The ring, tone and time pulses come from a pair of relay-sets which jack-into a separate metal case to form one unit. This complete unit was known as a Unit, Auxiliary Apparatus No. 78 and was mounted on an extension to the MDF ironwork. The relay sets were very similar to the U5 versions. Again no dial tone was provided. This UAA also incorporated the fault test number, which gave the same alarms as the U5 version.
The line circuit has three relays per line. When the line is looped this causes a start condition to be applied to the allotter circuit, which causes a free connecting link linefinder to hunt for the calling line. When the linefinder has found the calling line the allotter steps on to the next free connecting link. If there are no free connecting links available the allotter steps to the 25th outlet, returns busy tone to the caller and operates the overflow meter.
The caller is now switched through to a free selector. The selector is now stepped vertically in response to the first digit dialled by the caller. If we assume the caller is making a local call, the digit dialled will be either a 2 or a 3. The selector steps up 2 or 3 levels accordingly and then releases, this first digit being absorbed as it only serves to indicate which of the bank contacts the final number is on.
The selector now responds to the second train of impulses, which again step the wipers up vertically, the third train of impulses stepping the wipers into the bank to the required number. If the called number is free, ringing current and ringing tone is applied in the normal way. If the number is busy, busy tone is returned and if the number is unallocated number unobtainable (NU) tone is returned.
The numbers in the 2xx range are allocated to private subs and single line businesses, with businesses that are expected to, or already have more than one number, allocated numbers in the 3xx range. The reason for this is that the selectors only have 2-10 PBX hunting facilities on the second hundred (i.e. 3xx) numbers.
The PBX facilities are afforded by the means of an auxiliary PBX screw-arc. The extra (auxiliary) lines, on the next bank contacts follow the main number for the PBX. This allows all the lines apart from the last in the group to have screws inserted in the PBX arc, which allows the selector to step on to the next free line. If all the lines are busy the selector is stepped on to the final line, and if this is also busy the lack of the screw in the arc prevents the selector from stepping on and busy tone is returned.
The U6 could have up to 20 junctions, these junctions are manual so no digit repetition is possible. These lines being terminated on levels 0 and 9 of the second 100 selector levels. As with the U5, two digit exchange codes are used, with the parent manual board being allocated the number 01. Therefore when level 9 or 0 is dialled the selector does not absorb the digit, but wiper switches to the second hundred bank. The second digit steps the wipers to the required junction. The junctions are usually allocated in groups, which allows the PBX action previously described to search for a free line. When a free line is found the selector applies ringing current to the distant exchange, thereby operating a calling signal (usually a lamp), and transmits ringing tone to the caller.
Local CCB calls are effected automatically. Outgoing junction calls from a CCB are provided with a discriminating signal, to indicate to the operator that the call is from a coin-box and that a
fee needs to be collected. This signal was the flashing of the calling lamp, and in this way the U6 set a standard that was followed on all subsequent UAX’s. The coin-boxes where, as with U5’s,
a prepayment type, supplied with a tele 242, which again allowed the digits 01 to be dialled without inserting money. I presume that calls to other manual exchanges allocated numbers in the 9x,
and 0x range would have had to be completed via the parent operator, as the dial would not allow these other digits to be dialled without a fee being inserted.
The U6 equipment would be installed in the exchange building in much the same way as the U5 was. This basically means that there would be either one or two Frames, MD 0/240 installed over the cable chaise, with the auto equipment butting up to the frames. If only one frame was provided to start with and there was a likelihood of the exchange growing to more that 100 lines, a space would be left between the first frame and the auto units to allow a second one to be inserted. The frames were lain out in the same way as the U5 frames, and again the cables to the auto units were lead covered to exclude damp (remembering that the buildings were not heated.) The jumpering of line circuits to final selector multiple numbers was again achieved at the top of the units.
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