Fixing a MCS 3249, Need a Source for Tactile Switches

Corbin Gross

Corbin Gross

Lost in Translation
At least I think that's what these are...

The foam springs for the Memory and Tuning+/- buttons has turned to goo. I think I'm going to need to replace these little switches. Can anyone help me identify exactly what to look for and tell me a good source for something like this? I found some stuff on a site called Jameco Electronics, but there's a lot of options as to volts and amps and whatnot.

Possibly I can get some alcohol and clean out the goo which will bring the snappyness back to the switches? I'm hypothesizing that there are little plastic domes in there, like in the Atari controllers, and possibly they've cracked or warn out as well. But if you guys think the foam goo has glued them down than I'll try to wash them out.

And thanks for the help, guys!
 

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Thanks for your reply. I actually came by your thread whilst googling around yesterday. You mentioned that the tuning bar worked, just didn't center. The thing is that my bar and other buttons on that PCB don't work at all. They don't "click". The tactile switches behind them just stay depressed, even with the PCB removed and with the switched uncovered and defoamed.

I was thinking along your lines though, to try cleaning them out first. But can you clean the insides of those switches. They don't seem to be serviceable, but there's enough play that I think I could get some cleaner to seep down in there.

Can I use alcohol? Or must it be "contact cleaner"? And if these switches are not likely to be salvageable than I'll just skip that step and order new ones.

I like your foam idea though, for returning the use of the external button covers, once the switches are fixed.

Thanks!
 
I used RadioShack contract cleaner/lubricant and pretty much soaked the board a couple times. Those switches seem to have about no movement in them. I pushed down those white tabs and felt next to nothing. My problem was that some buttons seemed to stick and would lock up the tuner if it were pushed. Others needed to be punched pretty hard to work. The tuning bar would sometimes stick and just keep running through the stations. Those buttons just seem to need a tap to make them work. Making sure the silver buttons can return seems to fix the problem pretty well. I do not use that receiver much, but about a month ago, I fired it up. The station memory had lost my presets, but the buttons still functioned properly. Yours seems to be the same receiver except for a few watts less power. I keep mine because I like the looks and it just sounds good.
 
Yeah, I've definitely got plans for this guy. I really do like the looks of it too, not more than anything I've ever seen of course, but I'm really in to thrift audio and this was under a Jackson and the GW.

I'll go ahead and take a shot at cleaning them and see how it turns out. I've been looking around at other DIY solutions, seems these tact switches are a common issue on electronic instruments. Most of those threads say they'll need to be replaced. Seems there's a "snap disk" that wears out. I'd be surprised if somebody selected all of these memory presets enough to wear anything out, but I know I have a couple of options anyhow.

And from looking a bit more at these tact switches, other vendors don't have voltage information like the first site did. So possibly they're all about the same? I'm going to go to Radio Shack at lunch to check what they have anyway.

Thanks!
 
I don't think you'll be able to save those. They look very much like a square version of the rectangular tact switches Tandberg used on their 3000-series tuners, the plungers are identical and the overall appearance looks like it came from the same supplier.

The failure on those is that the rubber part inside falls apart at the folds; think of a small rubber puck with conductive paint that's suspended from the surrounding rubber by a "V" or "W" fold. They break along one crease of the fold, and the puck falls into the bottom of the switch and sometimes ends up shorting the contacts in the process though thumping it a little can get the loose part to move and open the contacts again.

I have a replacement switch for those that looks like it should drop right in, if you can get me the details on the housing dimensions (I'm guessing 4mm square), the total height of the switch (measure from the top of the plunger while pulling the plunger out away from the board, to the face of the board), and what the lead spacing is, I can confirm whether these will work.

John
 
That's kind of what I thought. The inner workings' description makes sense, they're surely worn off.

It'd be great if you have the parts. The switches are 7mm square and 9mm high. There are two leads, centered on opposite sides at the edge. They poke through the two holes and are soldered on the back side of the PCB.

They did have one switch at Radio Shack that would fit the dimensions, but it had four leads at the corners. I could probably get the to fit, but I don't want to do a lot of modifications if I don't have to.

Thanks!
 
Part of that matches, but the height is a little lower than I expected you'd say. Did you pull up on the plunger to make sure you were getting the full, normal extended height? When the rubber diaphragm inside breaks, the plunger will usually settle 1-2mm into the body.

I had a brain fart when I posted that previous message though, my switches are 8mm square, not 4mm. 4mm is the typical small tact switch used on more modern stuff, with the 4 leads. The ones I have are 13mm tall but I have a fixture to knock that down by 2-3 mm which fits what I use them for.

Check the lead spacing on the solder side - these come as 5mm pitch, but I mode them to fit 10mm spacing. The ones I'm replacing are 8x12mm though, you may be ok with the standard 5mm spacing.

John

P.S. here's a pic showing these switches, installed on a similar board from another brand of gear:

attachment.php
 

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Try looking in an old VCR or junked "all in one" stereo system, the front panels usually have switches like these.

Mark T.
 
All fixed! Thank you!

Hey, guys. Just wanted to post to say the unit is all fixed up and ready to go.

I ordered a dozen of those switches you recommended. Cost about $3. Getting the old ones off was a little tricky since I don't have a vise to hold the PCB, but I just hung a pair of vise grips off of each button as I desoldered it. Getting the new ones on was easy.

The only issue is one if them isn't perfectly flat against the PCB. So when I reinstall the unit, it presses against one of the button covers. I have to back off the screws just a bit and then it works perfectly.

Thanks for all the advice. I learned how to solder here, and this is about the 10th piece of equip I've brought back to life with your help.
 

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Thanks for the thread. I am having the same problem with my 3249. I noticed that there is a orange strip on one side of the switch that match the silk screen on the board and spaces for diodes that weren't used. I was worried that there was a diode in these switches. Couldn't get much of a reading testing these on a meter. I feel a little safer now replacing them.
001.JPG
 
Perhaps the diodes were intended as part of a sense matrix to prevent phantom signals if you hold down several buttons. On PC keyboards, you see diodes used that way.
 
For anyone joining this party late and is unable to find a replacement , digikey has part number P8085SCT-ND that fit perfectly, work as intended, and feel good. No need to shim, space, or add spring. I believe they are the same ones that are linked above, but are in stock as of this comment.
 
patra710 said:
I am having the same problem with my 3249. I noticed that there is a orange strip on one side of the switch that match the silk screen on the board and spaces for diodes that weren't used.
Click to expand...

The unused diodes on that board are for LEDs used on the 3260 (same board).
 
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