HP 53131A Power Off Mod

Important Update: due to a mistake in my thinking, I had to upgrade the fuse I added to a 220V, 3A model to match the fuses on the internal switching power supply.  My original plans were to mount the fuse on the power supply PCB and use it only to fuse the small power adapter used to power the relay.  But, when there was not enough space, I decided to use the in-line fuse adapter, instead.  Completely overlooking the fact that the in-line fuse would then need to handle the full power load.

I recently purchased a used HP 53131A Universal Counter 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 10 MHz clock reference source, I don't really need this feature.  So, I decided to see if I could modify the 53131A to have a true power off switch.  Note: this mode should also work with the HP/Agilent 53132A.

IMPORTANT: This projects involves working with potentially dangerous AC power connections, so please don't undertake making these modifications unless you have experience working with these potentially dangerous voltages.

Gary Sweeney accomplished this by hacking in a power switch, but I wanted a mod that I could easily undo and which wouldn't require me to remove the existing power switch in the front panel.  After looking over the schematic, I determined that the power switch (actually it's labelled as a power/standby switch) is DPDT switch that's only used to internally enable +12 volt and -12 volt DC power.  It does this by pulling two control lines to ground when in the On position.  The actual power supply is wired directly to the AC entry connector.  The signal lines from the switch enter the main board via pins 17 and 19 on a 26 pin ribbon cable that's located on the side of the main board that abuts the chassis, as shown below.  So, my mod involves rewiring the power supply so it's controlled by a relay that's controlled via the front panel switch using a small, 12 volt, encapsulated power supply.  When the modded 53131A is "off" this new 12 volt power supply will be drawing power, but less than 0.075W according to the data sheet. 

To disconnect the pins from the switch from their normal connection on the main board, I designed a simple PCB that breaks them (and ground) connection out to a separate, 3 pin connector and hard wired the signals they used to connect to (the +/- 12 volt enable lines) to ground.  As indicated by the note in the photo, when we get to installing this board in a later step, you may need to bend the three transistors down a bit to make sure they don't make contact with the pins on the bottom side of the switch break out board (more on this later/)

The top edge of the board contains a male, .1 header connector designed to into where the header connector on the end of the ribbon cable used to connect to the main board and the connector on bottom edge contains a header connector that accepts the plug on end of the ribbon cable.  All signals except those on pins 17 and 19 pass through, as mentioned above. You can order the switch breakout board from OSH Park by clicking this link, or you can download the Gerber files using the links at the end of this page.  Note: the connectors use .1 inch spacing for the pins, so you could also make an equivalent adapter using .1 inch perf board rather than using my PCB design.

I designed another PCB to hold the relay and 12 volt power supply.  It's sized to mount inside the 53131A using three 3mm holes located in the rear of the unit and near where the 26 pin ribbon cable connects to the main board, as shown below.  Note: it's possible these holes may not be present on all models of the 53131A, so please verify your chassis has these holes.  If not, you may need to come up with an alternate mounting plan if your chassis design is different.

This board contains a small, 12 volt encapsulated AC power supply module, a relay and a pluggable connector where the AC lines will attach to the PCB.  The overall design, including the switch breakout PCB (left side) and the power supply/relay PCB (right side) is shown in the following schematic:

The following picture shows the PCB layout, which has a Phoenix Contact  3 Pos, 5mm pitch, pluggable terminal block to accept the incoming AC connections.  12 Volt power for the TE Electronics OJE-SH-112HM,000 relay is supplied by a Mean Well IRM-01-12 encapsulated power supply.  You can order the power/relay board from OSH Park by clicking this link, or you can download the Gerber files using the links at the end of this page.

Parts List

You may already have some of the parts needed to build this mod (such as 3mm nylon stand offs and screws), but the following parts list contains all the components needed except for the two PCBs and possibly some 12 gauge stranded wire for hooking up the board to the AC input lines in the 53131A.  Note: the 3mm red LED and a 1K resistor, which are an optional feature I added for testing the power board and are not needed.  You can also click here to be taken to a page at Mouser.com where you order all the parts listed.  Note: these parts were all available from Mouser a at the time I made this list, but this may not be true when try to order them, as availability can change on a day to day basis.

In-LIne Fuse Holder and Fuse

In addition, you'll need an in-line fuse holder and a mating,  250V, 3A Fuse.   I happened to have these parts in my junk box, but here's a list of suitable parts and part numbers you can order from DigiKey:

Modifying the 53131A

The first step in modifying 53131A is to assemble the switch breakout PCB.  Be careful to make sure the notched "key" in both connectors is oriented correctly and match the positions indicated on the PCB.  Also, the XG4H-2631 connector should be mounted on the bottom side of the board (pins up) and the XG4A-2632 connector should be mounted on the top side of the board (pins down.)  In addition, you'll need to solder some short, flexible wires (about 4 to 6 inches long) to the "GND" and "SW1" pads, which will later connect to the power supply PCB.  Note: the SW2 pad is not needed, but you could wire it in parallel with the SW1 pin for redundancy.  The next step is to unplug the ribbon cable and plug in the switch breakout board but, before doing this, you'll need to make sure the pins on the under side of the board will not come into contact with the any of the tabs of the three, TO-220 style transistors next to the ribbon connector on the main board.  I did this by carefully bending each transistor away from the connector and closer toward the main board while being careful to maintain a gap between each transistor and the others, as the following photo shows. 

Click for larger view

Note: once the switch breakout board is installed and before we perform the other mods, it's good idea to verify the function of the switch using a multimeter's continuity check function.

Next, assemble the relay/power supply PCB by soldering all the components into place and then clipping off any excess wire that protrudes from the connections.  Note: before soldering in the PCB side of the pluggable terminal block, be sure it's oriented so that the screw terminals on the mating connector will face away from the IRM-01-12 power supply module otherwise you may not be able to plug it in once the PCB is assembled.

Then, using short lengths of stranded, flexible wire connect the GND and SW1 connections on the switch breakout board to the GND and SW connections, respectively, on the power board.  After this, use some 8mm M3 nylon spacers and 6mm M3 nylon screws to mount the assembled PCB to the chassis using the three 3mm holes shown in the photo above.  Then, the last step is to do a bit of rewiring on the AC input connector, as follows:

After making all the AC wiring changes, you should be ready to test your modifications but, just to be sure, carefully look over everything carefully one more time taking care to make sure there are no loose wires, or wires or connections in danger of shorting.  Only when you are completely satisfied you've checked everything carefully and have replaced the cover to the unit  should you connect AC power and test your modifications.  Here's what the completed mod looks like in my 53131A: