5 Apr
2016
5 Apr
'16
10:05 a.m.
About a month ago I was given a very nice AT&T 4425 terminal, which
was AT&T's OEM of the Teletype 56D. Alas, the terminal suddenly died
on me last week and I'm going to have to dive in and figure out why.
It appears to be something going wrong with the digital board, not the
analog, since sometimes I can get strange characters to show up on the
screen (but not always).
I did my normal searches of BitSavers and Google, but I haven't found
any schematics for the 4425. I don't suppose anybody has them?
At least it looks easy to work on. Kudos to whoever designed the
innards, it comes apart very easily and cleanly.
-Seth
--
Seth Morabito
seth at loomcom.com
6 Apr
6 Apr
9:08 a.m.
One more quick question about this.
Here's a close-up of the CPU and IO support chips on the terminal main
board:
http://i.imgur.com/6ZXwcLm.jpg
The "SL0227 / 404780" part appears to be a Zilog Z80 CPU of some kind,
but I've never seen those markings. The MOSTEK parts are Z80 SIO and
Z80 CTC equivalents.
Has anyone ever seen the SL0227 before? Any idea what kind of Z80 it
is? It's not in the data book I have access to.
-Seth
2:11 p.m.
SL0227 is a house number, and isn't likely to mean anything to anyone.
It might be a Z80, but it could just as easily be a mask-programmed
Z8.
4 p.m.
* On Wed, Apr 06, 2016 at 03:11:54PM -0600, Eric Smith <spacewar at gmail.com>
wrote:
SL0227 is a house number, and isn't likely to mean
anything to anyone.
It might be a Z80, but it could just as easily be a mask-programmed
Z8.
This is good information.
I did measure a 3.3MHz clock signal on pin 6, which leads me to
believe it's a Z80 or derivative, but I will proceed with caution and
try not to make any assumptions.
Voltages look OK, I see the aforementioned clock, and my cursory
inspection for cold solder joints and broken traces didn't show
anything obvious, so next I'll dump the ROMs and see what secrets they
reveal.
-Seth
--
Seth Morabito
seth at loomcom.com
9:25 p.m.
The "SL0227 / 404780" part appears to be a Zilog Z80 CPU of some kind,
The '404780' number looks a bit like a Teletype part number...
IIRC the Z80 has very odd positions for the power and ground pins, you
should be able to verify that it is a Z80 from that. Then stick a 'scope
or counter on the clock input pin to see how fast it is being driven, that
will tell you what type (plain, A, B, H) is needed.
-tony
8:12 p.m.
New subject: AT&T 4425 (Teletype 56D) repair - FIXED
Well that was fast and easy. Thank goodness.
It turns out the culprit was the Astec power supply. It had a dry
solder joint on the "DC OK" line. A few drops of fresh solder later
and the terminal is good as new (well, as good as any 35-year-old
terminal can be, anyway)
While I was inside, I removed a very old but thankfully non-leaky
battery. Need to figure out a new battery if I want to preserve
settings.
If anyone is interested, I did dump the terminal's ROMs while I had it
open. They're here:
http://www.loomcom.com/3b2/att_4425_rom
-Seth
--
Seth Morabito
seth at loomcom.com
3181
days inactive
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