On 2025Oct 13,, at 2:13 PM, Rob Jarratt <[email protected]> wrote: >> On 2025Oct 12,, at 3:28 PM, Rob Jarratt <[email protected]> wrote: >>> >> … >> >>> I measured AC OK by adding a pull up resistor, but if I am honest, I >>> forgot about this in my latest measurements. I have also looked at >>> Power OK, knowing that AC OK needs a pull up resistor, and it is >>> logically false, so it is definitely saying the power is not OK. >> >> What is ‘logically false’? 0V or –V? > > No, its about 7.5V. The comparator +ve input is 6.3V and the -ve input is > 10.4V, making the comparator output high impedance. I measure about +4mV on > the gate of the JFET. I think this means the JFET will be conducting and so > pulling the AC OK H down to 0V, which is what I see when I include a pull up > resistor on the AC OK H output. What you write below would seem to confirm > this.
There’s some confusion here somewhere. Those input V's would imply the comp. output should be loZ to Vsupply– pin, around –12V; not hiZ, +7.5V. The output for these comparators is an open-collector NPN transistor with E on the Vsupply– pin. Behaviour is: inputs V output output output relation trans’r Z logical ===== ===== ==== ==== +in > –in OFF hiZ 1 +in < –in ON loZ-to-Vsup– 0 Where Vin are a signed voltage. So for the input V's you mention, what should be occurring is: E1a.+in = +6.3V E1a.–in = +10.4V (these would be saying ‘power-is-good') ==> +in < –in ==> E1a.outTran = ON ==> E1a.outV = –12V ~ (because E1.Vsupply– = –12.9V ~, per your prior measurement) ==> JFET.G < 0V (negative by some number of volts) ==> AC-OK = hi-Z = logic.1 = +V with pull-up R. >>> Vstart is 11.3V and Vz is 6.4V. I have not been able to find where Vz >>> comes from on the schematic either, if I elaborate the KiCad >>> schematics further I will try to find its source. >> >> TDS = Tony’s schematic >> DRM = DEC rainbow manual per Wayne’s ref. >> >> Vz is in the lower left corner of TDS.pdf.5. >> Vz is apparently “V PR” in DRM and should be 5.1V [DRM.pdf.261-7.4.2.1]. >> Which is to say the zener there is some 5.1V type. >> 6.4V is rather high. >> I would suggest finding that zener on the board and measure right across it. > > I will locate this diode and measure it and also check the 7812, but it > might be a couple of days before I get time to do this. Note the 5.1V comes from the DRM and it’s conceivable DEC adjusted the design and put a 6.2V zener in there without updating the DRM. IMO it’s something that should be sorted out as Vz is used for a bunch of control refs, though not the main Regulation ref. Ideally the diode type would be visible.
