We consider a multiple-access system in which each user is given q out of Q available subchannels (q ≪ Q). We investigate capacities of two channels emerging in the system under consideration in the case of single-user reception. These channels can be considered as modifications of the A-channel. In the first case we assume that a signal transmitted by a certain user is always registered by the receiver. Thus, activity of other users is the only obstacle to correct reception of information transmitted by a certain user in the multiple-access system. In the second case we assume that the output of each subchannel is inverted with probability p. Analytical expressions (both asymptotic and nonasymptotic) are derived and investigated.