Ludlum HV issue

[Dennis P Brown]

Any thoughts on why my Ludlum (Model 3, I believe) went from working (I was calibrating it) to not? The main issue is that I cannot detect any high voltage output from that connector (where the detector plugs in via co-axial cable.) I did checked the input voltage to the voltage multiplier stack of that device on the circuit board and read no real voltage (either AC or DC but would a simple hand held voltmeter work or should I use a scope?); I do get a few 100 milli-volts out of the HV connector; and when I adjustment the counter's high voltage pot this tiny voltage does vary but that is a trivial output.

I have a circuit diagram. I see that there are transistors that feed into the two x-former circuits - are these transistors the likely culprits? I notice that the two x-formers and transistors are two sub-circuits separated by a chip (LM 358. Is this a g specific nomenclature created by the company? It appears to me to be just a std Op-amp nomenclature.) That chip is just a black box to me in regards to testing. Also, are the transistor ID's also general or company specific (ie their internal ID?) Just want to be sure before I try and locate parts.

The x-formers are marked T-1 and T-2; the one marked T-2 appears to be for the system that creates the voltage for the HV multiplier; T-1 appears to be (mostly) secondary or not directly part of that system (does have a connection to LM 358.)

Any thoughts on what tests should be performed - mainly voltages that should be there (esp. for T-2) and type: AC or DC before the x-former/chip (I know that a voltage multiplier and x-formers all require AC.)

Also, where can new such specialized parts be obtained? I have an oscilloscope, by the way. Not sure that these parts are the issue nor if this means much since I have no idea what to really look at/or what to look for. Any ideas on what to check or what to look for?

Thanks for any input!

[Richard Hull]

Ludlum HV oscillator transformers are traditional failure culprits, but so are the semiconductors in its circuit. A regular meter is no good to measure HV. You would need a meter with an impedance of 50megohm or more to do this properly. Measuring voltage at the connector is tough due to the series resistor to the counter circuitry being in the 3.3 megohm range and stealing voltage from any meter attempting to measure the voltage there.

An O'scope is rather worthless as its range is low as is its input impedance.


The only thing you can do is measure the transformer ohmically to make sure all terminals have a matching return terminal or terminals that are ohmically connected. Desoldering the transformer may be necessary to be sure. If the transformer is good then you are in luck as a semiconductor is at fault, most likely.

Finally, before doing anything, check every solder joint with a magnifying glass in and around the transformer and its circuitry. It might just be a solder joint.

Check the solder joints on the adjustment pots as well. Move the wipers back and forth several times over a wide range, returning them to there original positions. Pots can get dirty and wiping over the resistance zone they were in can clean that section up a bit and the wiper, as well.

If something was working and your were adjusting something and did not short something out, you might have distrubed a pot wiper or loose solder joint causing a failure.

Always hook up the probe move it away from the open instrument and place a very hot source near the window.......Wiggle wires, tap pots and components........If the thing works just for an instant, then, whatever you tapped is the culprit. Resolder.

Richard Hull

[Dennis P Brown]

Thanks Richard, your post was both informative and helpful; the unit now works. It was, in deed, related to the soldering but exactly how is just not apparent - all the solder looks like it was all done yesterday! Still, the battery voltage wasn't going to the power transistors for reasons that, while not clear, had to be due to the solder along the path of that part of the circuit. Cleaning and small bending of the board apparently fixed the issue. I also discovered that the high voltage (HV) wire at its junction was nearly gone so I removed it, cut off the bad end, stripped of some insulation and re-soldered it onto the board. The unit is working again very well. Did finish the calibration from a known Beta source (as in counts per minute.) So, all the pots are adjusted, as well as the HV.

(Aside: you are right about not being able to measure the HV output but I did manage to discover that the unit was working after my cleaning when my hand brushed a HV section - that method, at least, still works when it comes to discovering the presence of HV!!! lol)

The display meter's 'swing back spring' isn't very good (old I guess) so I removed the cover so that I can manually reset the pointer to zero after each reading. Since this device uses counters to deflect that analog meter, replacing that meter may or may not be possible; in any case, may not be worth bothering since this is only for radioactive (X-rays, mainly) tests right now.

I do have the Op amp and both power transistors on order (cheap and may come in handy any day (!) since the unit is old. Have extra's if anyone needs one of these parts (for the HV supply section only. The other chips are very special and if one of those goes, not likely to ever be able to figure that out anyway much less obtain one ... .)

[Dennis P Brown]

Well, the Ludlum once more stopped working but since I have all critical replacement parts, I decided to replace the primary Op-Amp as a first step. So I removed the old one and installed a mount for the new chip. In this manner I can install/remove a replacement Op-Amp without soldiering in the future. Also, took the time to re-soldier a few connections I was a bit suspicious of due to old flux issues. The detector works nicely, now.

Whether a bad connection or the Op-Amp, don't really know (or care.) Until something else fails, I will leave the secondary Op-Amp and the two oscillatory transistors alone nor install mounts for these. I have replacement diodes as well.

A nice learning exercise in digital electronics (very elementary one, through) and reading of circuit diagrams/tracing some signals (just DC & AC voltage) but overall, something I'd rather not have had to do. But the effort does turn a $10 surplus Geiger counter that again failed back into a working detector. The original, as received, had really terrible battery corrosion (had to literally dig the old batteries out with a hammer and chisel) and also had issues with the battery connectors and wires (of course.) Those I had fixed and the unit worked until recently. Hopefully, these repairs will be all I need for some time ... .

Guess I might start on the cheap CD Geiger counter I have that also stopped working (the reason I decided to tackle and get the Ludlum working in the first place.) The CD unit is simpler (read: cheap) and the effort may be a waste of time since it is very likely the x-formers died (and I'd guess really aren't replaceable.) But the transistors may be an other matter since these aren't critical and substitutes may be available.

That is another project that will be x-fered to the future "to-do" list after I get the fusor back up and operational (needs a new electrode and window; the old electrode was too long (allowing too much current flow to the chamber base) and also started to leak; the electrodes can't be replaced without replacing the entire window (bad design on my part but did save big $$$ on the chamber) and a new window takes a few weeks to get from the supplier. Also, still need to finish my 100+ kV positive voltage multiplier for the accelerator - since the 50+ kV unit worked so well, really need to finish that since I have all the parts. That will take priority once the fusor is back in operation.

Of course, project three will be a formal neutron detector effort but that is on hold - no fusor no reason for that project. I do have a Ag foil mounted with a neutron modulator next to the fusor. Will definitely check that for activity once I get the fusor working - that is, once I am able to get a stable plasma (for longer than 10 seconds!) at 25 - 30 kV and 20 - 40 milliamps. That is far harder than I expected; the electrode leak was a major issue as well as the electrode length. Always issues ...I am starting to appreciate the fusion community's issues with the tokamak's ...lol.

[Dennis P Brown]

I should add that the Ludlum meter (this older unit) has a really easy to understand HV system (from battery to HV output) that was really well built. The company was very careful to make easy access built right in and in a very clever manner. As a result, very easy to work on and test even with power on. Very glad some key electronics components are still available for the unit. An excellent unit to have.

Again. thanks for the repair info and the great circuit diagrams here; made understanding and working on this unit very doable - as a result, I have decided to study in more depth a few basics of Op-Amp and battery (DC) powered digital high voltage AC systems - a rather interesting subject. While this is a complex subject requiring a bit of physics as well as electronic knowledge, it is a lot more understandable then I had ever bothered to consider before. I have now beard-boarded a small transistor/Cap/x-former based DC to AC converter to study.