[RFI] DC power supply RFI

[Van Fair]

I purchased on ebay  for $10 a new OEM DC power supply. 120 AC in 24 DC out at 2.5 amps. I want to use this to power my SPID DC rotator. The unit is a HITRON HSH60-13. It is heavy duty and weighs about 3 lbs. with one good size transformer. The unit size 4" X 4" X 2" high and in a steel case. The quality looks good. When only connected to AC there is no RFI noise on 40 and 80 meters. As soon as 1 wire going to the control box is connected an S5 noise appears and the RCVR scope shows spikes every 10 KHZ. When both wires are connected the RFI goes up to S6. With a 3 foot unterminated wire connected the RFI can only be heard at the noise level so I suspect the RFI is traveling into my system on the input or output power supply wire and not through the air. Can anyone advise me a simple way to eliminate this RFI. The power supply is junk as it now is. Many thanks for all help. Van W4GIW