eric6.gif (7121 bytes)MINIRAX Mk II



A self-contained 24-volt unit auto exchange for 20 extensions with three both-way junctions and four connecting links. Can operate as a discriminating satellite with or without S.T.D. or as a dependent exchange on a parent manual, auto-manual or auto exchange. The local battery is trickle charged over a physical pair junction during idle periods. Other junctions can be carrier circuits. If there is a local mains supply, all three can be carrier circuits.

Diagram showing the principle of Minirax used as a dependent auto exchange

The latest (Mark II) version of Minirax, which is a single-unit self-contained auto exchange for 20 extensions with three bothway junctions and four connecting links, is more than ever acceptable to administrations requiring reliable auto telephone service for remote areas, since it now offers facilities similar to those of the larger public exchanges, including provision for S.T.D. Thus Minirax can be used as a discriminating satellite interworking with other automatic exchanges in the area and having access to the national network if required. It can, of course, still be used simply as a dependent exchange on a manual, auto-manual or automatic parent exchange.

General view of the Unit

Operation is from a 24-volt 12 ampere/hour battery contained in the unit and automatically trickle charged over a physical pair junction during non-conversational time. The other two junctions may be carrier circuits, in which case the physical junction is the last choice.

Alternatively, all three junctions may be carrier if there is a local mains supply to which a battery charger can be connected.

For dependent exchange working, Minirax extensions are numbered 20 to 39; access to a junction is obtained by dialling 8, 9 or 0.

For discriminating satellite working, a two or three-digit prefix is added to suit the numbering scheme in the area. Seizure of a junction and the switching equipment at the parent exchange is immediate on pick-up.

The maximum loop resistance of subscriber or junction lines including the telephone is 500 ohms and the minimum leak resistance 50,000 ohms.


  1. Automatic connection between subscribers on the Minirax.
  2. Automatic connection from the Minirax to a parent exchange.
  3. Physical or carrier junctions to parent exchange.
  4. Alternate testing of 1st and 2nd choice junctions.
  5. Coinbox discrimination with visual or audible signal to parent exchange operator.
  6. Single-fee, multi-fee and periodic metering.
  7. Manual hold.
  8. Visual and audible fault alarms.
  9. Visual indication of a fault condition extended to a panel in the Minirax building or in an adjacent building.
  10. Alarm test.
  11. Ringing and tones periodicities to administrations’ requirements.
  12. Battery charging over junction.
  13. One of the four connecting links can be reserved for incoming junction calls to prevent isolation from parent if other links are engaged on local calls.
  14. Junctions are guarded against follow-on calls if calling subscriber clears first.
  15. Route discrimination and barring.
  16. Subscribers are prevented from dialling back over junctions. This allows a common group of incoming selectors to be used at the parent exchange.
  17. Common equipment is not held if the subscriber delays dialling or fails to replace the handset or if there is a permanent loop fault.
  18. Conversion from ‘dependent exchange’ to ‘discriminating satellite’ working is effected by strap adjustment and the use of relay sets appropriate for the type of junctions and metering.
Minirax Mark II Unit with doors off

The equipment has full tropical finish and p.v.c. insulated conductors. It is accommodated on a single-sided dustproof unit of pressed steel construction with two lift-out doors at the front. The unit is enamelled opaline green on the outside and cream inside. It is self-standing and supported 4 in. (102 mm.) from a wall.

The upper shelf of jack-in relay sets at the bottom of the unit accommodates circuits for four connecting links, a ring tone and meter pulse generator, and the alarm and miscellaneous equipment. On the lower shelf arc three junction relay sets and three discriminators or route restriction and metering sets.

The unit has capacity for the following items, those marked * being provided as required.

Subscribers’ line circuits 20
Junction relay sets (Physical or E & M) * 3
Discriminators or route restriction and metering sets * 3
Connector switches and relay sets * 4
Protectors and fuses for subscribers’ lines * 20
Ditto for physical junction lines * 3
Junction overflow meter 1
Subscribers’ meters * 20
Ringing and tones relay set 1
Alarm and misc, relay set 1
Alarm display and low volts adjust panel 1
Battery jacks 2
Test line jack 1
24-volt battery * 1

The connector circuit includes heavy duty uniselector (B.P.O. No. 2 type) line finder switches and connector switches, the subscribers’ and junction lines being terminated on the linefinder bank contacts, which are multipled to the connector bank contacts.

Plan view of Unit showing the 24-volt battery in acid-resistant metal box freely positioned on mounting.

Relays are N30000 (P.O. 3000) type; the line relays are strip mounted, the rest being fitted on jack-in mountings. Apparatus on the upper part of the rack is on hinged gates.

The battery is housed in a separate ventilated steel box fitted at the top rear of the unit where it is easily accessible.

Wiring is provided for 20 line circuits irrespective of the number equipped. Junction circuit wiring is suitable for either physical line or carrier relay sets.

Junction terminating equipment appropriate for the types of junctions and the type of exchange is provided for installation in the parent exchange. The equipment is normally of jack-in type and if rack accommodation is not available, a suitable mounting is supplied.

State whether the Minirax is to be a discriminating satellite or a dependent exchange and give the following information as applicable:

  1. Type of parent exchange.
  2. Number and type(s) of junctions.
  3. Number of connecting circuits.
  4. Whether metering is required. If so, what type.
  5. Whether local extension of alarms is required.
  6. If the 24-volt battery is not required, give details of local mains supply.
  7. Numbering scheme for the area.

Type - N22525B1 Stock ‘Minirax’ Unit equipped to order
Width - 33 in. (813 mm.)
Height - 61 in. (1,549 mm.)
Depth - 12 in. (305 mm.)
Weight - 240lb (245Kg) (Fully equipped)

Taken from the Etelco Product Catalogue No. 60, Section 9 (Plessey issue 2) 

Click here for an article on the Minirax I

Click here for an article on the Minirax II


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Last revised: November 08, 2020