Film-Tech Forum ARCHIVE: Ascraft Cinex Lamps and Rectifiers

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Posted by Robert John Hodge (Member # 2444) on 08-21-2014, 09:56 AM:

Hi,
With the unfortunate passing of Bob Troop, I have become the new house projectionist for the Capitol Theatre in Rome NY.

Recently I've had the challenge of diagnosing lamp problems that previously didn't exist.

Both lamps use 11 mm x 20 inch positives and 3/8 x 9 plated negatives. And the 2 rectifier's are unchanged (arc voltage and current) as they were when they were put in service 4 or
so years ago. 90-100 amps on both meters.
But the arc is unstable and it wanders around on the negative with all the image problems that go along with it. The only way I can achieve stability is to keep the gap between the positive and negative smaller that the arc scope indicates it should be. Arc current when stable is around 115 amps.
Both feed rate rheostats are set about where they always have been. And since I've long believed that having a control set wide open or almost off indicates something else is wrong means that adjusting them for those conditions isn't an option.

All of the carbon drive motors have serviceable brushes and there are no drive difficulties with the feeds. The contacts are in clean , polished condition.
All of the connections in the rectifiers and lamps are tight and there are no problems with the AC supply to the rectifiers.
Could the rectifier stacks (original to the units) be developing high internal resistance which is causing problems with the current availability to the lamps? The service manuals don't address servicing the stacks.
Selenium stacks in power supplies can go high- but I read these aren't selenium.
I'd appreciate any thoughts on the matter.
For the last Capitolfest, I had to regulate both lamps by hand. And not with 100% success rate.

Also, the factory manual for these specifies a 11/32 x 9 negative but we have used 3/8 x 9 for a long time. Can that be enough to make a difference? Why do I think not. But why would National make a carbon with that small a difference if it didn't matter.

The rectifier units are the only common factors between the 2 machines. And I doubt the 2 step-down transformers are to blame. If I was to take a DC resistance check of the 2 bridges, what would a typical reading be? I'm used to reading silicon diodes and other semiconductors with a DVM, but these are outside my range of experiences. An open or shorted selenium stack in vacuum tube gear is simply replaced with an appropriate dropping resistor. Not an option w/ these!
I have a couple of physically very large GE Silicon bridges with hefty heat sinks that might work - but I would need to determine that before trying them, naturally. They are new old stock that came out of a theater supply warehouse.

Sorry for the long winded post. But those are the facts.

Many, Many Thanks!!

Bob Hodge

Posted by John Eickhof (Member # 323) on 08-21-2014, 02:56 PM:

what make and model are the rectifiers? Nameplate input voltage, phase, current,and output voltage and current? Has the local power company done any upgrading to the power lines, transformers, etc. recently? It has been known to have the power company replace a 120/240 transformer with a 115/208 which would cause reduced input voltage and require the taps on the rectifier to be adjusted to suit. Can you read the input voltage with a meter and see how close it is to the nameplate voltage? Check each phase leg across and to ground ? Another thing is too much exhaust draft, did someone close the draft control dampers on the exhaust stacks? Has the exhaust fan been replaced?? Easy check is to strike a lamp and slowly pull the exhaust stack off the top and see if the arc stabilizes, if it does, you have too much air being drawn through the lamp. Somewhere I have exhaust CFM guages for the old trashcans... or get an HVAC contractor in there to adjust the draft to the CFM spec in the manual.

Posted by Robert John Hodge (Member # 2444) on 08-21-2014, 03:56 PM:

The draft is what it always has been which was never enough to suit me. But we've never broken a reflector so it's probably enough.

But the pole pigs changeout is very possible. I will look into the AC component.
That might explain why the problem seemed to occur as it did and on both units.

Posted by John Vincent, Jr. (Member # 1791) on 08-30-2014, 09:18 AM:

Robert,

We have a large cache of Ashcraft parts here at Wellfleet DI.

We ran Super Cinex's (RCA Dynarcs) from 1957-2012, with one still operational. A Barco 32B took the place of the other.

Additionally, my former projectionist Dave Mead might be able to help you with the problem.

