How to work out unknown PSU replacement

Tony Duell ard.p850ug1 at
Fri Nov 2 23:22:00 CDT 2018

On Sat, Nov 3, 2018 at 1:09 AM Adrian Graham via cctalk
<cctalk at> 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: <>
> 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).


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