Fixing a completely dead Nest thermostat heat link

.Spoiler - it wasn't a capacitor.
If attempting a repair like this then take extra care to ensure the power is off to the PCB. It has a lot of exposed live tracks on it.
Do you remember when thermostats were cheap, simple and lasted for over 30 years? Welcome to the future where they are expensive, crammed with electronics and may not last a tenth of that.
I think this one managed to scrape by its one year warranty period before ceasing to sense.
I would categorise my test-bodge as a temporary measure to identify the fault. It would be better to get the proper component, although it's not an easy one to desolder. It may require more aggressive heat pen use and probably some careful masking with Kapton tape to protect adjacent components.
In use the unit seems to run pretty cool. The two hottest components visible to the thermal camera were the PSU diode and the PSU primary side MOSFET, and neither of them was particularly hot.
If you enjoy these videos you can help support the channel with a dollar for coffee, cookies and random gadgets for disassembly at:-
www.bigclive.com/coffee.htm
This also keeps the channel independent of KZbin's advertising algorithms allowing it to be a bit more dangerous and naughty.
#ElectronicsCreators

[@davidcoghill8612]

It's not just a case of the hardware physically failing, you also have to contend with the fact that often these things only work as long as the manufacturer supports them and at any point they can choose to brick it.

[@digitalsparky]

"Internet of sparks" ... shockingly good wifi? :P teehee

[@johnjones4825]

After having a dead computer motherboard for 5 months, I decided to try a repair. Found 3 caps slightly swollen. In our little town there is no supplier of discrete components, so I went to a rather dodgy little shop that "repairs" computers. I got one cap of the same rating (1800uF, 6.3v) and 2 others of 2200uF (all 2nd hand). Due to my lack of desoldering braid, I had to make long jumpers to bring to the "wrong" side of the board and have the new caps hanging in the breeze. What a pleasure, I'm typing on that (12 year old) PC now, 3 weeks and so far so good!

[@MladenMijatov]

I love these repair videos. They clearly show what kind of analysis should be made and how to solve the issue.

[@paulstuhlfelder9819]

Mine failed recently, there’s a guy on eBay who repairs them for £30- a new one is over £200 as you have to buy the thermostat as well 😡😡. There is a quick test he told me to do before sending it away, connect the micro usb cable, if it lights up he said he would be able to repair it, which he did, and it was the diode, looks like a very common fault on these units.

[@robinwells8879]

Don’t know you have ever watched STS Telecom and his skills and steady hands? Probably because of of the power of the algorithm you have!

[@PhoneVidoes]

Brilliant job, yet again a 10p component causes £££'s repair bill for the average consumer. Reminds me when I re-populated our fridge freezer control board with fresh caps, new board was £450 ! ! full set of caps costing about a £5 and an hour replacing them and the fridge freezer went on for another 7yrs before failing seals which I couldnt replace teminated its 20+years service.

[@jonnyduncan7056]

I'm a Sparky and installed three or four of these and had to go back to all of my customers after the Heatlinks and one main Nezt 3rd gen actually 'thermostat'/transmitte (wifi chip) failed). Google, now owners are helpful and cover them even if bought over a year ago, but it still takes 2 weeks getting a replacement here in UK! It's a shame as the customers are left without heating and hot water for that time unless, we re-fit the old hard wired units. At home, I've left my.old back plate, in circuit, so I can fit the old 2 channel programmer/

[@simonastro]

despite my appalling soldering, you've just saved me £70. many thanks indeed

[@stepheneyles2198]

Great to see you're back home Clive, and nice to see that scorched desk again!

[@theelmonk]

The worst weakness is that it's dependent on the manufacturer finding it profitable enough to keep the servers going

[@highpath4776]

You need some T Shirt merchendise - "There's More Screws' , 'Did Some Preliminary Probing' , ' Just Squirt Some Flux Over It' , 'It Sticks Up A Bit'

[@chrissavage5966]

Love the 'mains to death' lead :) Forgot to say before, your bench makes me feel at home. I had a hardboard topped workbench in my workplace where I spent probably the best part of 25 years at the same bench. I even managed to obtain one of it's brothers when I retired (there had been 8 of them specially made and as the years passed, less and less were needed). Mine too has a few witness marks :) Probably the least pleasant witness mark was perpetrated by a French student we had on work experience with us one time. He had a bit of a night out then proceeded to barf on my bench. I was off shift that day and my colleagues did a pretty good job of cleaning up the mess. Happy days....

[@quarteratom]

That's so awesome, you can fix anything. You just test what component broke, and replace it from your endless collection of electronic parts. That's a great skill to have, I probably could have repaired some of my broken electronics, if I knew more. You are a magician.

[@TheCod3r]

Always love to see repair content from you Clive. Keep 'em coming

[@roberthuntley1090]

That is a Heat Link unit, necessary on UK systems to interface the low voltage switching in the thermostat to the 240V switching traditionally used in our heating systems. Just for information, the T1 & T2 terminals supply a nominal 12 volt DC supply which can be fed down the old thermostat wires to supply power to the Nest thermostat, charge its battery etc. At the thermostat end of the cable it doesn't matter which way round the cables are connected, so there must be a bridge rectifier in the stat.

[@punksci6879]

Pro tip you can use two soldering irons to lift a single component like that in a densely packed area. I've had to do that for replacing a load of boards after a bad batch of SMT caps.

[@danyf3116]

BIG Clive: " ...internet of sparks!"

[@Birchall88]

Just checked the dead one I had lying around, and it is the same diode that has died on mine. Thanks.

[@christopherwhitehead4042]

Just wanted to say a big thank you! Had this exact same issue today on a 3yr old Nest and as a regular follower assumed it was a capacitor. Never would have known about the diode without this. You've just saved me £150!!