+/- 15v supply question - SN-voice

[synchroton]

Hi, working on putting together my SN-Voice. I found a +/- 15V supply PCB that I took out of a mixing console. It looks pretty straightforward, based on 7815/7915 plus some caps, rectifiers...

Anyway, I'm measuring +16.94v/-16.5v at the output. I find this peculiar because the 7815/7915 are sitting comfortably at 15v. The only components between them and the power supply output appear to be some diodes and a couple of resistors...

It's got four output pins, two of them being ground. I made my connection to ground by bridging the two grounds and soldering my connection there. Could this have anything to do with it? Not sure, but it doesn't appear as if they're linked on the board. I don't understand...

SN-voice seems to be working marvellously, however, I'm trying to calibrate it now and I can't get the triangle to center around 0v with the trim pot. I guess my options here are changing the size of the trimpot (or R21?) but I'd much prefer to get my power supply working properly. I imagine there may be other weird issues with bias voltages or what have you...

[synchroton]

OK, I guess I'm making more sense of it. Gnd and the ground of the regulators have the corresponding voltage difference... But why...?

[magman]

Not sure if this helps, but one trick with 78/9 series regulators is to put diodes in the ground leg of the regulator to raise the output voltage. Each diode gives 0.6V of boost, so if this is the case you should see 3 diodes in series with the ground leg of each regulator to give the voltages you mentioned. If present, shorting out or bypassing these diodes will restore the regulators to their normal voltage.

If the diode's are on the output, these would normally be used for reverse polarity protection and would drop the output voltage by 0.6 V as well.

Regards

Magman