on membrane keypads

[Alex Eisenhut]

So I have a Samsung microwave with the classic SE error meaning there's a problem with the keypad.

After a bit of cleaning, I noticed the keypad works perfectly fine for hours at a time as long as it isn't in the microwave. Clearly the problem has nothing to do with the keypad itself or steam accumulating on the connectors. As soon as I screw the module back, it stops working.

Is there a RF shield or something that's usually slapped over membranes? There's a weird little loop of aluminum-coated plastic that seems to touch the metal bracket, could that be the "rf ground" connection?

If it's not clear, I'll take pictures.

[Alex Eisenhut]

it's really aggravating me, I thought the flex pcb was rubbing against the sheet metal and making a short (there's a crease on the flex pcb that matches up), but putting tape there changed nothing.
Then I saw the flex rubbing against the metal case of the oven, so I put a spacer, no effect.

The keypad works hanging out, but when it gets within one inch of being closed up the error pops up.

 

[TheBay]

Sounds like its pressure rather than interference/moisture, maybe a cold solder joint somewhere, is there an IC on the board?
When you say it's okay till you tighten it sounds like broken contact or short.

[Alex Eisenhut]

Sounds like its pressure rather than interference/moisture, maybe a cold solder joint somewhere, is there an IC on the board?
When you say it's okay till you tighten it sounds like broken contact or short.

I tried poking with a wooden stick and it did not do the error.

I looked at all the videos and stuff about ripping off the membrane and peeling off layers and I didn't like that.

Instead, I think I found the root cause(s).

The PCB is single-sided and it connects electrically to the metal case by a single screw. The bottom of the PCB has a star pattern with solder for contact to the metal case. Well, solder flows over time so I guess the contact got worse, so I re-flowed lots of solder on there.

Then I looked at that stupid loop of conductive plastic. Looks like it was meant to touch the metal bracket and make a contact, but it's just sitting there like an idiot with nothing forcing the contact, just the lightest touch.

So I soldered a wire to the loop and soldered the other end to the metal bracket.

Now it seems to be working fine inside the oven. We'll see tomorrow.

These new appliances are so aggressively cost reduced it's embarrassing. It's like engineers have no say in a company but some accountant can say "this will save 0.1c today" and that's it, we build cheap! Geez, a star washer won't break the bank...

[Alex Eisenhut]

So much for that, it started again. Looks like the root cause is that Samsung makes a crappy microwave.

[TheBay]

So much for that, it started again. Looks like the root cause is that Samsung makes a crappy microwave.

LG are even worse, buy a Panasonic Inverter and call it a day

[Alex Eisenhut]

So much for that, it started again. Looks like the root cause is that Samsung makes a crappy microwave.

LG are even worse, buy a Panasonic Inverter and call it a day

Heh, yeah. I'm trying to avoid having to yank out this over-the-range monster. I'm also upset that I'm defeated by a crappy one layer consumer board.

[Richard Crowley]

So make a new control/button panel.  With proper switches.

[TheBay]

So much for that, it started again. Looks like the root cause is that Samsung makes a crappy microwave.

LG are even worse, buy a Panasonic Inverter and call it a day

Heh, yeah. I'm trying to avoid having to yank out this over-the-range monster. I'm also upset that I'm defeated by a crappy one layer consumer board.

We don't have over the range models here, microwaves seem somewhat disposable, they are about £30+

Make a control panel from some Cherry MX switches with big buttons like this

[Alex Eisenhut]

It's like the stupid membrane is a Theremin. I saw another video where the guy just separated the membrane where it plugs into the PCB, there's an overlap.

That seems to work also. Maybe there's enough crosstalk in that little section to mess up the scan sequence?

What a piece of junk.

[LabSpokane]

Have you tried looking for a replacement keypad on the online appliance parts vendors? 

[Alex Eisenhut]

Have you tried looking for a replacement keypad on the online appliance parts vendors?

Well sure but I'm not paying 30$ + shipping if all it takes is a piece of paper to space out a stupid membrane.

Which seems to be working this time. I'll let it settle for a few more days and then decide.

[Alex Eisenhut]

I'm not breaking out the Glenfarclas just yet but it seems to work now. Who knew scanning a keypad could be so sensitive to a few pF of stray capacitance...