I’m trying to fix a home theater receiver for a neighbor. I don’t know how it happened exactly, I don’t even know how it could happen, but his son was unplugging it, and somehow shorted the hot and neutral line together on the power cord. There was a pop and now there is no action at all from the receiver. No standbys no lights, completely dead.
I have looked and looked and cannot find a service manual or a schematic, so I’m going through and seeing if I can find any bad components. I checked all the obvious like fuses and such but I am working on this standby power supply board.
All the diodes and transistors seem to be working, but I am onto testing this optocoupler. From what I understand, you can put a little bit of voltage across the internal LED and measure resistance on the other side. Using a Peak Atlas DCA75, the led side measured good with a forward voltage of about 1.15v. The other side had a forward voltage of a little over 10 volts.
I’m injection voltage to the led, + to anode and - to cathode. I’ve had resistance go down to 49 ohms, but then I’m hitting current limiting on my power supply.
I at first set the limit of current to 100 mA, but hit that limit quickly. So I raised it to 200 mA, but hit that with only 1.77 v going to the led. That seems like a lot of current draw for a led. I didn’t take it any higher for fear of damage and it was indeed getting warm.
I tried a limiting resistor of 2k (not having a schematic sucks by the way) but no luck. After the testing I measured the led side with the peak again and it still measures exactly the same.
Does the optocoupler seem to be behaving oddly? Is there a better way to test it?
Thank you,
Dan
I have looked and looked and cannot find a service manual or a schematic, so I’m going through and seeing if I can find any bad components. I checked all the obvious like fuses and such but I am working on this standby power supply board.
All the diodes and transistors seem to be working, but I am onto testing this optocoupler. From what I understand, you can put a little bit of voltage across the internal LED and measure resistance on the other side. Using a Peak Atlas DCA75, the led side measured good with a forward voltage of about 1.15v. The other side had a forward voltage of a little over 10 volts.
I’m injection voltage to the led, + to anode and - to cathode. I’ve had resistance go down to 49 ohms, but then I’m hitting current limiting on my power supply.
I at first set the limit of current to 100 mA, but hit that limit quickly. So I raised it to 200 mA, but hit that with only 1.77 v going to the led. That seems like a lot of current draw for a led. I didn’t take it any higher for fear of damage and it was indeed getting warm.
I tried a limiting resistor of 2k (not having a schematic sucks by the way) but no luck. After the testing I measured the led side with the peak again and it still measures exactly the same.
Does the optocoupler seem to be behaving oddly? Is there a better way to test it?
Thank you,
Dan
Use a circuit like this:
https://electronics.stackexchange.com/questions/444486/opto-coupler-circuit-selection-of-resitor
Also, never apply a voltage source to a diode directly. Always use a series resistor, start with 10k.
To test a PN junction (diode or transistor) use the diode test function on your DVM.
Test both forward and reverse. Test both BE and BC junctions.
https://electronics.stackexchange.com/questions/444486/opto-coupler-circuit-selection-of-resitor
Also, never apply a voltage source to a diode directly. Always use a series resistor, start with 10k.
To test a PN junction (diode or transistor) use the diode test function on your DVM.
Test both forward and reverse. Test both BE and BC junctions.
Thank you for that, I will retest using that circuit. The reason I chose a 2k current limiting resistor was because that was the only resistor I saw in series with this device, it’s a 1.8k
I did some more looking and I don’t think the optocoupler is my problem. Of course without a schematic I’m poking around pretty much blind, but the large HDMI board has something to do with the power supply. There is a small 7 wire ribbon cable that goes between the standby board and the hdmi board. Without the cable hooked up I don’t have standby voltage on the little standby/ps board.
So I took a quick look on the HDMI board and after a bit I found a damaged IC, it has a hole blown in the side of it.
It looks to be an FR9889, synchronous DC/DC converter. I am going to order the parts, unfortunately looks like I can only get it really directly from China. There is one US seller which I’m sure gets them from China, but they’re asking about 5x the average price. It would be faster, but was hoping to get at least 10 so I could have some extras in case.
One worries y I have is that this board has so much logic on it, I wonder what the potential is that some was damaged.
Still worth replacing that converter and checking the surrounding circuit. Any decent replacement for the FR9889 that might be more easily obtained? From mouser or similar?
Dan
So am I doing this right? Im guessing it’s good. I made up the circuit, using a 10 Kohm resistor in series with the anode. In the circuit example it says to use a 9v battery, but I was getting no voltage reading with the probes either way on the transistor side. But once I increased the voltage on the supply to about 10.5v I’m guessing led lit. So I had a reading.
I get zero reading if I reverse the probes on the transistor which should be correct.
You say to test both the BE and BC junctions, how do I do that if the base is internal to the device?
Dan