Patt 765 Followspots not striking (a question for older members?)

[Hilary Watts]

Our theatre has two Patt 765 follow spots.  They have not been used for the last two years as a result of the pandemic (and may not have been used for some time before that) and on inspection neither has a lamp in it.  We have obtained a new lamp from a reputable supplier but it fails to strike in either of the follow spots.  I have downloaded the technical handbook from the Theatrecrafts website and have checked the values of the components of one of the units  (including the choke which is in a separate enclosure) all of which are within spec and appear undamaged.  The internal wiring is in good order.  The relay which provides the starting pulse is vibrating but I don’t have the equipment to check whether the 10Kv starting pulse is actually being generated.

 

My questions are:

 

1.         How quickly would you expect the lamp to strike after pressing the ‘Start’ switch (ours are the older model 765 with manual start switches – the later models had a relay- actuated start once they were turned on)?   The manual advises against letting the starter run for more than five seconds and suggests a number of shorter duration attempts but doesn’t say what would be a ‘reasonable number’ before concluding that something is defective.

 

2.   In the experience of members is it likely that the new lamp which we have bought is defective?  I have attached a photo which shows a small blemish on the lamp but I don’t know whether this is significant or whether there was generally a high failure rate for such lamps..

 

3.         Does anyone have any suggestions as to further fault-finding steps in the absence of high voltage test equipment? We don’t have an alternative lamp to try and are reluctant to buy another unless we know there is a reasonable chance of getting the units going.  (I thought I might replace the 3 capacitors and 2 resistors on the basis that while they appear undamaged and produce the right readings on a cheap multimeter they may react in a different way at mains voltage or higher.)

[Dave m]

It's been years but....

 

I would at least give it a proper 5 second blast on the starter. They didn't always strike first time, so keep at it having allowed a minute or two between strikes?

Also maybe leaving the kit plugged in a while so the capacitors have a chance to recharge? They could have just died.

Do you get the buzz from Hell on striking? 

[Hilary Watts]

Yes, I forgot to mention but we do get the buzz so the starter relay is clearly kicking in.

[adam2]

The presence or absence of the high voltage starting pulses may be determined with "voltstick" or similar non contact voltage detector.

The high voltage starting pulses will operate such a tester at a distance of several centimeters, unlike the close proximity required for ordinary mains voltage. Only a crude test as starting pulses of say half the required voltage will operate the voltstick.

Even a new lamp might be defective.

Or there might be insulation breakdown between the starting circuit and the lamp. ISTR that ordinary mains voltage flex was used for this connection and therefore operated briefly at far beyond the design voltage and perhaps vulnerable.

[delicolor]

I seem to remember that you would see a small arc inside the CSI lamp if it didn't want to strike confirming the presence of HT, provided the pip hadn't popped. (Unless I'm remembering what Xenon lamps do, I had a projector decide not to strike a few hours into lamp life once but it was fine with the spare). 

One other thought which is probably a red herring- 765s were originally designed for CSI lamps and you have a CID lamp there. I don't recall if they were entirely backwards compatible with the early models although later data sheets show 785s as CSI/CID. The CID apparently gave less light but looked brighter due to the spectral distribution. I suspect they probably were backwards compatible as not being so would have hopefully stuck in my mind. However we are talking 40+ years here...

[Ynot]

1 hour ago, delicolor said:

One other thought which is probably a red herring- 765s were originally designed for CSI lamps and you have a CID lamp there. 

From the Strand datasheet... (courtesy Strand Archive)

[paulears]

Check the wiring int the ballast connector - there were two incompatible types and sometimes they just arc in there too, sapping the oomph at the right end, same again in the lamp socket. other than that, I suspect it's the ballast. We have two complete ones, but only one now functions, the other does exactly what you are getting and swapping the ballast showed where the problem was. Oddly - CSI or CID lamps both worked fine I found?

[Hilary Watts]

Thanks for all the replies.  To save time for people who don't want all the details I'll say in brief that the conclusion we've come to is that the new lamp is faulty.

For those who are interested a friend and I had another good look at the situation over the weekend.  We double checked all the internal wiring and the action of the starter relay in making and breaking the circuit to the high voltage coil.  I also had a look at the wiring to the plug and socket connecting the ballast and this was all in good order.   We have two spotlights which have both worked without problems in the past; they and their ballasts appear identical but at some point (before our theatre acquired them) some one has replaced the original ballast connector plug and socket on one of them with 16A CEE connectors so it's impossible to mix up the ballasts.  The lead from the starter coil on the unit we dismantled has heavy duty insulation unlike the rest of the wiring and there is no sign on arcing or tracking either in the lantern gear tray or in the ballast .  Thanks for the voltstick suggestion - that confirmed there is a voltage appearing at output of the starter coil and it appeared to be significantly higher than mains voltage (it's possible to do a direct comparison as one of the terminals on the terminal strip to which the lamp is connected is normally connected via the ballast to mains live - however if the ballast is unplugged then this simply leaves the other terminal which receives the higher violtage from the starter coil when the 'Start' switch is pressed).

