Advantages and Further Repair of a Ludlum Mod 3 Meter

This area is for discussions involving any fusion related radiation metrology issues. Neutrons are the key signature of fusion, but other radiations are of interest to the amateur fusioneer as well.
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Dennis P Brown
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Advantages and Further Repair of a Ludlum Mod 3 Meter

Post by Dennis P Brown » Sun Nov 05, 2017 2:19 pm

From time to time, old surplus (for a reason!) Ludlum survey meters become available on ebay rather cheap; often without the Geiger-Muller tube. I did in fact obtain such a meter - a Ludlum Model 3 (very common as surplus.) The batteries were so corroded that they had to be both drilled and dug out. Further, the meter had intermediate operation (after the battery holder was partially repaired.)

With this meter, both the power regulator chips and some of the amplifier transistors are easy to obtain (standard and still available!) After tracing the circuit (except for the counter system), the main circuit for the high voltage generation and detector circuits are extremely simple to analyze. Besides my metter needing two power regulars to be replaced (and I added chip mounting holders since I was de-soldering anyway) I also replaced one questionable transistor and fixed a bad high voltage lead connection. This got the meter up and working very well.

I post this not to just discus the repair but point out these meters, unlike many, have a lot of readily available components/chips in an easy to understand circuit, but more to the point, this meter has few special or custom chips. As such, this meter is well worth considering if one is available surplus and offered as is, no returns (of course, for a trivial or extremely low cost compared to working meters.)

I obtained my for $10 plus shipping - a steal considering how simple the circuit repair turned out to be (not so for the battery holder.) Also, the voltage out put is adjustable from the front via a small screw and this allows this old analog meter to work with many available and inexpensive Geiger tubes, as well as a scintillation tube I have (for gamma/X-ray.)

After putting up with issues from the battery holder for far too long (often stops making contact with the still damaged internal contacts), today, I realized that such an issue is silly to put up with since one can make a holder and externally connect the wires to the meter (ok, a duh, there.) Not really an issue drilling an access hole in the case for the battery wires to connect. Since I only use this meter indoors this really was a silly problem to have put up with (not that this mod prevents out door use.)

In summation, these surplus meters should be considered; of course, working old meters (and other or similar models) as well as more modern meters that can be returned are sure bets but often, this results in unacceptably high cost (for many beginners, especially; these meters are generally only useful for x-ray detection and hence, are a nice to have but not essential.) Since the Ludlum model 3 is so simple a unit,this is certainly a meter to consider if one has any elementary understanding of electronics and is not afraid to solder small chips.

Well, one more "to do" project finally dealt with ...not too many left, at least, on the fusor front ...lol.

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Richard Hull
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Re: Advantages and Further Repair of a Ludlum Mod 3 Meter

Post by Richard Hull » Sun Nov 05, 2017 7:50 pm

Oddly I am working on a long term series of battery tests. I have relegated all alkaline batteries to single scenario usage only! In short, and to avoid a long text here, alkaline batteries love to leak and what they leak will destroy all contacts and much associated circuitry. In any instrument of value, no alkaline battery whould ever be left in the device for long periods of inactive use. The single use scenario spoken of is one where the device is of a type where heavy drain on the contained batteries is the norm while in active, turned-on, regular service. Portable devices of this type that are used in remote service where batteries are not avaialble, will benefit from alkaline battery technology when spares are brought along. Once the immediate need for such a heavy demand instrument, used remotely, is over and a long period of storage is anticipated, all alkaline batteries must be removed.

This sounds so simple and logical, but is almost never the case. The result is tears when the device is critically needed once again.

I have several new D cell and AA alkaline cell bulk packs of batteries made by "Duracell" which are just within the "use by" date and a number of the cells have leaked and eaten thru there box packaging. These never saw service and were inside their "use by" dates. I consider this totally unacceptable.

