Just got an EG&G Sensitometer
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.
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/2003-1...sAS/index.html) which has a schematic (http://www.sas.org/E-Bulletin/2003-1...974-08-09.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/ed...dgertonWW2.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/2003-1...sAS/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.
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/ed...dgertonWW2.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:
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.
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...
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....