How to fix/ modify your Starrett 721 Electronic Vernier Calipers REALLY! I did it!
OK Guys; after reading lots of "throw it away" posts by machinists who know WAY more than I do about the instrument, how to use it and other really good stuff I acquired a beautiful 721 which cost me practically nothing and for good reason; the battery box on the back was destroyed and the front of it was unbolted and needed reworking. I fixed BOTH and here is how.
Not being a top-class machinist myself I AM an electronics engineer who fixes 2-way radios which use exactly the same LCD display in them that Starrett does/ did. SO if you are just fixing the "only displays half the digits" problem then first I will help with that.
Open the front case (as I mentioned; my 721 had no rear battery case so I have no practical knowledge about what it looks like or how it goes on or off) BUT let's focus on the FRONT.
Take out the 4 screws and don't lose the little guys; I think they may be like 1-72 or similar thread; 2 mm doesn't fit and neither does 2-56 or 0-80 so perhaps one of you know; I could use one or two myself but moving right along:
the LCD foam interconnect block is literally "conductive foam rubber" which is an amazing material BUT it stops conducting the multiple contacts between the PC board and the LCD module by sneezing in the next room (just kidding here) but it takes almost nothing....a speck of dust for example to break the minute contact points SO:
Get a new (really! New!) #2 pencil with unused eraser and it has to be VERY new and quite supple, fresh, pliable call it what you want. Not fooling here. Rub the GOLD contacts clean on both sides (LCD and PC board) with the eraser until nice & shiny, Do NOT use any cleaners, liquids, don't touch it (one fingerprint and it is toast) and then gently blow the eraser particles out of the housing and prepare to reassemble the front cover to the body.
Squeeze the plastic housing on so it puts pressure between the front case (LCD) and the PC board (body) and tighten the screws back in place. Do NOT let go until you have at least 3 screws in place; pressure is critical to insure a good contact on all of those tiny gold plated PC board terminals via the foam block. The LCD has probably 20 of the connections to the board via the foam and if one of them isn't happy when you reassemble it then you will probably need to use some dry air in a can (used for computer cleaning) to blow off each side of the foam block.
Frankly I bet the foam pad is "most likely" the same one we get inside of handheld 2 way radios and we get these in all the time so if for some strange reason your foam pad won't work I can probably help you with that too.
NOW If you either don't have batteries or are tired of buying expensive batteries or are in bad shape like I was and perhaps don't even have a battery box at all then keep reading. This will make your baby work again!
At this point it needs a little more TLC. I bought some #22 gauge red/ black tiny 2-conductor "zip" cord electronic wire. This stuff used to be at every Radio Shack but now that they are dwindling in size you will have to hunt up a "radio-TV-Electronics parts house" (online, Ebay, Google, Yahoo, whatever you have to do) and get around 3-4' of it. You will also need either a source of low voltage but highly stable 2.5-3 volts by way of a battery box or power supply.
NOW since the issue is that your wonderful 721 is toast anyway, anything you do to help it live again is a plus eh? So with no battery box at all I stripped the RED wire and prepared it for soldering to the tiny stub of wire that is insulated and coming up from the inside of the PC board protruding out the back.
I soldered on a tiny ring-end wire lug to the end of the BLACK wire (negative) and used one of the little screws and a flat washer to provide the ground (- voltage) by tightening it into the back where there were 4 leftover holes that used to secure the battery box.
If you are tired of replacing expensive batteries then it may be worth your while to yank off the one YOU have and then do this modification which WILL allow you to revert back to the factory original configuration any time you want.
NOW you will need a source of stable 2.5 volts. That can be arranged several ways. I think the original batteries were like 1.5 volts each right? I never did find out BUT if you hit up that electronics parts house and get a 2-battery holder that will hold a pair of AA or even AAA batteries (they are 1.5 volts each and 2 makes 3 volts) so I "assume" that this is what the original used. REPLIES are appreciated! I don't have a manual for mine either and no idea what actual batteries it takes so I am overly cautious and only put 2.5 volts on mine (so far).
I put two "Banana plugs" on mine to plug into my regulated variable DC power supply (I am after all an electronics engineer, so we have like 4 of them here!) and I don't need or want to move away from the working area to use the instrument SO I am just as happy to not ever buy any batteries ever again BUT you may need to take it out "in the field" (garage, whatever) and use it near the machinery so the battery box idea is probably right for most of you.
Use the RING end wiring lug to be the strain-relief for the tiny wires and of course you do have to handle it with the care it deserves just like any other fine tool.
Want to talk about it? My office number is 310.534.4456 and I am in California so plan on any time-shift from where you are and call after 9 AM Pacific time. If you have a manual and can send me a copy I would really love it; CALL me to discuss me providing you my email address. Bob Burchett Comments WELCOME and see the image I attached to help you understand what I did overall.
ORIGINAL MESSAGE IS HERE:
I have a Starret 721, 6" vernier caliper. I haven't used it in a while and when I powered it up and "zero'd" it. The display started bouncing between 0.000 and 0.0025...And continues to bounce. I thought maybe batteries were low and replaced them, but that made no difference. Any one have any experience with this? Am I hosed and going to have to send in for repair to Starrett?
Thanks,
Bob
