I have just successfully repaired one of these. It was stuck in standby.
The problem is not the PSU board as commonly supposed, but the 10 x 220uF 16V through-hole capacitors on the mainboard. These are the notoriously unreliable Sam Young type which seem to last only until the end of the warranty. All 10 were significantly out of specification.
On this model, and several others in the Humax line-up, the main PSU (internal or external) only supplies 12V. The other required voltages are generated on the mainboard