Help me fix my kitchen hood - Page 1
Forum section: electronics / beginners

author: RiRaRi
author_rank: Regular Contributor
timestamp: September 21, 2023, 11:57:19 pm
content: Hello. My 5 years old kitchen hood suddently stoped working, completly. These are two boards that were inside. I tested everything i could. On PS board i tested relays, caps, therimstor, fuse etc. Big e-cap measures 0.2 ESR and almost identical capitance as written on cap.On other board i tested voltage regulator and other things i could but for that i can not guarantee that i tested everything as it should be tested.Transformer when unpluged from everything measured 17.5V, but when plugged 3V and it heats very fast and it becomes very hot in shirt period of time.Weird thing that i noticed: when everything is plugged and i plug half-way the PS PCB in the switch board kitchen hood and lights turns on and switches work as they should. When i plug it completly it stops working. To be precise, not half way but just a little bit so probably only some of the pins acctualy connects.I cleaned all plugs very good and tested for continuity when plugged and they all have continuity. What to test? What could be a problem? At least if anyone can tell me which board to replace?

author: RiRaRi
author_rank: Regular Contributor
timestamp: September 21, 2023, 11:58:07 pm
content: .

author: RiRaRi
author_rank: Regular Contributor
timestamp: September 22, 2023, 12:00:49 am
content: Back side of PCBs

author: Swake
author_rank: Frequent Contributor
timestamp: September 22, 2023, 08:24:00 pm
content: Any markings on that transformer?As you measure 17.5VAC I guess its rated voltage will probably be something like 14V.The fact it only measures 3V in circuit indicates it is likely overloaded. The quick overheating indicates this too.You have to find what is consuming all that power.Have you checked the small square device on the bottom? I think it is a bridge rectifier. But it is likely ok as you mention things are working when half plugged.There are 2 other 3-pin devices, likely transistors to switch relays.Do you have a lab power supply that you can use to power the LED board with?Careful, there is a solder smudge on the word Warning.

author: u666sa
author_rank: Frequent Contributor
timestamp: September 23, 2023, 08:58:47 am
content: Quote from: RiRaRi on September 21, 2023, 11:57:19 pmIt's not weird. It's how they break!You will have switches on your kitchen vitishka. I don't know what kind of switches you have, I have slide switches, and basically what happens is this -- fat from cooking gets into those switches and you have condition when pressing half way or fiddling with the switch will make your hood turn on. It's finicky.Either replace the offending switch, or try to clean it using flux off.

author: amyk
author_rank: Super Contributor
timestamp: September 23, 2023, 05:48:25 pm
content: Quote from: u666sa on September 23, 2023, 08:58:47 amNo doubt the use of low-voltage circuitry makes the problem worse. Mine is over 50 years old and has no ICs, no PCBs, just mains-voltage motors and switches.

author: RiRaRi
author_rank: Regular Contributor
timestamp: September 24, 2023, 07:41:48 pm
content: Thank you all. As can be seen, mine has soft touch panel. And yes, kitchen hoods can be so simple and jet they compicated it. When it is "half-way" plugged everything works including switches. They work flawlesly so it is not dirty switches.Also, transormer is rated 8V

author: RiRaRi
author_rank: Regular Contributor
timestamp: September 25, 2023, 05:02:40 pm
content: What should i test here? I used ESR70 for testing big cap (100khz) that is placed before rectifier. Should i test it with LCR on 100Hz?

author: RiRaRi
author_rank: Regular Contributor
timestamp: September 25, 2023, 10:25:35 pm
content: Check out the video of my problem. What should i do? Just replace switch board?https://youtube.com/shorts/QU3ucTrkRTA?si=gHeKJnS6PDDdxk_x

author: Nusa
author_rank: Super Contributor
timestamp: September 25, 2023, 10:57:35 pm
content: To me it appear you have either a damaged connector on the switch board or a damaged connector on the cable. Or both.Examine both closely. It's possible you can scrape/clean the appropriate contacts to make it work again. Otherwise, you're looking at replacing connectors and/or cable. Or the entire cable and/or switch board.

author: RiRaRi
author_rank: Regular Contributor
timestamp: September 25, 2023, 11:11:32 pm
content: I pluged everything and made continuity test and everything is perfectly fine. Also, when switch board is connected transformer heats very much (and as can be seen nothing is working). But when i disconect switch board transformer stops heating.Also, i cleaned everything with contact cleaner. All contacs are cleaned and if it was problem transformer wouldnt heat .

author: RiRaRi
author_rank: Regular Contributor
timestamp: September 25, 2023, 11:44:15 pm
content: I also discovered that 4 out of 6 pins on switch board plug are fully connected somehow. That might be unusual.

