PCB cleaning is easier than repair - Page 1
Forum section: electronics / repair

author: rhb
author_rank: Super Contributor
timestamp: January 07, 2025, 12:19:10 am
content: I've been using a 2005 production Samsung 204T in portrait mode since new. Lately it started failing to come on. It would flash briefly and go black until power cycled. After lots of tries it would eventually work with no issues of any kind. Only a problem if power cycled. Classic symptoms.I took it apart and photographed it to send to @tom66 for advice. He kindly sent me the datasheet for the chip. While I had it apart I sprayed IPA on the PCB, scrubbed with an old toothbrush, blotted with paper towel and repeated. I then set up to monitor the chip at power on with my scope and a DIP clip. Critical level on the OVP pin is 2 V and I was seeing 220 mV. So far as I can tell it is "fixed" simply by cleaning the PCB and reseating the connectors! I could see almost no residue on the PCB, but did scrub away a few blotchy spots. The underside appears to have a conformal coating in addition to the plastic HV shield. The wash did make it look different, but not the way removing flux does.If it's actually failing it should do it soon. If it's a "Clean me!" that's important. Non-smoker, so nothing but normal household aerosols. But not far from the dryer.This would seem to have been dust that stuck to condensation in a humid climate over the course of almost 20 years. Blows my mind that would disturb the operation.This makes an absurd amount of stuff I've "repaired" just by cleaning the PCB! It's crazy!!! Everything from consumer electronics to top tier T&M kit.It's rather like reseating connectors. Electronics apparently also likes a bath from time to time in addition to a stroke or two ;-)Have Fun!Reg

author: Shock
author_rank: Super Contributor
timestamp: January 08, 2025, 03:27:39 am
content: Tin whiskers are a thing as well. I've done many repairs by just cleaning, resetting defaults, unstucking a button, enabling the output, replacing or unplugging the battery. Yes connectors oxidize.

author: rhb
author_rank: Super Contributor
timestamp: January 08, 2025, 02:15:25 pm
content: This has been a rather intense discussion with @tom66. The tube is about 500k Ohms, so contact resistance is too low to affect the ignition. That leaves surface leakage paths. The monitor has been about 10 ft from the dryer in a pantry turned office for 16 years. PSU is marked 750 V @ 1.5 mA. Even a megohm or two leakage path would result in an over current situation. Nothing obvious on the PCB other than some whitish stuff. Superfine lint? I don’t know and as I didn’t expect cleaning to have an effect, didn’t measure the OVP pin until I had cleaned the PCB.This is simply the most extreme case of PCB cleaning restoring operation without any other work. Normally it’s the logic level stuff that gets unhappy about surface dirt. Solder flux can dramatically exceed the capacitance beyond the drive limit. That is the dominant failure mode I have encountered. It is so common, that it’s my initial assumption if consumer kit fails. I have found that flux under the chip can be an issue, so I use lots of IPA and paper towels on large SMD chips.I strongly suspect tin whiskers as the culprit in CD/DVD drives dying. I’ve saved quite a few for the optics, so I plan to investigate some of the “dead” drives the next time one “fails”.Have Fun!Reg