FX-6A flashtube availability and data

According to Joseph Gwinn :

And based on the manuals (discussed below) pins 1, 4, and 9 are actually used. (1 being one of the "probes", 4 being the anode, and 9 being the cathode). All other "probes" are only getting signal capacitively coupled by the adjacent runs in the ribbon cable (perhaps

1-1/2" long).

If so, the "capacitors" are in reality parasitic capacitance in that ribbon cable, as only the one is actually connected to anything, at least on my 1531-A. The photo (Figure 4-10, page 26) in the manual for the 1538-A shows the same construction, FWIW.

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Looking at the full schematic for the 1531-A, The anode and cathode are respectively connected to +400V and -400V (developed from a

310V winding on the power transformer in the 1531-A), and in the 1538-A, there appears to be a one-shot triggering the primary of a small transformer as a DC-DC converter, which gives +800V.

I don't find complete schematics for the 1538-A, though the

1531-A has the complete set. Both have the ghost views of the PC traces. And, apparently, IET still services (and supplies parts) for both models.

I wonder how much of GR's line IET took over? Or was it just a case of renaming the company for whatever reason?

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Making it rather difficult to check out.

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Great!

I see that the FX-7A is suggested as the current replacement lamp. I wonder what the prices are for that one?

Enjoy DoN.

You could get white light from three (RGB) LEDs and have the high speed. It could even have many LEDs to get the desired brightness.

Tom

You would need a uP scanner with a program for delay and dwell between and on each diode. That or buy perfect spectral set of eye sensitivity level.

So not so easy but doable.

Martin Martin Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net NRA LOH & Endowment Member NRA Second Amendment Task Force Charter Founder IHMSA and NRA Metallic Silhouette maker & member

Tm wrote:

Why would it need to be perfect? I would think close to white would be fine. You could just use the correct number of red, green, and blue LEDs and current drive them. You could even dump a cap into them with a SCR or power mos device to get high peak power. A processor could do the display and timing.

T

For the ribbon cable to parasitically couple to all pins, each with ~22 pF capacitance, the driven wire would have to be connected to every other wire in the cable. Is this how it's wired?

Yes.

IET bought the manufacturing rights:

"Since 1976, IET Labs, Inc. has had a long-standing commitment to conform the instruments and standards we offer to the customer's needs rather than to have the customer settle for what is available. Our ultimate goal is customer satisfaction.

In the year 2000, IET Labs acquired the GenRad standards, decade box, audio and strobe product lines and now continues to service and support these "In the Genrad Tradition..." Our experienced engineering and service staff make it possible to offer: "

From .

General radio was based in Concord, MA, while IET is based in Westbury, NY.

I've been reading the old patents listed in the back of "Electronic flash, strobe", and two key ideas have emerged.

A little inductance in the discharge circuit (flash capacitor and flashlamp) causes the discharge current to oscillate slightly, and the resulting reverse voltage helps deionize the flashtube. I've seen a 20 microhenry inductor with a 500 microfarad flash capacitor in one circuit diagram, giving a resonant period of 100 microsconds. Given that the discharge current is easily 1,000 amps, the inductor will need to be air core.

An inductance in the charging circuit (HV power supply and flash capacitor) delays the charging current just after each flash just enough to allow the the tube to deionize. I have not seen a component value on this, but my rough computations indicate that something of order 300 millihenries could work.

I bet it's the same.

Joe Gwinn

The diodes come in so many mcandles of light. if all were the same then the Green would blast the eye, then red and blue depending on the age of victim.

Look at the sensitivity curve of light for the eye - it is complex.

Martin

Martin Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net NRA LOH & Endowment Member NRA Second Amendment Task Force Charter Founder IHMSA and NRA Metallic Silhouette maker & member

Tm wrote:

So just use a few green LEDs and more reb and blue ones.

Again, so what if the color is off a bit.

Tom

According to Joseph Gwinn :

Only the center wire of the ribbon cable is connected to the trigger module. It is possible that all of those 22 pF caps are inside the stem, which has no apparent method of disassembly. (And the manual simply shows how to *replace* it, not to repair it.

I could dig up the little digital capacitance meter and see what it looks like, but I am a bit tired of this at this point. I'm now trying to get a 1558-AP octave band noise analyzer working -- and while the nicad packs still take and hold a charge (to my amazement, as I've had it sitting around for several years since I got it from a hamfest), the meter movement displays nothing. It would appear that one of the movement's spiral springs to take current from the mount to the coil has been damaged. Since I had to do some straightening of the folding handle and one of the sideplates associated with it. Those are working fine, but I have no idea what the sensitivity of the meter movement is. Time to watch for another at hamfests, and see whether I can combine them to make a working unit.

