How to work out unknown PSU replacement

Mike Stein mhs.stein at
Fri Nov 2 22:01:43 CDT 2018

----- Original Message ----- 
From: "Adrian Graham via cctalk" <cctalk at>
To: "General Discussion: On-Topic and Off-Topic Posts" <cctalk at>
Sent: Friday, November 02, 2018 9:09 PM
Subject: How to work out unknown PSU replacement

Hi folks,


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.
Are you taking into account that unlike positive 78Lxx regulators the pinout of the negative 79Lxx regulators is:

1 - Gnd
2 - Input
3 - Output


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