Ludlum Model 2A 500K cpm mod



I recently bought a Ludlum Measurements model 2A survey meter on eBay from Obtanium. This is a nice meter which features the well known rugged Ludlum design, adjustable HV and 3 ranges of 500, 5000 and 50000 counts per minute.

Using a scintillation probe the meter easily saturates on the 500 cpm scale, and the 5k and 50k ranges are also overloaded with certain sources.

So I thought of adding a 500k cpm range. I already read the manuals and associated schematics to find out how to do this.

Ludlum model 2 and 3 offer about the same functionality and basically share the same PCB, circuit, knobs and switches. Depending on age and model there are certain differences, but a lot of the stuff and functionality is the same.

As far as I know all model 2/3 ratemeters have the same calibration board mounted underneath the front panel on the left. The board has a row of precision potmeters for the various ranges, HV and alarm adjust.

To add an extra range, I soldered in an extra 1M precision potmeter. Since the board seems to be used in Model 2 and 3 it already has traces and holes for this component. I mounted a switch on the front panel to choose between the x100 /50k cpm and the extra potmeter for the x1000/500k cpm range. I cut the traces to the potmeters and connected the trace going to the x100 pot to the middle pole of the switch and from the switch to the two potmeters.

The front panel already has a template for the extra hole, so I simply drilled it out. I added some labels to the switch and calibration hole.

Shown below are the mods done to the calibration board:



To calibrate the new range I fed a 8000 Hz, 1% duty cycle signal from a DSO Nano into the meter through a HV capacitor. This is not  exactly the same as  the Ludlum procedure but the signal generated the expected meter deviations on lower ranges. I turned the potmeter until the meter showed 480k cpm.

The result is a model 2 that can measure up to 500k cpm and more or less functions the same as the much more expensive (even secondhand) model 3.



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