Help & Advice needed swapping PCB switches around

BobTPH

Joined Jun 5, 2013
9,296
I suspect that the switch / resistor network that produces a different voltage for each switch has some resistors in more than one path, so swapping resistors could have unintended effects.

The way to go from here is to map out the entire circuit around all the switches on all three boards.
 

Thread Starter

john2k

Joined Nov 14, 2019
219
I suspect that the switch / resistor network that produces a different voltage for each switch has some resistors in more than one path, so swapping resistors could have unintended effects.

The way to go from here is to map out the entire circuit around all the switches on all three boards.
That's what I thought at first and I wired all three boards up and tested it and the pin that is giving ghost readings is not linked to the same path of the ones I modified. Also isolated just that one single small board that was causing the ghost readings and on its own it was giving ghost readings until I desoldered the resistor linked to the top button which essentially disables the top button and now it seems to be good. But I need to decide if I should just keep that button disabled so the rest of the system continues to work or if I should try and swap that resistor.
 

Thread Starter

john2k

Joined Nov 14, 2019
219
I replaced that resistor that I removed to disable that one button with a new resistor to see if that resistor was faulty but as soon as I put new resistor in, the ghost readings immediately returned. So I think I will just keep that button disabled as I never use it anyway
 

MisterBill2

Joined Jan 23, 2018
19,466
I replaced that resistor that I removed to disable that one button with a new resistor to see if that resistor was faulty but as soon as I put new resistor in, the ghost readings immediately returned. So I think I will just keep that button disabled as I never use it anyway
OK, as removing the resistor opened the circuit, that points a finger at the button. If you are able to probe that button then you may see the varying resistance value. And if you are able to somehow clean that button the problem could be solved. The alternative may be to replace that button. At any rate, a close examination of that button on both sides of the PCB is in order. Possibly a cleaning with something like starting fluid? Done OUTSIDE, because the stuff is rather dangerous and combustible.
 

Thread Starter

john2k

Joined Nov 14, 2019
219
OK, as removing the resistor opened the circuit, that points a finger at the button. If you are able to probe that button then you may see the varying resistance value. And if you are able to somehow clean that button the problem could be solved. The alternative may be to replace that button. At any rate, a close examination of that button on both sides of the PCB is in order. Possibly a cleaning with something like starting fluid? Done OUTSIDE, because the stuff is rather dangerous and combustible.
So the actual switch itself can cause the jumping resistance values as if it's being pressed? but it's not reading the actual resistance value of the resistor it goes through. By removing the resistor I've essentially stopped that buttons output path right? I'll try and play around with that button and see if I can get a replacement
 

MisterBill2

Joined Jan 23, 2018
19,466
The ghost resistance value is always greater than the measured resistor value, and the switch is in series with the resistor, and series resistance values add. That is why I am saying that the switch has a problem. Quite possibly a soft drink spilled on it by a previous owner, as I think that the system was purchased used. Spilled soft drink residue will absorb moisture eventually, and cause problems. And it can get inside things like those switches and stay a long time.
 

Thread Starter

john2k

Joined Nov 14, 2019
219
The ghost resistance value is always greater than the measured resistor value, and the switch is in series with the resistor, and series resistance values add. That is why I am saying that the switch has a problem. Quite possibly a soft drink spilled on it by a previous owner, as I think that the system was purchased used. Spilled soft drink residue will absorb moisture eventually, and cause problems. And it can get inside things like those switches and stay a long time.
It's not a used item. It's actually a brand new Chinese aftermarket system. The button orientation for the temperature was made for the LHD market thats why I needed to change it over. And I upgraded the backlight LEDs from a horrible green colour to white. Looking at the buttons itself, while changing the led i can see a little mark on the side of that button where the solder tip must have touched but it's only marked the plastic itself and isn't deep so I wouldn't think that would cause any issues.
 

MisterBill2

Joined Jan 23, 2018
19,466
Measure across the button contacts with your ohm meter. My guess is that the button is damaged "just a bit." If the button uses a membrane internally there could be a problem with that small touch.
 
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