Bell 103 Datasets

Hi It was not mandatory to use BT modems but they were suppposed to be approved with a (BATB) sticker on the bottom. Both Hayes and USR produced UK versions. Plus there were some UK made modems eg Modular Techology & Dowty. Priot to that UK acoustic couplers where available. I remember connecting to a time sharing service (RSTS on an 11/70) using a ModTech coupler whilst working on VDU design circa 1973. Rod Smallwood -----Original Message----- From: cctech-bounces at classiccmp.org [mailto:cctech-bounces at classiccmp.org] On Behalf Of Tony Duell Sent: 20 December 2006 23:59 To: cctalk at classiccmp.org Subject: Re: Bell 103 Datasets I beleive so, since everything connected to the phone line had to be rented from the GPO. I can't help with dates, but I do have a few old ex-GPO modems in the collection. The Modem 2B is a large box (about 16" square and 7" high from memory. It's 300 baud, original and answer. The front panel opens by undoing 2 captice screws at the bottom and then hinging it up (there's a little 'stay' to keep it open). Insider are 4 plug-in modules -- PSU, Demodulator, Control, Modulator, with testpoints, fuses, and adjustments on the front panels. The modulator appears to be a VCO with switched control voltage. The demodulator has 'tobacco tins' which cotnain complex LC filter networks, and appears to work by mixingthe incoming signal with a lcoal oscillator, extracting the 'sum' frequency, and feeding that to a discrimiator circuit (similar to one of the standard ones used in FM radio receivers, I forget which). It's all discrete transistors and relays (in the control module), no ICs. The Modem D1200A is similar in construction and design (and seems to be the same case) but is 1200/75 baud (I forget which way round, it may well be the 'host' end of a Prestel link). The Modem 13A is somewhat later. It's a plinth that's screwed to the bottom of a type 746 telephone (one of the standard desk telephones of the 1970s). the phone has buttons to select voice or 'data' in front of the handset rest. The circuitry seems to use several metal-can (10 or 12 lead TO5-like cans) ICs that I know nothing about, and alas the schematic diagram inside the phone just shows the wiring to the extra switches and to the modem unit, no details of the internal circuitry of the modem itself. Again it seems to be 300 baud, probably originate only. -tony