Television with no standby light, no voltages at the output of the smps transfor - Page 1
Forum section: electronics / beginners

author: robsims
author_rank: Frequent Contributor
timestamp: September 17, 2024, 12:37:03 pm
content: Hi,I have a television with no standby light. When i measure voltages at the input pins of the smps transformer i measure 179 DC volts on two pins. The other two pins are negative, so I assume the voltages on the primary pins of the transformer are good.When i measure voltages at the output pins (secondary side) of the transformer i get zero volts. If i measure between the ground pins and the other pins i get zero voltage. Is there something wrong with the switching IC on the primary side? Can someone explain why i don't get voltages on the output pins of the transformer. MAny thanks in advance

author: madires
author_rank: Super Contributor
timestamp: September 17, 2024, 01:32:54 pm
content: I guess you won't like my comment, but I'd recommend to let someone experienced in SMPSU repairs fix it.

author: robsims
author_rank: Frequent Contributor
timestamp: September 17, 2024, 02:42:53 pm
content: Madires sure i don't like you're comment. What is this blog for?. To learn or what? I'm learning myself tv repair. I can already fix backlight problems. If you don't want to help or can't help please don't comment

author: wasedadoc
author_rank: Super Contributor
timestamp: September 17, 2024, 02:49:50 pm
content: I agree with madires especially as the OP does not seem to have enough intelligence to give any details of the TV.And posting a repair question in the beginners section instead of the repair section.

author: tunk
author_rank: Super Contributor
timestamp: September 17, 2024, 02:53:06 pm
content: Photos please.I'd guess that the standby PSU has to work in order to switch it on.

author: Sorama
author_rank: Frequent Contributor
timestamp: September 17, 2024, 03:17:27 pm
content: There is a standby power supply.Try to find that one (in proximity of the standby LED).As long as the standby PS is not working, the other smps won’t start.If standby ps is not working, one can inject the same voltage by means of a bench power supply.(If there is no short).

author: robsims
author_rank: Frequent Contributor
timestamp: September 17, 2024, 04:56:58 pm
content: I added some pictures of the powersupply. It doesn't have the standby voltage. I think the 12 will power up the standby light

author: robsims
author_rank: Frequent Contributor
timestamp: September 17, 2024, 04:58:12 pm
content: The capacitors are good. Forgot to mention that i replaced the mosfet on the hot side because it was shorted, as a result of a powersurge i guess. My guess is that the chopper chip is bad, but i don't have an oscilloscope to test it

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: September 17, 2024, 07:58:06 pm
content: Magic smoke released here (on attached image), or just dust? What voltage do you measure at these bulk input caps? For the SOT23-6 IC on the primary side (IC9101), can you get a datasheet on it? If so, measure VDD/VCC and see if it's within the specified limits (probably around 9-16v if I were to guess). Also measure the voltages on those SMD caps on the primary side and see whether any are shorted or 0v. Based on the blown primary side FET (but it not blowing after replacement), I do wonder whether the faulty component has now gone open-circuit.This is a pretty simply power supply with only a single optocoupler and everything on the PCB labelled. I think it should be fixable.

author: BlownUpCapacitor
author_rank: Frequent Contributor
timestamp: September 18, 2024, 07:45:19 pm
content: I think if OP injects a positive voltage at the pin labeled on/off, OP could potentially measure the voltages then to isolate the problem of the PSU not working simply because it has not received a command to turn on yet.Quote from: LooseJunkHater on September 17, 2024, 07:58:06 pmIt does look quite suspicious too, but where could the source be? If a capacitor were to release its magic smoke, I would expect less dark residue. I don't see any other potential components of suspect.

author: robsims
author_rank: Frequent Contributor
timestamp: September 20, 2024, 01:29:33 am
content: Fixed this problem. The PWM chip was damaged. Replaced the chip and got standby light and the tv worked again like it was never broken. So madires if you read this, i'm now experienced in SMPSU repairs. No need to bring tv's to someone experienced in SMPSU repairs to fix it

author: Bryn
author_rank: Regular Contributor
timestamp: September 20, 2024, 06:55:02 am
content: Quote from: robsims on September 20, 2024, 01:29:33 amSorry to nitpick, but you can't call yourself an expert simply by fixing justonething (especially if it's the first time you've done this), at least that's what your post is suggesting...Good that you got it fixed anyway, even if it's as simple as replacing the chip, but that no way makes you a "qualified" TV repairman should you wish to go down that route in the future.I'm sure others would agree with what I said, even if you don't like it as you did for Madires' post earlier.