Alternator screeching when under load

[radek at istar.ca]

Hello group, need your help.

My 88 90Q has been doing this for some time now.  When the electrical load increases (like
having the 
headlights on and switching on defrost) the alternator would start making the most awful,
high-pitched 
screeching noise and stop charging (voltmeter drops from 14V to 12V or so).  First, I
suspected the belt 
slipping but tightening it would not solve the problem.  Next, I suspected high electrical
resistance so I 
took out the alternator, removed the voltage regulator, checked the brushes and cleaned
all the 
contacts.  Still no help.  Yesterday the screeeching got worse and on my way back from
work the belt 
broke.  

I know it looks like the belt is slipping but I don't think this is the case.  It is
definitely moving.  The 
sound seems to be coming from inside the alternator.  Bearings?  Old age?  Time for
rebuild?  I'm 
planning to take the alternator apart on the weekend.  How easy/difficult would it be to
replace the 
bearings and how to find the correct ones (don't want to pay the dealer).
Any insight will be appreciated.  Thx.

Radek
88 90Q
91 V8Q
5-sp.