[cctalk] Re: Rainbow H7842 PSU

On 2025Oct 12,, at 3:28 PM, Rob Jarratt &lt;robert.jarratt(a)ntlworld.com&gt; wrote: … What is ‘logically false’? 0V or –V? DEC used the JFETs to drive AC-OK so that no-power/power-off state would present low-Z-to-GND at AC-OK (i.e. 0V / LOW). If they used a bipolar, it would need power (base current) to get low-Z / 0V / AC-is-not-OK. Catch-22. The JFET conducts by default (gate JFET.G=0V), it needs V to pinch off conduction so it can go ’high'. But that JFET.G V must be negative (relative to source terminal JFET.S) to pinch off. And that’s why E1 has GND-pin at –V, so it’s output “Power-OK” can drive the JFET.G below JFET.S which is at circuit GND. So: E1a.+ E1b.– Power-OK(E1b.out) AC-OK ==== ==== =============== ===== power good: +5V ~~ +15V ~~ –12V ~ hi-Z power-bad: +5V ~~ < E1a.+ >+6V ~ low-Z power-off: 0V 0V 0V low-Z ~ = approximate (Above is supposed to render as a tabbed table) 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. 11.3V also seems a little low for output from a 7812. TDS.Vstart is apparently DRM.Vbias. I would still suggest checking the two “–12V” sources for connection. I expect they are not connected, and that E1.GND goes to the 2K7 “startup" –12V [TDS.pdf.2-lower-left-quadrant]. Measuring E2,E3.GND at –0.7 is a little odd if they are connected directly to circuit GND, you might want to look if there is a diode dropper involved for those pins.