unusual circuit blown up need help - Page 1
Forum section: electronics / repair

author: peterepair
author_rank: Newbie
timestamp: June 03, 2025, 12:46:37 am
content: HI, the attached photos are from a Breezair evaporative air conditioner water dump circuit that suffered water damage and catastrophic damage to part of the board, which i am attempting to repair.The damaged area is controlled by the transistor part of the 4n28 optocoupler and directly controls the ac mains circuit.The output controls the 24volt ac dump valve. I cant really work out how this operates. I have traced out the circuit as shown.I really want to identify the burnt and missing components ideally from an existing board.There is an odd black component with no markings which i need to identify.traced out circuit in next postany help is appreciated

author: peterepair
author_rank: Newbie
timestamp: June 03, 2025, 12:48:15 am
content: traced out circuit of blown up area

author: Poroit
author_rank: Frequent Contributor
timestamp: June 03, 2025, 07:04:33 am
content: G'day peterpair,Those missing resistors are probably part of the Voltage Divider to drop the 240VAC to 24V AC to operate the pump.http://hyperphysics.phy-astr.gsu.edu/hbase/electric/vdivac.htmlDo a bit of research and things may gel.Could the Black thing be a Polyester Capacitor?https://www.altronics.com.au/p/r3025b-0.1uf-100v-mkt-capacitor/

author: JoeyG
author_rank: Regular Contributor
timestamp: June 03, 2025, 08:47:30 am
content: If you have all the AC model details - you could.1. Search for replacement boardsFrom spare parts companies2. Contact Seeley for an original boardhttps://spares.seeleyinternational.com/partsSpare parts dealershttps://spares.seeleyinternational.com/orderTime = Money = Effort so it depends how much effort and time it is worth to you.

author: peterepair
author_rank: Newbie
timestamp: June 03, 2025, 10:25:06 am
content: Hi everyone, i can buy a replacement board but i really want to repair this one. It is a learning experience and i would learn nothing from buying a new one.

author: inse
author_rank: Super Contributor
timestamp: June 03, 2025, 11:44:42 am
content: How would you be able to recreate the circuit with so many parts missing?

author: BILLPOD
author_rank: Frequent Contributor
timestamp: June 03, 2025, 01:25:07 pm
content: Quote from: peterepair on June 03, 2025, 10:25:06 amGood Morning PeterRepair, having 2 identical boards, side by side, with one working and one not, is the ultimate troubleshooting situation. Power them both up and probe away with your DMM, and/or O-Scope. I use this method whenever possible. Good luck, and don't forget to post your results.

author: temperance
author_rank: Super Contributor
timestamp: June 03, 2025, 03:13:01 pm
content: QuoteToying around with non isolated mains voltage and especially with an oscilloscope is bad advice for a newbie.

author: BILLPOD
author_rank: Frequent Contributor
timestamp: June 03, 2025, 04:37:49 pm
content: [Toying around with non isolated mains voltage and especially with an oscilloscope is bad advice for a newbie.Yes, I agree, and didn't think my reply through.Great care should ALWAYS be practiced when probing PCBs with mains voltage. Thanks for the reminder, Temperance.

author: fzabkar
author_rank: Super Contributor
timestamp: June 03, 2025, 05:52:16 pm
content: The circuit doesn't make sense to me. You have a 450V capacitor connected in parallel with a 24V valve.

author: peterepair
author_rank: Newbie
timestamp: June 04, 2025, 01:20:20 am
content: Thanks everyone for your replies.yes ideally i would like to have an identical board to follow.the whole circuit doesn't really make sense to me, i can't work out how the transistor can switch the valve on.i forgot to put the value of the top horizontal resistor, it is 56k.

author: mikerj
author_rank: Super Contributor
timestamp: June 04, 2025, 06:45:44 am
content: The transistor in the optoisolator is only rated for 50mA continuous, there is no way this is powering the solenoid valve directly. I strongly suspect the "black thing" is a triac and one of the legs has corroded off.

author: temperance
author_rank: Super Contributor
timestamp: June 04, 2025, 12:47:44 pm
content: I have the idea that the left side of the 1N4007 in the provided schematic was soldered onto the tab of a TO220 triac.ok, checking the images again, you can see that the middle pin of the triac was not directly connected to the PCB but trough a 1N4007 diode.

author: peterepair
author_rank: Newbie
timestamp: June 05, 2025, 12:23:24 am
content: Thankyou mikerj and temperance for your replies.it is starting to make a lot more sense now.i have had a closer look at the unidentified black thing which has in fact 3 wires, the middle one completely missing. It looks to be a triac, but i have never seen one in this package and it has no markings. i originally thought it was a capacitor.I have redrawn the circuit now per attachment.the missing components are probably resistors just because of the lead spacingi was told that the dump valve is a 24 volt ac synchronous motor driven but think now that it is dc?

author: peterepair
author_rank: Newbie
timestamp: June 05, 2025, 12:29:09 am
content: forgot to add image link to circuit