I also have an old Eveready Carbon-Zinc collectable D cell with a use by date of July 1943 that has not leaked, of course, it is dead. I have recently pulled two dead carbon-zinc cells of the old aluminum foil covered Ever Ready style from an old flashlight purchased at a yard sale. They said use by Oct 1965. No leaks. I also recently bought an old PDR-27 Gm counter filled with the last batteries placed in it. No leaks. (see photo)

Carbon-Zinc is still a good technology.

Richard Hull
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Re: Advantages and Further Repair of a Ludlum Mod 3 Meter

Post by Silviu Tamasdan » Sun Nov 05, 2017 10:00 pm

Yes, it looks like the ammonium chloride electrolyte is less corrosive to rubber gaskets and whatnot than sodium hydroxide. I don't trust alkaline batteries either - especially those made in recent years when it looks like manufacturers have started cutting too many corners.
(e.g. smoke detectors in my house. Normally you shouldn't need to change the batteries in them more than once a year. I purchased recently a pack of 9V batteries; they have an expiration date of 2025. I changed the battery in one smoke detector in July. Last week it was making noise and I had to change the battery again, after only 4 months. I expect to have to change it again in March)
There _is_ madness to my method.

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Re: Advantages and Further Repair of a Ludlum Mod 3 Meter

Post by Jerry Biehler » Sun Nov 05, 2017 11:52 pm

Use lithium primary cells, they have a 10 year shelf life and i have never seen one leak under normal conditions. Expensive, yes, but the last a long time especially if you use it sporadically.

You can get them in 1.5v or 3v, I have a bunch of D size 3 volt ones and I used 3 in my PDR-27R instead of 6 normal cels.

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Dennis P Brown
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Re: Advantages and Further Repair of a Ludlum Mod 3 Meter

Post by Dennis P Brown » Mon Nov 06, 2017 10:05 am

All excellent battery advice; fortuitously for me (but not the agencies involved), that smart battery advice/guidelines are/were ignored resulting in the surplus availability of very inexpensive Geiger counters due to their non-working nature thanks to this very talked about battery corrosion. Now if someone would just make a battery powered neutron detector system that the government would need to buy en-mass ... .

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Re: Advantages and Further Repair of a Ludlum Mod 3 Meter

Post by Richard Hull » Mon Nov 06, 2017 5:06 pm

Note to PDR-27 owners. For quite some time the chain driven, range change, 50ua meters had a major issue with their magnets within the d'arsonval movement weakening significanty. I have gone through as many as 16 PDR-27s of a number of models. Some of the 50ua meters would only go full scale with over 100ua! This would make the instruments read lower radiation than calibrated for while in the hands of the military. I am told by a qualified specialist who calibrated these for the navy that that issue ended with the chain driven meters and the later model PDR-27s (transistorzed) were OK.

I check them by removing only one wire from one meter terminal and hooking up a circuit to supply exactly 50ua to the meter. If it does not go full scale, you have one with a bad meter in it.

The earliest PDR-27s used large, thick, silver dollar diameter RM-4 mercury cells for the filaments and 67.5 volt batteries to power the B+ for the tubes. a rare few mounted a 22.5 volt battery internally as well. The big, ugly, old PDR-27 series, over many modifications and model designations, may be the longest serving GM counter ever made for military service as the Norfolk Naval shipyard still maintains a few in good working order.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

Silviu Tamasdan
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Re: Advantages and Further Repair of a Ludlum Mod 3 Meter

Post by Silviu Tamasdan » Tue Nov 07, 2017 2:05 am

Dennis P Brown wrote:
Mon Nov 06, 2017 10:05 am
All excellent battery advice; fortuitously for me (but not the agencies involved), that smart battery advice/guidelines are/were ignored resulting in the surplus availability of very inexpensive Geiger counters due to their non-working nature thanks to this very talked about battery corrosion. Now if someone would just make a battery powered neutron detector system that the government would need to buy en-mass ... .
My neutron detection setup is battery-powered. It runs off the battery of my laptop through USB. Yes I know that's not what you mean. :)

What I really would like to have though is a plutonium thermoelectric "battery" like they used to put in cardiac pacemakers in the '50s and '60s.
There _is_ madness to my method.

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