I have an old Mcal that needs descaling.
How do I do it?

I would still recommend white vinegar diluted to about 1.0M (4.5% or so.) It is a weaker acid than citric acid, and does not form insoluble salts as citric can under some conditions. You might also follow the advise of an earlier post on this topic to inspect the inside of the boiler by illuminating it with one of the special lights for illuminating rifle barrels. You should be able also to examine for pitting. These boilers seem to be made of brass. Stronger acids are really a bad idea with brass because they can induce "dezincification".

Sometimes people use sulphate containing water for making espresso and end up with CaSO₄ scale especially when they steam a lot of milk. You can tell CaSO₄ by the fact that it will not dissolve in citric or acetic acid solutions. The normal CaSO₄ descaling solutions are apt to damage a brass boiler, I suspect. (The safe way to remove it is to make espresso with purified water for several weeks, and if you do not like the taste of pure water espresso add sodium or potassium bicarbonate. Use about 100 mg/litre). Actually there is dramatically more Ca and Mg in the coffee beans than in the hardest water, but generally little bicarbonate.

I would examine the top of the group for pitting. Avoid using water with much chloride in it.

When I got a new MCAL there was a sheet accompanying it warning that Elektra will NOT guarantee the machine against any kind of damage from poor quality water.

4.5%, so 45 ml in one liter. I put it in the boiler and then what?

If you purchase vinegar at a grocery it is already near the proper strength, some is already 4.5% others about 6 or a bit more. So you can just use 4.5% acetic acid vinegar as it comes from the bottle. 6% you can dilute a bit, perhaps two to one. (If you have access to pure "glacial" acetic acid, add 45 mL to one litre.)

Drain the boiler completely. Then fill it completely to the top with the vinegar diluted to give you about 4.5% acetic acid. It is best to let it stand for a while, it is a good idea to turn on the machine just enough to raise the temperature to perhaps 35 or 40 degrees right at the start. You should see bubbles rising from the liberated carbon dioxide. I have always let it stand for several hours and then drained the boiler and looked inside to check what is left. Then you can pour in a second batch of the vinegar (dilute acetic acid) if there still be CaCO₃ deposits.

Rinse with tap water at least four times, preferably five, to remove the last traces of acetic acid. It does not take much residue to make espresso that is like salad dressing.

If you find residue that does not dissolve it could be calcium sulphate. It dissolves to the extent of about 2 grams per litre in water. It usually results from doing a lot of steaming. The only safe way I know to remove it is to use Calcium and sulphate free water for a few weeks. I had an used 1978 La Pavoni that was filled with calcium sulphate. I used pure uncontaminated water for a few weeks and it slowly dissolved completely. (Note that light roast coffees, and certain coffees can taste like club soda when doing this, use a coffee that is not acidic should this be necessary.)

Thanks for the reply. Just arrived hope and will be taking care of the machine.
So I am going to the shop today to buy some vinegar.
When I turn on the machine, how hot can I make it and do I need to have the cap on or can it be open?

When I turn on the machine, how hot can I make it and do I need to have the cap on or can it be open?
Also for how long should I have the vinegar in the boiler? Just to be sure

Think the answer is already in the second response by rpavlis, heat up to 30-40°C, would say without the top as it mentions you should be able to see bubbles and leave for several hours, repeat if necessary.

Ok. Should I also soak in the cap as it has a built in safety valve?

If that cap sits in the boiler above the water I would reckon not as steam is pretty much as clean as distilled water, but I'll leave this to the folks who know this machine...

You're right about it being as clean as distilled water...

1+

Since distilled water, is boiled > evaporated > condensed

The boiler boils the liquid, evaporates it to steam and it will condense somehow (arguably the steam wand has some condensation, hence the rule of purge-ing exists)