ET-3400 Revision Questions

From: Patrick Rigney <patrick_at_evocative.com>
Date: Thu Oct 24 12:08:00 2002

Guys,

I just got an ET-3400 that was partially built. On average, the
construction seems to be fairly good. I haven't yet plugged the rascal in,
I'm still doing a visual checkout and comparison to instructions and
schematics, and trying to find the point or points at which the kit has
remaining steps. Mechanically it seems to be in great shape: the labels for
the case haven't been applied, and the case is completely unscratched.
Soldering is generally good, but flux hasn't been removed so it looks
untidy. The keys still have the thin plastic film over their caps, and it's
obvious that this unit has had little if any use. Perhaps the builder had a
problem and wasn't able to diagnose it, I don't know.

I have a manual with schematics, but it's apparent from inspection that the
manual refers to an older revision (part numbers different, parts changed or
absent, etc.). The main board is part number 85-2712 (-2). Unfortunately,
I don't have the x-ray views of the board, and the component side is
completely covered with a white plastic mask that carries various labels and
graphics, so traces on that side are all but invisible.

Before I break out the probe, I've got a couple of questions, for those who
may know:

The 6800 in the schematic is a 6802 on the live unit, and the 6875 in the
schematic is absent altogether, and there's no trace (no pun intended) of a
location for it. This seems to make sense; the '02 has its clock generator
on board, correct? Where is its 128 bytes of RAM mapped? There are also
two 2114's on this board, rather than two 2112's and two empty sockets for
additional 2112's.

There's a jumper soldered between U4 (a 74126) pin 5 and the nearest
resistor (part of the 8-LED assembly). Not on the schematic, but it seems
like it quite possibly could be a pull up or down (pin 5 is an input to a
tri-state buffer, pin 6 is the output to the DBE signal on the MPU, at
least, according to the schematic I _have_). Again, I can't tell easily in
this area of the board because the component-side traces are covered with
the white plastic mask.

The ROM is 444-364, so I suspect the source listing in the manual I have
(for ROM part number 444-17) won't nearly cut it and routines have probably
moved. Does anyone have a listing of this ROM they'd be willing to share?

IC1 is listed as a 'LS241 on the schematics I have, but there's a 'LS240 in
the socket. I can't visually verify that the change is appropriate, or just
a bad substitution by the builder, because the component-side traces are
covered by that white mask. It seems plausible that Heath would, for
whatever reason, decide to invert the sense of the lines buffering the eight
binary display LEDs (not the 7-seg displays on the right, the eight FLV117s
on the left), and this would be an appropriate substitution. Of course I
can quickly find out with a logic probe or some of Heathkit's infamous
"short wires", but I was just wondering if anybody knew for sure?

Does anyone have a GIF or JPG (or other format) of the x-ray of the main
board (even a newer or older revision), or a copy of the "Illustration
Booklet" that they'd be willing to share? For that matter, does anyone have
a copy of the manual for this (or near) board revision? The manual I have
is from Raymond Sarrio, and it was listed on his site as the Illustration
Book, but it turned out to the Manual (I'm going to contact him as well).
I'm happy to buy/trade...

Thanks! --Patrick
Received on Thu Oct 24 2002 - 12:08:00 BST