Desisti "Rembrandt" 12K HMI

[Bob Hart]

On the strength of the 12K lamp working with a near end-of-life globe in it, I ordered a new globe in. On arrival, this globe appears to have a liquid stain within the envelope. The old globe had lots of stain and melted bits within and spots of dust and rust burned into the outside of the envelope which is why I am not too keen to keep on using it.

Explosions seem to be fairly violent, judging from previous damage within the lamp housing, sharp-edged gouges in centre of dents inside the bodywork, on the reflector and some small chips out of the fresnel ridges inside.

Is a stain inside a new 12K double-ended globe normal? I imagine the chemistry which vaporises to affect the colour temp of the light has to reside somewhere.

[Bob Hart]

Over winter, the lamp has been in storage. This year there has been a rat infestation which proved impossible to control by non-chemical means until an electric rat trap which actually worked was discovered. Poisons are not an option due to endangered wildlife being collaterally poisoned from eating the dead rats which stagger outside looking for water

Unfortunately the beasts managed to get inside the lamphouse and have eaten the woven heat-shield around heat resistant cabling and two inches entirely out of the small cable to the door switch.

Whilst I am about pulling the whole thing apart just to get at the damage, I am minded to replace the thick lampholder cable shield material as well. One piece was entirely missing on arrival when I first received the lamp, maybe a prior misadventure with rats in its former home, the US.

Replacement materials are proving hard to locate unless one wants to buy entire commercial sized reels of the materials offshore.

Any advice as to sourcing smaller quantities of woven heat resistant wiring sleeves and heat tolerant 18AWG cabling would be much appreciated. The original wiring in the lamphouse is old-style woven fabric insulation which was discontinued out here very many years ago.

[Bob Hart]

THE 12K LAMP SAGA.


So here's the rub. The lamphouse was made by Desisti. It is put together around a metal frame. Internal panels are fixed with screws. Light-trap vents are fastened with screws. The outer skins are attached with rivets. Superficially it represents the way aeroplanes are put together.

I really don't like to take on things unless I do them properly. However, do I drill out 40 mongrel rivets, remove the skins, neutralise the rust in the contact faces like a good airframe engineer?

Next, refasten with new rivets, or just leave it and paint over it to entrap the oxide floaties, which may yet still escape to get on the quartz bulb and shorten its working life if the joints are disturbed in transport?

The lamphouse appears hand-built. Except for some generic components and louvre slots, it is something you could put together in your shed with a welder and tin-snips.

However, that said, the Italian manufacturers of the times and maybe still, had a passion for their engineering doings.

There are nice little touches like hand-threading all screwholes in the metal frame instead of using speed screws which cut their own threads with sometimes haphazard results.

Then there was their practice of through-bolting a sub-assembly to the frame by threading the holes in the sub-assembly and adding a locknut on the end of the bolt.

You won't see this thoroughness in Chinese knockoffs.


A final cautionary note. If you import a pre-used large appliance, there are requirements that it be treated for vermin and insects. There were residues inside the electronic ballast and the lamphouse. I think that the people may have used aprocarb as the taste began to ermerge in my mouth when I started cleaning, sanding and wiping out the rust in the lamphouse.

So take care humble peons. Wear gloves and long sleeves and a dust mask if doing up appliances sourced from offshore. Also, if anything electrical on a circuit board looks matte-finished and white, treat it with intense suspicion. Don't fracture it or cause it to make dust. Some older heatsink materials of the vintage of this lamp contain beryllium oxide which is a potent carcingen.

[Chris Medico]

Originally Posted by Bob Hart

Any advice as to sourcing smaller quantities of woven heat resistant wiring sleeves and heat tolerant 18AWG cabling would be much appreciated. The original wiring in the lamphouse is old-style woven fabric insulation which was discontinued out here very many years ago.

Is something like this suitable?

http://www.amazon.com/High-temperatu...mperature+wire

http://www.amazon.com/CS-Hyde-Temper...V016YCMENA8MMW

[Bob Hart]

Chris.


Most excellent assistance. Thank you Sir.

Given I have the lamphouse stripped down to the bare skeleton except the two riveted side panels, I might as well do the job properly and eliminate as many possibilities of catastrophe as possible.

The main leads to the carriage look good. Only the sleeves themselves appear beaten. That fabric tape is also what I was looking for but could not find.

Lightmaker ballasts are apparently becoming harder to have fixed except back in the US, so the lamp needs to be the best it can be.

Needed for a small indie shoot in December - January near Jarrahdale, south of Perth.

This is the really good side of dvinfo at play again. Thanks Chris.

[Bob Hart]

THE 12K LAMP SAGA - CONT'D

Sometimes negative happens-chance has its upside. Yes I am vexed about the rats shredding the wiring. It is a major-league p---off. However in the lowest innards of the beast I found three foreign objects which are not lamp parts.

One is a thrust bearing, another a brass wingnut which fits nothing, the last is a small 4mm x 40mm machined shaft. My guess is somebody used the top of the lamp as an emergency workbench and the small parts fell in through the top vent.

There were smokey signs that sometime, something went across via one or the other of these metal intruders. I have been intrigued by some outward-facing dents in the rear cover. The cause is now apparent.

Somebody in the past appears to have tried to stop the hour meter or get a stuck one working again by giving it a smack or three or four through the guts of the lamp-house with something thin, hard and heavy.The lamp used to be a Panavision rental.

