Baltimore and Ohio Railroad
Color-Position Light Signals

CPL_signal head
This is a complete (four-aspect) color-position light (CPL) signal head.  Have you ever wondered what was inside of one of those lamp units?  Well, thanks to an old website that had the 1937 General Railway Signal (GRS) Company catalog available, we can take a look under the hood (pun intended) of a GRS Type U lamp unit.

The diagrams and list of parts shown are from the catalog; the photographs are mine, taken of one of the lamp units from my CPL (shown in the lead photo), with the various prts numbered so that you can compare them to the drawings and see what they actually look like.

CPL signal parts 4 CPL signal parts 3
CPL signal parts 2 CPL signal parts 1

Note: In drawings 2 and 3, the red lines indicate the background, or target, of the signal.

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Part no.
Description
Part no.
Description
1
Screw, no. 10-32 x 3/16" fil. hd. for fastening ring (27) to bezel ring (20)
21
 Screw, no. 6-32 x 5/16", with clip, for fastening lens (25) to light unit
2
Spring for stud (11)
22
Hood, for light unit
3
Screw, no. 10-32 X 3/4"fil. hd. for fastening hood (22) to bezel ring (20)
23
Incandescent bulb, 13.5 volt, 17 watts, for light unit
4
Bushing for screw (9)
24
Lens, outer doublet hot spot, 8-3/8", 4" focus
5
Bushing for screw (9)
25
Lens, inner doublet, 5-1/2" dia. green, for light unit
6
Nut, no. 14-24 x 1/8", hex head, for terminal posts
25a
As above, except yellow
7
Nut, no. 14-24 x 5/16", hex head, for terminal posts
25b
As above, except red
8
Washer, for terminal posts
25c
As above, except lunar
9
Screw, no. 14-24 x 2-7/16", sq. hd., used f-or terminal posts
26
Gasket, for lenses (24 and 31)
10
Adjusting nut, for light unit
27
Bezel ring, as shown, for light unit
11
Stud, for adjusting light unit
28
Pin, 3/32" x 3/4", for locking stud (17) to bezel ring
12
Plate, for closing opening in bottom of light unit
29
Screw, no. 6-32 x 11/32" hex hd. with washer, for fastening sighting plate (33) to light unit
13
Screw, no. 8-32 x 3/8" fil. hd., for fastening plate 912) to light unit
30
Roundel complete, includes roundel clip and spring assembled together, for light units figs. B to B4 onlyB
14
Screw, no. 6-32 x3/16", for fastening sighting strip (32) to shield (16)
31
Lens, outer doublet 30° spredlite 8-3/8" dia., for light units figs. B to B4 only
15
Cover, complete, includes clip and chain, for light unit
32
Sighting strip, for light unit
16
Shield, for light unit
33
Adjustable sighting plate, for sighting light units
17
Stud, 3/8"-16, for fastening light unit to background
34
Bracket, for lamp receptacle
18
Nut, 3/8"-16 x 3/8" hex, for stud (17)
35
Screw, no. 14-24 x 1-3/8" sq. hd., for clamping bracket (34) to stud (11)
19
Spring for stud (17)
36
Receptacle, only, for lamp receptacle
20
Bezel ring for light unit
37
Plunger, for lamp receptacle
38
Spring, for plunger (37)
39
Block, for receptacle
40
Adjusting nut, for receptacle

Major components of a CPL Lamp unit CPL lamp details

12a in the first photograph is the wire channel that 12 covers; however, when the plate (12) is applied properly, a small looop of wire extends forward of the target and must be protected.  This where plate 15a comes into play.   It is attached to the bottom of the bezel ring (27) to protect that loop of wire.   Two photographs, below shows these two plates in place; plate 12 is on the left amd 15a is on the right.   In photo 2, the small nuts and bolts shown in the aqua-colored boxes are used to hold the lamp assembly together; the bolts pass through the socket and socket block downward for the nuts and bolts (to attach the wire, on the left, and to adjust the position - forward and backward - of the lamp socket to adjust its focal point on thr right). The second wire connnects to the bottom of the plunger/socket assembly (37, 38 and 39) These wires connect to bolts (9), to which the wires from the power source also connect.

Plate 12a in place Plate 15a in place

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Last updated Suday, May 8, 2016