alarm question - Drew20
I have an alarm and immobiliser fitted to my golf and, being a top notch muppet, managed to short the alarm unit out yesterday.
This meant that the immobiliser could not be disengaged but that the alarm was still fully functional etc.

I have fixed this. I took the alarm unit apart and traced the imobiliser wires through the car (it was fitted between the ignition switch and the 15 power rail in the fuse box and hence isolated the coil). I then traced the circuit until I found the burnt out piece of circuit, this was then bridged by soldering in a small piece of wire.

Bingo! I thought and patted myself on the back but...
now the siren doesn\'t work, the immobiliser is back on line so the fix worked but in the process of dismantling the alram unit, soldering or reassmebly I have damaged something related to the siren or amplifier.
The siren itself connects by two wires, and a small plug, to the alarm\'s printed circuit board. I disconnected this when soldering.
when soldering I did acidentally touch a capacitor on the PCB witht the side of the iron, could this have damaged the capacitor in some way? How would a capacitor be involved in an amplifier? What should I check to locate the fault?

thanks, sorry its so long

just to clarify the siren was working fine after I shorted the alarm and only stopped working after I reassmbled the alarm unit
alarm question - Altea Ego
>when soldering I did acidentally touch a capacitor on the PCB with the side of the iron

Depends on the type of capacitor and the length of the touch. A millisecond touch with a soldering iron on capacitor will not harm it, but a 5 second \"lean\" while you solder the tracks probably will (again depending on the type - is it a small can type or a small bead type?)

My fave would be the plug - make sure its connected properly, the right way round, and you did not pull a wire out of it.

Also make sure your resoldering of the track did not break the solder joint of another adjacent component

Yes most amplifiers do need capacitors. In laymans terms they are used to smooth voltages, keep bits charged ve or -ve, and decouple (keep potentially dangerous and competing bits apart) components.
alarm question - frostbite
Also, it is to be hoped that you have a good quality soldering iron, and not one which leaked something nasty into a delicate ic.
alarm question - Drew20
I think if I\'d trashed an IC then I\'d be throwing it away, but I would then expect the symptoms would be slightly more severe than a very quiet siren.

I am not an expert electronic engineer but would consider myself competent to fix obvious faults on a PCB, my problem is that I have somehow caused a non-obvious fault whilst fixing the glaring burnt out PCB track.

The fix itself was v fiddly as the burnt track was inside a sandwich of three PCBs, kinda like those games where you move a hoop of wire down a bendy coathanger. I was too busy watching the tip of the iron and not looking out for capacitors which were obviously in the wrong place

alarm question - Altea Ego
AH a clue - the siren is quiet. Are you saying it makes a noise but not as load as it was? if this is the case the last stage of the amplifier has gone kaput, the siren is shorted down in some way (if it stays that way it will kaput the last amp stage)or has high resistance or incorrect connection.
alarm question - Drew20
thanks, will check the plugs. I wasn\'t unreasonable with them, especially the to pin one for the siren, this was loaded before I dismantled the unit, ie when the unit was open but the siren was still working then...
I disengaged it carefully so...always a possibility.

the capacitor is a barrel one, and is quite big: about a cm high and 5-6mm in diameter. I\'m not sure how long the iron was in contact but long enough to leave a small bit of melted plastic on the iron. It wasn\'t the tip but I suppose even the shaft of the iron gets quite hot.

I\'ll check for continuity on the surrounding solder joints, the botch was near the edge of the PCB on plug contacts and mini-relay contacts so the solder joints were well spaced. If they were any closer I would not have attempted the fix. I am relatively happy with the actual fix, i.e. the joints were quickly made without excessive heat or solder being used, unusual for me!!
so my chief culprits are a kackered capacitor (preferable as an easy fix) or pulled plug contacts (easily checked by measuring resistance across the siren unit though the plug, any idea what ohms an alarm siren should have?

thanks again
alarm question - Altea Ego
Siren? assuming its just two wires and a glorified speaker 4,8 or 16 ohms

Capacitor? can type electrolytic from the sound of it, and you just have a smidge of plastic on the iron it should be ok, but changing it wont hurt, they are pennies only (they are polarity sensitive so make sure it gets soldered in the right way, it will be marked)

Good point about the quality of soldering iron, if its not screened or earthed you could have leaked some ac voltage into a circuit
alarm question - Gen
I don't have the answer but you get my respect for managing to find the problem to disengage the immobiliser. What year car is it?
alarm question - Drew20
hmmmm, by earthed I take it you mean more than just the earth pin in the power plug?
Don't know is the answer but it sounds nasty and its too late now. The points soldered were a plug, ie no circuit attached and a relay which would have been open so, fingers crossed.

ah yes sorry I really should have mentioned the quiet siren rather than silent, I thought I had. It really is very quiet (ear by speaker quiet) indeed.

Final stage amp sounds likely, thanks. What would this look like on the PCB? how could I test it?

thanks everyone, much help to me. And cheers Gen, that's the kind of reply I like. ;-) its a 91 golf, though the alarm is after market. To be honest the immobiliser and ignition switch were easily bypassed just by connecting the X circuit to the 15 circuit on the relay board. Temporary though
alarm question - Altea Ego
Final stage amp will be (depending on how state of the art it is)

Recent - will be an IC or chip, with a heat sink, includes pre and out stage of amps combined

Older, will be a couple of matched pairs of power transistors (flat or cans) with heat sinks,

Will be connected to the siren output pins, possibly decoupled by capacitors/resistor


Its entrely possible the siren is an oscilator/speaker combined.

Sorry without seeeing it in the flesh its very hard to advise
alarm question - Drew20
OK thanks, you've reasured me that the whole alarm isn't busted, the siren is easier to live without than the immobiliser

will have a look for possible shorts and damage to wires and plugs around siren. Its fiddly to trace the circuit in 3D but I will see if the capaitor at issue is in any way connected to the siren.
Can't remeber the PCB, I only remember two big voltage regulators and an IC or two
alarm question - Deryck Tintagel
Another think you could do ...

I used to have a Foxguard alarm / immobiliser on my previous car and the siren went kaput - in fact it was the connection to the loudspeaker coil that was faulty. Anyway ..

The useful feature of this alarm was that it had a wire than went to 0V in alarm so I used a siren under the dash. Not knowing what you have in your car you could find something similar. This makes it rather uncomfortable for anyone trying to steal the car and probably more use than an external siren! Maplin Electronics is a good supply of all things electronics
alarm question - Drew20
thanks but I'm a little confused. Your siren broke so you wired a replacement one under the dash, yes?
and the bit about a wire that went to zero in alarm. Does that mean that the siren was switched at both ends, ie the siren was only grounded when the siren was on?

alarm question - Altea Ego
In this case, i think it means that the alarm had a signal that went to ground. You cable a self contained siren or something that makes a noise when it has 12 volts applied to it. Cable one side to permanent 12v feed, other side to ground supplied by alarm. When alarm provides ground, circuit complete - noise.
alarm question - Deryck Tintagel
Yes, that's right. The alarm had a dedicated output wire and I connected a cheap siren for use in burglar alarms between the output wire and the battery supply to the alarm.

I think that the original purpose of this wire was for another add-on module (can't remember what) but it has sufficient drive capability for a small self-powered siren

Value my car