My phone # is 508-349-2450

Posted by Tom Horsfall (Member # 6138) on 08-30-2014, 10:15 AM:

I worked with many different models of Ashcraft lamps in the 60s and 70s and the symptoms you describe remind me of the time in the late 70s when I was working in a drive-in using Ashcraft Super Power lamps with a 10mm x 11/32" trim and a motor generator. Those Ashcraft lamps always had a very steady arc and ran flawlessly until one night I noticed the tail flame wandering around, I was able to stay on the screen by using the negative lateral and vertical adjustments. After that reel I looked at the negative carbon and noticed there was no core material in the core. I put in a new negative after checking it for core material and everything was normal until I ran into the same problem about every tenth carbon. I and others noticed the quality control of National Carbons wasn't what it used to be. I suggest you examine the negative carbon closely. We also ran into the "no core material" problem in some areas of the positive carbons too. That was a BIG problem, we had to change the positive to continue. I ran into this quality control problem in other theaters too.

National General Theaters switched to Marble Double Eagle carbons right around that time and the quality was better.

Posted by Robert John Hodge (Member # 2444) on 09-15-2014, 09:44 AM:

Thank You John Eickhof, John Vincent and Tom Horsfall!!

It's in the bag!

The problem was the line voltage which fell from 220v. to 207 across all 3 phases. You could have knocked me over with a feather after finding out that the AC had fallen that much.

Resetting the taps on the rectifiers got the current back to where it should have been without manual carbon adjustment. The flame is now textbook perfect and as stable as it always was before all this happened- Back when the voltage was 220 volts.

I'll be in touch with you John about your parts. BTW, I've been to your drive in for the Sunday flea market. But of course the booth wasn't open. I have a friend that has friends with a cottage not far from you.

Many Thanks, Sincerely Once Again!

Bob Hodge

Posted by Monte L Fullmer (Member # 2797) on 09-15-2014, 03:12 PM:

Glad to find the issue. When I did a drive-in with the same Ashcraft setup and the same size of carbons, I couldn't get the light needed to the screen and had the wandering flame.

Found out that the arc beam mirror was way out of adjustment making the trim way out of focus to the reflector.

Zeroed that all back in and got my steady light back and a much brilliant picture.

I was a relief operator that found this issue. The Union guys there couldn't (or didn't care to) figure this one out.

-Monte

Posted by Robert John Hodge (Member # 2444) on 09-15-2014, 03:40 PM:

Good Work!! I hope they appreciated it! Even if the crater was a little bit out of the proper position would be bad. And it sounds like it was more than a little bit!

Best!

Bob

Posted by David Buckley (Member # 2600) on 09-15-2014, 04:09 PM:

quote: Robert John Hodge
The problem was the line voltage which fell from 220v. to 207 across all 3 phases. You could have knocked me over with a feather after finding out that the AC had fallen that much.

For a three phase supply, the correct voltage is 208V, so your supply is now correct, whereas previously it would have been high.

220V is normally seen in single / split phase.

Posted by John Eickhof (Member # 323) on 09-15-2014, 04:39 PM:

actually David, in the USA there are two different 3 phase electrical systems. The older one is a 120/240 volt DELTA system that provides an even 220-240 volts across any two of the phase legs (X,Y,Z), and 120 volts from X & Z phase leg to Neutral or ground (Earth) and the Y leg is known as a stinger that outputs approx 180-200 volts to ground. The more modern system is the 120-208 volt WYE system, it supplies three equal legs of approx. 110-120 volts to it's own neutral or ground and an even 208 volts between any two of the three legs. (X,Y,Z) The original Delta system was used so that lighting and lighter loads could be fed from the same 3 phase service yet the 240 volts was a more robust supply voltage and a slight reduction in conductor size was also a benefit.

Posted by Monte L Fullmer (Member # 2797) on 09-16-2014, 01:06 AM:

Like my building: got 480 delta coming in and heads to a transformer to reduce it down to 208 WYE, then out to the sub panels which then feeds the individual panels.

Posted by Louis Bornwasser (Member # 3063) on 09-16-2014, 09:12 AM:

Several cinemas have a transformer at each panel.

Posted by John Eickhof (Member # 323) on 09-16-2014, 10:01 PM:

When I built my shop 15 years ago, I have 480 volt DELTA feed from the road to my building then transformered to 208 WYE the 800 foot run of copper was way more affordable at 480V instead of 208 0R 240 Volts! Plus, by code I only needed three conductors for all the hot legs as the transformer produces it's own neutral leg. Also it qualified me for an agricultural rate for my power! Very low cost!