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So -- I should see what they charge for the meter? It appears to be the same meter which is used in the 1551, which I think that they still supply/service.

Nope -- I will still try contacting them, however.

O.K. That could have been the purpose of the air-core inductor in the supply which I duplicated. The wire was about #10 ga bare copper, wound in a groove turned in a plexiglass cylinder the full width of the case.

O.K.

Probably.

Enjoy, DoN.

OK. Maybe someday.

One can still buy meters, and there were only a few standard sizes, so a retrofit is certainly possible. What will be lost is the device-specific artwork (including scales) on the meter face.

How many turns? What is the diameter and length of the winding? There are standard formulas to compute the inductance given this data.

What was this supply for?

If the 300-mH coil is properly oriented, the 1000-amp pulse from the discharge-circuit coil will induce a bucking voltage in the charging circuit, helping to deionize the tube. Or at least not defeat the 20-uH coil. At least in theory - the pulse is very short.

Joe Gwinn

According to Joseph Gwinn :

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The scale is a DB scale, and not the common range of a VU meter. It is -6 to +10, plus white zones (on a black background) for "CAL" and "BAT", so swapping in another meter will take some artwork.

The face shape of the meter is also uncommon, except in GR equipment.

And nothing tells me what the full-scale current (or voltage) is, other than the fact that there are three forward-biased diodes in series across the meter, so it should not be any more than 2.1V FS.

I've done a bit of exploratory work with a stereo zoom microscope, and the movement out of the case. It turns out that the top hairspring is not firmly attached at the axis of the coil, though it is attached at the outer end, and the bottom hairspring flexes as would be expected throughout the range. I'm going to try my hand at re-soldering the inner end of that -- after unsoldering the upper end and lifting away the bridge with the jeweled bearing. It is certainly the kind of work which is best done under the stereo-zoom microscope (it turns out that mine is the AO version, not the B&L, FWIW.)

At the moment, if I turn on the voltage check function, and then tap the meter face, it makes contact long enough to send the meter to beyond full scale occasionally, and to have it vibrating at a lower level most of the rest of the time -- about 5/8th scale. The reason that it goes beyond full scale is because only one hairspring is resisting the force generated by the moving coil, so it moves beyond where it normally would.

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This is from about 1966 or so, IIRC. You expect such details?

The coil form was long enough so it just barely fit inside a 19" cabinet rack with about 15" of vertical panel space. Since there was some space left at the ends to avoid breakdown to the mounting hardware, that limits the coil to perhaps 15" to 17". Coil form diameter was about 3", minus 1/2 the diameter of the wire to allow it to bottom in the turned groove. Something like about 5 to 8 turns per inch, so that would make it 75 to 136 turns.

Laser pump for Nd-Yag optically pumped laser. Max rep rate of

10 PPS. It was intended to be an illuminator for a night vision scope which was gated on when the pulse was at the right distance, and off afterwards to mimize the brightness of light sources within the field of view. It was viewed by a three-stage image intensifier system before the changeover to the micro-channel ones which could get the same gain in a single tube.

I probably did calculate the inductance back when I made it. The original design was to have it tapped for the ten 20uF 1KV capacitors along its length to stretch the pulse, but that turned out to not work well, so the pulse was going through the entire length of the inductor from all of the capacitors in parallel.

It was charged from two big gray transformers made by UTC to be able to get the caps back to full voltage within 100 mS.

In the lab, it was good for erasing print on paper. :-) The power supply also lit me up rather spectacularly -- *once*. Between left finger and right elbow (which was leaning on the case of the scope on a cart.

Agreed.

Are these diodes only for protection, or do they also implement the log function? There were meter movements with overly strong springs, so it took a significant voltage to lift the pointer off the "zero" stop. Such a meter could indicate the voltage across the diode string, which is logarithmic in current.

And a very steady hand...

Sounds right.

I rest my case - you do remember, well enough to compute approximate inductance.

This tapped inductor with capacitors is a lumped-component transmission line, one kind of pulse-forming network. These were widely used in WW2 radars to generate the radar pulse, where one wants a constant power for a fixed length of time (rather than the exponential decline of a simple RC network).

I recall a passing note in Edgerton's book saying that lots of people had tried to use pulse-forming networks to drive flashlamps, but the wild variations in flashlamp impedance during the flash undermined all their efforts.

Ten 20-uF caps at 1000 Volts is 100 Joules. Ouch! Good it was not from arm to arm. A defibrillator is 400 Joules.

