Troubleshooting a heating controller.

This is a faulty Honeywell heating controller sent by Dr Pipe, who's channel is linked below.
It's exhibiting a common fault with these units where they keep resetting, preventing the heating systems from running.
The usual fix is to swap the entire front unit or replace it with a new controller, but as with many of these devices, the fault can usually be fixed by replacing one or two components.

For a professional heating engineer where time is money, it's going to be faster and easier just to swap the unit. But if you have one of these in your own home then it's viable to fix it yourself, either by loosening the two screws at the bottom of the unit and plugging in a new one or doing a small repair on the PCB.

Be aware that when the unit is lifted off its backplate there are exposed live connections. Turn the heating system off completely and always assume that contacts are live until proven otherwise.

Here's Dr Pipe's channel:-
https://www.youtube.com/channel/UC0hk5bxzJjD_vyA5qHs93uQ

I'll probably make another short video showing just the fix on its own. I tried desoldering the faulty surface mount capacitors, but even by SMD standards they are not easy to remove. They're not glued, but the solder connections extend underneath for far enough that it would require heat on both pins simultaneously to remove. There are also other smaller surface mount components in close vicinity that make hot air removal harder. They're probably best left in place, as attempting to remove them can easily result in track damage.
The easiest fix is to add a standard through-hole 47uF 63V capacitor to the spare SMD capacitor position closest to the dropper capacitor, and laid flat over both the capacitor positions.

I'll also test the dropper capacitor to see how low the value can go before it causes excessive voltage drop off. The dropper capacitor value can be tested in circuit due to its position in the circuitry. Despite its age, this one had held its value very well.

This unit was old enough to have very brittle plastic. Be careful when separating the back from it. There are four clips that can be gently released with a thin screwdriver. Using excessive force may snap them off and prevent the unit being reinstalled in a safe manner.

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