Tuesday, 3 May 2016

Another follow up to Marmitek/Haibrain SC9000 PSU (PS90U) failure

Here we go again, I've had another SC9000 power supply failure. For those that have been following my blog before you'll know I use an SC9000 X10 alarm system, and last year the power supply PS90U failed on it.
The power supply fails in an odd way, it still provides power to the alarm system, so everything appears fine, however when it fails it stops sending or receiving X10 commands, which is critical as the X10 commands trigger the siren/sounders and show the alarm as triggered.
I noticed that the timed lights weren't coming on and off when they are supposed to. These are controlled by the timer light functions on the SC9000, and so this made me suspect it yet again.

Sure enough, checking the PSU out, I found it wasn't sending/receiving the X10 commands through it. So time to check what's wrong.
Firstly. This is the 2nd unit I've had to purchase. The first failed in the same way, and now this one, less than one year since I bought it (Purchased 04/06/2016, failed 17/04/2016). So these units really aren't reliable. Originally I thought this was due to heat and so put heat vents in this PSU last year, however it looks like this hasn't helped.

Opening the unit up, again it has the stench of burnt electronics. They are comprised of a large transformer, some mains filter circuitry (and DC step-down) and then the X10 circuitry.

The first photo shows the transformer and the mains side of the circuit. You can see to the lower part of the board the discolouration of the circuit board, which is where the transformer sits, so the transformer obviously runs hot.

After stripping away some plastic covers/heatshrink, I revealed a glass fuse (I was expecting a resistor that was being 'used' as a resistor). Other components on there such as the capacitors, etc, looked OK, they weren't bulging and putting the meter on them showed the 'correct' resistance rising up to infinity, so they appeared OK.
The larger yellow component (visible in the photo) MPX275-X2 is a surge AC capacitor, small blue circle (to the right of the first photo) 
Various transistors are there 2SC667 which is an NPN transistor used for switching high current from a low current source (So most likely output from the chips on board)

Anyway, after various tests, none of the components appeared to have failed, which was odd. So I then started probing parts of the board for continuity, thinking heat may have fractured a trace. I wasn't getting much continuity from the mains leads (red and black in the photo) to anywhere, and then I tested the fuse. It was open circuit, so it had blown. It's a small 250mA 250v fuse, so a very small value.
Following the wiring of it, I realised that the mains wires (red and black) come in, directly connect to the two brown wires (to power the transformer) but then go through a fuse, choke, etc, and into the circuitry. So the transformer IS NOT protected by the fuse, but the small electronics ARE! How strange. So what was happening was the transformer was still working and stepping down/providing the low voltage to power the SC9000 alarm, but the fuse had blown and so the X10 circuitry in the PSU weren't getting any power. Hence the failure mode of these units, when they fail I suspect that is always the weak point, but rather than stop the whole thing working (A much better failure mode to me, in that you'll know it's failed. Rather than waiting until you need the alarm, and it not sounding!).

I did a small test, I bridged the fuse with a replacement, and hey presto, it started working. Now why the fuse failed I'm unsure, it was a very small current (250mA) which makes me think even a small electrical surge would cause it (The fuse was directly on the mains input, not after the smoothing capacitors, etc) so that's what I'm thinking. Especially as I've found that this does seem vulnerable to electrical surges.

So a quick replacement fuse and it's working again. But for how long?!

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