Strand (and Furse) Pattern 23 mirror spot refurbishment
Sooner or later, most theatres will be faced with PAT (Portable
Appliance Testing). Although this is meant to ensure electrical
equipment is up to scratch (obviously a Good Thing), as with many Health and
Safety issues, a load of myth and bulls**t has also crept in. Some
independent PAT testers might insist on older equipment having niceties like
internal earth bonding and OTT cable clamping. Although such equipment might
still be perfectly safe, it was never designed with this in mind since no
requirement existed at the time of manufacture.
LADS (Ledbury Amateur Dramatic Society) had this problem with a few of
its stage lights. In some cases, this was a good time to consign items
past their use-by date to the skip and get something more modern.
(Before we did so, we stripped them of anything useful to keep as spares!)
Strand Pattern 23 Mirror Spot
(and the Furse copy), of which we have six,
are simply too useful to throw away, even if they do date back to the
1950's! They were quite advanced when they first hit the stage: die-cast
body (when everything else was heavy steel) and remarkably efficient optics.
Although the 23's cable clamping left a bit to be desired, simply
pulling a zip-tie tight round the cable just inside the entry hole was
sufficient to stop it pulling through - this arrangement was fine for years
but it wasn't good enough for our PAT man. Also, 23's don't have
'cross-bonding', which in simple terms is internal earthing connections
between all exposed metal parts, so earth continuity doesn't rely on hinges
etc. to make good electrical contact (as if you'd ever fire up a lantern with
the door open - and when it's locked shut the locking screw ensures a good
contact anyhow).... Most spots and Fresnels have a movable lamp carrier,
which also needs to be bonded to be really Kosher. The following tells
you how to future-proof a 23.
There isn't much space inside a 23, so devising a clamping arrangement
that would keep our PAT man happy was a bit challenging. We didn't use
proprietary cable clamps since most of them are too big - and plastic ones
would probably melt. Most of our lanterns were fitted with immersion
heater cable, which is far too thick. 23's run very hot, so the cable
must be a 'hot' type - we bought a 25m drum of a silicone insulated type from
RS Components (part no.1927934) for this and future repairs.
Please note: As with most cables of this type, silicone is more susceptible to mechanical
damage, eg. abrasion, than PVC or butyl rubber.
Other items needed to do this job are:
Both RS and Farnell supply all these bits, but they were cheapest (at the
time of writing) as shown above.
DISCLAIMER - All advice given on this web page is offered in
We do not accept ANY liability for any damage or injury that might be
by carrying out any modifications outlined below.
- Remove the lens barrel. Open the back door, take out the lamp
and then remove 4 screws holding the baseplate and cable assembly. Put
these to one side.
- Remove both mirrors (4 screws each), the front light baffle (4 screws)
and the rear light baffle nearer to the hinge (2 screws). This is a good
opportunity to clean out the lantern internally and re-spray the baffles with
matt black heat resistant paint.
- Remove the screws holding the lamp carrier, then the lamp holder and
cable. If the cable is not between 6mm and 7mm diameter, damaged or not
heat resistant, replace it - a 1.2m length should suffice.
- If the lamp holder is broken, that might be an end to the matter since
P28 lamp holders are no longer made and are difficult to find. One solution
to this would be to make a converting block to the more modern GY9.5 (bi-pin)
holder (which takes T18 lamps). A drawing of this is
here (not sure why the M4 holes are included, though
they could be intended for earthing). Note: There are two sets of
mounting holes, one pair of which accommodates Strand bases (as we have).
The other pair is intended to accommodate an Aldis base, which has
different fixing holes. The solution for our one broken holder was to
cannibalise an old Pattern 45 Fresnel (which also failed the PAT test owing to
lack of cross-bonding, so it was going to be slung anyhow).
- If reusing the P28 base, ensure the Paxolin insulator isn't
- The lamp carrier and baseplate need to be modified as in the photo below
(positioned relative to each other so rear of lantern would be to the right):-
- Drill a 3.2mm (or 1/8") hole in the lamp carrier and countersink it from
the side with the bosses.
This must be sufficient to lose the head of the M3 screw (so the lamp carrier
will slide easily over the baseplate when reassembled).
- Cut away part of the original cable clamp boss from the baseplate as
shown (bottom left). Ensure you do not damage the threaded (2BA) hole.
