My old RCA WP-702A power supply (c. 1971) has crapped out. The voltages on each half of the supply measure fine until you put a load on them. A load on the left-hand unit causes it to oscillate, while a load on the right-hand unit causes it to drop to 0V. So've I've opened it up to see how the circuit was put together. So far as I can tell, the basics of the circuit, minus the regulation, is as in the diagram below.
I haven't looked at a lot of power supplies, but I was struck with how the AC from the transformer is being rectified. They've taken the center tap as the positive and they are half-wave rectifying the top and bottom halves of the secondary to form the negative. This gives them the same pulses they would get from a full-wave rectifier attached to the ends of the secondary (at half voltage), but saves them two diodes.
So my question is why do this? Were diodes that expensive when this was designed? Does it reduce manufacturing costs on the transformer somehow? Or are there other advantages?