TV power supply problem. - Page 1
Forum section: electronics / repair

author: Inari
author_rank: Contributor
timestamp: November 19, 2020, 07:22:44 pm
content: Try it here one more time..I please some help to figure out one component in FSFR1700 psu circuit (comes from Thomson 55UA7706 TV)Part of this circuit which I draw is in below (I draw it because no service manual). Drawing is bad, because of only MS Paint, sorry. But all components is placed correctly, all marked caps/resistors is measured and their values is marked - maybe this may help to figure correct D404 value? This is small glass MELF zener and therefore no code to identify its value.In FSFR1700 datasheet there is no this kind of circuit application example.How I can calculate this value?And what happens when this value is not correct or lower/higher than desirable?I dont want to just "try" because I have only one new FSFR1700 ic and this takes very long time if I need to buy another.Sorry my bad language skill.Thanks for all!

author: fzabkar
author_rank: Super Contributor
timestamp: November 19, 2020, 09:23:00 pm
content: The datasheet stipulates that the voltage at the AR pin should never exceed LVCC (see absolute maximum ratings). This suggests to me that you should use a zener which clamps the voltage below LVCC. In fact LVCC is derived from a zener regulated supply, so that zener voltage should be the upper bound.

author: Inari
author_rank: Contributor
timestamp: November 19, 2020, 09:56:07 pm
content: What would be optimal?If this 2 other zeners is 18V (this values is measured Zener voltage - they are 18V), may I replace this damaged one also 18V or is better to decrease this value, for example 17V?

author: Manul
author_rank: Super Contributor
timestamp: November 19, 2020, 10:07:56 pm
content: Hello. Similar/same schematic. D404, 18V zenner.

author: Manul
author_rank: Super Contributor
timestamp: November 19, 2020, 10:14:08 pm
content: By quick looking I do not understand why they need it. It would make some sense if it would be lower voltage, then it would affect something about auto reset timing (as far as I understood the datasheet).

author: Inari
author_rank: Contributor
timestamp: November 19, 2020, 10:24:34 pm
content: Thank you!Where you get this? Do you share me full circuitry? This seems very close but some component values is different.I am also wondering why they use this zener...probably some kind of commercial fooling, dont know.TCL? What model? This Thomson which I have - there is TCL panel, virtually TCL, Thomson, Hisense and many more seems only different brand but same inside?

author: Manul
author_rank: Super Contributor
timestamp: November 19, 2020, 10:33:14 pm
content: Quote from: Inari on November 19, 2020, 10:24:34 pmI do not have it. It was just 1 minute google image search by IC. Already closed this tab.

author: fzabkar
author_rank: Super Contributor
timestamp: November 20, 2020, 07:40:13 pm
content: Attached is a Toshiba chassis with a similar circuit, but without zeners.Here are 3 TCL PSU circuits, 2 with zeners, 1 without:http://monitor.espec.ws/download.php?id=136372https://www.iwxzl.com/uploads/2018/10/TCL%E7%8E%8B%E7%89%8C01-E421C8-PWE1XG%E7%94%B5%E6%BA%90%E6%9D%BF%E5%9B%BE%E7%BA%B8%E7%94%B5%E8%B7%AF%E5%9B%BE.pdfhttps://www.iwxzl.com/uploads/2018/10/TCL%E7%8E%8B%E7%89%8C01-E421C5-PWB1XG%EF%BC%88PE421C5%EF%BC%89%E7%94%B5%E6%BA%90%E6%9D%BF%E5%9B%BE%E7%BA%B8%E7%94%B5%E8%B7%AF%E5%9B%BE.pdf

author: Inari
author_rank: Contributor
timestamp: November 22, 2020, 02:57:22 pm
content: Tried to 18V zener (BZV55C18 from DC components - only which I get fast, this is cheap also) and replaced also FSFR1700 (purchased from ebay but seems genuine), new LED stripes lit, now need to open panel and put new stripes place. At this moment seems like 18V zener is right choice. If this zener shorts, then also FSFR1700 is dead and need to be replaced. One good thing is that if this IC fails - no "big fire/bang" happen but sometimes at first glance a bit difficult to identify what parts is bad.