Philips TV PSU HV output, how close is good enough? - Page 1
Forum section: electronics / repair

author: erikbrenn
author_rank: Regular Contributor
timestamp: February 15, 2025, 06:29:33 pm
content: I have a philips TV that does some strange flickering in the first 10 minutes after turning it on. My guess was a faulty cap or a bad solder joint somewhere, so I checked all cap ESRs and all relevant resistors, all seems ok.Then I noticed that the high voltage output was only 33V, while the table on the PCB says that it should be 44voltI even checked the rails by loading them as specified, and they seem to be dead stable. As the next step I plan to reflow the BGA video processor chip, but would appreciate any hints!TV model: Philips 46PFL8008S/12

author: erikbrenn
author_rank: Regular Contributor
timestamp: February 15, 2025, 06:44:07 pm
content: Voltage output of transformer is 150 Vpp between, which is rectified and fed through the smaller transformers ..

author: benj38
author_rank: Frequent Contributor
timestamp: February 17, 2025, 09:43:00 pm
content: The title of the thread indicates that you may be wondering if 33V is "close enough" to the specified 44V. It is definitely not!Don't reball anything before resolving this first.

author: CaptDon
author_rank: Super Contributor
timestamp: February 18, 2025, 10:13:53 pm
content: Why would you start reflowing (a.k.a. screwing up) the BGA device before getting the 44 volts back to where it belongs. Do you think your 120 volt house would be happy at 90 or a 240 volt home happy at 180?? I bet a 12 volt car runs like shit at 9 volts sort of like your t.v. The only way 33 volts could be correct is possibly if the same motherboard is used in different size sets with a small set not requiring the full 44 volts but that is a long shot. Kind of like back in the days of the Philips J, K and L C.R.T. chassis for 19", 20" and 25" T.V's. It was the same chassis, a couple of value changes and radically different flyback transformers. The changes were footnotes called out in the SAMS Photofact service information.

author: erikbrenn
author_rank: Regular Contributor
timestamp: March 03, 2025, 07:33:57 pm
content: Apology for late followup.Quote from: CaptDon on February 18, 2025, 10:13:53 pmBecause I couldn't get the 44v. I tested everything on the power board, including desoldering all pwr resistors for test. I tested all the rails with my electronic load, they all performed fine (ampere wise)Quote from: CaptDon on February 18, 2025, 10:13:53 pmNo need to be sarcastic. TV is only flickering during first 10 minutes, so obviously it can work with the mentioned voltage. I should have asked more specific, was just hoping someone could share some experience or knowledge about this.Thanks anyway.

author: inse
author_rank: Super Contributor
timestamp: March 03, 2025, 07:41:36 pm
content: Did you observe any difference on the voltage in case it flickers and in case it has stabilized?It sounds like a thermal problem, you can try to narrow down the suspects with the help of a hair dryer/hot air gun and cold spray.Such behaviour is typical but not exclusive for bad electrolytic caps.

author: MathWizard
author_rank: Super Contributor
timestamp: March 03, 2025, 11:04:39 pm
content: What are the SMPS controller IC's, what do their feedback pins see ? Has anything in the feedback path drifted a lot, or are caps wearing out? Or would some opto be giving out ? Is something pulling too much current and pulling the rail down? Or can it just not 'get it it up'.I'd use the datasheets and map some of the circuit, and run it in a sim.

author: erikbrenn
author_rank: Regular Contributor
timestamp: March 04, 2025, 01:57:44 pm
content: Quote from: inse on March 03, 2025, 07:41:36 pmGood idea, will try some cold spray after it has stabilized and see if I can get et fault to show again

author: erikbrenn
author_rank: Regular Contributor
timestamp: March 04, 2025, 02:16:47 pm
content: Quote from: MathWizard on March 03, 2025, 11:04:39 pmyes, I was also thinking that circuitry around the optos and the FB circuit may have detoriated, will check.I actually found the service manual, and it contains full schematics for the main board, showing all voltage rails etc. Such manuals never cover the pwr supply unfortunately, which is often made by third party. Anyhow, will check power rails on main board also