author: jpanhalt
author_rank: Super Contributor
timestamp: September 25, 2023, 11:55:23 pm
content: My thoughts are along the line that Nusa suggested. Is there any place on the switch board that you can measure voltage coming from the PSU? Then do your plug test. You should see that voltage as you start to plug in the connecter, and then when fully inserted, it drops or disappears. I am thinking either the connector or a bad solder joint that make contact when the units are unplugged from each other, but with slight distortion of inserting the plug entirely, you break that contact. I have not seen exactly what you describe, but I have seen bad solder joints that were temperature sensitive, and vibration too, of course..

author: RiRaRi
author_rank: Regular Contributor
timestamp: September 26, 2023, 12:20:30 am
content: Okay. I will try to give a little push with soldering iron to resolder them a little bit. But why would transfomer heat very fast when switch board is fully plugged. When it is unpluged it doesnt heat almost at all?IMHO something is shorted on switch board and when it is half-inserted one of pins doesnt connect and then everything or almost everything works correctly. But when it is fully inserted something comes short and transformer starts to heat.Also it is not exactly half-way inserted, it is like one angle when half pressed

author: jpanhalt
author_rank: Super Contributor
timestamp: September 26, 2023, 09:54:17 am
content: Loss of voltage could be due to poor contact or a short. The latter would explain the PSU transformer overheating.I would do the experiment(s) first before trying a cure. You know, symptoms>signs>diagnosis>treatment>cure is a typical logical approach. Shotgun treatments can work, but it is far better to pinpoint the problem first. In your case, the symptom is it didn't work. As significant sign is that it works with partial insertion of the plug. Now diagnose why.

author: RiRaRi
author_rank: Regular Contributor
timestamp: September 26, 2023, 12:58:50 pm
content: Thats exactly why i made post in this group. To get a help. Also, it is under begginer section of forum so i really dont know why i deserved this kind of answer. Switch board is full of very small IC and it is multi layer board and i really dont know how to test everything..

author: jpanhalt
author_rank: Super Contributor
timestamp: September 26, 2023, 01:17:15 pm
content: Sorry to offend you. That was not my intention. My suggestion was to focus on identifying the problem before doing anything like resoldering. The key point is that the thing works apparently normally as you start to insert the plug. All of the work you did previously is fine, as you found no other obvious defect.

author: RiRaRi
author_rank: Regular Contributor
timestamp: September 26, 2023, 10:32:42 pm
content: Okay, no problem. When it is half inserted switches doesnt work but fan and lights turns on dependig of angle. Maybe the best solution is to replace switch board?

author: jpanhalt
author_rank: Super Contributor
timestamp: September 27, 2023, 12:07:25 am
content: The switch board is probably expensive, if even available. The problem seems to be related in part to the plug and receptacle (part with male pins). They seem to be typical connectors, maybe JST (jst.com) or Molex (molex.com). Both are available from common distributors.Pin number and spacing ("pitch") are important to find matching replacements. You have an ohmmeter and voltmeter. I would still try to determine the actual problem before replacing any parts, much less the entire board, unless it is very inexpensive.

author: RiRaRi
author_rank: Regular Contributor
timestamp: September 27, 2023, 12:30:28 am
content: The board cost around 40EUR. There are two things why i think it is not problem in plugs beside checking continuity:Transformer is heating when switch board is connected which IMHO indicates short somewhere on switching boardWhen i half insert it switches still doesnt work but at one angle LEDs work, when fiddeling more fan starts working and when fiddeling more both are workingIf you think that still the problem might be bad contacts i can try to solder them directly to try.

author: Nusa
author_rank: Super Contributor
timestamp: September 27, 2023, 02:23:43 am
content: Switch board looks to be two layer (top and bottom), so all the traces should be pretty visible. Your pictures aren't good enough for me to follow them, however.So far as those six contacts go, looks like two of them are power and ground to the switch board (shouldn't be too hard to identify those), and the other four are switch board outputs to control the relays on the power/relay board. They're probably connected to processor output lines.

author: RiRaRi
author_rank: Regular Contributor
timestamp: September 27, 2023, 12:56:51 pm
content: I will take closer photo when i come home. And yes it is two layer board.I will also try to indentify those

author: RiRaRi
author_rank: Regular Contributor
timestamp: September 27, 2023, 06:54:42 pm
content: Here is one side

author: RiRaRi
author_rank: Regular Contributor
timestamp: September 27, 2023, 06:55:23 pm
content: And other side

author: RiRaRi
author_rank: Regular Contributor
timestamp: September 27, 2023, 07:59:55 pm
content: Please check out this video where i connect my PSU after the gretz (where it converts to DC) and when i plug switch board it overloads. Transformer is rated at 9VAC but when pluged in kitcheen hood, after the gretz it measures 23V DC. Is that okay?https://youtube.com/shorts/r7lAetzZ0N4?si=qzwVk1qtf9heKmR2Also i dont know what this small 3 leg component is right after grety connection but it measures 22.99V on every leg. What is this?

author: RiRaRi
author_rank: Regular Contributor
timestamp: September 28, 2023, 01:19:37 pm
content: what should i do next?