2 Nov
2018
2 Nov
'18
9:22 p.m.
On Sat, Nov 3, 2018 at 1:09 AM Adrian Graham via cctalk
<cctalk at 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
