On Sat, Nov 3, 2018 at 1:09 AM Adrian Graham via cctalk <cctalk@classiccmp.org> wrote: > > Hi folks, > > This week I managed to get my paws on a machine that I only ever saw in > ‘coming up!’ type magazine articles in the mid-80s. It’s made by a UK > manufacturer of Viewdata set top boxes and home micro modems called Tandata > who were a split from Tangerine, the company that gave us the Microtan 65 and > eventually the Oric 1 and Oric Atmos. > > Documentation on the Tandata PA is zero, if you search for it you get my > Binary Dinosaurs page and nothing else so tonight I set about trying to work > out the power inputs from its 4 pin socket. Going clockwise pin 1 is > definitely GND/0V and pin 2 is not connected. Pin 3 goes to the input of a > 79L05 -5V regulator which via a capacitor seems to be used as the GND pins > for 3 CMOS 74 series chips. Pin 4 goes to a 7805 5V regulator. > > I’ve never seen a -5V reg be used in a GND circuit so before I continue > searching am I barking up the wrong tree? The trace literally goes from > socket to 79L05 pin 2, output goes to a capacitor then to the GND pins on a > CD74HC74E, CD74HC86E and CD74HC4066E. There’s a VARTA battery nearby too. > > Board pic is here: http://binarydinosaurs.co.uk/tandatapa-13.jpg > <http://binarydinosaurs.co.uk/tandatapa-13.jpg> > > Any insight much appreciated!
Remember that a capacitor will not pass DC, which is what comes out of a 79L05. (OK, I am overimplifying things, but that will do for the moment). It would, however be very usual to have a decoupling capacitor connected between the output of such a regulator and ground. Are you sure the output of the 79L05 doesn't go anywhere else (to analogue ICs, or the modem board, say)? At first glance this looks like a conventional circuit to produce +/-5V. So pin 1 of the connector is ground, pin 3 would be about -9V from the PSU, pin 4 would be about +9V (to clarify there would be about 18V between pins 3 and 4). -tony
