24 Aug
2014
24 Aug
'14
8:56 a.m.
In any event, once the data LEDs were acting "normal", I went ahead and
plugged-in the CPU board and tried it out. It was running at about 100V of
line when I noticed that something in there was now pulling about 350mA..
almost 35W! Without the CPU board, current was almost nil..
Then I caught a slight whiff of "that" smell, and switched-off, to hunt
down the hot component.. which seems to be on the D/C board, not the CPU
board. The D/C had a fire in the past, but I did replace the associated
chip & damaged components. Maybe I missed a shorted cap or something, but
how would the insertion of the CPU board create a new, high-current path to
the D/C board?
And that's when I packed it in for the day. If you had seen the state of
this thing when I got it, it's almost like a miracle that it's doing
anything at all right, lol. A genuine basket case, it was.
On Sun, Aug 24, 2014 at 10:48 AM, drlegendre . <drlegendre at gmail.com> wrote:
Well, they were /functionally missing/. So many little
parts all crammed
together, all alike, and no clear pin numbering in sight - I couldn't tell
whether or not the 330 was present until I pulled the board back out,
flipped it over and started counting. Then I found the bunk joint where #41
should be soldered..
As things moved along, I was able to measure the 470 to gnd, tho, so I
knew that much was there.
Now back to your original point, the one you felt I was missing..
Pin #41 had the 470 to gnd, but no 330 to VT+, so it was at a true logic
low, pulled down by the 470. This causes the 7404 output to go high,
turning off the LED. When I connected the 330 part, Pin 41 came up to
2.8VDC (about 1/2 of VT+) which I guess the 7404 sees as a high (?)
switching off the 7404 output and turning on the LED.
All good so far?
if so, then I don't understand why you were stressing that a typical TTL
chip would see a floating input ("undetermined") as a logic hi. Since all
of the bus pins in the D0-D7 series connected to the 7404s had either 2.8V
or 470 to gnd, none were just floating, were they? Or were you speaking of
some other lines left floating that I should be concerned about?
See what I mean?
On Sun, Aug 24, 2014 at 3:52 AM, Brent Hilpert <hilpert at cs.ubc.ca> wrote:
On 2014-Aug-23, at 11:12 PM, drlegendre . wrote:
in
On Sat, Aug 23, 2014 at 11:57 PM, Brent Hilpert
<hilpert at cs.ubc.ca>
wrote:
> No, you're missing the point.
>
> You said the D2 LED is OFF, which by your logic implies the 7404 input
is
> LOW.
>
> But, as I've stated twice now, an undriven TTL input (which should be
the
> state of the line without termination
resistors) should be interpreted
by
> the inverter as HIGH. Thence (as per your
desc) the output of the
inverter
> would be LOW and the D2 LED ON.
>
> This input behaviour is a characteristic of TTL logic at the electrical
> level. It may seem counter-intuitive but an open or unconnected input
in a
> physical logic implementation is not
inherently a logical low. Pulling
TTL
> inputs high with resistors is not done for
the sake of setting the
input to
> a logic high, it's done for noise margin
and speed issues.
>
> There is other stuff in the front panel connected to the DIn lines,
but I
> don't think any of it would be driving
the lines in the idle state, so
all
> the data LEDs should be on with no boards
plugged in. You might
confirm the
data
switches are all in the same position and toggling them has no
immediate effect on the LEDs.
Then I'm not sure what to say, because when I fixed the bunk connection the term. network (for pin 41 / D2) the LED came
on just like the rest
of
them.. shrug.
Well, that's good. If the 470-ohm R to ground was intact and the 330 R
to +5 had a bad connection that would explain the symptoms. Your earlier
messages left the impression the D2 Rs were missing.