The original cable entry hole will be used for holding the P-clip, so part of the
side of the boss will also have to be removed to accommodate its width.
- Remove paint from the top of the boss, and also from the three
countersunk holes on the other side (not shown).
- Drill a 7mm dia. hole as shown (this is the new cable entry).
Make sure this isn't too close to the edge, which would foul the flange
on the body when re-assembled.
- Cut a piece of braid that will connect the door via the body to the
baseblate. Ensure it is long enough for the door to open fully (see
- Remove one of the three 2BA screws that hold on the lens barrel carrier
- use this to ensure the holes in the braid (below) are the correct diameter.
Remove any paint or corrosion from this screw and around its hole.
- Make 3 holes in the braid by teasing apart the strands - at one end to
clear the self-tapper holding the rear baffle, at the other and part way along
to clear the 2BA screw removed earlier. This is best done using a pair of
circlip pliers as a mandrel. When the holes are the right size, saturate
them with solder. This means you don't need to fit ring-tags.
- Refit the 2BA screw through the braid as shown above. Remove any
paint or corrosion from the fixing holes of the rear light baffle and refit it
with one screw through the braid as shown.
- You can now refit the front baffle and mirrors. Ensure the braid
is routed between the front baffle and the body, and isn't trapped by the
- The baseplate is re-assembled as below:-
- Cut a short length of braid as above. Make terminations at each
end as before to clear M3 and 2BA respectively. Fit this to the lamp
carrier with the M3 nut, bolt and washer.
- Thread the cable through the new hole and connect it to the lamp
holder. If you've replaced the cable, you'll need a new 2BA ring
tag for the earth wire. Re-use the heat resistant sleeves over live and
neutral wires. NBB The live must be connected to the centre
pin of the P28 lamp holder.
- Although it doesn't seem to matter optically which way round the lamp
holder is fitted, we fitted all ours with the larger flange to the left (see
photo) - which results in the live connection being shorter.
- Refit the lamp holder to the carrier - don't forget the Paxolin
- File about 1mm off the side of the P-clip that will abut the boss.
- Fit the P-clip over the cable and then the lamp carrier to the
- Fit the P-clip to the baseplate using the M6 * 12 roofing bolt.
- Fit the 2BA screw that was the original cable clamp through the earth
ring tag and both braids to the hole in the boss. All parts will now
be properly cross-bonded.
- Ensure the lamp carrier can be moved over its full travel.
If not, reposition the braids as necessary.
- Test the lens barrel safety chain as shown below.
- Refit the baseplate and lens barrel to the body of the lantern.
Refit the lamp.
- Check, with an ohmmeter, you have good earth continuity from the open
trap door to the plug earth pin.
Check also there is greater than 10Mohm resistance between Live and case,
and between Neutral and case.
- PAT test it!
Lens Barrel Safety Chain
There is (or should be!) a small chain between the movable lens barrel and
the rest of the lantern. One end of this is screwed, via a tag, to one
of the colour frame mounts. The other is linked via a large ring tag
through which the barrel clamp screw passes. The chain and tags are made
of brass: each link is formed into a circle with the ends brazed
together. All very well until a chain link gets iffy, someone mounts the
lantern pointing down (good for artistic effect but shortens lamp life) - and
then fails to tighten the barrel clamp sufficiently....
The inevitable happened - fortunately without personal injury.
Sod's law said it was the only dodgy chain we had (though two others were a
One way to test if the chain is OK is to remove the barrel clamp bolt completely and then
withdraw the lens barrel. Firmly hold the large ring tag in one hand and the barrel in
the other. Sharply lift the ring tag whilst letting the barrel drop.
(Do this over a soft surface in case the chain does break!)
Do this two or three times and carefully inspect the chain. If any link looks elongated,
replace the chain as below:
- Use 'chain stitch' type chain (as used in many loos with high-level cisterns)
as in the photo below. This is made up by threading one link through the
holes in the one before it, meaning no welds or brazes that could fail.
- Elongate the hole in the large ring tag through which the original chain passed (see
- Dismantle some links of your new chain and thread the first through the
Repeat to add further links (9 or 10 should suffice)
- Fit a small washer to the self-tapping screw and thread it through the hole in the last
link: then refit the screw to the barrel (see photo).
- Repeat the 'drop test' as above.
||Steve Glennie-Smith Feb 28th 2013
Updated Feb 27th 2017 to fix broken weblink.