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 earth bonding and OTT cable clamping.  Although such equipment might 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 our 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, 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 are approaching 50 years old.  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.

Other items needed to do this job are:

• Heat-resistant P-clips for cable clamping.  (RS Components part no. 2987987 - bag of 25.)
• Braid, for cross bonding.  The 30A rated one we used might be a bit OTT, but it's easy to terminate for connections.  (Farnell part no. 1231000 - 25m reel.)
• An M6 * 12 roofing bolt and square nut - available from most DIY stores, eg. Homebase.
• An M3 * 10 countersunk screw, nut and washer - from a model shop, for instance.

Both RS and Farnell supply all these bits, but they were cheapest (at the time of writing) as shown above.

Procedure

• Remove the lens assembly.  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 difficult to find.  It might be possible to graft in the more modern GY9.5 (bi-pin) holder (which takes T18 lamps).  Our 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).  Ensure the Paxolin insulator isn't damaged.
• 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 photo below).
• Remove one of the three 2BA screws that hold on the lens 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 mirrors.
   
• 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 insulator.
  • 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 baseplate.
  • 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 are now properly cross-bonded.
• Ensure the lamp carrier can be moved over its full travel.  If not, reposition the braids as necessary.
• Refit the baseplate and lens assembly 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 no short from live to case.