Just got an EG&G Sensitometer

Discussion in 'Darkroom Equipment' started by ic-racer, Apr 6, 2007.

  1. ic-racer

    ic-racer Member

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    I just got my EG&G sensitometer today. I currently have an older Wejex and would like to put together a buyers guide comparing the two. If I can pick up an X-rite green/blue sensitometer for cheap I would like to include that in the comparison.

    Before plugging in the EG&G I took the cover off to inspect the capacitors and visually check the components for signs of damage.

    Everything looked OK so I fired it up. The electronics seem to work OK except the three buttons for flash duration do not seem to work in the correct order. The 10-4 button gives the shortest flash but the 10-3 button gives the longest flash, with the 10-2 button falling somewhere inbetween. I'll have to take the cover off again to check this out. The cover had been off previously and the power cord had been replaced.

    It looks like there is a diffusor screen missing from the exposure chamber. If someone out there has an EG&G I wish they could comment.

    My light path is as follows: xenon bulb then a clear plate with black border over it. Next was a Wratten ND filter just placed over the clear plate. Next was the exposure chamber which is empty. Next is the rectangular clear glass on top of the exposure chamber and taped on that was a plastic step wedge.

    It seems like I am missing a diffusor in the light chain.
     
  2. Kino

    Kino Member

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    I think I can zerox you a copy of the user's manual on Monday. In fact, I probably should scan it and post it on this site; any problems with this Sean or other admins?
     
    Last edited by a moderator: Apr 6, 2007
  3. ic-racer

    ic-racer Member

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    Thanks, I will gladly pay for your time/effort to the extent of what the "camera manual" web sites charge. We can PM for the specifics. If there is some way this could be posted in PDF in the public domain this would be great also.
     
  4. ic-racer

    ic-racer Member

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    Figuring out how the circut works

    It has been a long time since I experimented with xenon flash circuts so I needed some review. I found this site (http://www.sas.org/E-Bulletin/2003-11-28/labNotesAS/index.html) which has a schematic (http://www.sas.org/E-Bulletin/2003-11-28/labNotesAS/art/1974-08-09.jpeg) for a high speed Edgerton flash and a description of how it works.

    Page 7 of this PDF (http://mit.edu/6.933/www/Fall2000/edgerton/EdgertonWW2.pdf) shows and explains a circut that more closely matches that in the EG&G sensitometer.

    Inspection of the EG&G sensitometer circut reveals mostly common compononts such as resistors, capacitors, chokes, switches and a power supply. There are two fancy coils, one is the ignitor coil and the other is some kind of 'in house' (stamped EG&G) coil that goes between the ground and the 'high speed' large discharge capacitor.

    The mains are connected to a special "regulated" power supply. This is a metal box with a fancy transformer on top and some hidden components underneath. There is a metal access door to the underneath components and it looks like there is a big capacitor can in there (perhaps a filter capacitor, however, I think the output is AC, not DC) I know for shure there is NOT a silicon/IC controlled voltage stabilization circut in there. Probably some clever analog circutry that would be rapairable if broken.

    The lines from the power supply go through the bottom of the chassis and travel to the main circut board. This is a great board, there are just posts and wires, NO traces. Very elegant. (the inner workings of this machine belong in a MUSEUM!)

    The bottom of the circut board has 6 blue chokes (or they look like chokes). They are arranged in a pattern that reminds me of a transformer schematic, with a central tap. This central wire leads to the top of the board. I suspect this is the voltage multiplier circut. I also suspect the power supply that feeds this is an AC supply.

    From here it looks like things get split up in to charging circuts for 4 capcitors.

