Boot issue? Wandel & Goltermann PSM-39 Selective Level Measurement Set

[youngplayer]

I've recently obtained a PSM-39, however, it does not seem to work.
When I turned it on for the very first time, it said "main memory corrupted, using default settings", or something along these lines. I also remember it displaying a dBm number (like it should). However, on subsequent boots, all I get is this. https://www.youtube.com/watch?v=l7dU29ve7fU&feature=youtu.be
I'm guessing the beeps are similar to the BIOS beeps, and there's a specific error tied to that beeping pattern.
I have popped the top cover off, inside is this seven segment display, which displays the parts of the boot process that pass (I think anyways). There's also a lithium battery (3.5V, 750mAh) inside, that one is really dead (about 0.1V @ 10MOhms). The whole memory corruption thing and an empty battery makes me assume that that battery was for holding user settings in volatile memory in between boots. However, that would not explain why it won't display the correct stuff after boot. I have also hooked up a current limited PSU to the battery terminals with 3.5V, just to see if that does anything. And it doesn't - it draws about 175uA, be the device on or not, so it's probably flowing into the battery - I have not tested this for longer than about 10s.

tl;dr:
Does somebody have experience repairing similar devices or "memory corrupted" issues?
PS: Does somebody have any kind of document on this device or it's sister models? I have searched the depths of the internet, and the only thing I found was this datasheet http://www.testequipmenthq.com/datasheets/WANDEL%20&%20GOLTERMANN-PSM37-Datasheet.pdf, but I need a service manual or operation manual.