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Beomaster 4000 power switch repair

A broken switch often starts off with some intermittent on/off switching.  Normally the resistance across the switch contacts is higher than expected (less than about 2-3 ohm I would guess is normal) or vary a lot (5-25 ohm in my case).

Unfortunately replacements are not available, unless you can find a second hand one. You can also try and hack a different switch in there but it is not going to be easy.

In this case I have decided to repair it.

Step 1: Remove the switch
https://forum.beoworld.org/blogs/beomaster_4000_restore/archive/2021/12/04/beomaster-4000-power-switch-removal.aspx

Step 2: Open the top of the switch

The switch is held together with 4 plastic rivets that runs through the switch housing. The rivets were melted on the one side to keep the switch together. Simply cut the melted part flush with the PCB using a sharp blade.


Step 3: Remove the plastic rivets

From the opposite side, remove the plastic rivets carefully by lifting them slightly with a blade and then pulling them out

 

Disassembled with rivets removed

Step 4: Remove contacts and PCB frame

Here you can see the damage on the contacts

 

Step 5: Cleaning up the moving contacts

Just to be clear, fixing contacts by filing them off is a last resort and not ideal.

These copper contact area must be cleaned and the surface restored. The copper plates can be removed from the carried frame while cleaning but I simply left them in place. With extreme damage you can make new copper contacts from copper plate.

I clamped the copper plates in a vice with soft jaws and restored the surface in 4 steps a) file with a fine needle file b) file with a diamond file (if you have a Leatherman, some of them have diamond files or you can raid your partners makeup bag for a fine nail file) c) 1000 emery paper d) microfiber or abrasive cloth. Do take your time to do this because a better surface will last longer. Photo below is halfway through the cleanup.

When done I place a bit of Deoxit on each contact with a earbud.(I prefer using an earbud to avoid getting any chemicals on the old PCBs)

Step 6: Fixing up the static contacts

The static contacts also needed to be cleaned. It is possible to file them as well but since they are symmetrical, I decided to simply flip them around and use the other side. This is done by removing them from the PCB. First remove all the solder and then pull them out or alternatively pull them out with a tweezers while heating them with the soldering iron.They can now be flipped and inserted into the opposite hole.

Step 7: Assembly 1

Assemble the 2 parts separately as shown below. Use masking tape to make sure everything stays in place
Make sure you insert the moving contacts and frame the correct way around. If the shell can not close properly, start again and make sure that the sliding contact frame is aligned with the slot in the housing and in the top PCB

Step 8: Join the 2 parts together

When joining the 2 sub-assemblies, take care to ensure that the bump on the white plastic slider is aligned and fits into the slot of the contact frame. (This is where the plastic slider grips to move the frame and copper contacts when switching.)

The long copper strips from the top contacts should also slot into the rectangular slots on the bottom PCB. Normally a bit of wiggling is needed to get them into the holes.

Step 9:

Before closing, test the switch by measuring it with a multimeter. Counter intuitively the contacts are closed (zero resistance) when the switch is out (normal position when not mounted in the frame). Measure both contacts. The resistance should be close to 0 ohm (< 1 ohm). (you should subtract the resistance of the leads which is about 0.4 ohm in my setup)

Step 10: insert the plastic rivets

I used a cable tie to keep the shell together and then inserted the plastic rivets again. I insert 2 rivets from one side (green circles) and 2 from the other side and did not melt them. I left the cable tie to enable disassembly if I have to open it again.


Posted Sat, Dec 4 2021 10:25 AM by OttoES
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