Advantages and Further Repair of a Ludlum Mod 3 Meter
Posted: Sun Nov 05, 2017 9:19 am
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.
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.