One channel gets weak in Mac Receiver 1700

Have you considered contacting McIntosh service and support. I have always found them to be extremely helpful and cooperative.

A tube is going. Simple fix, about $20.
You should have a pair in there. You can switch them from side to side and see if the problem moves.
Don't touch when hot, definitely replace in pairs.
Great vintage receiver.

Sounds like the tuners section getting weak on one channel.
Try the tubes as Elevick mentions or tuner cleaner. I think the 1700 has the gang type tuner? If so the cleaner may solve it, if not you might also check with AudioClassics.com they are McIntosh people that know the older components quite well.