Modem 13A

[Tony Duell]

I also came across that Modem 13A that I mentioned a few days ago, the 
one that's a plinth under a normal 746 telephone.

The phone part is pretty normal, apart from having 2 buttons on top, in 
front of the handset rest, labelled 'TELE' and 'DATA'. They are 
interlocked so that presing one releases the other, and both are released 
buy the heandset rest. The 'TELE' button operates a microswitch, the 
connections of which go nowhere, the 'DATA' buttom operates a multi-pole 
changeover switch, again most of the contacts aren't used. The rest of 
the phone is what I'd expect in a rotary-dial 746.

The base of the plinth comes off by 4 captive screws. Inside is a PCB, 
component side down (so you can see the components when the base is 
removed), also hald in by a further 4 captive screws. There are 8 wires 
from the modem PCB up into the phone sectionm and a label inside the base 
seems to indicate that 2 of them are used to externally power the modem 
(it can be line-powered) -- these wires go to 2 otherwise unused 
terminals on a terminal strip, 4 of the wires are to be linked in 2 pairs 
(again done on said terminal strip) and the remaining 2 wires go to the 
line (presumably via that DATA switch.

There are 4 terminals on the PCB that go to a cable that comes out the 
back. The label gives the 'CCITT circuit numbers' for each terminal, I've 
not looked them up yet, but they're going to be normal RS232 signals. 

The PCB contians a number of discrete transistors, a number of 8-lead 
TO99 cansm which I susepct (from the part number and connections) to be 
748 op-amps and a single 14 pin DIL device, which might be something like 
a 7403 (???) And a lot of pot-core inductors and strange-value 
poiystyrene capacitors, presumably for filtering.

-tony