Bizzare behaviour from bench PSU

I've got a great bench PSU that's err, not so great...

THANDAR TS3021S 30v 2a bench PSU with current limit.
With the output switch off, the voltage and current controls act as expected, allowing full control. As soon as the output switch is enabled the voltage plummets to 400mV, as though there's a short ( voltage verified with DMM) and it goes into current limit mode as indicated by an indicator on the LCD panel. The voltage stays stable. The current kicks in at the set level (900mA) then falls towards zero, goes negative (weird i know) and keeps going increasingly negative until -1999 mA, then indicates -1 whic I believe is 'out of range' This fall takes maybe a minute, and can be instantly recreated by briefly switching off then back on again.
There is no load connected and the sense links are tightly connected.

Thanks

as though there's a short

Click to expand...

That's what I'd be looking for, starting with the output switch itself.

Presumably there's a reverse polarised diode across the output?, that could well be S/C.