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?
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)
> ...
> 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.
11.3V also seems a little low for output from a 7812.
TDS.Vstart is apparently DRM.Vbias.
> The supply to E1, E2 and E3 is 11.3V,
> referenced to the GND output, the GND pin on E1 is -12.9V and the GND pins
> on E2 and E3 are -0.7V
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.
> Let me know if I have missed anything.
