Component repair of house alarm box

[carrera]

The house alarm has started to play up, all the warning lights came on alongside a mains hum. It's ~33 years old, wired, mains with step down transformer.

Isolated, dismantled and found a damaged component, the board is black, on the reverse too

Any idea what sort of component this is, so I can try & find a replacement

[wrighar]

Gut would say a small (2w?) wirewound of metal film resistor....  but I could be wrong.

something like this thats burnt its coating off


Unlikely a fuse given it's location and fixing.

[carrera]

I was thinking a wire wound resistor too, but there's no band to establish rating

Definitely not a fuse, you can see the top of two fuses at the bottom of the picture

I'm fairly sure this is where the mains hum was coming from, with all those diodes around I was wondering if it was some part of a rectifier circuit. There is a separate transformer. Maybe 240v AC > low voltage AC > low voltage DC

[phoenix]

It won't be part of a rectifier per se but it may be being used to drop the voltage for another part of the circuit. That may be the "obvious" sign of a fault but odds are it's another component that's failed which has caused it to burn out.

[carrera]

OK, I have not removed the component yet, is it likely that a rating will be on the underside

I thought the "wire wound" marking might be indicative of the type of component, perhaps not as obvious as I thought

Any ideas what the "47" indicates

I'm used to R1, C1, D1 etc, grasped out of that air that it might be indicating a 47 ohm resistor, but that just seems too simple and unexpected

[wrighar]

Probably depends what is written around other resistors.

I see E1, guessing electrolytic...

is there a 45, 46, 48, 49?   if so then component number....

search interweb for any oard numbers to see if you can find one..

I'd say at 33 yrs, some of the capacitors will certainly be bulged/dried, significantly reduced in capacity....  which will lead to other components failing.

[weetabixface]

It's in with a bunch of diodes, 4 diodes suggests a rectifier, capacitors & inductors are used to smooth rectifier output

Coil symbol ~ inductor

If you do remove it the marking on the PCB will help with ID process

[carrera]

That's great I will need to get the soldering iron out then.

Would that also explain the mains hum it was exhibiting, not always, but sometimes. I thought it may be related to the charging of the back up battery

[phoenix]

An inductor that small won't have a significant effect at 50Hz. I still think it's a resistor that was dropping voltage for another part of the circuit. The hum you will have heard is because something has gone short and is pulling excess current, causing noise in the transformer windings. Can you take a picture of the entire board?

[carrera]

Here you go - there are no components on the reverse, the keypad is removed, to provide access to the damaged component

[phoenix]

Ah, OK. Likely suspects are the 7805 (5V) and 7815 (15V) voltage regulators in the top right. If it's old you may also want to consider replacing the main capacitor which could have dried out and be ineffective.

[carrera]

Cheers

The voltage regs look poor because the metal keypad support goes between the heatsink & the board, so they are floating.

Those can be easily sorted, but I'm still not sure what I need to order to replace the greeny component. I put a meter across while I was doing the photo, it showed 56 ohms

The 12v battery is only showing 3.75v.I'll try & charge that up & see if it will hold a charge. Maybe that failed, maybe it pulled down because the charging circuit has failed ?

[phoenix]

56 Ohms doesn't sound unreasonable. If it's a mini wirewound then despite it's appearance it could still be functional. Battery sounds like it's died but they normally go open circuit. If you disconnect the battery what voltage do you see across the wires that connect to it?

[carrera]

I haven't checked voltage at the cables. The box is hardwired to the mains, so it's a fuse removal to be able to work safely on the system again

Easy stuff first, check the battery

Thanks

[sgould]

How old is the battery?  I have a contract and the guy checks it every year.  It’s a gel type and needs replacing every four or five years.