2 Nov
2018
2 Nov
'18
6:09 p.m.
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!
--
adrian/witchy
Owner of Binary Dinosaurs, the UK's biggest home computer collection?
t: @binarydinosaurs f: facebook.com/binarydinosaurs
w: www.binarydinosaurs.co.uk
2 Nov
2 Nov
6:27 p.m.
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!
3 Nov
3 Nov
2:59 a.m.
On 3 Nov 2018, at 01:27, Paul Berger via cctalk
<cctalk at classiccmp.org> wrote:
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?
Hi Paul,
The trace goes from power socket to pin 2 of the 79L05 and a small value cap which is
connected to the GND plane along with pin 1 of the regulator. Pin 3 goes directly to a
decoupler then straight to those 3 chips which are isolated. Today I?ll break out the ?tin
foil covered sponge? method of tracing any other points on the board that I might have
missed last night.
Cheers,
--
adrian/witchy
Owner of Binary Dinosaurs, the UK's biggest home computer collection?
t: @binarydinosaurs f: facebook.com/binarydinosaurs
w: www.binarydinosaurs.co.uk
4 Nov
4 Nov
12:15 p.m.
----- Original Message -----
From: "Adrian Graham via cctalk" <cctalk at classiccmp.org>
To: "General Discussion: On-Topic and Off-Topic Posts" <cctalk at
classiccmp.org>
Sent: Saturday, November 03, 2018 4:59 AM
Subject: Re: How to work out unknown PSU replacement
...
Today I?ll break out the ?tin foil covered sponge?
method of tracing any other points on the board that I might have missed last night.
---
"Tin foil covered sponge" ???
1:48 p.m.
On Sun, Nov 4, 2018, 16:23 Mike Stein via cctalk <cctalk at classiccmp.org wrote:
----- Original Message -----
From: "Adrian Graham via cctalk" <cctalk at classiccmp.org To: "General Discussion: On-Topic and
Off-Topic Posts" <
cctalk at classiccmp.org Sent: Saturday,
November 03, 2018 4:59 AM
Subject: Re: How to work out unknown PSU replacement
...
Connect one lead of the multimeter to some pin/net you want to trace. Then,
connect the other lead to a foil-covered sponge (or metallic brush) and
brush it over the PCB backside, stopping when you hear the continuity
alert. Not useful generally for big nets that connect everywhere, and not
very useful for SMT boards; good for through-hole stuff though.
Kyle
Today I?ll break out the ?tin foil covered
sponge? method of tracing any
other points on the board that I might have missed
last night.
---
"Tin foil covered sponge" ???
1:56 p.m.
----- Original Message -----
From: "Kyle Owen via cctalk" <cctalk at classiccmp.org To: "General Discussion: On-Topic and
Off-Topic Posts" <cctalk at classiccmp.org
Sent: Sunday, November 04, 2018 4:48 PM
Subject: Re: How to work out unknown PSU replacement
On Sun, Nov 4, 2018, 16:23 Mike Stein via cctalk <cctalk at classiccmp.org wrote:
----- Original Message -----
From: "Adrian Graham via cctalk" <cctalk at classiccmp.org To: "General Discussion: On-Topic and
Off-Topic Posts" <
cctalk at classiccmp.org Sent: Saturday,
November 03, 2018 4:59 AM
Subject: Re: How to work out unknown PSU replacement
...
--
Today I?ll break out the ?tin foil covered
sponge? method of tracing any
other points on the board that I might have missed
last night.
---
"Tin foil covered sponge" ???
Connect one lead of the multimeter to some pin/net you
want to trace. Then,
connect the other lead to a foil-covered sponge (or metallic
brush) and
brush it over the PCB backside, stopping when you hear the continuity
alert. Not useful generally for big nets that connect everywhere, and not
very useful for SMT boards; good for through-hole stuff though.
Kyle
----------------
Neat trick; surprised I'd never heard of it before in my decades of trouble-shooting.
Then again, at my advanced age and senility I might just have forgotten... ;-)
Thanks!
2 p.m.
On Sun, Nov 4, 2018, 16:56 Mike Stein <mhs.stein at gmail.com> wrote:
Neat trick; surprised I'd never heard of it before in my decades of
trouble-shooting. Then again, at my advanced age and senility I might just
have forgotten... ;-)
Thanks!
Hah! Probably also best to discharge the caps before proceeding; I could
imagine some major damage resulting if voltages were still present! :)
Kyle
2 Nov
2 Nov
8:01 p.m.
----- Original Message -----
From: "Adrian Graham via cctalk" <cctalk at classiccmp.org>
To: "General Discussion: On-Topic and Off-Topic Posts" <cctalk at
classiccmp.org>
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
m
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
2202
days inactive
2203
days old
8 comments
5 participants
participants (5)
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ard.p850ug1＠gmail.com -
binarydinosaurs＠gmail.com -
kylevowen＠gmail.com -
mhs.stein＠gmail.com -
phb.hfx＠gmail.com