Well, quite OT actually

I'm filling in a few days before my PIC programmer arrives by constructing a
power supply. Simple job. Couldn't be easier. Doesn't work.

I thought it would be handy to have +/- 1.2V to 20~30ishV to hand, with the two
rails independently adjustable. I'm using a 317 and 337, as you'd expect.
Individually, connected to an unregulated source of +20V or -20V (respectively)
they do what they're supposed to. The 'adjust' pin looks at a voltage divider
formed from a 330ohm resistor and a 5kohm pot. Twiddle the knob and
Vout = 1.25(1+Variable/330). Lovely. Then I connect them together; they share
the series-connected dual windings of a 15V transformer (to give 30V), a bridge
rectifier, and earth. Now each 'tracks' the other, so that if I've got +20V and
-20V 'dialled in' on the two pots, when I reduce to, say +10V, the negative rail
slowly approaches -10V. Now if I dial in -5V, the positive rail reduces to about
+5V. The way the 'uncommanded' voltage changes looks like a curve of a cap,
charging (or discharging) through a resistor. There is a 10uF cap from the 'adj'
pin to ground 'to improve ripple rejection'.

In the words of Marvin Gaye, What's Going On?

Sorry to be so long-winded, but I've stared at this for several evenings, and I
don't know where else to turn (sob).

Regards

John Midgley