    1) The '10-2' capacitor, which is a shiny aluminum cylinder.
    2) The '10-3' capacitor, which looks identical to above.
    3) The '10-4' capacitor, which is the large oblong capacitor on the right of the chassis.
    4) The small axial capacitor for the ignitor circut

    (If you are confused, '10-2' is an abbreviation for scientific notation and implies "1 times 10 to the negative 2" or 1/100 of a second)

    The above mentioned PDF describes high speed capacitors as needing low internal resistance.
    This is a quote from (http://www.sas.org/E-Bulletin/2003-11-28/labNotesAS/index.html)
    "...capacitors of extraordinarily low internal inductance .... Such capacitors are made by interleaving sheets of conducting foil with sheets of insulation and connecting the many alternate conducting sheets at the edges. The fabrication technique is costly, and the capacitors are also expensive"

    My intuition would say that the shorter duration flash circut (10-4) would be the smallest capacitor, however, it is in fact the largest on the chassis. My impression is that the capacitor is a special 'high speed' capacitor, as described above.

    The other two capacitors look like standard 'can' capacitors.
     
  5. ic-racer

    ic-racer Member

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    The ignitor circut

    I had forgotten, but it has come back to me now, that when the capacitors charge up, they are directly connected to the xenon tube. That is: there is no 'power switch' or relay to connect the voltage to the tube. Once the capacitor is charged, the xenon tube is also energized (so be carefull).So, at rest, the resistance across the electrodes inside the xenon tube is too high to allow any current to flow from the big capacitors.

    So, how does it fire? There is a third wire to the xenon tube, and this wire leads to a coil (like a small automotive ignition coil) which leads to the ignition capacitor, described above (#4). When this circut is discharged it sends a pulse of current to the xenon tube which lowers the resistance in the tube, allowing the actual flash capacitor to discharge through the tube and make the bright light.

    Some interesting things about the circut. The three 'flash speed' circuts are color coded, red, orange and green. The actual flash voltage travels THROUGH the little push button switches contancts. The wires carrying all that voltage are very small (I guess because the current is small or short duration)
     
    Last edited by a moderator: Apr 7, 2007
  6. ic-racer

    ic-racer Member

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    What is wrong with MY unit?

    When I first took the unit out of its packing I immediatly noticed "THAT SMELL" of fried components. My initial exploration did not yield any thing that looked burned up, though. The fact that the flash durations did not match the switches was curious, though.

    On second exploration, my NOSE and an infrared temperature probe led me to a resistor on the main circut board. The board is labeled "R4" at that position. This looks like a 1/2W resistor. Color code shows Gold (5%) and 150K resistance (15 x 10k). Close inspection does reveal some discoloration, and the infrared temp. gun shows that this thing heats up pretty fast when the power is turned on.

    Also, noted was the fact that the flash was not firing all the time. I could see a small 'puff' from the ignitor (when looking at the xenon tube) but the thing would not fire every time.

    Here is a quote from (http://mit.edu/6.933/www/Fall2000/edgerton/EdgertonWW2.pdf)
    "The trigger switch is initially open, and the trip
    capacitor is charged to a voltage Vs. Vs is determined by the voltage divider composed of R1
    and R2 and E, the voltage of the power source:
    (equation here)
    When the switch closes, the energy in C1 is pulsed into the spark coil, and a voltage is applied to
    the trigger electrode wrapped externally around the flash lamp. This excites the noble gas in the
    tube (typically xenon) and decreases the breakdown voltage of the gas. When the voltage across
    capacitor C is greater than the gas' breakdown voltage, the gas in the tube will ionize and create a
    brief flash of light."

    So, the bum resistor turns out to be on the "10-3" flash circut (the one that seems to be firing TOO bright). I suspect that a derangement in this resistor is also causing a problem with the ignitor capacitor charge voltage.

    Anyway, today I need to go to Radio Shack and get a 100K and 47K resistor (thats all they carry) and put them together in place of R4 to see if the unit will work correctly. If so, I'll order a correct 150K from an online source.

    I guess this is just another reason not to play the "e-bay game" unless one likes tinkering with stuff.
     
  7. magic823

    magic823 Member

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    The copy of the manual I have has a schematic for the mark VI.
     
