Oh no, not my fob! Anyone with electrical knowledge?

[viivo]

Sorry if this isn't the correct forum but it seemed to be the closest I could find for my problem.

My keyfob (this one: http://www.ipmart.com/main/product/Viper,479V,Responder,Remote,246994.php?prod=246994) stopped working. I took apart to see if anything was burned or disconnected but everything looked fine. I decided to try it without the case and found it only works when I'm touching a certain part of the circuit board:

Am I completing a circuit? There are two black connectors connecting it to the board below - could the pressure be pushing those together?

 

[PottedMeat]

i would look for cracked solder joints on both sides of the connector pins

 

[Cogman]

The diode in the top left hand side of the photo looks like it has some pretty crappy solders.

Other than that, I don't see any obvious problems. I would need to thing to be able to find the problem.

 

[PottedMeat]

actually looking closer - i think those 2 joints might be bad. the dark area around the pins may indicate the pin separated from the solder.

 

[viivo]

I have a small soldering iron that should fit there. Should I just put some solder over the current blobs and keep them separate?

Thanks for the help by the way. My other fob doesn't have a button combo for remote start and the temperature has been in the double digit negatives the past week.

 

[William Gaatjes]

I do not hope this is the case, but sometimes these glass diodes fall apart. This one is positioned on the corner of the pcb and in the direction of the longest side of the pcb. This could mean that when the pcb is ever bended, the glass diode may crack more easily. This is also the case for ceramic capacitors. Brittle components do not like it when the pcb is bend. There is nothing you can do about it, but if you are ever going to design a pcb yourself, you know what to do .

Besides all the other good advise given in this thread, also check if there is no short circuit between components of the top pcb and the lower pcb.

 

[Cogman]

http://www.aaroncake.net/electronics/solder.htm

Give this a read.

 

[Cogman]

I do not hope this is the case, but sometimes these glass diodes fall apart. This one is positioned on the corner of the pcb and in the direction of the longest side of the pcb. This could mean that when the pcb is ever bended, the glass diode may crack more easily. This is also the case for ceramic capacitors. Brittle components do not like it when the pcb is bend. There is nothing you can do about it, but if you are ever going to design a pcb yourself, you know what to do .

Besides all the other good advise given in this thread, also check if there is no short circuit between components of the top pcb and the lower pcb.

I doubt that the diode is cracked. He has intermittent functionality (when pressure is applied.) Diodes don't work when they are cracked.

 

[William Gaatjes]

I doubt that the diode is cracked. He has intermittent functionality (when pressure is applied.) Diodes don't work when they are cracked.

Well, i have experience with it and believe it or not, it was the case and not the only one. A zener diode when applied pressure worked fine. When released it stopped working. When i heated 1 side up, i had 2 parts of the zenerdiode, because it sticked to my solder iron which was at the right temperature and professional stuff. I had it also with regular small signal rectifier diodes.


For the interested this is about MLCC capacitors:
http://www.ami.ac.uk/courses/topics/0179_fmcc/index.html

With 0603,0402 i have not yet seen this happen but with the larger 0805, 1206 and larger case sizes it is common if not taken into account the proper guidelines.

 

[C1]

You will need to explore the PCB thoroughly, but carefully by flexing the PCB and probing components with a PLASTIC tool such as a newt stick. Determine if it is just a solder joint or a damaged component. If you decide to attempt a solder repair that involves the terminal of a component, be sure to heat sink the component and get on/off the solder joint quickly to minimize damaging the component (eg, a small diode) via over heating.

 

[ScottMac]

Sounds more like the antenna connection (probably a strip-line antenna .... a trace on the back of the board) is broken and you're becoming the substitute antenna.

*CAREFULLY* flex the board a little and see if you can see any separations of a trace, a joint, or a component. Try to find the PCB traces that are acting as the antenna and follow it/them to where they are fed from the RF driver.

Also, since it's in the middle of the board, give the IC and connections a good close look too. When you are pressing both sides it may be causing a separation of a broken connection.

You can concentrate on the area around the activation button ... that's what takes the most physical abuse (i.e., repeated pressing, maybe a bit too hard, flexes the PCB or stresses the contacts.

