Modifying a partially damaged chrono-thermostat - Page 1
Forum section: electronics / beginners

author: twizar
author_rank: Newbie
timestamp: September 28, 2014, 05:48:40 pm
content: Hello to all EEVBlog Forum users.I have a wireless Baxi BCT102-RX receiver for the chrono-thermostat of my heating system. The receiver is malfunctioning and having it professionally fixed is almost as expensive as a new one so I'm trying to modify it.The wireless part is working correctly and the two units "talk" to each other without problems, but the relay never gets activated. I've tried to open it up and see signs of something faulty, but sadly it passed the eye/smell test. I have no idea what to measure, because I can't find schematics for it. The relay is in working order, but no current is getting to it. Also it's all made with small SMD components and that makes it even harder for me to find something wrong.So, because I consider it to hard to fix I'm thinking of using the working part and adding something to it. The device has a LED that comes on when the relay should be activated. I'm thinking of taking the signal from this LED and driving a relay with it. Now, I'm pretty sure that the current supplied to the LED is not enough to directly drive a relay so I need to boost this current and probably voltage to more common values for relays.The output of the relay should be able to drive 6A at 230V and the input whatever is easier to make.For powering this board I have a 5V ~500mA mobile phone charger and some with higher voltages if 5V is not enough.If someone could help me with some schematics for this add on board that I want to make, it would be great.I have attached photos of the problem chrono-thermostat board.If my information is unclear or incomplete please ask anything needed.Thank you.

author: twizar
author_rank: Newbie
timestamp: September 29, 2014, 06:05:08 pm
content: On further inspection I found a blown resistor (101 ohms) under a MOV (at least I think the blue component is a MOV). Should I replace the resistor and see if anything else will release the magic smoke? Or is there something else that I should check before?Close-up picture attached.

author: SeanB
author_rank: Super Contributor
timestamp: September 29, 2014, 08:01:48 pm
content: That is a 100R resistor, and it is next to a capacitor. the most likely thing is to check the relay driver transistor Q1, next to the 3 big 15k resistors, it most likely is shorted and thus the relay has no drive. Check the relay coil resistance, if it is under 400R the relay or transistor is suspect.

author: twizar
author_rank: Newbie
timestamp: October 01, 2014, 07:06:47 pm
content: Thank you SeanB for making me poke around a little more. The transistors were fine, the relay was fine. Knowing this I watched closer exactly what happens when the relay should click and a saw that the LED went a lot dimmer. This made me think that there is something wrong with the power supply, but the design was different from what I'm used to, so I did not know what components to check. I checked all the capacitors and they were all good, at least their values were good.Then I started to search online about problems in similar receivers and saw that the X2 capacitors went bad, making all sorts of problems. I checked mine and it was 0,08uF instead of 0,33uF like it should be. I had no ideea that an X2 capacitor could be used in the power supply part because I saw them usually as filters. Anyway, I replaced the X2 capacitor with a 0,47uF one from an old tv board that measures 0,48uF event after about 25 years or more. I also replaced the 100R resistor with 120R one that I had. Now it all works perfectly fine, so this topic is solved and there is no need for the modifications.

author: SeanB
author_rank: Super Contributor
timestamp: October 01, 2014, 08:08:29 pm
content: Good, yes those caps often go bad ( well, pretty much all of them with time and enough mains transients) and low in value.Funny enough i had a board yesterday with a bad relay, everything working electrically ( power led, operate led but nothing from the motor) and the relay worked after a sharp tap from a screwdriver back. Suggested that the relay should be changed, and to keep a spare one or two as well. Also suggested a new multimeter, seeing as the $5 one there was not exactly happy, voltage readings wereall out on all ranges, and resistance readings were all out as well, giving a reading when leads were open varying from 1000 to 1500 counts. I left it as a kit of parts and some suggestions for either a slightly better or substantially better meter. Price dictates he will buy a slighly better one.......

author: twizar
author_rank: Newbie
timestamp: October 01, 2014, 08:29:40 pm
content: A bad multimeter is the worst thing ever, basically you are trying to fix something with a "broken tool".

author: SeanB
author_rank: Super Contributor
timestamp: October 02, 2014, 06:13:42 pm
content: That is why it was left in kit form, I think some of the screws fell on the floor though, possibly also along with the range switch contacts. Still had the original OHL 9v battery in it.