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!)  However, the 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.

Procedure

DISCLAIMER - All advice given on this web page is offered in 'good faith'.
We do not accept ANY liability for any damage or injury that might be sustained
by carrying out any modifications outlined below.

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 bit suspect).

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:

[Back] Steve Glennie-Smith   Feb 28th 2013
Updated Feb 27th 2017 to fix broken weblink.