The SWTPC 6800 Repair Quest Continues

From: Wirehead Prime <wirehead_at_retrocomputing.com>
Date: Wed Mar 18 10:41:19 1998

Ok, so I swapped out the 7474 with a socket (checked) and a 74LS74 and I
get the exact same behavior...all the input signals are the appropriate
voltages but the outputs are still reversed.

This means, of course, that either there are some 'sneaky fast pulses' on
one of the input lines or that Q on the 7474, which is connected to pin 2
(not halt) of the 6800 is grounded. I've checked and it's not physically
grounded. I'll explain:

Is it possible for the 6800 to have an internal problem that would short
pin 2 to ground when it's powered on but not when it's powered off?

Second question:

If the problem is fast sneaky pulses, how do I detect this? What's the
easiest way and, barring that, what's the most effective way even if it's
difficult?

Anthony Clifton - Wirehead
Received on Wed Mar 18 1998 - 10:41:19 GMT