Desisti rembrandt 12k hmi - repair notes.
 

The following notes are personal observations, neither factory endorsed nor written by a qualified engineer. Therefore whoever reads should consult others and conduct research of their own.

The author accepts no responsibility for the ill judgement of others who attempt self-repairs of any similar lamp they own. Readers by continuing to read these notes accept personal responsibility for their actions, the dangers they may expose themselves to and the consequences of that exposure..

These notes should not be taken as a service guide but an informer as to what is now a vintage lamp. Because they have been often of rental inventory, now aged, a few of them may be coming into the hands of low-budget film-makers or aspiring gaffers.

Because there are dangerous voltages involved, unless people have the appropriate electrical trade qualifications and experience, electrical work should not be attempted.

Furthurmore, HMI globes of these powers are in themselves both vulnerable to damage, yet also capable of inflicting serious injury. They should not be handled or installed by untrained people.

Readers are encouraged to offer challenge to any of the comment below and correct any misinformation which has slipped through.

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DESISTI REMBRANDT 12K HMI LIGHT.



GENERAL MAINTENANCE.

The lamp is comprised of a fabricated steel frame made of square and rectangular channel, clad in thIn pressed steel sheetmetal which is either permamently fixed lanels secured by pop rivets or removable panels secured by metric screws.

This sheetmetal is covered in a simple paint system. There is no galvanic protection of the steel. Due to fair wear and tear this paint will be injured and the sheetmetal will rust, therefore the paint finish should be frequently repaired before flaking rust sets in as the material is thin and will become pitted through. Later models of Desisti lamps were clad in stainless sheet steel which ended the corrosion issue.

Inside the lamphouse, heat resistant fibre panels are fastened with screws, which are also screwed into threaded holes in the frame. One panel is are attached to a removable upper rear end cover. These panels are finished in heat-resistant black to protect the material from ultraviolent deterioration. They are positioned to allow a barrier flow of air to prevent the outer skins from heating up to a point of injuring operators or buring off the paint finish.

The top and bottom of the lamphouse contains light trap vents. These are also secured to the frame by screws.

There are several smaller light traps, two fastened inside the lamphouse at each side of the front access doorway, two in the base area attached to a ribbed vent panel.The access door doubles as the Fresnel lens holder and holder for the barn doors which are mounted on a ring frame..

lnside the lamphouse there is a base frame, also made of steel square and rectangular channel. This supports the carriage assembly and parts of the flood-spot focus mechanism.. The carriage is supported by two round chromed brass rails which float in holes drilled through the base frame.

The rods have been inserted through the base frame and through four white teflon bushes which locate in cutouts in the moving carriage frame. Both rails are retained in the frame by several "E" clips. The right rail serves double duty as a rail and rotating axle for the spot and flood motion of the carriage which is achieved by use of an "endless" cable and winch arrangement.

A lower rear end cover supports a power supply socket, in the case of the US lamps, of the Cannon ITT Veam type, an hour meter and a small switch panel.This is also secured by screws. There is no heat reasitant panel attached to it. Heat shielding is provided by a panel attached to the rear of the internal base frame assembly.

Under the left rear of the lamphouse, attached to the left side panel with external screws is an electrical sub-assembly cover. The electrical sub-assembly itself is attached with three small bolts which fasten into three small threaded pillars welded to the lamphouse frame. Unless a particular order of dissembly and re-assembly is followed, servicing of this area can be an awkward task



APPARENT DEFICIENCIES.

This lamp model appears to be an early first-of-kind, mostly hand-built with simple cut pieces of industrial material stock making up its mechanical structure. The assembly of components and sub-assembles in the lamphouse seems to have been driven more by what could be conveniently attached where, than by systematic layout to facilitate easy servicing. Most likely due to bumps, abuse and thermal cycles, the alignment of screwholes through panels and vents to the frame seems to be imperfect. Best practice seems to be to match each screw with the hole it came out of rather than allowing screws of identical type to mix. Only two screws of the whole set had deformed threads in the lamp I serviced but two are enough to give you a real bad day if you have not matched them.

The pivot bolts which support the lamphouse to the yoke are installed through captive nuts welded to the inside of the lamphouse frame. Welded captive nuts versus larger metal pieces with threaded holes, whilst an easier method can be a problem. Due to the thermal cycle of welding, the nuts may distort and the metal soften. The two bolts on the lamp I serviced had both stripped their matching nuts and these had been drilled out and re-threaded with keyed threaded insert bushes, a method that predated the helicoil system but is not as robust.

Furthurmore, the bolts which should have been fastened through the nuts to firm tightness had in this assembly been passed through holes in the heat-shield panels and pulled down tight upon them. This material both crushes relatively easily and frets rapidly in wear. Both bolts were loose and moving within the panel material as the outer fasteners were loosened or tightened. To remedy this, clearance holes were cut through the inner heat-shield panels and the bolts tensioned down onto the frame itself, then the panels installed around them. These bolts serve as axles to support the full weight of the lamphouse. Leaving the outer "wingnuts" which tighten the yoke loose in shipping may well have damaged the welded nuts trough the yoke being allowed to vibrate.

