Sound from my turntable is veiled (like behind a curtain)

 Be very careful about putting pure alcohol directly onto your stylus. Alcohol may dissolve the glue that holds the stylus to the cantilever, and soon you will be playing LPs with a cantilever and no stylus. Styllast ,the company, makes products specifically designed to clean styli. You could buy a bottle of stylast very cheaply. Also there are other products like magic eraser and ultrasonic cleaners That do a good job cleaning a stylus. But alcohol is a nono.

Toshiba almost certainly needs maintenance if it’s from the 70s or 80s.  Your own report should tell you that at the very least the switches are dirty and probably also the volume controls. That is why you are occasionally losing the output from one channel or the other. That alone could cause the sound to be veiled. But having said all that I agree with others that there are any number of modern inexpensive outboard phono stages that would probably be superior to the Toshiba, even if the Toshiba is working 100%. 

+1 ggagnon! I clean the styli on my cartridges after every side. I have used isoprophanol on the Discwasher brush for decades without any damage! The use of a cleaner from AT or LAST is a good recommendation! Before playing a side I like to apply a lubricant (Stylast). Claimed to reduce wear on the diamond stylus!

@hifiman5 The setup of the table was done by a professional.  The store where I bought my cartridge.

@lewm and @roberjerman I will definitely look for a solution to clean the styli more often.

I am convinced now to find a good phono stage, any recommendation will be appreciated, knowing my cartridge is and MC High Output.

Thank you all for you answers. 

Rob, The only Discwasher brush I know of is designed to clean LPs before play, not styli.  Discwasher makes a solution to be used with that brush that does probably contain a low concentration of isoprop, but the OP seems to have used undiluted isoprop, which is another kettle of fish entirely.  Furthermore, I think the Stylast cleaners actually meant for stylus cleaning may also contain diluted alcohol.  My point was not to use UNDILUTED alcohol to clean a stylus.  But above all else, I think dirt on the stylus is not the primary cause of the veiled sound, maybe a contributor but not the major issue.

ggagnon, There are dozens of highly touted phono stages in the under $1K cost category.  Unfortunately, I have heard none of them and cannot help.  I was fairly convinced by Michael Fremer’s review of it that the iPhono2 must be very good, for $500, and handles LOMC cartridges fine.  You’ll get dozens of other suggestions.

@lewm The alcohol I used was 70%, so it is not pure.  But I will follow the advice of using special solution.  I didn't think that could damage the styli itself, I was very careful to not put alcohol on the cartridge plastic casing.