Secret Scotland

If it's secret, and in Scotland…

LNT – Beware habits and assumptions

I have to confess to spending a ridiculous amount of time fault-finding a small remote control (radio) mains switch this week, probably a couple of hours per night, beginning over the weekend.

One of a set of four, all used occasionally over the past few years, this particular one just ignored commands to turn on.

A pig even to open and get in to – the manufacturer had filled all six screw holes with a very effective white compound (not only filled the holes but also bonded), and hidden a couple of security head screws under it too. Just to make things even ‘better’, a bead of this stuff had been used to seal the case shut. The only good thing was this stuff was not a solvent, so the case halves were not welded together, and could eventually be split without damage.

I’ve tended to avoid working on things with capacitive droppers, as opposed to a mains transformer (which provides isolation), but you can only oppose change for so long, so in I went.

For some reason, the board had an LED to show if it was ‘ON’, but the case was solid, so this was useless unless you’d opened it (yes, I did drill a small hole so the LED could be seen, and modified the other three as well).

At first, I thought the fix was going to be easy – a visual check, there was an onboard fuse (fine), then an electrical test showed NO mains reaching the PCB. I remade all the solder connections, restored mains to the board – but, more subtle problems were waiting.

The LED showed the RF section was working, as it came on when the button was pressed, but immediately faded out (so the output relay didn’t energise), and wouldn’t trigger again unless the switch was unplugged, and plugged in.

So, I suspected the output relay, but removing and testing proved it was fine.

Interestingly, with the relay removed, the LED showed the circuit was responding to the remote, and now turning on, and off, in response to the button – but when the relay was put back, the fault symptom returned.

I suspected a component failure in the regulation of the voltage on the board, but while I confirmed a mere 7 -9 V available for a circuit with 24 V relay coil to drive, no components failed on test.

I even cracked open one of the other switches (to confirm actual voltages), and sure enough, there was 19 V to be found there.

I tried a bit of reverse engineering, and drew out part of the circuit, but it was all pretty standard.

Eventually, there was only one component I hadn’t pulled – an X2 capacitor used as a dropper.

I hadn’t suspected it (since it was an X2 type, less likely to fail by design), and most caps I’ve had fail at mains voltages tend to need a small brush and shovel to tidy up behind them.

Not this one – it checked out fine as a capacitor, but when I dug out a meter and measured the value…

It had turned into a 20 nF device, instead of the 330 nF (0.33 μF) declared by the marking.

Small wonder the circuit died once the transistor switched the load of the relay coil into it.

I confirmed the failed cap was the cause of the problem since I had a similar in my junk – but the final fix will need to wait.

The handy test cap is 5 mm longer than the original, so not only doesn’t fit the board (which would be easy to fix), but is physically just too big fit into the available space in the box.

Oh well – at least I didn’t make the mistake of continuing to assume that cap was unlikely to have failed.

Their rating and construction makes it unlikely, but NOT impossible.

330 nF X2 Safety Capacitor

330 nF X2 Safety Capacitor

Funny, I missed the days when TV sets had ‘hot’, or live chassis, connected directly to the mains, and killed many a TV repair man when he touched the chassis.

I did work on a few though, and it was scary stuff – one wrong move, one slip, or one wrongly wired plug (or even bad house wiring), and you could be history.

We’ve almost gone full circle, with many devices powered via capacitive droppers, meaning they’ve lost the isolation provided by even a small mains transformer.

While they may not have anything like the currents or voltages found in those old CRT TV sets of the mid-20th century onward, they can still bite, and depending on the circuit/current path, they can still kill, so need to be treated with respect.

I’ve left a few colleagues behind along the years, attended funerals, and have no desire to follow them down that road.

Update

Of course, my usual (bad) luck applied, and not a single X type capacitor was the of the value needed.

I did end up with the choice of some suitably rated caps (for voltage), but even that was between 470 nF (too big and would have stressed, or blown the switch), and 220 nF, which I guessed was too far from the intended value, but decided to try, since it was the right size to fit the available space.

But, as I expected, it was just too small. It didn’t provide enough current/voltage to pull in the relay, and the original symptom returned – the LED lit, then died.

They’re hardly expensive, even buying just one, but I no longer have any shops ‘At the end of street’ where I can pick one up quickly.

Some of my old ‘Trade Counters’ will still deal, but that usually comes with a minimum order – and £20 for one 30 p capacitor doesn’t seem to be a very good deal.

Time for a bag of 100 from China for 50 p, three weeks delivery, and free carriage :-))

 

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19/09/2019 - Posted by | Civilian | ,

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