The rear lower panel which supports the power supply cable socket, is of the same light and thin steel as the rest of the casework. There is a small pressed metal stiffening dish into which the socket is bolted. However the panel is compliant to the weight of a heavy cable and the necessary motions of insertion and removal. This movement transfers to internal cabling at the joints into the socket pins. There is risk of wires cracking and fraying at the pins and strands straying across to adjacent pins and shorting. Best practice will be to wrap the wire-pin joints or immobilise them by encapsulation.

Vermin can gain access to the interior of the lamp via the vents. Damage to wiring is near impossible to detect except by dismantlement.

Two small heat shields carried in the carriage are intended to cast shadow over the teflon sliding bushes and the chromed brass rods. These heat shields are closest to the radiance of the bulb and become brittle. Another small heatshield of the door safety switch is also similarly damaged. There is a modern material which is a composite of mica flakes and a high performance binder which performs better in this environment than the original black-painted glass-fibre reinforced resin.

Single 6mm metric machine slot-head screws fasten the holders for the barn door ring frame to the door with no other pinning or keying except conformity to the circular shape of the door.. These screws cannot be adequately tightened. The holders become loose, twist and baulk the barn door support ring from inserting or rotating. Pressure upon crew to perform urgently will attract abuse. These slot-head screws are best replaced with Allen screws, which can be tensioned more effectively. No one really wants to go near a $1,000 fresnel lens with an impact screwdriver.

The two small mini rocker switches in the rear panel are of the spade connector type. The wiring has been soldered onto the spade terminals which are thicker than solder tags, are slower to heat and take solder. They hold heat for much longer. These switch bodies are made of thermoplastic and are easily damaged by the heat of soldering. Take extreme care to heatsink the spade connector at the switch body when unsoldering. The switches are of two types, "normally off - momentary on" and "normally on - momentary off". This second style of switch has been impossible to find worldwide thus far. Desisti may have had a limited custom batch made.

If there is interest, I will prepare an recommended order of dissembly and re-assembly.

Re: Desisti rembrandt 12k hmi - repair notes.
 

Footnote to the preceding post.

The vents/light traps are frames in which pairs of overlapping "U" sections are spot-welded. The vents are a tight fit in the welded steel lamphouse frame. Shape change of the frame and/or the vent/light trap over the years meant that there was only one way either the top or bottom one would fit back in with any hope of the screwholes matching.

There is probably a little labour to be saved by slightly widening the clearance holes around the screws in the vent/light traps and adding flat washers

Re: Desisti rembrandt 12k hmi - repair notes.
 

The two mini momentary rocker switches on the rear of the lamphouse which trigger the ballast to switch on and off remotely are a type which is not as readily available as the typical on-off switches in the same style.

These are "momentary" switches. One is a "normally off - momentary on" type. This triggers the ballast to power the lamp. The other is a "normally on - momentary off" type which triggers the ballast to kill the lamp. The second type of switch is very hard to come by.

I eventually sourced a "normally off - momentary on" type which can be opened up and conveniently converted to the "normally on - momentary off", be moving a little white internal plastic bridle and its spring to the opposing end of the switch body WITHOUT altering the arrangement of any other components.

The moving rocker is hinged in the switch body by two small pillars cast into the rocker which locate in two holes in the switch body. These two pillars also retain the rocker in the switch body. For factory assembly the rocker is simply pressed in and two ramps cast into the pillars wedge the switch body wider until the pillars find the holes and the sides of the switch spring back in to secure it in place.

Two small jewellers' screwdrivers I used to wedge the switch body wide enough to allow the pillars to clear from their holes and the internal spring popped the rocker up out of the switch body. To remove the white bridle and its spring requires a good metal tweezers and lots of patience in installing into an existing matching slot in the opposite end of the switch body.

Once all the tiny bits are in their correct places, the rocker is simply pressed back into the switch body and clicks into place when the pillars pop into their holes.


This switch type is :-

Marqardt 1801.1202 Rocker Switch SPNO.

Re: Desisti rembrandt 12k hmi - repair notes.
 

FOOTNOTE TO ABOVE:


Finding a vendor can be a bit convoluted and tricky. Being a slow moving item, the switches may not be held as stock by all vendors, therefore likely to be back-ordered and re-shipped with consequent delay.

This may contravene dvinfo guidelines, so Chris, if it does, I have no problem with the vendor's name being deleted or this appended post for that matter.

The vendor I found was "Safe Components & Equipment", which lists on eBay. I had a few back and forth emails with the vendor who was very helpful, has an understanding of the issue involved and need to convert the switches for the "off" function of the lamp.