There's a tiny spring on the power switch held between two hooks. One hook is the slider and it's usually white.
You can try pushing the white hook a little in the same direction the spring pulls and I'm sure that
the power switch will release. (Don't work inside the Beomaster with mains connected!).
However, the next time you switch off, it will probably seize again.
In some cases, the spring will have lost a bit of tension and it can be tightened slightly to make it work again.
This may, however, only provide for a temporary working solution.
The correct thing to do is to replace the power switch but it's no longer available.
Next best thing, and it works just as well if done properly, is to take the power switch apart, clean it and put it back together,
which is where the job gets a bit fiddly.
Inside the power switch are the four stationary contacts that has the mains leads soldered at their outer ends, two
moveable contact points and a slider with a spring with two nylon bushings. It's all within about 1cm2 and parts
will escape to all sides when you take the switch apart (doing it inside a large zipper bag or similar will help you
keep track of all stray parts).
The contact points (six in total) and the holding points for the moveable contacts should be cleaned off oxidation
and they will usually have quite an amount.
Inside the switch housing you will most likely find a nice amount of fiber dust from the wearing of the housing and slider.
This dust is easily removed with a tiny brush dipped in IPA.
Resist the temptation to lubricate the slider, it will only bring headaches later (don't ask how I know).
The contact points etc. can be cleaned wiith good results using a glass-hair brush.
Don't use contact cleaner in a mains switch, clean the contacts manually.
Contact cleaner sprayed into a switch may shift the dirt around a bit but it will not remove anything.
When it dries, it will allow the dirt to settle down where it just happens to sit now and in the best of
cases the contact cleaning agent will evaporate completely and not add to the mess, that's rare though in that
most contact cleaners contain a little amount of lubricant, silicone or whatever and that will only attract and
hold on to more dust and dirt.
Never use any WD40, 3-in-1 or other similar wonders and rust loseners in a stereo, it belongs in the garage.
The switch housing is held together with small melted plastic "rivets" which will have to be broken off and the housing
closed again using a tiny amount of superglue but not until you are sure that it works as intended.
A temporary fixation can be made using nylon cable ties (which can be left in there as an extra support if there's room).
Gaining access to the switch and having it dismounted from the Shadow-type switch bank is in itself a task.
Basic electronic tools and skills are required. A good amount of patience, handling of small parts (tweezers etc.) and
some soldering is involved as well.
No, it's not easy but yes, it's worth it and very rewarding too.
Any good technician can do it.
For reference, it normally takes me about an hour (not counting the time spent looking for stray parts).
The above applies to mains switch problems in Beomaster 2200 and derivates Beocenter 3300 and 4000.
A similar switch with similar problems is found in Beomaster 1600 (the caterpillar version) and 1700, it
can be repaired the same way and accessing the switch in these Beomasters is much easier.
Martin