[sdiy] Problem with 723 PSU

[jorgen.bergfors at idg.se]

Hi all power supply gurus.
I have built two power supplies from the Formant schematics.
They each consist of two 723-based regulators, where the negative output of one is connected to the positive output of the other, to form -+15 volts and ground. So it is really two identical positive regulators, connected together.
Anyway, this works fine as long as the transformer is connected directly to the units.
But I wanted to put the transformer in a separate enclosure, to keep 230 volts out of the main unit.
The cable from the floor unit is two metres long and this is evidently enough to cause some instability in the negative regulators.
There is a small transient, with accompanying ringing. It is synchronized to the main frequency. It is only around 0,2 volts, but it is enough to cause a slight buzzing sound in some modules (both VCF).
If I break the ground connection between the sections, the problem goes away.
Loading the output reduces the ringing, but it doesn't go avay completely.
I tried changing the freq comp capacitor and several other parts, but nothing seems to help.
Any ideas?

/Jorgen