The problem is that the # key is part of the 12 numeric keypad buttons which are allocated to the SOLO function, either with live source coming in or in TEST mode (for SOLO, GROUP/MUTE, ALL). And the way they implemented the SOLO functionality (live or TEST) means pressing any one of the solo buttons solos that speaker which presumably has been associated with a virtual speaker (either the hard-wired buttons in the upper part of the remote, or the 12 numeric keypad buttons) triggers THAT speaker only, even if no speaker is associated with one of those 12 numeric keypad buttons. And that's the case with the # key , which has no default speaker assignment. But if you press it in "live SOLO" mode there's no speaker to solo so there's no sound. Alternatively, if # is manually assigned to solo some speaker, well that speaker's sound would be heard.
This is as it should be, but clearly prevents the # key from also functioning as "write BMP to SD card" when it's actually part of the set of keys assigned to SOLO, either live or TEST.
I think the documentation is incorrect here. Good idea to have an "instant screenshot to SD card", but it's definitely not going to happen with this key.
Well, I got an answer from James about how this BMP functionality actually works. It turns out it IS associated with the # key, but it must be manually activated. As factory delivered default it is deactivated.
Settings > System > Factory tests > Press enter to proceed > navigate down to "Save LCD screen as BMP" item and press ADJ+/- to turn on (switch will change to green). Then BACK out. The # key is now activated as the BMP-generator key.
I did this, and then performed some experiments to learn exactly how this works. I also wanted to understand how the seeming conflict between # used as a BMP key and its second use as a SOLO key (for a manually assigned speaker if you want, or just left unassigned as factory delivered) resolves.
(1) When you then press # key with anything on the LCD display screen the BMP file is written to the \Realiser folder on the SD drive. And when outputting to the SD drive there is a little light inside the drive that you will see flash for a second or two, indicating WRITE in progress. That is the visible confirmation that the screenshot is taking place. For these few seconds the LCD screen also "freezes", until the BMP output is complete after which the screen behavior returns to normal.
(2) It is also true that the # key absolutely IS a SOLO key, no matter whether a speaker is assigned to it or not (as it is not from the factory). Turns out the apparent conflict with the now activated BMP function is resolved by the fact that the two operations ARE BOTH PERFORMED... BUT IN SEQUENCE, first one and then the other!
So the BMP is written out first by that functionality for the # key, and then the SOLO function is performed by that functionality for the # key. So BOTH results are achieved. Not what I expected, but it works perfectly.
Note that this actually does kind of produce a somewhat unexpected result, but obviously there's no way around it if you want both functions to be operative for the one # key. It can only be resolved as they've done it, doing one then the other second. But be advised, since # is actually a SOLO key either for (a) no pre-assigned speaker, or (b) a manually assigned speaker, when the SOLO function kicks in second (after writing the BMP out to the SD card) you will either have (a) NO SOUND, since there's no assigned speaker to SOLO, or (b) only sound from the one manually assigned speaker. You will then need to press ALL to restore sound to all speakers, or perhaps press another SOLO speaker if you want to do that. However you decide to exit from this SOLO mode, I'm afraid you're always going to have to do that whenever you use # as your BMP output key... because it also triggers the SOLO function for #.
No way around this given that they programmed two functions for one key. So just be prepared for # to get the BMP, which will also result in SOLO of that speaker (or no sound if there's no speaker assigned), so you must then also next press ALL to get back to normal. I guess I can live with that for the rare instances where I really to want BMP images of the screen.
So, I took a few BMP screenshots. The function works with ANYTHING on the screen, i.e. with all display outputs. And the file names contain the date/time.
Here's what my \Realiser folder now looks like after four BMP screenshots.