> -----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. Regards Rob
