There have been a few power on switch modifications for the HP5313X frequency counters and I initially just put a SPST toggle switch on the back of mine in series with the A.C. power and that worked fine. However reaching around back to fumble for the switch grew old pretty fast.
Gerry Sweeney (
) modified the front panel circuit board by removing the existing switch and putting a mechanical switch button in that location with a bent rod to activate a DPST switch wired in series with the A.C. power so the on-standby feature of the counter was eliminated. The new mechanical switch was also placed where my high stability oscillator (opt 10) is mounted and that mod would eliminate the OCXO from being powered to stabilize while having the display off.
Wayne Holder (
https://sites.google.com/site/wayneholder/hp-53131a-power-off-mod ) made a more complicated modification that accomplished the same thing but his could be undone if you decided to sell the counter as stock.
The modification I made does involve drilling one hole in the front panel just under the channel 3 input jack and using a bent rod to clear the 3Ghz (opt 30) board to activate a SPST push on-off switch I recovered from an old 1970s dead DMM. The rod is coupled to the switch shaft with a short piece of small rubber air hose like is used for aquarium air pumps. The switch is wired in series with just the hot side of the IEC power connector on the back which is the common way to wire a power switch in the States. The bracket for the switch did also require drilling 2 small mounting holes in the side rail.
What I ended up with is a modification that allows the counter to operate as intended with OCXO and channel 3 options installed but I can completely power the counter down if I use an external time base and don’t want to have the counter partially powered in standby mode. Here are some photos of what I did.
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