On 2017-Aug-26, at 9:57 AM, Rob Jarratt via cctalk wrote: > I have replaced several parts in the area of the monitor sense circuit, > including the comparator and the buffer it is driving, but the comparator > does not seem to be doing what you would expect. I have reverse engineered > the circuit and I simply don't understand how the circuit can behave in the > way it is behaving. I have posted all the details here: > https://rjarratt.wordpress.com/2017/08/26/decstation-220-monitor-sense-circu > it-problem/ > > Does anyone have any ideas what could be going on?
You have some of your reasoning reversed regarding comparator behaviour. Comparators are typically: +in > -in --> output transistor off --> output TRUE / HIGH with pull-up R. The 176 outputs are connected to the comparator -inputs. The two 0V -ins are thus below the +ins at 0.36V and their output transistor will thus be off / not conducting to GND. Of course that doesn't solve the problem, the 3rd, oscillating, -in should be toggling its output transistor and the overall output should be toggling rather than steady +5. Is power good at the 339 supply pins (including gnd)? Is the oscillating 176 output high long enough (pulse width when high) for the frequency response of the 339?