  8. ic-racer

    ic-racer Member

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    Would it be possible to post a scan of the schematic?
     
  9. ic-racer

    ic-racer Member

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    Resutls of Resistor Replacement

    Well, it turns out the resistor number 4 is just what I think is a 'saftey' resistor that drains the capacitors to ground when the unit is turned off. This resistor connects the charging circut input voltage (430DCV) and the ground.

    I poked around with the voltmeter and found out that the power supply is AC and looks like it is split to about 400 v when it enters the series of blue "chokes." Well these are really probably diodes and they form a rectifier circut, because the output is about 430 DC volts. So, I had that wrong.

    Anyway, after getting rectified to 430 DC volts the voltage is distributed evenly to all three exposure circuts (ie "10-2", "10-3" and "10-4" all have the same input voltage). Each circut then has its own set of resistors that then connect right to the respective high power capacitors.

    Also, all three main capacitors are charged to the same voltage, irrespective of where the 3 position switch is set or if it is in neutral.

    The 3 position switch, then, determines which capacitor is physically attached to the xenon tube. When the 3 position switch in in 'neutral' there is no voltage at the xenon tube. Pressing any of the 3 buttons applies 430 volts to the red wire of the tube.

    My ignition capacitor charges up to about 117 volts and seems to fire OK. I did test the output of the ignition coli and it is well over 1000V. The xenon tube does show a faint blink, but there is not full discharge. This is very frustrating because there is certainly full voltage across the leads to the bulb socket and the capacitors are all charged appropriatly (I presume 430 volts is appropriate).

    So I have narrowed it down to:

    1) Not enough voltage on the ignition capacitor (may need more than 117V?)
    2) Ignition coil weak and not 'sparky' enough (not high enough output voltage, need more than 1000V?)
    3) Bad xenon bulb (though why would it sometimes seem to work fine)
    4) poor bulb socket connection

    At this point, after testing all the above, I could not get ANY full discharge from the xenon bulb. Just a faint flash from the ignition circut.

    Still seems repairable, I just need to think about it a bit more...
     
  10. ic-racer

    ic-racer Member

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    Shotgun aproach to repair: Just replace the suspect components

    I checked out the bulb and indeed one of the leads was not soldered well to the base. I re-soldered but still I have intermittent operation.

    Next I soldered jumper leads right from the leads on the back of the socket to the bulb and still intermittent operation (the ignition voltage always makes the tube glow a little, but no full discharge).

    Some strobe web sites indicate an old tube may be hard to fire. New tubes can be purchased from B&H but they are expensisve $100 to $300.

    This pdf page from mouser electronics has a nice replacement strobe tube and a new ignition coil/trigger transformer for just a few dollars apiece: http://www.mouser.com/catalog/629/127.pdf

    Note flashbulb 'C' and trigger transformer 'A' these both look similar to what I have now and would probably make good replacements.

    One last thing to try before replacing the tube and or the ignitor coil is to try to put some extra wire around the tube to see if it will ignite better....
     
  11. ic-racer

    ic-racer Member

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    The flashtube is probably bad

    I wound some extra coils of wire around the original flashtube and still was not able to get good ignition.

    I did have an old Rollei E36RE flash with a bum/shorted capacitor. I took the tube out of this unit and jumped it to the back of the tube socket on the EG&G and it works perfectly. Fires every time!!

    This tells me the flashtube that came with the device is worn out.

    One remaining problem is that the flash intensity on the settings 10-2 and 10-3 is almost exactly the same. 10-4 is much less intense to the eye and is probably working correctly.
     
  12. ic-racer

    ic-racer Member

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    Radio Shack Flash Tube Makes Good Replacement

    I wound up just going to Radio Shack and I got the Flash Tube replacement they sell. I re-soldered this to the base that the original flash tube was connected to. Making shure to get the polarity correct. The Radio Shack tube has a little red dot on it. This flashtube cost less than $10 USD. Now my unit fires perfectly.
     
