[time-nuts] Fwd: HP 53131A Power Off Mod

Wayne Holder wayne.holder at gmail.com
Mon May 7 19:08:41 EDT 2018

I recently purchased a surplus HP 53131A and was surprised to see that it's
designed to stay partially powered on, with the fan running, even when the
power switch is in standby.  The manual says this is for timebase
temperature stability but, since I plan to use with my Trimble Thunderbolt
as the reference source, I don't really need this feature.  So, I decided
to see if I could change this.

But, unlike how Gary Sweeney accomplished this,
<https://www.youtube.com/watch?v=xyFBBYZ6jXI&t=271s> I wanted a mod that I
could easily undo and which wouldn't require me to hack up the existing
power switch.  After looking over the schematic, I determined that the
power/standby switch is DPDT switch that's used only to enable +12 volt and
-12 volt DC power by pulling two control lines to ground.  The actual power
supply is wired directly to the AC entry connector.  The signal lines from
the switch enter the main board via a 26 pin ribbon cable on pins 17 and
19.  So, I designed a simple PCB that breaks out these connections (and
ground) to a separate, 3 pin connector and then hard wires the +/- 12 volt
enable lines to ground.

The connector on top is intended to plug into the ribbon header connector
on the main board and the connector on the end of ribbon cable then plugs
into the lower connector.

I also designed another PCB that's going to mount inside the 53131A using
3, 3mm holes located in the rear of the unit near where the 26 pin ribbon
cable connects to the main board using nylon standoffs.

This board will contain a small, 12 volt encapsulated AC power supply
module, a relay and a pluggable connector where the AC lines will come and
go.  The Mean Well IRM-01-12 power supply module I selected has a no load
power consumption <0.075W, so I think that's about as close as you get to a
true "off" switch without making a more invasive mod like Sweeney's.  The
overall design is shown in the following schematic:

I'm waiting to get the PCBs back from OSH Park so, if anyone is interested
in implementing my approach in their 53131A, stay tuned and I'll let you
know how this all turns out.  BTW, the parts count (excluding the PCBs) is
about $17.  The switch breakout PCB costs $7.50 (for 3 copies of the board)
at OSH Park and an additional $15 for the power supply/relay board.  But,
one could easily build these on .1 inch perfboard to save a few bucks.  Or,
if I don't screw anything up, I'd be happy to pass on one of my 2 spare
PCBs to anyone interested.

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