HP 7907A woes

I have just pulled my HP 7907A out of storage and tried to power it up. No joy, it emitted a couple of squeaks from the power supply and that was it. I have taken a look on bitsavers and HP museum but can only find general service information for this unit. Does anyone out there have more detailed service information for this box? The PSU itself is a switcher, manufactured by SSI (Switching Systems International) - model SQV250 - 1421 manufactured 6/89. Perhaps someone has the schematic for a similar PSU? Does anyone have a copy of the user manual for the 7907A? The unit has been powered down with a cartridge installed - will the heads have been fully retracted? Is there a way that I can power the unit up that ensures the heads will remain retracted so that I can remove and check the media? Peter Brown