 

[PsiStar]

Sounds more like the antenna connection (probably a strip-line antenna .... a trace on the back of the board) is broken and you're becoming the substitute antenna.
.
.
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I think this. And this is assuming you touched the circuit board lightly.

Your finger could also act somewhat like a capacitor also. Which sort of suggests what ScottMac gets into.

Before you opened it up, did it not work at any distance ... such as standing next to the car?

 

[viivo]

The device itself turns on when I insert the battery. Normally when I would press a button, like lock doors, a little antenna icon would flash and the doors would lock. Now, the icon still flashes when a button is pressed but nothing happens unless I have it out of the casing and am touching that spot on the bottom. I checked the antenna and the solder joints look fine, but obviously I'm no expert in that area. I'll get some high res shots of that area.




edit:

 

[William Gaatjes]

Do you have a multimeter ? Put it on measuring resistance, please. It is not uncommon to have some disconnection in the pcb. Check if the antenna actually makes contact. It can be that the copper trace is not connected any more to the antenna because of mechanical stress. Check if the crystal (23.512) is not shorting anything.

 

[bobdole369]

as a former EE bench tech - I'd resolder the entire board. SHould only take a few minutes. Just reflow solder on each joint.

 

[William Gaatjes]

as a former EE bench tech - I'd resolder the entire board. SHould only take a few minutes. Just reflow solder on each joint.

True and possible the best option, the question is : "Does he want it fixed or does he want to know what is the problem ?" I think he just wants it to work.

 

[SpatiallyAware]

It looks like many of the through hole components have bad solder joints. I'd let your soldering iron heat up and melt them so they reflow (unless you have a solder sucker or braid). You don't want much more than a 30w iron, and don't heat it up too much.. Just enough to let it melt

If that works it will probably be permanent, but worst case you'd need to buy a solder sucker and redo it.


I'd avoid the SMC surface mount components because they're delicate - plus from what I can tell they don't look bad

 

[jamesmcuk]

Check if the antenna actually makes contact. It can be that the copper trace is not connected any more to the antenna because of mechanical stress.

This is quite likely and so hence it maybe using you as the antenna when you touch the circuit.

 

[SpatiallyAware]

FWIW I've had similar units with the same problem and it would still work, just only within 5ft of the car..


It sounds like he had to physically take the case off, then press the circuit board..

 

[Modelworks]

the diode is polarity protection in case someone installs the battery backwards. If it has power then the diode is fine.
Get a small length of copper wire, about 4" long. The wire length needs to be short, don't just use any long piece of wire because that will effect the transmitter in a negative way and may cause it to transmit garbled or off frequency data to the car. touch the antenna solder point and see if it helps. Then follow the path on the board going backwards joint by joint touching the wire to each solder point and trying the buttons until you find where it starts to work. That will help to locate the problem.

 

[PsiStar]

as a former EE bench tech - I'd resolder the entire board. SHould only take a few minutes. Just reflow solder on each joint.

Normally I would agree with this approach, but with all surface mount parts this would be a quick way to kill the device.

I am thinking that a tuning cap just drifted ... hmmm, so try to get the fob very cold (in your freezer) and then very warm (hair drier), try it after each temperature extreme.

Modifying this process a bit, test it while you are holding the hair dryer is blowing at the FOB & try this hair dryer test first.

If it does start to work, then this mostly confirms a component has drifted. If you can isolate precisely 2 locations that your finger is across that makes work, then the culprit component can probably be isolated.

Having said all of that/this, unless you already have some (*any*) knowledge of soldering much less surface mount soldering ... good luck :'(and then you also will have to figure what value to use ... usually a trial & error since a schematic is not available.

Ugh ... just get another one for $100 now rather than put it off.

 

[Rubycon]

Wow that part is over seven years old at this point.

What about the pads under the keys? Have you tried cleaning those as well? If it's become harder to use over time and you've had to press the buttons harder this puts more stress on the pcb as well. If you have a decent soldering station and basic soldering skills you can start with going over some of those solder joints. Make sure to clean up the pcb well when finished. A Q-tip soaked with Brakeleen does a decent job.

 

[William Gaatjes]

The last photo posted of the antenne pcb, next to the ic with marking 34xx,LMC64,82IM and above the orange/red resonator/filter. There is a green component. Is the component cracked or is it just a piece of housing material stuck on that component ? I think it is a SMT inductor.