Power supply issue, wrong voltages.

[Heemis]

I know this is primarily a stombox forum, but this issue is just really bugging me and I was hoping some of the brilliant minds here could help!

I recently bought an Allen and Heath RPS9 power supply off ebay to use with an early 90s Spectrum board. When the PS arrived, I immediately tested the voltage. To my dismay, I was getting +27v on the +15v rail, and -20v on the -15 rail. +48v seems to be fine.

Does anyone know where I should start tracking down the problem? I have a fair amount of electronics experience working on amps and pedals, but haven't had to deal with voltage regulators and the like.

If it helps, the voltage on the secondaries of the power transformer are reading +22v AC where the schematic calls for +18.5v.  This is putting + and - 30v DC  at the inputs of the regulators.  I figure this is due to my home mains voltage being 125v instead of 120v.

Here's a link to the schematic for convenience: http://www.allen-heath.com/media/RPS9-SCHEMATICS.pdf

Thanks!

[tubegeek]

If a power supply isn't connected to its load it will often read high.

Try connecting it to the equipment and reading the voltages then.

[Heemis]

I did think about that... but isn't this supposed to be a regulated voltage?  If I remember correctly that should mean the output voltage should be constant regardless of load.

[armdnrdy]

Scroll down to the bottom of the page. (the area highlighted in yellow)

http://www.musicfromouterspace.com/analogsynth_new/WALLWARTSUPPLY/WALLWARTSUPPLY.php

[Heemis]

Hmm... says that the load thing is a characteristic of the negative regulator... could that be the case for the positive reg as well??  I guess I need to measure it at full load!

[slacker]

You can temporarily connect about a 470 Ohm resistor from the negative output to ground. That should provide enough load to pull it into regulation, and you should then get about 15 or 16 Volts out.
You could try the same thing with the positive side but I'd be a bit suspicious of them. I wouldn't have thought the Voltage would be that high even if they weren't regulating.

[Heemis]

Would it be ok to use a 1/4 watt resistor?  Do you think it's a bad idea to try the same trick on the positive rail?  The regulators are getting 30v DC at their inputs, so maybe that 27v isn't that crazy?

[slacker]

15 Volts through 470 Ohm is about half a Watt. You could probably use a 1/4 Watt if you don't leave it connected for very long.  Two 1k resistors in parallel would be better if you've only got 1/4 Watt. No harm trying it on the positive side.

[Heemis]

Found a 3 watt 

So I tried it on the negative rail and it worked like a charm, brought the voltage down to -15.  Hooking it up to the positive rail ended up still giving me +26v... so I think I've got a couple faulty positive voltage regulators.