I had wondered about the difference between the original CSI lamp and the later CID lamp but the GE Lighting catalogue seems to show the same technical data for both apart from the fact that as mentioned the CID lamp has a lower output in lumens and none of the literature I've found mentions any incompatibility. 

Finally we did look up the lens tube (after opening the iris!) and there was no sign of an arc when the starter switch was pressed.   This might suggest the absence of a sufficiently high voltage but the most compelling factor is that the blemish shown in the photo attached to my original post appears to be slightly worse than at first thought and on close inspection there seems to be a flaw or possibly even a hole  next to the electrode at the junction of the glass and the ceramic base.  This and the fact that the lamp shows no sign of striking in either lantern leads us to the conclusion that we need to try a second lamp before doing anything else.  We're sending the lamp back in the hope we can get a replacement.

I'll report on the eventual outcome and in the meantime thanks to all for accompanying us on a trip down memory lane.....

[Whiskers]

you could replace the lamp with 2 bent copper wires with points set to about 3mm apart, the starting pulse if it exists will jump that, pins from a 5A plug may well fit the lamp holder, please please please remember this is deadly high voltage. 

lid open interlock switch is a sure fire stopper, sad twistlocks on the top can mess all that up   

the early ones had a fuse holder sticking out of the back panel, this was overstressed and had to be field replaced with an mcb that was also the on off switch.

WHEN the big capacitors in the ballast deteriorate with age the spot will draw far to much current, even in good condition you would get 10 plus amps

I think getting at the igniter circuit involves sliding the tray backwards, you have to disconnect the lampholder tails first, I think from the striker not the holder.   

 

 

Strand documentation / puzzle books are at

https://www.theatrecrafts.com/archive/documents/patt765csihandbook.pdf

https://www.theatrecrafts.com/archive/documents/patt765exploded.pdf

 

AGAIN THERE ARE SOME VERY HIGH VOLTAGES AND CAPACITORS - - - I DID NOT SIGN YOU OFF FOR HV WORK - - - IF YOU GET KILLED DON'T COME BLUBBING TO ME

 

 

 

 

 

 

[Hilary Watts]

Thanks - that's an interesting suggestion.  Strangely, when we had the insides opened up at the weekend we did discuss (in jest)  the possibility of powering a Jacobs Ladder from the starter circuit. 

I think we have two early models.  There's no interlock switch and there's a simple fuseholder on the back panel - no mcb.  The control circuitry including the igniter coil is all in a shallow metal tray under the lamp housing and it simply unscrews for access.

I'm puzzled by your reference to the big capacitors in the ballast.  The circuit diagram simply shows a .005uf capacitor in parallel with the coil(s) and in the two ballasts I  have opened up it is small ceramic disc like this which measures as expected on a multimeter and there is no sign of any deterioration.  

Finally, I am happy to confirm that if I kill myself I won't come blubbing to you .... although I can't say the same for  my nearest and dearest!

[Whiskers]

The fuseholder problem was that the lamp drew more than 10 A but some standard or other decreed that 240v rated 1 1/4 fuses may only go up to 10 A, people and experts substituted automotive fuses which go higher, then the holders started to fry.  the mod kit might even have been free  so its a bit of a surprise to find unmodded examples.

The big  capacitor in the ballast box;  The ballast is not shown on the diagrams I showed. The ballast is a large series choke [inductance] that is in series with the arc, an arc is effectively zero ohms when burning and the choke in series limits the lamp current to prevent melt down.  however this generates 'extra' current in the circuit, google 'reactive power' and 'power factor'  The cure for this was a large AC rated capacitor, these do fail with age often with smoke or a bang, experts then remove them as the spotlight works just as well, but drawing lots of 'extra' current. It is also possible that Strand omitted the capacitor, my addled brain cannot remember everything. 

   

[Hilary Watts]

On 2/14/2022 at 3:53 PM, Whiskers said:

you could replace the lamp with 2 bent copper wires with points set to about 3mm apart, the starting pulse if it exists will jump that

Sorry, it's taken a little while to take up your suggestion but we've now done that and pressing the start button produces a consistent stream of sparks across the gap (it was fairly easy to set it up safely by disconnecting the lampholder and setting up the copper wires on the  terminals attached to the circuit board).  This seems to confirm our theory that the (new) lamp was faulty.  We're presently waiting to hear from the supplier.