Bad power supply? - Page 1
Forum section: electronics / beginners

author: gaddy996
author_rank: Contributor
timestamp: February 03, 2014, 08:21:10 am
content: Hi, i have TLC L32m61AZ tv that wont turn, ive taken out the power supply, and im reading +20.21v on the +24v rail. Could this be the problem? If so what would be causing it?

author: Rigby
author_rank: Super Contributor
timestamp: February 03, 2014, 01:32:01 pm
content: Could be.Test all the electrolytic caps after looking for obvious signs of destruction. If you have no way to test, power it on for a while and see if anything gets crazy hot. Let us know what you find.

author: electronics man
author_rank: Frequent Contributor
timestamp: February 03, 2014, 05:09:24 pm
content: Do you have an esr meter if so its the esr of the caps you are looking for and not the capacitance. And also test the resistance between +24v and ground

author: valentinc
author_rank: Regular Contributor
timestamp: February 03, 2014, 09:16:06 pm
content: If you have an oscilloscope check the ripple on the +24V rail... If it's more than ~ 100 mVpp then certainly it's a faulty capacitor on that rail...Also, something that happened to me in the past with a power supply, was the voltage reference for the feedback circuit... Which wasn't working correctly, and that made a 12V power supply to output about 10V or so... Also check that...

author: gaddy996
author_rank: Contributor
timestamp: February 08, 2014, 07:18:51 am
content: Sorry for the late reply, i have test the rails with and without load, this is what im getting. And sorry there is no sign of any bad caps. Sorry i don't have an oscilloscopeWithout Load12v Rail = 13.4424v Rail = 20.36With Load12v Rail = 10.9524v Rail =25.6Are these results normal, i usually though voltage goes down with load?

author: electronics man
author_rank: Frequent Contributor
timestamp: February 08, 2014, 10:50:04 am
content: They don't look good

author: valentinc
author_rank: Regular Contributor
timestamp: February 08, 2014, 01:42:16 pm
content: No, they are not normal... It's normal for an unregulated power supply that the voltages drops with load... But most switching power supplies are regulated... (your's sure is, because the load of the circuits of the TV is not constant) Either by sensing the current in the primary of the transformer, either the voltage in the secondary, or both in many cases... Try to check the voltage reference in the secondary side, see if it's within specs, and that it's stable (regarding of the load current)... If there is no sign of bad caps, most probably they are ok...There are many possible causes like: the voltage reference, the optocoupler, the switching transistor... What IC does it use ?

author: gaddy996
author_rank: Contributor
timestamp: February 08, 2014, 10:44:30 pm
content: I think i have located the voltage reference IC in the power supply. Its a VIPer22A, is this the voltage reference IC? How can i check that is working?Thanks

author: valentinc
author_rank: Regular Contributor
timestamp: February 08, 2014, 11:50:31 pm
content: The VIPer22A is the control IC, with integrated switch... Measure the voltage at pin3 (feedback)... It should be between 0V and 1V and it should be steady with a constant load...

author: gaddy996
author_rank: Contributor
timestamp: February 09, 2014, 12:03:15 am
content: Yeap im getting 0.6v DC, on that pin. Does that mean its fine? What should i try next?Thanks Valentinc

author: valentinc
author_rank: Regular Contributor
timestamp: February 09, 2014, 12:11:01 am
content: Does the decimals after 6 vary ? If yes, by how much ?

author: gaddy996
author_rank: Contributor
timestamp: February 09, 2014, 12:29:55 am
content: Its starts at about 0.605v without load. Once tv turned on (with load), it goes down to 0.456v and plays around between there and 0.725v. Sorry i didnt check it properly the first time.Thanks

author: valentinc
author_rank: Regular Contributor
timestamp: February 09, 2014, 01:23:02 am
content: It varies quite significantly... Try to locate the voltage reference in the secondary side... Behind the transformer is an optocoupler, the voltage reference is connected to the "secondary" part of the optocoupler, the led to be exact... And then measure that...

author: gaddy996
author_rank: Contributor
timestamp: February 09, 2014, 01:53:16 am
content: I think i have located the voltage reference ic on the bottom side of the power supply. Could you verify which pins i need to test against?The picture shows the ic in red and the optocouplers in blue.Sorry i cant read the writing in the ic at all

author: gaddy996
author_rank: Contributor
timestamp: February 09, 2014, 01:53:56 am
content: That is the only ic on the secondary side btw

author: valentinc
author_rank: Regular Contributor
timestamp: February 09, 2014, 02:22:15 am
content: The IC must be a TSM101... The output of the reference is at pin1 (with respect to pin4, which is ground)... It must be 1.24V

author: gaddy996
author_rank: Contributor
timestamp: February 09, 2014, 02:43:30 am
content: These are the readings im getting:Pin 1 = 0vPin 2 = 0vPin 3 = 0vPin 4 = 0vPin 5 = 4.97vPin 6 = 4.97vPin 7 = 0vPin 8 = 2.456vDoes that mean this IC is dead or blown?Thanks

author: valentinc
author_rank: Regular Contributor
timestamp: February 09, 2014, 02:47:15 am
content: Not necessarily... It may mean that it's not a TSM101 chip... TSM101 pin8 is Vcc, which should be a lot higher than 2.4V... Can you try to use some alcohol to clean the chip and see the markings ? Or is is scratched ?

author: gaddy996
author_rank: Contributor
timestamp: February 09, 2014, 02:54:37 am
content: Sorry i cant read it at all, the writing on the chip is really faint and unreadable at all. You can see very faint writing but its definitely unreadable. Is there any other way i could i identify this chip?

author: valentinc
author_rank: Regular Contributor
timestamp: February 09, 2014, 03:03:29 am
content: I looked up for the TL431, a common voltage reference chip used in switching power supplies, it comes also in a SOP8 package, but the pinout does not correspond to what you have measured... Of course it can be identified... Try to google SOP8 voltage reference, but there are a ton of them... And not all are intended for this purspose... There are also precision voltage references in this type of package... If I find out someting, I'll let you know...

author: gaddy996
author_rank: Contributor
timestamp: February 09, 2014, 03:04:46 am
content: Alright thanks heaps valentinc.Im just really keen on repairing this power supply