My UK power adaptors all say 7.5V output on the label.  No load output from my pyramid bases is 5.65V.  I would be a bit careful about buying a charger with 9V on the label - perhaps it is for something else?

Be aware that a original* BC6000 Mk2 cannot be charged on a Mk1 charger due to additional plastic elongations around the charging pins  There's a photo on the forum somewhere. 

* As opposed to a Mk1 that has been modified to become a Mk2 by fitting a new keypad and motherboard

And here's the thread with the photos of the Mk1/2 charger differences: 

Thank you! I was thinking that MkII handsets were reverse-compatible with MkI pyramids but guess it must be the other way around... Only problem is I can't tell the difference between different versions of the pyramids, since most ebay sellers don't specify or show close-ups of the charging pins. At first I thought the indicator light was unique to MkI pyramids but then I realised the pyramids *without* an indicator light all seem to be table chargers, not PSTN bases with charging capability, necessitating the more costly purchase of a separate PTSN base.

Did B&O remove the PTSN feature from pyramids to force customers upgrading to MKII handsets into purchasing new separate bases?

I suspect you're right about the seller having confused the 9v power adapter, and it actually belonging to something else. I won't bid on it.

ccarriee wrote:

Did B&O remove the PTSN feature from pyramids to force customers upgrading to MKII handsets into purchasing new separate bases?

I think that the separate PSTN also gave additional functions, such as support to more handsets (and other models of handset, like BeoCom2.  There's some useful info here: 

I appreciate your ongoing advice - this is really helpful!

Guy wrote:

It's well worth getting a Mk2 rather than Mk1 Beocom 6000 - more reliable and more functions.

Love the colours for older handsets which is my main motivation for choosing an MkI base to be honest, however I'm aware of the RF chip problem and that does make me a little nervous buying secondhand...

Guess a compromise, albeit costly, would be to purchase a separate PTSN base and initially try an MkI handset + pyramid. Leaving me with the option of changing to an MkII handset + pyramid if the MkI handset proved itself unreliable.

The older (larger) separate PSTN base appears to be available more cheaply than the newer (smaller) separate PTSN base - other than the number of handsets you can pair, is there any significant advantage of spending more on the newer one? Thanks again.

If I was starting again, I would go for the larger PSTN base, together with a Mk2 BC6000 and compatible charger.  I can't see any particular advantage of the newer PSTN in a simple setup.

My PSTN is built into my Mk1 pyramid charger in my study, and I use 3 Mk2 Beocom 6000s with it.  Luckily, one of the Beocoms is a Mk2 that was converted from a Mk1, so it sits in the PSTN/charger just fine.  The other 2 Beocoms have to charge in their own separate non-PSTN bases.

All my Beocoms connect fine to the pyramid PSTN, despite it being at SW version 1.4.  For some reason the FAQ says that SW version 2.5 is required, but this is not my experience.