    Last edited by a moderator: Apr 8, 2007
  13. ic-racer

    ic-racer Member

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    Comparison of Wejex and EG&G

    Eventually I would like to compile all this info into a buyers guide. But for now I will just post some things as I think of them.

    Light Source:
    Wejex-incandecent small bulb running off of about 5 - 10 VAC
    EG&G-xenon flash tube

    Exposure Duration:
    Wejex-one second, controlled by a shutter driven by a clockwork motor.
    EG&G-1/100, 1/1000, 1/10000, controlled by fixed capacitor size and fixed resistors.

    Lid:
    Wejex-sturdy metal, but spring loaded so need 3 hands to load film. Opens TOWARD the operator.
    EG&G-flimsy plastic, connected to trigger so chance of premature firing. Opens AWAY from the operator. Stays open for loading.

    Place to put filters:
    Wejex-yes
    EG&G-yes

    Method of adjusting exposure:
    Wejex-variable resistor for light intensity
    EG&G-3 choices of exposure duration. Intensity must be controled with ND filters

    Power supply:
    Wejex-simple transformer, runs from alternating current.
    EG&G-complex 700volt filtered and regulated AC power supply with a rectifier circut. Unfiltered DC output (no filter capacitors to replace or to blow up)

    Mfg. Specifics:
    Wejex-now made by Tobias; still in buisness, though support for the older white light models may be NA.
    EG&G- still in buisness(?), though support for the, no longer made sensitometer may be NA.

    Shop manual or owners manual on the internet:
    Wejex-no
    EG&G-no

    Light chamber:
    Wejex- built in
    EG&G- removable

    What I like best:
    Wejex- I like the fact that it is so simple. It can be copied or rebuilt from simple components (older white light model). Newer models may be more complicated.

    EG&G- Excellent workmanship and circuit layout. More complex than the Wejex, but still primitive enough to be fun to work on.


    What I don't like:
    Wejex- little light bulb may be too yellow even with 80A filter in place. Color temp probably changes with lamp voltage. Though, these factors probably don't matter much in real life film testing for gamma.

    EG&G- some components are specific to this unit and are probably unobtainable (like the monster super-fast discharge capacitor). Suitable replacements ARE available, but may alter the function of the unit. Though, who really needs exposures of 1/10,000 sec in fine art photography. Those fast speeds were for matching film sensitivity data to high-speed xenon flash subject matter.
     
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  15. ic-racer

    ic-racer Member

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    Why you need a sensitometer

    The need for a sensitometer is similar to the need for an exposure meter. One does not really NEED it but it can help out. With experience its use becomes less important, but when faced with the unnusual or out of the ordinary, it is a neccessity.

    People learned photography years ago with instruction on how to use a light meter. Sensitometers were not deemed neccessary (in my opinion) simply because they were so EXPENSIVE. A current unit from Agfa cost $24,000. They are so expensive because they can be calibrated for ISO standard determination. Unless one is running an ISO standards laboratory, this is unneccessary.

    THE MAIN REASON THE FINE ART PHOTOGRAPHER NEEDS A SENSITOMETER IS TO DETERMINE A STARTING POINT FOR DEVELOPMENT TIME.

    If you use the same film and developer combination all the time you DON'T need a sensitometer. Likewise if you shoot in bright sun all the time you DON'T need a light meter. I was like this for many years, however, the film and developer that I like may soon be extinct.

    UNKNOWN FILM/DEVELOPER COMBINATIONS ARE THE MAIN INDICATION FOR SENSITOMETER USE

    For generations one had to guess at a starting point based on mfgs tables or the experience of others or trial and error.

    Use of a sensitometer (and densitometer) puts some order into the process and allows one to zero in on a development time by comparing the slope of the new film with the slope of a known good film/developer combination.
     
  16. ic-racer

    ic-racer Member

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    How To Do It...

    If one has never processed film or printed film, a sensitometer can give some guidance, but perhaps not much more than tables or experince of others.

