Central lock car key repair

[Morsus]

Hello folks.
My neighbor asked me if i can see whats going on with his central lock and alarm car key. He told me he tried to fix it himself but he actually made a mess. So the board contacts are ripped off on some places and I have no idea what should be connected and what shouldn't. I am guessing some of you already knows these small circuits and if you could give me a hint that would be great. So, all of the 4 micro switches work fine. Battery (12V) works too. It's just circuit connection problem I guess. The funny thing around SW3 is actually glue (he used it to fix the switch in the place) and the SW1 had shorter pins so I used wire and solder to lengthen them creating a mess. I will buy new one like tomorrow. Here are the pictures. Thank you all.

[eccerr0r]

Re: Central lock car key

What *was* wrong with it, and I suspect it was made worse...
What did it do before, what didn't it do before. What does it do now?

LED still blinks?

These tend to be somewhat critical RF circuits and adding crap like glue to the board will affect capacitances and thus affect operation.

Sometimes there is a reason why to ask a professional before attempting DIY. And likely buying a new one might be cheaper than repairing...

[Morsus]

Re: Central lock car key

Of course the same thing I have said. One of the switches (SW1) was broken, and he replaced it but messed up the board. LED does not blink at all.

[stj]

Re: Central lock car key

the original fault was corrosion of the tracks from liquid damage.

[redwire]

Re: Central lock car key

I say toss it in the garbage, once the PCB pads are damaged for a pushbutton switch, it will just keep breaking off no matter how much glue.

[stj]

Re: Central lock car key

no, it's do-able.

remove the switches,
cleanup the mess,
fix the track damage,
check all the through-hole plating,

then fit new switches, clean the flux off and test it.
if it works, put a coating of epoxy over it to hold the switches and keep water off the board.

[eccerr0r]

Re: Central lock car key

I'd say make sure you can get the LED to blink again first (and otherwise work), then deal with jury rigging buttons... no sense fixing a device not worth fixing...

I'd be a bit worried about water damage on RF devices, especially one that does not appear to be crystal controlled...

[Morsus]

Re: Central lock car key

no, it's do-able.

remove the switches,
cleanup the mess,
fix the track damage,
check all the through-hole plating,

then fit new switches, clean the flux off and test it.
if it works, put a coating of epoxy over it to hold the switches and keep water off the board.

This is what I had in mind. But I am not really sure about the damaged tracks. I can't really figure out witch ones are damaged and witch ones should not have contact. :/

eccerr0r: Yes, I would focus on getting the LED to blink.

[stj]

Re: Central lock car key

between the red & black buttons is a fat track, above it is a thin one that is water damaged.
you can tell by the dark colour.

[eccerr0r]

Re: Central lock car key

so sw1 is the one that's nearest the IC?
What's up with SW3 or I cannot tell what is what because of the angle... is the red switch SW3 or SW4? There is also some novice solder job on the switch nearest D2, what's the story behind that, if only the switch nearest the IC was attempted at replacement?

Gawd whoever designed that PCB was not making it easy to repair, then again that was probably their objective. Using that through hole via under the LED to pass power is a fsckup.

Well, there is apparently water damage, was it damaged in this manner?

[Morsus]

Re: Central lock car key

between the red & black buttons is a fat track, above it is a thin one that is water damaged.
you can tell by the dark colour.

Oh I see it. Just can't figure out where should the upper (thin) line lead to? Second pin of the SW3? (Image1)

so sw1 is the one that's nearest the IC?

Yes

What's up with SW3 or I cannot tell what is what because of the angle...

It is glue-fixed to the board...yeah yeah I know xD

is the red switch SW3 or SW4?

Red one is SW4

There is also some novice solder job on the switch nearest D2, what's the story behind that, if only the switch nearest the IC was attempted at replacement?

Not replaced, just glue mess, should be fine :/
The replaced SW should be SW1

Well, there is apparently water damage, was it damaged in this manner?

I am not really sure. It could be but the owned would probably notify me about that.

[R_J]

Re: Central lock car key

This trace also needs to be repaired, it supplies the voltage to the common side of the buttons, then the other side of the button goes to a pin on the ic. It looks like they are using 2 diodes as a matrix to get 4 buttons (maybe 2 buttons need to be pressed at the same time to work a function) The diode shown in the diagram would be the led

[Morsus]

Re: Central lock car key

Okay I did my best to make it as prettiest as possible xD
The big mess at to top actually does not have contact with huge track above it witch Image 2 proves.
The problem is that I have no idea where track on the Image 3 leads to. :/

Edit: The broken track on Image 3 is actually okay, just protection layer came off and its connected to the D2.
Tried it again, no blinking.

I added an image 4 where you can see all +12V readings while negative lead is on the battery.

[stj]

Re: Central lock car key

resolder the green inductor, and check the plating where tracks pass through the board to the other side.

[eccerr0r]

Re: Central lock car key

Also some stupid checks like whether the IC is getting power, etc... are the voltages on each pin changing when the buttons are pushed...

[R_J]

Re: Central lock car key

The points I marked in yellow should be connected together and have no continuity to ground. The ic pin (1) should be connected to ground
The points you marked that have 12 volts supply voltage to the R.F. amp section via that `U' shaped trace (antenna)
In image3 I marked a point with a blue arrow, does it connect to one of the diodes (in blue square)
I made an image with the one side flipped so the feed-thru's make more sense, I think there may have been a feed-thru where I marked the arrows? That trace might go under that one switch and connect to one of the diodes which might supply the voltage to the ic (I said might)

[Morsus]

Re: Central lock car key

The points I marked in yellow should be connected together and have no continuity to ground. The ic pin (1) should be connected to ground

The connection was bad and I soldered it a little bit better so its fine now. ic pin1 is connected to the ground.

In image3 I marked a point with a blue arrow, does it connect to one of the diodes (in blue square)

Nope, not any of the diodes - any side.

I made an image with the one side flipped so the feed-thru's make more sense, I think there may have been a feed-thru where I marked the arrows? That trace might go under that one switch and connect to one of the diodes which might supply the voltage to the ic (I said might)

This is correct. There is a connection between the points marked with the arrow.
Should I connect the marked pin on not-existing C6 to the D2 via jumper or something like that?

[R_J]

Re: Central lock car key

I don't know where the line from the c6 location goes, it might just goe to that switch. I was just guessing, the switch at that location would need to be remove to follow where the trace goes, it might go under the switch to a diode it might not.
D1 cathode looks like it connects to 5 of the ic, do you know where the anode is connected? Can you see where D2 is connected? I can't tell from the pictures

I don't know how... but that ic need to get 12 volts on pin 8, and it most likely happens when a button is pushed, and other circuits supply it through a diode, although it could be there all the time but not usually. the ic inputs use internal pull low resistors so to activate the function the button supplies a high (12 volts)

[Morsus]

Re: Central lock car key

Okay I took it off. Here is the image. Had some trouble soldering it back since the SW pins are so short they can barely get through the half of the board. (less than 1mm)

[Morsus]

Re: Central lock car key

I have just noticed the 5th pin of the ic should be but isn't connected to the SW1 and I connected it with a jumper, so that is fine now. And also the Q1 pins are shortened like on the picture. Is this good or bad?
Thank you so far.