I should also mention that is is not likely the transformer that is the problem. This is because the two higher voltages look good. The transformer is usually wired such that these higher voltages are wired as series turns of the 8V winding. So the transformer will usually have one center tapped winding for both phases of the 8V. The +- rails would be two additional windings stacked on top of these low voltage windings. The plus and minus separated by the diodes polarity so each looks like a full wave relative to the center tap.
So all totaled, there are two separate high winding attached to two ends of the center tapped low voltage winding. The AC voltage on each of the windings should be around 5.8volts, give or take a little.