How to work out unknown PSU replacement

[Paul Berger]

Are you saying that there is a capacitor between input? output? of the 
79L05 and the ground pins of the chips?  If so that would seem pretty 
normal as it is usually recommended that you have capacitors to ground 
on both the input and output side of a 3 terminal regulator.  Are the 
ground pins on the three chips you mentioned isolated from the ground of 
the rest of the board?


On 2018-11-02 10:09 PM, Adrian Graham via cctalk 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!
>