Maybe if the Hobbs meter has become stuck, the user can then haggle over an estimation of the burn time. But then again, I may be unfairly ascribing deceitful behaviour.

[Bob Hart]

Where its at.


Given the panel work on the lamphouse is thin metal, there is not much scope for rust to continue untreated before the panel becomes shotholed someplace. There was also flaking rust found in the bottom of the lamphouse after it was opened up.

Loose rust and dust do not do a lot for the longevity of hot globes.

I put the stripped parts in for sandblasting. I was going to have them powder-coated but was advised the material would not tolerate the heat in the immediate working area of the globe and might give off smoke and vapors which might shorten globe life.

The carriage and frame of the lamp holder have therefore been finished with automotive high temperature paint to avoid rust recurring. The original finish appeared to be something similar or might just have been simple stove black.

The rest of the interior of the lamp-house beneath the internal heat shields is being done with conventional enamel finishes.

No all I have to do is put it back together without having any spare parts left over and wait for the replacement heat resistant wiring and sleeves to arrive.

[Bob Hart]

THE 12K LAMP SAGA.

Cometh the time to deal with the heat shields. They're made of some sort of woven fibre and resin stuff which looks like munted printed circuit board on roids.

They are a bit tatty and being a little asbestos-paranoid, I don't really like the look of free fibre here and there on perished edges. For sake of re-assurance it will be finished over with high-temperature heat resistant automotive paint.

I shall have to shove a bunch of industrial lights inside the thing to cook it off before I light up an expensive quartz globe in it. If there are any lighting guys out there who see me making a craven fool of myself, please feel free to convey advice, which will be much appreciated.

[Sander Vreuls]

My military vehicle had asbestos around the exhaust pipes, which can get pretty hot, you can get modern fiberglass style protective stuff to rewrap exhausts now.. it might help here too :)

[Bob Hart]

Sander.


Thank you for your response. I was able to find on Amazon after following a lead given me, some heat resistant fibreglass wrapping tape, which should work for wrapping bare ends and chafe points.

[Chris Medico]

FYI - The ceramic fabrics are AWESOME! Like really awesome.

[Bob Hart]

THE 12K LAMP SAGA. CONT'D.

Some stuff has arrived so the rest likely is due soon.

As for the lamphouse difficulties, it gets bettered and betterer. Some time in the past it seems somebody might have done a Tarzan on the big turnscrews which lock off the lamp tilt. The captive nut threads have been stripped and replaced with some cruelbad self-threading steel bush, an early evolution of the helicoil principle and what a dog it is. Leastways it is no longer waiting in well hid ambush to fail. I always wondered why it seemed loose. Now I know. Very gratified it is not going to come unstuck and come down on some poor sod's dome. A flat head is not a good look. Not quite sure how I'm going to fix this one. There's now too much missing diameter for a modern helicoil solution. If I go near it with a welder, then that new heat paint is for nought.

[Chris Medico]

How hot does that part get with the lamp in operation? If it doesn't get more than a few hundred degrees F you may be able to fill it with a high strength epoxy such as "JB Weld" then drill and tap a new thread in there. Alternatively you can use the "JB Weld" to fuse in a thread insert.

I have used this stuff on engine blocks to repair cracks and leaks. Even a refrigerant leak in my heat pump has been repaired with it. Pretty amazing stuff. The temp limit is listed as 550F.

J-B Weld Twin Tube | J-B Weld

[Bob Hart]

Chris.


After using a but of heat and two pairs of vice grips I managed to extract the bolts from the threaded insert.

Interestingly it appears to be an old technique. a threaded sleeve carrying the old bolt thread is fed into a widened hole. I may have been misguided as to whether this thing tapped its own thread inside the original stripped captive nut.

After cleaning it up and carefully restoring the threads on the bolt and the insert which had become galled and deformed, there were to be seen four little square-cut channels of about 0.7mm. In each of these was a sheared off miniature woodruff key. It seems that once the threaded sleeve repair was screwed in, the woodruff keys were driven in to lock the sleeve.

When the original repair was done, the bolts were installed through holes drilled through the resin/fibre heat shields on the side walls. The downside of welding ordinary nuts onto a frame as captive nuts is that they both soften with the heat and shrink narrower. They are more likely to seize and strip threads.

If a tiny dag of weld flies in and fuses, then as the bolt is screwed through against what is thought to be the resistance of shrinkage, the threads on both will be permanently modified. The bolt and nut will continue working but only as a matched pair.

The bolt heads in the original repair had been pulled down onto the heat shield material which is not metal-hard. Use has fretted it and the bolts have become loose enough to wobble but fortunately not enough to turn.

I have re-assembled it as originally repaired with hardened shoe tacks as wedge keys and tightened the bolt head firmly down onto the inside of the frame where it should have been in the first place. That arrangement seems to be holding tight. There is a tiny dose of Loctite in there for good measure. The bolts are in effect fixed studs through the frame and facing outwards.

I imagine that in original assembly the yoke went on before the bolts were inserted and pulled down from the inside. Getting it on and off by springing it over the "studs" is grunt labour but it can be done. If the threaded sleeve eventually works loose again, I'll capture it with a small spot weld.

[Bob Hart]

Here it is in its skeletal state