Priority Calling Modifications
My description of priority calling in yesterdays post wasn’t quite accurate. I’ve dug out the Operational Bulletin and checked. It says:
3.4 Priority Service
An official having this service may speak instantly to any extension on the exchange, whether the extension is already engaged on a call or not.The caller dials the required number and the Link Selector wipers extend the call to the required line.
If this line is free the bell rings and the called extension answers. If the called line is engaged no Busy Tone is heard and the caller is instantly switched into the existing conversation.
The call at this stage is not private; when privacy is required both engaged parties must replace their handsets, ringing current is then automatically connected to the required party’s bell and the call proceeds in the normal manner.
The circuit diagram notes say this:
7. Priority Facility
The priority facility enables an extension to speak to an engaged extension when the line that is dialed is already engaged.An extension line with priority has tags K and PC strapped on the tagblock. On completion of dialing the two digits, relay G operates if the line is engaged.
Relay G operated:
G5 completes a circuit to operate relay E (NEG. 800E, C2, G5, (TL6) (NPB) S1, T1 & 2, U3, FS bank tags PC to K, K2, L1, FS (P) bank, U27, B6, T9 and 10, POS)Relay E operated:
E1 completes the circuit to operate relay D (NEG. 200D, 500YB, G3, NR1, E1, B1, POS)
E4 disconnects Busy Tone from the calling line.Relay D operated:
D2 & D3 complete a speech circuit across the called extension lines and the caller can speak and hear.If required the priority extension may request the extensions to clear the call in order that he may speak in secret to the required party. In this case on the line circuit of the required line being restored, NEG is replaced on the PN wire from 1300K, in the line circuit and relay H is now operated from (NEG, 1300K, PN bank, 900H, C7, E2, NR4, N2, B1, POS)
Relay H operated:
H6 releases relay G followed by relays D and E.
With relay E normal, ringing current is connected to the called line at E3, and the call proceeds in the normal manner.
So that’s how it’s supposed to operate. How does the modification I’ve got change this operation?
This diagram should ideally be read in conjunction with the original diagram – as the diagram above doesn’t show the whole circuit, just the PC modifications (with a few relevant bits of circuitry just so you can orientate yourself on the main diagram!) As far as I can tell, the new sequence of operations is:
Relay PC operates:
PC is operated as follows: (NEG. 2000E, (TL6) (NPB) S1, T1 & 2, U3, FS bank tags PC to K, K2, L1, FS (P) bank, U27, B6, T9 and 10, POS)PC2 completes a circuit to operate relay E (NEG. 800E, C2, G5, PC2, (TL6) (NPB) S1, T1 & 2, U3, FS bank tags PC to K, K2, L1, FS (P) bank, U27, B6, T9 and 10, POS)
Relay G operated:
G5 completes a circuit to operate relay ERelay E operated:
E1 completes the circuit to operate relay D (NEG. 200D, 500YB, G3, NR1, E1, B1, POS)
E4 disconnects Busy Tone from the calling line.Relay D operated:
D2 & D3 complete a speech circuit across the called extension lines and the caller can speak and hear.If required the priority extension may request the extensions to clear the call in order that he may speak in secret to the required party. In this case on the line circuit of the required line being restored, NEG is replaced on the PN wire from 1300K, in the line circuit and relay H is now operated from (NEG, 1300K, PN bank, 900H, C7, E2, NR4, N2, B1, POS)
Relay H operated:
H6 releases relay G followed by relays D and E.
With relay E normal, Int Ring Tone is connected via PC1 to the called line at E3, and the call proceeds in the normal manner.
I have no idea why you would do this.
Int Ring Tone isn’t designed to ring a bell (it’s a tone!) and as far as I can tell, the only conditions under which PC1 is operated are when the called party is off hook – so I’m at a complete loss as to what this mod was attempting to achieve. Having documented it thoroughly, and spent a number of hours headscratching trying to work out what it’s for… I think I’m going to declare it useless and reverse the modification.