Reset-me Zephyrus

A while ago I posted a picture of a reseteable Zephyrus tone decoder in my Facebook profile - I received a couple of questions regarding the what for and the how to so here it goes with some detail... the story behind the modification.

In the daily use of the 380 series decoder, having the display latch and hold the status of the last tone received is a no issue, but maybe you're troubleshooting a system in which several tone sequences are used and all tones are the same one - unless you keep a close ear to the decoder board you'll miss the reassuring click of the relay each time a tone is decoded... the 7 segment display remains on the same though. If the circumstances of the testing enviroment impede to be close to the decoder, the latched display will prove to be unhelpful; there is a way to reset it to something else: power cycle the box... convenient for one or two shots but can grow to be a tedious task if the troubleshooting effort requires more attention. There has to be an easier way.

There is. Within the logic of the decoder board there exists an MC14538B Dual Precision Monostable multivibrator. With a just-waiting-to-be-used reset pin. Lacking a schematic for the decoder board I just traced the reset pin I needed from the IC to a convenient point I could safely solder a wire to - that would either be a feed-through eyelet on the board (preferred) or a pin on a support component (a resistor preferably).

For one end of the momentary switch I chose to attach the wire on the "power bus side" end of a 100k resistor as seen on this picture (click on them for a bigger view):

The connection can be made on either side of the resistor, but it is technically wiser to have the contact closure to go through the resistor and not directly onto pin #3 of the IC. The other wire for the switch can go into any conveniently located electrical ground point, and I found a board pass through eyelet right by the 7 segment display suitable for this:

The next task was to play test the modification, before doing anything else. Then clean up the soldering job (the pic above prior to that task ok?). As with all mods, anything done should be very easy to get undone... I doubt Zephyrus would appreciate receiving a unit for service with an undocumented feature installed in.

I had a nice cable hold down clip available and used it to secure the momentary switch to the front end of the board. Whenever I am in need of troubleshooting any of our air chains in regards to the sub audible tones, I just remove the plastic cover on the decoder and press the button every time I need the readout go back to zero.

And this is a bird's eye view of the whole 5 minute retrofit:There you have it. A quick and useful mod.

Should I worry about putting a fine-print footnote that this post makes no guarantees nor endorses undocumented modifications to industrial equipment? that it is the sole responsibility of the person who attempts to perform such modifications and none resides on this blog author nor provider?. I think we're all grown ups here. And good engineers at that :-)