C A I R O

Dual Operator Box

Construction Details

The Dual Operator Box has such a prominent role in so many CAIRO configurations that it should be constructed for the highest possible mechanical integrity and durability, whilst preserving its functional and aesthetic simplicity.

Constructors who work solely from the interconnections diagram, may find that the internal hook-up wiring soon resembles a "rat's nest" and that it becomes increasingly difficult to work with a soldering-iron inside the box, as it nears completion. This can lead to joints which may be unsound but which defy close inspection, and it also leaves unresolved the problems which might be encountered in the future should it become necessary to replace a damaged component; a socket, potentiometer or switch. However this should be an unlikely occurrence if high quality items from a reputable source, are selected and used from the outset.

Investigations and trials have now shown that the construction of the complete Box can be greatly simplified if it is undertaken as a regime of elementary repetative preparations. These preparatory techniques include steps that save time and also ensure good mechanical integrity with reliable electrical isolation between the signal wires. Inevitably though, some of these steps require attention to mechanical details at the minutae level, and require descriptions which may be more reminiscent of a cooking recipe than the usual summary style adopted elsewhere in the CAIRO documentation.

CD .1 Historical Note

Origionally called The Dual Operator Outlet Box, this was one of the first hardware items to be constructed and evaluated in the formative years of CAIRO.
Towards the end of 1983, the conversion of an old ambulance into a Forward Communications Vehicle - FCV, as a joint venture between Raynet and St John Ambulance, in the West Midlands, led Bill Williams, G8AVX to consider the needs of two radio operators seated side-by-side. From the start of that project, it was clear that operating space would always be at a premium and so there would be little merit in having an outlet provision if it became as bulky as the transceiver itself !  
Indeed, CAIRO itself emerged from the simple observation that the concept of any Dual Operator provision or facility could only become a practical reality if any two people, who might form a "buddy-pair", could be drawn (at haste) from a pool of volunteers who all had 'standardised' plugs on their otherwise personal preference accessories. If not, such a 'box' would have required a plethera of alternative sockets!
So Bill very diligently studied the typical activities of an Operator-Logger "buddy-pair" and constructed three almost identical prototype Boxes for installation in that (now-demised) FCV. Each was based around the 114 x 89 x 55 mm diecast box with its outlets and main controls in much the same positions as those which are declared here. Only very minor changes were made in subsequent prototypes, since the basic layout and operation met with the instant approval of all who encountered it, in those early days. It is therefore a testimony to Bill's inspiration, then, that such precise details for its construction can be given, now.

CD .1X Operational Note

The CAIRO assignment of DIN-pins has allowed for a 12V interlink to exist between the pin-7's of all sockets in the Dual Operator Outlet Box so that a 12V supply may be injected at the Auxiliary Socket for any accessory that requires it. However, this 12V interlink is so seldom used in practice, that it is permissible for a constructor to omit this provision (perhaps after prior consultation with fellow users). If so,remove the pin-7 from all sockets before commencing the construction and ignore all references to brown-wire hook-ups in the following construction notes.

CD .2 Volume Controls

In general, all CAIRO volume controls should be 100R, 2W cermet potentiometers for a smooth loudness control into a typical 8 ohm loudspeaker, as the lowest permitted single load in the CAIRO scheme. This control is equally satisfactory whenever two or more higher impedance devices occur in parallel, e.g. a pair of 32 ohms earphones. All three potentiometers for the Dual Operator Outlet Box have been previously declared to meet this specification even though, in practice, the front pair almost always only control headphones or headsets. Thus they are generally used as an occasional pre-set adjustment rather than a full and frequently altered control. If a loudspeaker is required, it is plugged into the rear auxiliary socket where the extra "pedestal" resistor (e.g. 6R8) with the rear potentiometer, ensures that it can never be turned fully off.

These observations suggest that the usual rule for potentiometers can be relaxed slightly just for these two front panel controls of the Dual Operator Box to use instead the 16mm, 1W, 100R cermet front-face, panel-mounting items. These come complete with an integral black knob (with white pointer spot) which encases the bulk of the device so that there is a minimum intrusion at the rear of a panel. If the item is pre-wired before insertion, it is easy to fit, it looks good on the finished Box (assuming this has also been painted black) and it leaves plenty of internal finger space for when the side sockets and switches are being installed. (It is also slightly cheaper than the combined cost of the 2W cermet potentiometer and suitable knob.)

Fig. 1

Interconnections within the Dual Operator Outlet Box

The above diagram depicts an anchored-tail as the means of connecting the Dual Operator Outlet Box to the Transceiver signals. However, the Constructor is invited to consider that a Loose-Tail (CI.1) connection renders the DOB an even less cumbersome item when packing it away (e.g. in a Grab-Bag).

