Theatre Design & Technology - Fall 1979 - 37

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Technical Reports

Rewiring Commercial 35MM Slide Projectors for Theatre Use
by David Mardon and Sharon Wade
Small commercial 35MM slide projectors are often useful in theatre
production work. If they are not modified, however, the projectors
cannot be used on dimmers, as the application of less than full voltage
to the fan motors causes them to stall and quickly overheat and burn
up. Many theatre electricians rewire the fan motor to operate on a
non-dim, thus allowing the lamp to be dimmed separately. Unfortunately, if the non-dim circuit is not turned on, or fails, the lamp still operates. The lamp will quickly destroy the projector and is a fire hazard.
This problem is eliminated if a relay is added to the projector, wired
so that if the fan current fails, the lamp circuit will be interrupted (Fig.
1). A double pole switch was added to enable the lamp to run from either the dimmer or the non-dim circuits. This enables the electricians
to turn on the projector without calling for a dimmer. This can be invaluable for focusing projectors in a multi-projector set up. It also allows the projectors to be used for conventional slide shows.
It is most important that a double pole switch and relay be used so

that there is no connection between lamp and fan circuits. It is quite
common for the line and neutral to be interchanged in stage cable, so
the circuits must be isolated to avoid possible short circuits. Of
course, the equipment must be grounded and, as it has been modified, the UL or CSA approval markings must be effaced. The additional wire needed should be high temperature insulated fixture wire,
type AF, GTF, or TEW
16 stranded. Any splices necessary should
be made with ceramic twist-on type connectors.
We put our relay and switch in the compartment for storing the remote changer; however this space may not exist on other makes of
projectors, and the circuitry might have to be put in a small metal box
that could be bolted onto the projector side or connected with a multi
cable. We have eight projectors modified in this manner and have had
no problems with fan failures. We have applied similar protection. using 20 amp contactors, to our 2KW scenic projectors.

Fig. 1 Circuit diagram for Kodak projector.

Fig. 2:

1. Relay; 2. DPDT Switch; 3. Wires to lamp; 4. Wires to fan motor.

5--10 amp DPDT, center off switch
R-DPST NO Relay, 10 amp contacts, 120 volt coil.

Use wire with high temp. insulation.

Non Dim

O>------------l'
Ground

A Si lencer Box for Kodak Carousels"
by
David Mardon and Art McKay

USITT IFali. 1979

For a recent production at Dalhousie, we needed two pairs of
Carousel projectors on the balcony rail. Because the fan and slide
change noise would have been unacceptably loud for the audience,
we decided to build silencer enclosures to hold two projectors each.
The enclosures are insulated, and use a set of baffles to force the projector cooling air to follow "S" shaped paths to and from the projec-

Theatre Design & Technology

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