power board busted, trying to identify two diodes. - Page 1
Forum section: electronics / beginners

author: ewcc
author_rank: Contributor
timestamp: August 10, 2020, 05:00:24 am
content: Hi, First time poster here though I've been following the EEVblog's YT channel.I am a fledgling electronics hobbyist.I'm attempting to fix a TV power board which had no standby power and noticed that I was getting 0 v on the 12 V rail that went to the digital board. (I also checked that there was around 2V AC.. so I'm hoping it didn't blow the digital board..)Here's what I did.1) I opened up the back of the LED tv and the first thing that I noticed was bad was a ceramic capacitor. I replaced that.2) The STBY5v still wasn't available, I went checked the power board and noticed the bridge rectifier was also busted. Replaced that as well.3) But still no STBY5v.I went online and downloaded the schematics for the power board and poked around with my multimeter.I came across a weirdness that lead me to believe that two diodes were busted/shorted. I tested the two diodes in circuit using diode mode. One orientation gave OL. The other orientation gave me a beep. Weird. So I took out the two diodes from the circuit and tested them out. Same results.Unfortunately, I couldn't identify the SMD diodes. I've searched online and haven't quite come up with what it is. I've used the schematic's info and it just points back to the schematic. I've looked at  and am unsure if I'm looking at it correctly. I have a bunch of SMD components that I 'think' are diodes at home that I salvaged from other items. I'd like to replace these busted diodes but unfortunately, the diodes I have I haven't fully identified yet; but at least some of them have some sort of identification.Here are the diodes in circuit.Here's the schematic part.Given all that, can someone point out how I can easily identify these diodes? The closest info page was ; but looking at the schematic, they are normal diodes (with the normal diode schematic symbol) and not schottky or zener diodes but it says in , the B list contains mostly zener and schottky diode info. So I'm guessing I'm looking at this wrong.Any help appreciated and thanks!ed

author: bob91343
author_rank: Super Contributor
timestamp: August 10, 2020, 05:16:22 am
content: Why do you want to replace the diodes? It seems they tested good, conducting one way but not the other.

author: Dabbot
author_rank: Regular Contributor
timestamp: August 10, 2020, 05:21:54 am
content: https://www.digikey.com.au/products/en?keywords=SJPB-L4Edit: Additionally, refer to the Marking Diagram section in the datasheet:https://www.semicon.sanken-ele.co.jp/sk_content/sjpb-l4_ds_en.pdf

author: ewcc
author_rank: Contributor
timestamp: August 10, 2020, 06:34:15 am
content: Quote from: bob91343 on August 10, 2020, 05:16:22 amTested good? I tested them and they didn't conduct one way (that's good); but they both beeped while giving me a 0.173.That said, I've tested through-hole diodes and they don't do that; but, I'm not familiar with smd components so smd diodes could react differently.So these diodes are good?Ed

author: ewcc
author_rank: Contributor
timestamp: August 10, 2020, 06:36:41 am
content: Quote from: Dabbot on August 10, 2020, 05:21:54 amI am an idiot. Thanks for that link.Ed

author: magic
author_rank: Super Contributor
timestamp: August 10, 2020, 06:40:24 am
content: If 0.173V is voltage drop not resistance they may simply be Schottky diodes and it doesn't necessarily mean that they are bad.

author: ewcc
author_rank: Contributor
timestamp: August 10, 2020, 06:54:40 am
content: Quote from: magic on August 10, 2020, 06:40:24 amOh, then why would the schematic show a regular diode symbol rather than use a Schottky diode symbol?Ed

author: magic
author_rank: Super Contributor
timestamp: August 10, 2020, 06:59:23 am
content: Dunno, but so what.

author: Dabbot
author_rank: Regular Contributor
timestamp: August 10, 2020, 07:01:56 am
content: Quote from: ewcc on August 10, 2020, 06:54:40 amThat's a good question for the author of the schematic.Maybe the circuit will work fine with either type.Maybe they didn't have a schottky diode symbol handy (unlikely).Either way, a schottky diode has been specified and installed (by part number), so you can take that over the absence of the appropriate symbol.

author: magic
author_rank: Super Contributor
timestamp: August 10, 2020, 07:04:56 am
content: What's the voltage that those diodes work with? If it's <40V then there is barely any chance of mistake.If you hve the diodes out of the circuit, apply some 20~30V in reverse to them and see if they blow up.