If so, ignore references to the CAIRO in-feed Line in the following pargraphs and instead, make provision for an Audio-5 chassis plug to be installed in place of that incoming line, by punching an extra "DIN-diameter" hole in the rear panel. When, in the following sequence, the five DIN-7 sockets are prepared with coloured leads and sleeving, also pre-wire this Audio-5 chassis plug.

CD .3 Marking and Punching

As noted earlier, the 114 x 89 x 55 mm diecast box is an ideal housing. Other than the controls on the front panel (a 114 x 55 mm side), the side and rear panel sockets and switches, and the rear panel volume control, should all be centered on the mid-line which, for purposes of a marking table, can be scribed at a height of 25 mm. Measuring from the base of the rear-panel box side, the two DIN-7 sockets and mute switch centres are at 20, 45 and 65 mm respectively on each of the two (89 x 55) sides. These DIN-7 sockets should be mounted vertically with their key-ways facing forwards (pin-2, the "12 o'clock" pin, towards the rear).

Fig. 2
Component Placement Centres
for the
Dual Operator Outlet Box
Please note that the sockets' lugs do NOT exactly align with the front-panel 15 and 35 mm centre-lines.

The rear panel has centres, measured from right to left, at 30 mm for the auxiliary DIN-7 socket which is mounted horizontally with its key-way facing down, 65 mm for the 100 ohms cermet potentiometer, and 95 mm for the signals tail. On the front panel, the operator selector switch is horizontally centered at 55 mm (measured at the base) and is 35 mm above the base, while the mini-pots are symetrically placed at 30 mm from their respective edges on a centre which is 15 mm above the base. Although minor variations in these positions can be tolerated, overall symmetry should be achieved for the front panel, and again for the side panels as a pair, noting in particular that the forward DIN-7 sockets on each side will have a tag strip between their upper (M3) anchor bolts.

Having marked all the centres, and assuming that the components used are those suggested here, the drilling requirements will be a 6mm hole for the two small (mute) switches, a 10mm hole for the three potentiometers and a 12mm hole for the large (operator selection) switch. The DIN sockets require a suitable clearance hole for a 16mm (minimum) circular sheet metal punch (which is actually a 16.5mm punch requiring a bolt clearance hole of 10mm). Finally, the line cable entry grommet requires a 7mm hole. When the box has been drilled and punched, run a file around the lips of the box and its lid to remove the burrs or roughness which remain from the casting.

Items Check List : metal punch [543-563]; grommet [543-197]

CD .4 Painting

The Dual Operator Outlet Box is a "significant" passive item in the general CAIRO scheme and should therefore be painted black. The matt black (heat radiating) aerosol is quick drying and leaves a very smart appearance. However it is essential to clean the diecast box thoroughly and apply a primer first followed by at least two coats of matt black for adequate resilience to the sometimes rough handling that will inevitably occur in service.

Items Check List : matt black (heat radiating) aerosol [567-345]; primer [567-749]

CD .5 Colour Coding

A tag strip (cut back to 14 tags) will be mounted between the side-panel's forward DIN-7 sockets to complete the wiring when all components have been installed. Prior to this, each component in turn may be pre-wired in a jig or vice and all this can be done between applying the paint layers to the fully drilled and punched box and setting it aside to dry. For this scheme to succeed, it is necessary to use hook-up wires that conform to the CAIRO plan so that they can be easily identified and soldered to the tag strip during the final stages. The colours are;

DIN-Pin

Signal

Colour

cuts (+1 if Audio-5)

1

Speaker

Blue Bl

12 (13)

2

Earth-Screens

Grey Gy

8 (9)

3

Mic.-High

White Wh

8 (9)

4

PTT

Red Rd

8 (9)

5

Mic.-Low

Yellow Ye

5 (6)

6

Secondary Audio

Violet Vi

9

7

12V d.c. power

Brown Br

5

For simplicity, sets of 7/0.2 wire sufficient for all components, can be cut to length beforehand, as tabulated above. With the exception of the grey (earth) lengths which should be cut to 125 mm, all other colours may be cut to 100 mm. At the same time it is useful to prepare the 50 (55), 10mm lengths of 3.2mm heat-shrink which will be required on all pins of the sockets, potentiometers and the two mute switches. (The six terminals on the operator selector switch would require a larger diameter sleeve but this may be ommitted because these terminals will be in clear space in the final box.) If no heat gun is available, the pre-wired components, with their sleeves in position, may all be set out on a metal tray and popped in a hot oven for a couple of minutes (150 deg.C, 300 deg.F, gas mark 2 - cookery indeed !).

