MIU

The Monolog Interface Unit (MIU) is mounted within the Telephone Exchange and sites between the Exchange and Line side of the MDF. The circuit schematic of the MIU is shown below. The purpose of the MIU is to improve the accuracy of Monolog when connected to long / high-resistance lines. The MIU contains a standard BT55 current sensing line relay along with a few other components to supplement Monolog's voltage sensing.

MIU Circuit schematic

It is technically possible to utilise a standard Secondary Line Jack Unit (LJU) in place of the MIU, although this is not recommended since it does not contain any of the current sensing components. In some of the larger exchanges where multiple Monolog units may be required, a shelf was usually fitted with a number of MIU's directly above, attached to a standard Strowger 3000-type relay mounting plate. Alternatively, in smaller, more rural exchanges, I have seen a wooden wall-mounted holder similar to that shown below that can host a single Monolog unit.

Shelf mounted MK1 Monologs

Wall mounted Monolog

Wall mounted Monolog holder

There were 9 different versions of the MIU as follows;

Model: Mk1

Identification: No logo on cover, Teltone relay.

Usual problem: Overhearing

Cause: Crosstalk between the Control and Monitored line

Solution: Change for later version

Model: Mk2

Identification: No logo on cover, Teltone relay.

Usual problem: Line is seized permanently

Cause: Relay contacts stick. Gives half volts for B-A

Solution: Change when fault appears

Model: Mk3

Identification: MONOLOG Call Logger logo on cover, Teltone relay.

Usual problem: Line is seized permanently

Cause: Relay contacts stick. Gives half volts for B-A

Solution: Change when fault appears

Model: Mk4

Identification: MONOLOG Call Logger logo (not underlined) on cover, ERG relay.

Usual problem: Incorrect or missing digits, no outgoing calls or meter discrepancies

Cause: A leg reversal on relay coil

Solution: Modify MIU

Model: Mk5

Identification: MONOLOG Call Logger logo on cover (underlined), ERG relay.

Usual problem: Incorrect or missing digits, no outgoing calls or meter discrepancies

Cause: Shorts and High Resistance (HR) from bad PCB

Solution: Change when fault appears

Model: Mk6

Identification: MONOLOG Interface Unit on cover, GTI relay

Usual problem: Incorrect or missing digits, no outgoing calls or meter discrepancies or permanent seizure.

Cause: Component damage and value of resistors R1 and R4 incorrect at 10Ω

Solution: Modify MIU

Model: Mk7

Identification: MONOLOG Interface Unit and new BT logo in Blue on cover, ERG or GTI relay

Usual problem: Incorrect or missing digits, no outgoing calls or meter discrepancies or permanent seizure.

Cause: Value of resistors R1 and R4 incorrect at 10Ω

Solution: Modify MIU

Model: Mk8

Identification: MONOLOG Interface Unit and new BT logo in Grey on cover, GTI relay

Usual problem: Line is seized permanently

Cause: Relay contacts stick. Gives half volts for B-A

Solution: Change when fault appears

Model: Mk9

Identification: MONOLOG Interface Unit and new BT logo in Grey on cover, ERG relay

Usual problem: No known problems

Mk6 / Mk7 fix

Mk4 modification

Monolog is connected to the Interface Unit via a short cable terminated with a standard BT Plug 631A on one end, and a male 25-way D-type connector on the other. The pin-out for this is as per the diagram.

Monolog connections for circuits using Line Reversal on Answer

Monolog can be connected to the subscribers line either within the Exchange or at any point on the line up to the subscribers premises, or even at the subcribers premises themselves, although this is not usually the case. For example, if connecting Monolog within a Primary Connection Point (PCP) cabinet found on the street, then a typical installation would look something like the diagram below.

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