Corrosive glue - Page 1
Forum section: electronics / repair

author: BurningTantalum
author_rank: Frequent Contributor
timestamp: June 01, 2021, 12:20:41 pm
content: I have been repairing a V-twin mower engine to replace a blown unit on my ride-on. After all the mechanical work I found that there was no spark. The ignition system is not a magneto type but HT coils and a switching module with reluctor pick-up coils triggered by a 'lump' on the flywheel.The ignition unit is a Denso type as fitted to a wide range of engines on mowers, snow blowers and mules etcFortunately the pcb is not potted in its case and, after a lot of struggling, I was able to remove it. There was no evidence of moisture ingress, but glue had been applied to the edges of the board where it slid on support slots on the sides of the case. This must have been liquid when applied as it had spread on the pcb and puddled in the bottom of the case. It had set extremely hard and was nearly impossible to remove. Wherever the glue had touched a component it had corroded the leads. This had caused the failure of the unit, but as the engine has done 2000 hours it must have been a slow process or a product of some chemical aging of the glue.I had to replace a 2SC945 and an unknown zener whose marks had gone and it had failed short. I took a guess at the zener which supplies the voltage to the circuit and the unit now works.I will have to remove more components to clean the board later to stop further problems.Has anyone come across this problem?The engine had died from mechanical causes but has since been standing outside in a mower for 20 years.The pic shows the dark glue on top and bottom edges of the pcb; it appears to have run until stopped by the hot-melt barrier. The zener is the top left component, and the to92 is to the right of the top electrolytic.BT

author: andy2000
author_rank: Frequent Contributor
timestamp: June 01, 2021, 02:15:40 pm
content: That was quite a common problem in products from the 1980's, particularly in VCRs. Hitachi, NEC, JVC, several others loved the stuff. It seems to be a reaction between the glue, and flux residue, accelerated by heat. I occasionally see it in newer products as companies repeat mistakes of the past. I think it's some kind of contact adhesive.You'll need to remove all of the glue since it's corrosive, and conductive.

author: shakalnokturn
author_rank: Super Contributor
timestamp: June 01, 2021, 02:21:49 pm
content: Same experience here as andy2000, the afflicted "newer products" being mainly high-end active subwoofers made in China, the glue is used extensively to secure from vibration.

author: BurningTantalum
author_rank: Frequent Contributor
timestamp: June 02, 2021, 02:28:29 am
content: Thanks for that info, both of you. I will depopulate the whole board and scrub it well.As I said: these units are quite common and seem to have had a long production life, but bootleg copies are readily available from China so that suggests that there are/were reliability issues.BT

author: Audiorepair
author_rank: Frequent Contributor
timestamp: June 02, 2021, 06:28:04 pm
content: They are STILL using this glue on powered studio monitor speakers from various manufacturers.I have found that this product is really good at loosening/removing the crunchy corrosive/conductive stuff, but unfortunately not much good at removing the stuff yet to go bad.Well worth buying though, will save a lot of time.https://uk.farnell.com/servisol/100019000/flux-remover-160-200ml/dp/3821470

author: BurningTantalum
author_rank: Frequent Contributor
timestamp: June 03, 2021, 01:06:09 pm
content: Thanks for that, Audiorepair. It is available in Aus but very expensive with shipping to remote areas.I didn't detail the type number for the ignition module, so for anyone searching later:Denso 21119 2120 igniter. Denso 21119 2157 is the later version (don't know the difference - maybe no glue !).Incidentally, the SIL chip in the unit is a Toshiba T2333 and there isn't a data sheet for it. It seems to be a dual comparator, and is found in other auto applications such as air-con controllers. Having a delve around it looks as if it is a renumbered TA7522S.BT

author: Audiorepair
author_rank: Frequent Contributor
timestamp: June 03, 2021, 06:32:09 pm
content: Quote from: BurningTantalum on June 03, 2021, 01:06:09 pmMaybe someone more knowledgable than I with chemicals could go through the ingredients to find why this product works on the crunchy stuff where most others don't.