Posted by David Buckley (Member # 2600) on 09-16-2014, 10:33 PM:

quote: John Eickhof
actually David, in the USA there are two different 3 phase electrical systems. The older one is a 120/240 volt DELTA system that provides an even 220-240 volts across any two of the phase legs (X,Y,Z), and 120 volts from X & Z phase leg to Neutral or ground (Earth) and the Y leg is known as a stinger that outputs approx 180-200 volts to ground.

Ah yes, I'd forgotten about ye olde wild leg system. Weird thing. Not the weirdest piece of USA electricity, that honour belongs to the four phase supply (is there any left?), but pretty weird none-the-less.

So it looks like John went from wild leg to "proper" three phase, and his supplier omitted to tell him... Nice.

Posted by Dave Macaulay (Member # 813) on 09-17-2014, 06:45 AM:

I think the Edison DC system takes the weird prize. A decade or so back there was still some DC service in New York City that you could trace in the street with a compass. I have no idea what used it, maybe transit?
Tesla didn't personally do as well as Edison but his AC system certainly won out for local power distribution. HVDC is still used for some long haul power lines.

Posted by Stephen Furley (Member # 1277) on 09-17-2014, 07:00 AM:

Dave,

I doubt that it would have been for transit, I don't think that they ever took d.c. from the public supply, but used their own rotary converters or rectifiers. The New York Subway only shut down its last 25 Hz. rotaries in about 1999, and the IND had some more modern 60 Hz. ones for a little longer.

For d.c. public supply in a city my best guess would be lifts (elevators) in older buildings. Cranes in industrial premises or dockyards would be another possibility.

Posted by Kenneth Wuepper (Member # 1174) on 09-17-2014, 07:08 AM:

Many years ago the electrical generation plant in Grayling, Michigan was a few city blocks from the theatre. The Diesel generator, in addition to the AC power, supplied DC to the projection booth where the ballasts limited current for the Carbon lamps.

The operator demonstrated how holding the Carbons together could dim the street lights outside. Now that was really fun!

Posted by Gordon McLeod (Member # 33) on 09-17-2014, 09:52 AM:

For many years IATSE required only DC for stage lighting on broadway

Posted by David Buckley (Member # 2600) on 09-17-2014, 05:08 PM:

quote: Dave Macaulay
A decade or so back there was still some DC service in New York City that you could trace in the street with a compass. I have no idea what used it, maybe transit?

I can remember when it was switched off, it was quite a big deal.

What was it used for: Elevators.

It is only of comparatively recent times with the development of variable speed drives (VSDs) that elevators with AC motors could have both have their speed controlled smoothly, and be capable of high speed. Which were the attributes necessary in tall buildings.

Before the development of VSDs, one had to use DC motors. To run those DC motors, one needed (obviously) DC.

Where there wasn't a DC supply, the best way to control elevators nicely was using the Ward Leonard system, but that needed a lot of heavy equipment (it was based on motor-generators), and was quite inefficient.

quote: Gordon McLeod
For many years IATSE required only DC for stage lighting on broadway

I've read about DC lighting being used in books, but never actually seen it myself. DC based control systems (some of them are, quite literally, Frankenstein boards) still exist in a number of theatres, long unused.

But... the same advances in electronics that made VSDs possible also brought the stage lighting equivalent, the SCR (originally thyristor, later triac) dimmers. This invention was the single greatest advance in stage light ever, and swept away all before it. That was the end of DC in theatre.

Posted by Randy Stankey (Member # 64) on 09-18-2014, 01:39 PM:

If Esther Williams ever knew how they lit those underwater scenes in her movies, she'd probably shit a brick!

They just threw a light bulb into the pool and let the water carry the DC current back to ground.

Posted by Rick Raskin (Member # 1561) on 09-18-2014, 02:35 PM:

The telephone plant runs on DC. Commercial AC feeds rectifiers which continuously charge batteries that float on the system. In the event of a commercial power failure the batteries continue to supply power until backup generators come online. Essential AC is produced by inverters that run on DC. There are some gargantuan Frankenstein switchboards in those power rooms. I remember in one CO seeing the ammeter register in excess of 6000 amps. One CO complex in Washington DC had 5 750KW turbines for backup but I'm told those have since been replaced with diesels.

Most of the theaters in that area ran MG sets for DC but I do remember there was commercial DC in a small area in the 1960s.

The old trolley system generated their own DC and I recall as a kid being fascinated by the power plant that was on Georgia Ave.