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Hobby user from Helsinki, Finland
Hi All,
I recently purchased an old Geodimeter 424 because EUR60 was cheap enough just to have a look inside and see how they work. Came with two batteries a hard case and nothing else. Opening the batteries was wise, both were dead and one needed gloves to clean out the corroded gunk.I hooked a lab supply to the battery wires and gave it 12V at 0.5A as estimated from the current draw. Did not work. I poked around in the PSU PCB inside the unit and discovered that the 12V rail was pulled low when I pressed the power button and then noticed that my lab supply was current limiting. I increased the limit to 1A and the unit woke right up and then reduced to 0.5A current draw as predicted. The start up current is about double the normal consumption.
After poking around and reading various manuals I was able to level it and take a range reading that seems to be correct to a few cm over 7m. It has an INFO26 warning about low batteries but they still have 3.05V so not altogether dead yet.
I got a slightly worn tripod and have pondered fitting 18650 Li-Ion cells into the battery pack. If I put in 8 modern high capacity cells with a charge/BMS circuit I could get 14 hours from one charge. 🙂 My concern is that fully charged they will have almost 15V and that is higher than the old Ni-Cd cells would have provided. Does anyone know what the absolute max battery voltage would have been.
I would like to also change the coin cells carefully soon. I will try to backup the RAM and then also investigate if there is any way to remove the INFO26 message later. Lots of fun to be expected.
I might survey the street lights on out street one day to add to Open Street Maps, just for the learning experience.
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8 Replies
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This is all way over my head, but neat to see.
Welcome, friend.
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Bill93 on this website is our resident electrical genius. Hope he chimes in.
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Sorry, I don’t have a handle on the over-voltage tolerance of that equipment. It might be fine or it could cause failure, depending on the components inside.
You might look at what other options the manufacturer offered for powering the equipment and see if any would have at times supplied more voltage.
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No expert here, but http://www.geoglobex.it/wp-content/uploads/2015/07/GDM-500-manuale.pdf
The pic shows where linked manual calls out 12v
I would note that for a lot of us connecting various survey equipment up to a car battery (nominal 12v) was pretty common prior to current battery tech. I have never used a Geodimeter, and am not an electrical engineer, so YMMV.
It is unlikely anyone published absolute maximum voltage…
-All thoughts my own, except my typos and when I am wrong. -
Welcome from the center of the US of A.
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Don’t know anything about Geodimeter, but I’ve done some battery re-cells on an older Topcon. It seemed to have a routine for sensing over-voltage. At one point, trying to get ready for some field work, I made up a temporary alkaline battery pack which supplied about 8.0 volts instead of the 7.2 volts from the original NiCad pack. The instrument wouldn’t initialize.
I did succeed in replacing NiCads with NiMH batteries. Voltage and charging characteristics were evidently close enough.
You might do better to stick with NiCads and the original voltage. New NiCads would likely provide a decent amount of run time. And some replacement NiCads might well have a larger power capacity than the originals.
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You write:
“After poking around and reading various manuals I was able to level it and take a range reading that seems to be correct to a few cm over 7m.”
That is less than 1 part per 250 parts. I looked at the ~$180 US Leica Disto brochure which reports an accuracy of 1mm in up to 10m.
Of course it seems that you are more interested in getting the unit to work. Good luck with that. Unfortunately I am unaware of any good resources to help you with your quest.
BTW, I spent about two weeks in Helsinki (with side trips) in 2019. Had a great time.
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Thank you all for your kind comments.
Unlike the GDM-5xx series This unit uses a 12V battery and 12V external power. A car will charge a lead acid battery at 13.8V nominal though it may reach 14.5V and providing that through the external connector should be safe. However the attached battery might be less tolerant as the designer would have had a strong expectation that the input would never exceed a fully charged Ni-Cd pack.
I have not found a schematic to inspect to estimate the maximum voltage limits. The internal PSU is sufficiently compact and involved that reverse engineering it accurately will be a lot of work that may result in accidental damage. If I one day find a spare parts unit I may sacrifice the PSU to find out. A crying shame that decades later the service manuals are still not public domain. One of the documents scanned by user robot has a few portions of the schematics for another unit to document revisions but that is the most detail I have seen.
I may just compromise and place a pair of diodes after the Li-Ion cells and drop the voltage to 12V and side step the issue.
The error in my range readings is from stretching a tape measure in the air from the door that my reflective arm band was hanging from to the unit sitting on a small table before I sourced the tripod. I expect that the accuracy is not compromised as yet.
Helsinki, like Finland in general is an ok place. Lots of trees and flat terrain that can limit how far we can see that can be a bit depressing but one gets used to it. I returned here 9 years ago after an absence of 39 years. My parents first left in 1965 because of the many closed minded people and I sympathise.
As this is in the back ground things will go slowly.
What is interesting is that the DE-9 Female connector on the unit is not wired up the way many would expect the PC-AT DE-9 style serial port connector is wired. When I start to make data and power cables for mine I will probably post what information I have found in my search.
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