Imlac progress!

From: Bob Shannon <>
Date: Mon Sep 3 11:00:11 2001

Hi Tom,

A late night with the Imlac gave me a good step forward towards fixing the
vector generation. I now know exactly what the problem is, although I've not
found and
repaired the cause.

Let me correct that, I've found at least one problem...

Apparently my Imlac cannot make any negative Y-axis displacements in short
(increment) mode. Any vector made with a -Y axis deflection is drawn with a +Y
axis displacement.

So the 'H' and 'E' of "HELLO" are totally misformed.

Probing with a scope (and the extender board) shows that the subtract signal
going into the Y axis accumulator LSB is stuck low. Checking this at the input
to the 8-bit instruction register (slot 220) shows that the input data for that
bit is toggeling as the 'hello' program runs, but the output from the 7495 latch
is stuck low.

I'm not sure that the latch is bad however, I've replaced the card in slot 220
with no effect. Perhaps that net is sorted, or some other device on that net is
pulling it down really hard. Assuming that every board I've swapped is good
(not a safe assumption) than then the cause of the problem would have to be the
input gate on the display program counter board, which has not (yet) been

Now then, there is another problem I'll have to look at. When I run the HELLO
program, I get a much too-tall, corrupted H and E, the two L's are fine...

But there is no O, nor even any attempt at drawing the O. If I'm lucky, this is
a display program counter problem that will go away when I fix the negative Y
axis deflection problem. I can't see how this (-Y) problem would totally
prevent the formation of vectors for the O, but I've not started looking at that
just yet.

Also, I've looked at the power supply (mechanically). Normally its been tucked
away under the corner of the dining room table, so the exposed transformer AC
leads are out of harms way. Its not nearly as tightly packed as I'd originally
thought, and its probably very simple to re-cap this supply. So at this point
my plan will be to get the other power supply section and rebuilt it with new
caps, then install it into the Imlac. Hopefully this will address the DAC power
supply noise thats making my vectors unstable.
Received on Mon Sep 03 2001 - 11:00:11 BST

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