Before I start, I want to give anybody that will read this in future a good tip that I wish somebody has given me before I started fidling with lenses: Use gloves and try not to touch any inner lens surfaces because you will never be able to clean them to the point they were before and if you try too hard, you'll end up with scratched coatings. You can see people opening lenses with their bare hands on youtube. They're literally ruining them. You can fix a lens in a way it looks like new, but only by working slowly and patiently. So take a deep breath first!
Other usefull tips:
For lubrication I recommend high temperature vaseline optionally mixed with a little vaseline oil for required thickness. They are neutral to plastics and optics. The actual lubricant used on helicoids is propably lithium grease although I know little of them and also I like to have light and fast focusing pull for action shooting and video. I don't know the exact term, but there is available "clean" or "technical" vaseline and vaseline oil in bike shops, also used in sewing machines and for securing gaskets.
For cleaning fungus and generally for inner lens surfuces use lens tissues. It's a special kind of paper that doesn't leave any fibers on the cleaned surface. Use it slowly in a controlled manner with straight sweeps and never use one part of a tissue twice. If all you need to clean is just a tiny bit of fungus, then consider living with it instead. As long as you expose it to light (ie use it) it won't grow and you won't be able to spot it's effect.
To the point... If I'm not mistaken it all applies to ML 50mm 1.9 as well. They look identical. I haven't got pictures yet, but let's start with main reasons to open a Yashica ML 50mm f2 from the front:
adjust focusing to infinity
fungus in front of the aperture
uneven, gritty or heavy focusing ring
Next there are things you will be able access from the back:
fungus on the inner surface of the rear lens
fungus on the front most element of the rear group (right behind aperture)
declick aperture for video
unspecified mechanical problems with aperture control (springs and such)
Now to do the following operations you need to open it from both sides:
oily aperture blades
fungus right in front of the aperture
other mechanical problems with aperture
I don't remember if you actually need to open it from both sides to get full access to focusing helicoid in case you want to relube it, so no advice here.
Now I'm not a specialist. I just experimented with my sample for long enought to come to conclusions. I suspect there may be better ways to do things, but here are simple, most basic steps to all above:
opening from front:
Focus to minimum distance so that you can grip the barrel nose firmly.
Give the barrel nose a strong twist counter clockwise. Do not try to lift the front nameplate. It's completely unnecessary and it doesn't give you access to anything, really. Also it's glued.
Before you unscrew the nose completely, be aware, that front lens group behind it is completely loose. Hold the lens face up and unscrew the front. You now have access to focus ring and front lens group.
Now you see three screws holding a brass ring along outer edge of the lens. unscrew them to gain access to focusing helicoid in case you want to change infinity point. If you just want to clean the lens elements, don't do it unless you have spare time to readjust focus!
If the front group doesn't fall out, you have to pry it. Use a sharp blade. Insert it between the piece of plastic the front element is in and a metal thread.
Now you can see the back surface of the front lens group. You cannot disassemble it any further, so fingers crossed fungus isin't there.
You also can see aperture but be aware you need to first open the back as well, or you can break it.
Opening from back is easier. What you really need to know is that you absolutely must use JIS screwdriver. I picked a philips bit by accident and almost broke my lens beyond repair.
Only remove the screws on the silver part of the lens mount. They may sit very firmly in place, so push quite hard on your screwdriver.
Lift the mount really gently. Right underneath there is that tiny aperture ring ball. It sits on an equally tiny nest, which really is a microscopic spring. If you are unlucky the ball and the spring might stick to the mount andyou'll have to put them back in their slot.
Rear lens element can now be easily accessed by unscrewing it's frame. But don't do it unless you double checked that there is fungus right on it's surfuce(very unlikely). If you're like me, you'll think the fungus is there, and you'll end up with dirty rear lens group. They are VERY hard to clean because of their frame.
Now notice there are two springs. One is on the mount and the other inside the barrel. We're interested in the latter. To access aperture unlink this spring from the little pole-shaped silver screw. Or is it screw shaped pole?
I think that's it. Correct me if I'm wrong. I'll think about pictures later if necessary.
Tua, Is the fungus big or positioned in the middle of the lens?
I'm asking because when the lens is in use, the fungus doesn't grow. I noticed my other lens has some fungus on the edges but it doesn't show in pictures. In an attempt of disassembling, you can damage your lens or make the wrong lens element dirty (it's hard to tell which surface has fungus on it). And you'll never make it as clean as new. If the fungus isn't too bad, then maybe just give it a name
Unfortunately I couldn't test heat the glue, as I sold mine. I just had a 1.9 and couldn't justify keeping both as they're too similiar. I have an idea though. You can try to unscrew the nameplate. Then use a few drops of acetone on the inside of the thread. The important part is not to use acetone on plastic or it will melt.