The TDC 116 Projector with lamps dimmed

Halogen Lamps for the TDC-116 Stereo Projector

Part 2: Illustrations and Discussion of the Modifications
Reproduced with Permission from the Victorian 3D Society

Introduction

Over the last 3 or 4 years there has been a dramatic increase in the number of Club members with projectors, from around 2 or 3 up to around 7 or 8, with many of these projectors being TDC models from the USA.

As most projectors coming from the USA need some form of modification to operate on our mains voltage, and given that it would also be an opportunity to bring such projectors up to date by using better lamps, this article describes in detail the modifications made to a TDC-116 projector to fit low voltage halogen lamps to it.

Modifications

Taking into account factors discussed in Part 1 of this article, it was decided to modify a TDC-116 projector so that low voltage quartz halogen lamps could be used with it. The modifications described here follow along similar lines to those undertaken by Max Hem on his TDC-716 projector, which has been used extensively at our Club meetings.

Lamp Sockets

The original lamps in the TDC-116 were a CZX-120Volt / 500 Watt. The lamps chosen to replace these are the popular EHJ-24Volt / 250Watt, which require a completely different lamp socket.

Fig1: The TDC 116 Projector Stripped ready for modification.

Figure 1 shows the TDC-116 stripped of all it's major components. The TDC projector is extremely well made and easy to work on. None of the components in the projector are overly complicated, and yet they all perform the intended function very well.

The new lamps require a "2-pin" socket with 6.35mm spacing. This is a standard pin spacing, and no difficulty will be encountered locating such sockets. However it may be difficult to find sockets rated for use with 250W lamps. In this particular modification some lamp sockets were salvaged from old mono projectors.

In designing a method for mounting the new lamp sockets it is important to allow some facility to adjust or fine-tune the position of the lamp. This requires an ability to move the lamp plus or minus a few millimetres from its nominal position. This adjustment should allow the lamp height, sideways position, and fore-aft position to be fine-tuned.

Fig2: Mounting for new lamp sockets and re-positioned mirrors.

Figure 2 shows a view of the new lamp sockets mounted in position on the top-deck of the projector. A flat aluminium plate 1.6mm thick was mounted to the top deck with just one screw. Since the mounting hole in the aluminium plate was a slot slightly larger than the mounting screw it is possible to provide the sideways and fore-aft adjustments with the one mounting screw. The mounting screw used was a 3mm metal thread, and the top deck was tapped with a thread for this screw. (However a 4mm screw would provide a firmer locking of the mounting plate.)

Initially the lamps were placed at the same position as the old lamps. The height was set as shown in Figure 3, which allows room for the filament image from the mirror to be positioned above the lamp.

Fig3: The nominal positions of the old and new lamps.

Lamp Cooling

Fig4: Less air was directed at the lamps.

The issue of cooling air for the new lamps is a little more tricky than for the old lamps. It is possible to "over-cool" quartz halogen lamps, where as the original lamps basically needed as much air as they could get. Also as the amount of heat generated is half that of the original lamps, less air is needed. Three factors have been altered from the original design that reduces the amount of air fed around the lamps:

• the flat plate the lamp sockets are mounted on allow less air through than the original sockets
• a baffle at the bottom of the first condenser was removed to allow more air around the condenser and heat filter assembly (see Figure 4)
• the new slide gate cooling took more air away from the available flow

So far these arrangements seem satisfactory.

Lamp Flue

Fig5: A rebate inthe base of the lamp flue allows for the mounting plate for the new lamp base.

To allow for the new lamp sockets to be fitted, a rebate of approximately 2mm was cut into the base of each flue surrounding the lamps. This is shown in Figure 5.

Slide Gate Cooling

Fig 6: Additional slide cooling vent.

A new ventilation hole was added in the top deck of the projector, just in front of, and beneath the slide gate (between the slide and the projection lens). Figure 6 shows the new arrangement. Note that a hole also had to be cut in the base of the black slide gate shroud. With this arrangement the slide gate temperature rises just 15 Celsius degrees above ambient, which is evidence of excellent slide gate cooling.

Mirrors, Lamp and Slide Gate Alignment

It was required to obtain satisfactory light coverage for slides up to 31.5mm wide x 23mm high. To obtain this performance some fine tuning of the alignment of the various components in the projector was required. In summary the following alteration were made:

Fig 7: Lamp alignment can be inspected visually with dimmed lamps.

• the lamps were moved forward approximately 5mm, so that the filament is now about 19mm away from the first condenser.
• the mirrors were moved forward a similar amount (5mm), which required four new mounting holes to be tapped into the top deck of the projector (see Figure 2). The new mounting holes were approx 7mm forward of the original ones. The holes tapped were 3mmx0.5mm thread which is different to the original American thread that is uncommon in Australia.
• the complete condenser assembly (but not the mirrors was raised 1.2mm, as it was found that the centre line of the mirrors were not perfectly in line with the condensers). This was easily done by placing an aluminium packer under each mounting screw.
• the slide gate was raised 1.6mm using aluminium packers locked into position between the base of the gate and the metal guide attached to it. The locating spring for the gate was re-tensioned.

The alignment of the lamp can be inspected by holding a simple low-powered positive lens between the projection lense and screen (eg: a pair of glasses). Figure 7 shows the correctly aligned lamp.

Electrical Components

A 500W power transformer is required to run the low voltage lamps, and for many this expense (and weight) will make this modification unacceptable. The transformer is external to the projector, and the power is fed to the projector via a cable 2m long, terminated in a 12-pin Cinch-Jones connector that replaces the original power inlet connector. See also the attached circuit diagram.

The primary of this transformer has several taps (260V, 240V and 220V) so that the lamps can be operated at low, medium, and high settings (respectively). Also a 100V and 90V taps were provided so that the original 110V/60Hz fan could be used. (As Australian line frequency in 50Hz, a 20% reduction in voltage will prevent the 60Hz motor from running too hot).

The primary circuit has a lamp dimmer circuit included in it (which can be bypassed with a switch in case the dimmer circuit fails open, and also to remove any residual voltage drop when turned fully on).

The secondary winding is nominally 50 Volts with a centre tap (25-0-25V). The lamps are run in series, and fed with 50 Volts to limit the current in the cable from the external power transformer. The junction of the two lamps is connected to the centre tap to balance out the voltage on each lamp.

Allowing for losses in the connecting cable, the lamp voltage can be varied from 23 to 25 Volts using the various taps in the transformer. Over running the lamp gives more output, but dramatically reduces the lamp life.