBC left to right with the flat side facing you, what means I should get a forward voltage drop and thus a reading on the diode function between E to B and C to B, however I get a matching reading from B to E and B to C instead as if it was a NPN. Also testing E to C or C to E does not give me anything either on any of the ones I test.The only thing I can be sure of is two of the transistors so far give me no reading when measured E to B, B to E, C to B, B to C.Think it may be time I get some ST-8550D's and just replace them. (If I cant get hold of them, what would be an alternative.)

author: Vito_R
author_rank: Regular Contributor
timestamp: November 08, 2014, 10:30:39 pm
content: The 2 that don't give you any readings look like they are open and defective. The other transistors should simply give you a forward voltage reading usually around 0.5-0.7 volts on your meter in diode test function depending on the type of transistor you are testing. As long as you get 2 voltage readings from from both sides of the transistor. Then reverse your probes around and they should show nothing, then this confirms they are working properly and not leaky. Hope this simplifies it.

author: Vito_R
author_rank: Regular Contributor
timestamp: November 08, 2014, 10:48:40 pm
content: Depending which country you're in I saw some 8550D transistors on digikey. Or else you can order direct from China or ebay seems to have a listing for them. If not you can try to do a cross reference and match the specs of a general purpose PNP transistor with these as a last resort.