sundance-communications.com/forum Forums Other TDM Phone Systems 1A2 and Comkey Telephones & Systems KSU power for 4A

Guys in this post https://sundance-communications.com/forum/ubbthreads.php/topics/565621/2/A_couple_4A_questions we discussed a way to send power to an 82B connecting block on the violet/brown pairs (24 & 49)of the run to the KSU and then briging them to the AC terms on the block.

What I have found is that while an 82B block with a wall wart transformer seems to work fine if I try to use the 18V from the G+/- and S+/- on the KSU it will immediately cause some sort of short that blows the 18V fuse on the supply. Its not a dead short it appears to be some sort of bridge to the DC supplies and happens when the 108 speaker is plugged into the block (meaning the short is not in the 680, the feed line from the KSU, the option relay or the telephone set.

So the question is: can I make a KSU supply of 18VAC from my 551B work? Is there something that needs to be lifted and stored in the 108?

At first I thought I had a wiring problem but I took an 82B and hooked up the AC terms directly to the G+/- and S+/- (actually one wire) and POP.

Thoughts?

It is a known fact (it's mentioned in the BSP) that the power supply for a 3-type speakerphone cannot be a grounded supply, and must be floating, preferably from a 2012B transformer. This is to prevent a ground loop from interfering with the speakerphone LK operation and causing weirdness.

I am not aware of anyone's trying to power a 4-type from a key system grounded supply, but I suspect that it is not allowed for the same reasoning.

The BSP for a 4-type is silent on this issue. That doesn't mean it can't be done, but your experience seems to indicate that it's not a good idea.

Yeah, I can see all sorts of weird ground loop issues being a possibility. I had 2 main reasons for wanting to do power from the KSU. I didnt really want wallwarts since they are ugly and I dont have a good place for them to live (hiding an 82B block is bad enough)> Aside from aesthetics its a power conservation issue. I have 4 of these things potentially and only one speaker unit is likely to ever be in use at a time and even then they arent used often.
I suppose I could use one wallwart and power all the sets via the brown and violet pair to the 82b blocks. Then at least they would be hidden in the equipment room....

And yes the BSP completely ignores the subject....