    The main utility is to answer the question of how long a new film developer combo needs to be developed WHEN ONE HAS ALREADY PROCESSED A FAVORITE FILM IN A MANNER THAT GIVES EXCELLENT PRINTS.

    One needs to expose the KNOWN film with the sensitometer, and process it in a KNOWN manner. The results are read with the densitometer and plotted. Then, using some indicator of slope or gamma (many ways to do it, just be consistent) find the slope or gamma for the known film.

    Then expose (with the sensitometer) and process the unknown film and measure its slope or gamma. Then alter one's development time to adjust the slope or gamma to match that of the known film. More development to increase slope/gamma and less development time to decrease slope/gamma.

    The utility of these methods will become more important as we photographers are forced into unknown film/developer combinations by market availability of products.

    There are many, many other uses of sensitometers and densitometes in fine art photography, but the above mentioned testing sequence is the main reason why I think almost everyone would probably benefit from a sensitometer.

    As an analogy, if one is taking pictures in the bright sun without a light meter and then one wishes to photograph inside a dark building, one does not NEED a light meter to get good exposures, but having one can really help. In a similar manner, when moving into an unknown film developer combination, one can get by without a sensitometer, but having one can really help.
     
  17. ic-racer

    ic-racer Member

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    X-Rite Sensitometer

    I am looking for an X-rite blue/green sensitometer to compare with the Wejex and the EG&G. I am starting to beleive that these relatively small battery operated, LED driven devices may have some advantages for B&W testing. The main drawbacks are that I think the exposure time is long (like a second or so, like the Wejex) but I am not shure. The on line PDF file for the 393/394 indicates that there are 7 different exposure settings but does not list the times in the specs.

    I even refreshed my memory on reciprocity by looking back at the Zakia and Todd book on Sensitometry and I still think that when one is just interested in "process control" information (specifically DEVELOPMENT TIME), the duration of exposure in the sensitometer does not make much difference.
     
  18. ic-racer

    ic-racer Member

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    I guess there is a point that I wanted to make clear again when I make the statement that I believe that all 3 types of sensitometers are probably going to work fine. The point is that I would never consider trying to mathematically match film data to a scene to be photographed. That is , I would not measure the 'range' of the scene and then look at sensitometric data to find out which curve will 'fit' that range. These 'process control' sensitometers are not going to be accurate in that type of setting.

    My method implies that one uses 'trial and error' to find out which film curve (ie which development time) fits the scenes range. THEN when one wishes to try a new film or developer on a similar scene, the 'process control' curves of the known and unknown film can be matched. This saves re-doing the 'trial and error' again with the new film or developer. Another way of putting it is that the sensitometric data that one is able to easily obtain is going to be too far removed from the actual scene to do a 'mathematical' matching of scene range to film curve. If one wanted to do a 'mathematical' matching one WOULD need to be concerned with spectral composition of the sensitometer light source, sensitometer exposure time etc. And even then one may need a different sensitometer and different test exposures for different scenes. Furthermore, the range of the scene is not consistently related to the range of intensities behind the lens, because of flare and such. So, some type of film plane reading would be needed. Again, at this point one is 'way out there' in unneccessary complexity.
     
    Last edited by a moderator: Apr 9, 2007
  19. Kino

    Kino Member

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    I have the manual in-hand and will attempt to scan it tonight. IC-RACER, please PM me your email address so I can send you a PDF.

    UPDATE: OUCH! It is 20 MB! Be forewarned!

    UPDATED UPDATE: Just found out my email server won't transmit anything more than a 15 MB attachment, so I had to do an ugly compression job on it for now. If anyone would like to host it on a site, I can ftp it to you @ 300dpi and 20 MB.
     
    Last edited by a moderator: Apr 9, 2007
  20. ic-racer

    ic-racer Member

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    Thanks for the EG&G manual. It did confirm some thing and clear up some confusion on my part.