Joe Gwinn

According to Joseph Gwinn :

That could indeed be what they are for. The zero adjust pin was broken off the zero adjust screw, so I guess that was intentional, rather than someone being ham-fisted in putting the meter face back on.

Of course. I've worked on camera shutters and iris diaphragms, as well as other meters, so I figure on a reasonable chance at least.

At least the whole moving mechanism can be removed as a unit from the magnet assembly, giving me a better chance at getting things right without a magnet grabbing a tool just at the wrong moment. :-)

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Yep! That was the intention when our "customers" asked for that feature.

Probably why this did nothing useful. :-)

But it *was* from arm to arm. Remember -- between left (index) finger and right elbow.

It seemed to take five minutes before I realized that the noise which was getting in the way of my checking for my heartbeat *was* my heartbeat.

Shortly after that, I pushed for our lab to send everyone who wanted it to a CPR class. These days, we would have to stock anti-AIDS mouthpieces for use just in case. :-)

I also once (intentionally) dumped that cap bank into a 10 ohm 2 Watt carbon resistor. Parts of that wound up embedded in the ceiling acoustic tile. :-) Yes -- I did make sure that nobody was in the path of the expected shrapnel. :-)

Enjoy, DoN.

If you can see the pole faces, does the magnetic gap vary with movement angle? This is the other way to implement a log meter.

I compute a 100 to 260 microhenries.

Yep. It worked pretty well with hard-vacuum tubes, like magnetrons.

Right. You were very lucky in the timing (with respect to your heartbeat) of the shock.

Did you explode any wire?

Joe Gwinn

Common indeed. The key to repairing a moving coil meter like that that's jammed from magnetic swarf in between the pole pieces, is to simply unbolt the magnet from the movement.

You MUST store it with a keeper.

Once it's off the movement, blow or brush the debris out of the way.

I've unstuck a bunch of simpson meters with hang-y meters this way.

The diodes BTW are protection diodes. They prevent the coil from blowing out by shunting any voltage more than 0.7 volts around the meter.

Jim

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Indeed so.

Well ... in this case, 2.1 volts, as there are three silicon diodes in series. (Even without looking them up, I know that they are not germanium diodes, because only the silicon ones have their glass cases painted black -- presumably to eliminate photoelectron generation.

Enjoy, DoN.

Hi Jim,

I hated those diodes in meters. Next to worthless trying to use one when working on a transmitter.

Read DC voltage, key mic to see if voltage drops, watch meter read backwards...

Either had to clip the diode train out of the circuit or go find a different meter without them.

According to Joseph Gwinn :

Well ... I just won the spare parts one on eBay for $24.95. As I expected, there were no other bidders, given the *obviously* missing parts.

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Yes -- but have mostly been used on aluminum, which at least tends to generate non-magnetic slivers. :-)

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Yes -- I had thought that the hairspring was firmly soldered to the coil, so I wasn't worried. *wrong*!. :-(

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U.S. Army Night Vision Labs (at that time -- it has since gone through quite a few name changes -- once three in a single year. :-) Location was (and still is Ft. Belvoir, VA.)

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Yes.

Enjoy, DoN.

Hope the meter is OK.

Yes, but still, a good vacuum cleaning and washdown is in order.

OK. But I bet they read the MIT Radiation Labs series (which laid out the technical basis for radar, as developed in great secrecy during WW2).

Joe Gwinn

According to Joseph Gwinn :

So do I. But if all else fails, it is at least a possible donor for two more hairsprings. :-)

From the photo, it looks as though the faceplate is slightly melted on the top right side -- but I can swap the faceplate from my other one into that. It *may* be simply another JPEG artifact, as the photo was rather heavily JPEGed. Or it could be dirt, or reflections. It is right next to the missing special knob, in any case. The special knob has a transparent flange with a white dot of its own, and two DB scales. The outer scale ranges from 50 to 140DB, and the inner one from

0 to -50. You start the measurements with both switches fully CW, and you adjust one knob if the meter reads below 0 dB, or the other if it reads above +10 dB. For calibration, you set both white dots pointing straight up.

If you care to look at it, the eBay auction number is

7612826976.

Aside from the rather special switch knob missing, there is also the clamshell case bottom/top and the linkage which joins them and allows opening/closing without ever letting the parts separate.

Agreed -- except a washdown is pretty difficult with so many large tools bolted in place on that bench. At least all of them are within my lifting ability -- unlike with the 24" DiAcro brake, for which I would have to get the engine hoist. :-)

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Quite likely. Or encountered it in their college classes recently enough so they still remembered it.

Enjoy, DoN.