On 9/25/20 8:46 PM, John Kennedy wrote:
>
> I unsoldered and replaced D6, D7 and the 74LS10.
> Interestingly the LOW signal at pin 10 of the 74LS10 is exactly as
> expected. However, when the chip is inserted, the logic probe won’t give
> a high or low - there’s a small voltage there. Where could it be coming
> from?

The 47K resistor in the wired-OR is great for CMOS logic but TTL, even
LS TTL, draws more current. A quick check of a data sheet shows input
current  when Vin=0.4V of -0.4mA. Which will create quite the voltage
drop on a 47K resistor.

You could lower that resistance but I can't help but be horrified by the
use of LS TTL with an 1802. 74HC or HCT if you must but please not LS.


--
http://davesrocketworks.com
David Schultz