    First, in the troubleshooting section, under "Intermettent Flashes" the first thing mentioned is "Flashtube FT-118 at end of life, hard starting." This was indeed the case, as replacement with the Radioshack tube cured it. I did find a place on the internet with a real FT-118 for about $30.

    Second, it was curious that the 10-3 position seemed brighter than the other two. My usual experience with almost all 'automatic' photoflashes for photography is that they use flash DURATION to control the light output. So I figured the brightest flash was the longest duration.

    This was NOT confirmed by the circuit which shows the 10-3 circuit discharging through a 2 ohm resistor and the 10-2 circuit discharging through a 50 ohm resistor. So the discharge through the 2 ohm resistor IS Faster, but it also burns the lamp brighter.

    This is all confirmed in the manual:

    10-2 light output = 800
    10-3 light output = 4000
    10-4 light output = 70

    Therefore the need for some special filters to even things out.
     
  21. sanking

    sanking Restricted Access

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    Just for the record, the Wejex that I have has an exposure duration of 2.5 seconds.

    Sandy
     
  22. ic-racer

    ic-racer Member

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    Thanks for the info. Can you post a picture of your unit? I wonder how else it is different from the one I have. Mine buzzes for about 2 seconds but the shutter is only open for one second. I know the current model they show on the Tobias web site is smaller than mine, and I suspect the inner workings may not be the same (http://ourworld.compuserve.com/homepages/tobiasinc/TAPages/prodother.htm)

    I really like the Wejex design and in fact was looking into parts availablility to do a little article about making a sensitometer based on some of the existing designs.

    Timing with the Wejex is very consistent because it uses a Synchronous motor in which the speed is dependent on the Hertz of the energy source, not the voltage. So it runs at the same speed at 117V and 121V etc. As long as the Hertz is the same (60Hz in the USA).

    I found a source for a 60 RPM synchronous motor. With a shutter shaped like a semicircle (180 degrees) it would produce an exposure time of 1/2 second. Then all you would need would be a microswitch, bulb and box, diffusor, stepwedge, etc.

    The other way to go would be to use LEDS and something like the 555 IC timing chip and battery power and base it on the X-rite style (though, with 'white' LEDs).

    The third way would be to copy the EG&G and just get a used standard photoflash with a Manual setting, and put this into a box.
     
  23. ic-racer

    ic-racer Member

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    For those with EG&G sensitometers that may be following this thread, I found a source for the FT-118 tube. http://www.electronicpartsupply.com/

    The Radio Shack tube was nice because I was able to get it in a moment, and able to verify the sensitometer was in working condition quickly. Now that I know I will keep the sensitometer, having the original tube for less than $10 will be great.
     
  24. GeorgesGiralt

    GeorgesGiralt Member

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    Hi Guys !
    For a long time now, I was considering buying an sensitometer. But the price tag is astronomical, even on Ebay.
    I was wondering if one could make good use for B&W material testing of dual color sensitometers used in X-Ray material testing like the X-Rite 383 which shows up from time to time ?
     
  25. ic-racer

    ic-racer Member

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    I have been trying to get an x-rite on e-bay for about 25-30 USD to use for testing, but two just got away. I want to compare it with the Wejex and the EG&G.

    I can't just put a green or blue filter in the Wejex because, I think the x-rite uses LEDS which will have a discontinuous spectrum that cannot be duplicated with a simple blue or green filter. I DO actually keep a BLUE 80A filter in the Wejex, but since the common light bulb in my Wejex is a continuous spectrum device, some red does get through.

    I really think that for 'process control' use (that is checking development times) the x-rite devices will work fine, but I want to test it first so I am not posting nonesense.
     
  26. ic-racer

    ic-racer Member

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    Just a follow up post on the flash bulb that I got from http://www.electronicpartsupply.com/
    . It IS a 1.5 loop tube, like the original, but the overall dimensions are slightly smaller. Otherwise it looks like a good replacement.