PCB from a TV – button acting as though it’s depressed, yet isn’t

Hello everyone!

I'm fairly new to electronics and I'm trying to fix a tv and stop it automatically going down in channels.

The problem seemed simple, the channel down button on the tv is acting as if it it is depressed. Yet physically it's not. You can push it and it depresses, like normal. Yet, once the PCB is inserted into the tv, without touching anything, the channels start going down.

I used a mini compressor to blow out any dirt particles from all over the PCB, especially that button.
I've replaced the button, thinking it was somehow dirty or faulty. It changed nothing.
I've cleaned the entire PCB.
I've looked over the PCB and can see no issues.

I've taken a couple of photos, the problematic button is the one with black ink near it.

I'd love some suggestions on how I can fix this issue.

Thank you.

Nathan - intriguing.  7 buttons over two wires and no IC.  At a guess, and looking at the different resistors next to the keys. it work by sending different resistances when different keys are pressed.
I would imagine you have checked the contacts of the problematic switch.  These switches are not beyond doing this sort of thing, I've had them doing all sorts of things.  If not, check across the switch with a meter and make sure it goes from almost zero to a fairly high resistance as son as it's released.  At a pinch you could try swapping the switch with one that is known  to work properly if you suspect anything wrong.I would also try measuring the resistance from ground for each key line on the connector and see if you see anything strange around the problem key -resistance not changing completely, etc.  You'd probably find that the keys are in two groups which work almost identically, so you may be able to compare.  you may also get better results putting +5V on the power line, 0V to ground, and measuring the voltages you get on the key0 and key1 lines for various key presses.  It may be that the key groups form a potential divider of sorts and put out different voltages on the key line for different key presses.  If one of the resistors is out of tolerance or has a bad joint, this could be the problem.  Have you tried resoldering all the joints - in mechanical components like this dry joints are not uncommon?
Cheers // David

Nathan...sorry...just saw you did replace the switch.  So ignore anything I said about that.  My other comments still stand...may give you something to try.The problem may of course be in the TV circuit to which this board is connected.....You could also try googling the TV make and model, there's a lot of people in the world and one of they may have solved this problem already...Cheers // David

NathanEven though you have replaced the switch may I suggest thouroghly spraying the board with contact cleaner ( and the waiting for ~15 minutes).  I can’t tell you the number of times ( mainly since I haven’t counted)  this technique has solved a puzzling problem. And it can’t hurt to try it. If you are in the UK or South Africa or maybe Australia(and possibly other places) a product called Servisol seems to have even more magical properties than oth er contact cleaners. 

Hello everyone. 
Many thanks for your responses. It has given me a few ideas and tests to perform.I neglected to mention that the scrolling problem isn't present when I disconnect the problem PCB from the tv. It doesn't automatically control. Which is the main reason I think the problem lies in this particular PCB.
I've now bought some contact cleaner - In the possibility that the cleaner I used didn't do the trick. No change.
I used the multimeter and tested the board. I tested each of the resistors, which had the following readings from right to left of the board (where the connector is on the left. 1.16 678 464 1.21 675 469I tested each of the buttons, which had the following readings from right to left of the board (where the connector is on the left. 1.18 1.86 2.32 1.2 1.88 2.33Each of the above buttons readings went down to zero when the button was depressed.The final button, to the left of the connector, the power button. Well, it seemed that my multimeter didn't go up high enough for it, it didn't want to get a reading. However, when the button was depressed the readings lowered down to zero. Any suggestions will be gratefully received. 
Thanks again.

Nathan... nice work!  Your resistors are near enough not to cause any problems.  Your results confirm my suspicions that the buttons are detected by the resistance or voltage on one of the connector pins - probably voltage as there is a power pin.  And if that is the case, the board is somehow putting a wrong voltage into the TV which makes the TV think the down button is pressed.  I'd agree with your thinking that the problem is on the board... but that's not an absolute, given that the TV will be detecting relatively small voltage changes on the Key pins.Is it possible to measure the voltage on each of the connector "Key" pins with the board in circuit, and with each button in turn pressed?  This may give you a further clue.  And if you could trace, and post, a circuit diagram of the board that would also be of help.What is that lump of white gunk covering the pins of the connector on the bottom side of the board?  It's unlikely to be a contributor to the problem but you never know.  Might be worth cleaning it off unless it has some structural function with the connector.Did you try retinning ALL the joints?Cheers // David

Hi David, 
Thanks for your reply. I'm unsure what you mean by measuring the voltage on each of the "key" pins with the board in circuit.
Do you mean whilst it's actually plugged into the tv?I had the same possible thought about the white gunk. It seems to be some sort of glue they've used on multiple connectors, I guess to to doubly ensure they don't fall out. Also, possibly for warranty claims.
No difference with it cleaned off completely. Great thought about retinning all of the joints. I'll do that right now. Could be something that isn't working/joining correctly. Thanks again.
Nathan

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