Items Check List : tag strip [433-781; hook-up wire [357-227] et seq.; 3.2mm heat-shrink [399-502]

CD .6 Five Sockets

A jig for the five sockets can be made by clamping a DIN-3 plug upright in a small bench vice so that each socket can be placed on it, in turn. In this presentation the pins are in mirror image, so pin-7 (for a brown wire) is at 7-o'clock (or it is an empty hole, if the 12V provision is being ommitted) and pin-6 (for a purple wire) is at 5-o'clock. The full clockwise order is; Brown (or none), White, Yellow, Grey (pin-2, 12-o'clock), Red, Blue, Violet. First, cut off the socket's earth lug and then solder the wire to each pin in turn, working either clockwise or anticlockwise according to your handedness. Ensure that sufficient solder is applied to completely fill the elliptical pin hole, even though each bare wire turn only occupies the upper half of the pin hole. When all wires are in place, go round trimming each turn of excess strands and check the colour assignment against the above tabulation or previously checked sockets. Thread the pre-cut lengths of heat-shrink sleeve and adjust them so that no part of a pin is visible. Keep the socket face down when transferring to the tray for oven heating, or apply a heat gun to each socket while still on the jig plug.

CD .7 Controls and Switches

The rear-panel volume control potentiometer should first be prepared by cutting its shank in half to a length of 7 mm so that it will accept the alternative knob that is now recommended for its black finish and large body. It should then be pre-wired with a blue wire onto the top terminal and should have a 6.8 ohms resistor soldered and sleeved in-line with a grey wire from its ground terminal ."Top" and "Ground" are found in the sense that clockwise rotation increases wiper output. The wiper terminal is left alone at this stage, for it to take the auxiliary socket's pin-1 (blue) wire directly inside the box. (This is the only connection which does not use a tag on the tag strip). The two front-panel mini-pots should be prepared with two blue wires, one to the wiper tag and one to the top tag, and a grey wire directly to the ground tag (no "pedestal" resistor).

The two mute switches should be pre-wired with one blue wire, on what will be its "lower" tag when fitted, and two purple wires on its common and "upper" tags. The large operator selector switch requires three red and three white wires, each from the set of three tags which are in the same plane as each other, when viewed horizontally.

CD .8 Installation

The precise order in which the components are fitted in place is not significant except that the rear-panel volume control and auxiliary socket (and Audio-5 plug) should be installed early in the sequence. This allows the blue, pin-1 speaker connecton to be made without too many other wires in the way. Fit the front-panel mini-pots next and then proceed to the components of the two sides, leaving the front-panel main switch as the last item to be fitted. As each socket is installed, make sure that the lower M3 bolt is fully tightened because access to it later will be difficult.

CD .9 Tag Art

Cut the tag stip between the fifteenth and sixteenth tag and remove the fifteenth tag by squeezing the grip lugs which hold it in its square hole, to leave fourteen tags and two mounting holes.

Take four M3 or 6BA solder tags and bend a right-angled turn just beneath their solder hole.

Solder them together in pairs, with each solder hole against the stem of the other, such that they form a right-angled bracket.

Attach these to the tag strip, on the same side as the tags themselves, so that the tags will "hang" below the strip in the box, and include two more M3 or 6BA solder tags on the right-hand end of the strip. This will become the single-point earth bond to the box chassis.

Now fit the tag-strip assembly behind the nuts of the upper anchor bolts of the two panel-centred DIN-7 side sockets such that the tag strip crosses the box, just below the lid.

Comb all component wires into an easily read fan and begin the tag connections from the left (Tag-1). At this stage, wires from close-by components which are excessively long, may be cut to a better length.

Wh | Rd | Vi/Bl | Bl/Vi | Br | Wh | Ye | Rd | Bl | Vi | Bl/Vi | Vi/Bl | Rd | Wh | Gy

Tags 1 | 2 | 3 | 4 | 5 | 6 | 7 | 9 | 10 | 11 | 12 | 13 | 14 | "15"

M+ | PTT | Sec | Pri | 12V | M+ | M- | PTT | Pri | ST | Pri | Sec | PTT | M+ | Gnd

Tags

Signal

Colour

from ...  

Left

Right

to ...

wires

1

14

Mic.-Hi

White

main switch to that side's sockets' whites

3

2

13

PTT

Red

main switch to that side's sockets' reds

3

3

12

Mute-Switch common

Violet

front socket violet and rear socket blue

3

4

11

Mini-pot wiper

Blue

mute switch blue, front socket blue and rear violet

4

5

12V interlink

Brown

all sockets' browns

5

6

Tail Mic.-Hi line

White

to main switch common white and auxiliary (rear) socket's white

3

7

Tail Mic.-Lo line

Yellow

to all five sockets' yellows

6

8

Tail PTT line

Red

to main switch common red and auxiliary (rear) socket's red

3

9

Tail Speaker line

Blue

to the three potentiometer "tops", blues

4

10

Auxiliary (rear) secondary

Violet

to mute switches' upper tag, violets

3

"15"

Grounds/Chassis

Grey

all Greys together with the Signals-Tail screens

9

 

Update:
99/06/08

©G 8 C Q H