On Sat, Aug 26, 2017 at 7:09 PM, Rob Jarratt <robert.jarr...@ntlworld.com> wrote: > >> -----Original Message----- >> From: Tony Duell [mailto:ard.p850...@gmail.com] >> Sent: 26 August 2017 18:39 >> To: r...@jarratt.me.uk; Rob Jarratt <robert.jarr...@ntlworld.com>; General >> Discussion: On-Topic and Off-Topic Posts <cctalk@classiccmp.org> >> Subject: Re: DECstation 220. Another Impasse >> >> On Sat, Aug 26, 2017 at 5:57 PM, Rob Jarratt via cctalk >> <cctalk@classiccmp.org> 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? >> >> My first worry is that you have measured the reference voltage as 0.36V, but >> according to your schematic, said point has only a resistor to +5V and a >> capacitor to ground. So I would expect it to be close to 5V. >> > > > Thanks, for some reason I was assuming a voltage divider, but of course you > need > *two* resistors for that. I will check to see if I have missed a resistor and > also check > the other board that works better in this area to see what the reference > voltage is there.
FWIW I would expect a voltage rather less than 5V there, otherwise the comparators will never switch. But your schematic wouldn't produce such a voltage, so I suspect an error. I don't know how easy it is to do it, but it can help to lift the output pins of the 3 comparators (desolder them from the PCB) and temporarily add a 4k7 (or so) resistor from each one to +5V. That way you can see what each comparator is doing separately. -tony
