THE LOST CAUSE
By Tony Chuter (Area 22)
Seated one day at the A/V Console behind my projectors and fade unit, I was not weary, nor ill at ease; but my fingers did wander idly over the switches and keys. Far from weariness, my mood could be described as euphoric as I contemplated the adulation and applause that was bound to erupt from the assembled company, when my slides appeared on the screen.
All this, and the beautifully modulated voice - in perfect syncronisation - that I knew would issue forth from the speakers when the show began.
Ron Croad gives the signal: "Switch 0n" ...... this I do, BUT NOTHING HAPPENS!!
My carefully prepared programme - double checked before I left home - did not materialize on the screen! Nor did my melodious tones charm the audience! Only sibilence!
Oh! CHARIGIN!..... `Where can I hide my head?`.... `What`s gone wrong?`
For the benifit of those members who were able to attend the A.G.M. at Didcot this year, I`ll explain this modest introduction; those of you who were there will find this a `bit old hat`, but bear with me... it could be your turn next!
The usual procedure at the A.G.M. is for most of the afternoon to be given over to members to show their slides, prints etc, You know the sort of thing.
In my case I had prepared an A/V programme for my two `Carousel` projectors and the whole programme was controlled through an `Electrosonic` synchroniser; the signal was fed from a cassette tape played into it from a four-track recorder. All perfectly normal stuff.
Now, I accept that projectors and their ancillary electronics are prone to moments of unexplained capriciousness and with this in mind it is my normal drill to check each programme very thoroughly to ensure everything is performing smoothly. `DIN` plugs I find, are a constant source of trouble, so I pay particular attention to these and connectors of all kinds.
Having satisfied myself with several `dress rehersals` I felt that I had everything right to expect that all `would be right on the night` at Didcot. I have already told you.... it was not!.
Fortunately, I was able to revert to hand operation for both the projectors and talk the programme through. But I was unable to explain why this should be necessary when all this electronic wizadry around me should be doing it for me. Very puzzling.
That evening when I had returned home I set about tracing the cause of my embarrassing let-down. Both projectors, tape recorder, speakers, amplifier and the `Electrosonic` pulser were all thoroughly tested. I could not trace any broken circuitry, malfunctioning `DIN` plugs; nothing at all. So I switched on.. youv`e guessed it.... it all worked as it should have done at Didcot!
Now this contradictory result warrented serious investigation and it was not until that evening that I recalled a chance (sotto voce) remark by one of the members who inferred that the cause of the trouble may be due to outside magnetic interference. My pulser and slide-changer`s voltage output is very low and requires amplification before the signal is strong enough to work the relays in the projectors; so any interference from an outside source would be likely to stop - or distort a low powered signal at this point. Obviously, it did.
From this point on I started to investigate and it would appear that my experience is by no means an isolated incident. The more inquiries I made, the more instances of embarrassing involuntary `shut-offs` were related to me.
To begin with I spoke to fellow A/V enthusiasts - both amateur and professional - and most of them had, at some time, suffered inexplainable difficulties with their electronics .... and they are still wondering what happened!
John Ruffell told me of an occasion when he was unable to proceed with his A/V show because of electrical interference in the hall where he was booked. It transpired that the hall was `loop-wired` to assist hearing-aid wearers and it was not untill someone remembered that the `loop` was switched on; it was then switched off (too late to give the show) that John`s equipment worked perfectly.
I must addmit the nearness of Harwell to Didcot stirred some suspicions in my mind but I couldn`t substanciate this line of reasoning.
Among the many people with whom I have discussed my dilemma is John Pearle. John is the son of the C.R.C. member Major Pearle, and through his considerable experience with the PO Communications, has given the correct answer to the problem. I am most grateful to John for the trouble that he has taken on my behalf and for permitting me to publish his analyasis here.
This is what John has to say"
I assume that you have eliminated causes such as leads and cables, (for example, extension cables or whatever that only get used at Didcot) or a simple intermittent fault and I appreciate that this is not always as simple as it sounds since you are not able to reproduce the trouble. I don`t know exactly how your equipment works or the details of the problem you had. However here are my general thoughts, generally on similar lines to your own.
It seems very likely that interference, probably at RF is at the bottom of this. This leaves the two questions (a) where is the interference coming from? and (b) How is it getting into my set-up?
On the first question there are many and ever increasing sources from dustrial plant such as welders and RF heaters to microwave ovens and personal computers but I doubt if Harwell, the Power Station or the Railway are the culprits, at least not directly, although the magnet power supplies for the 7GeV syncrotron `Nimrod` used to cause the mains to go up and down a bit, I don`t suppose is still operating after over twenty five years.
The most likely source I feel is a point to point microwave communications link. These are in use all over the place by users who have large communications requirements. Look at the rooftops of the taller office blocks and you will see the ubiquitous dish. If your equipment is fairly close in the line of fire of such a link you may well get trouble.
There was a link terminal within a few hundred yards of the meeting place just across the railay line, operated by Mercury, the phone company. However even if you know the source it doesn`t help much unless you get out of it`s way. This brings us to the question (b). How is it getting into the equipment? Because if you can make your equipment immune - your`e home and dry. It is possible, and indeed preferable to design equipment to be resistant to this kind of trouble. There are two routes into your equipment. The first is direct pick up of radiation by the internal connections and components of your equipment. Careful screening will reduce this but can be awkward with plastic cases etc. It is worth making sure that all earth connections to the various parts of the equipment such as covers are crrect. Aluminium foil is often used to line a plastic housing and fixing screws relied on relied on for earth connections. If you have a good look inside your equipment you will see any likely problems.
The second way is to the outside world. The most obvious is the mains lead but speaker cables and controll cables can also suffer. These can be fitted with filters to attentuate unwanted signals. I suggest that you start by fitting a mains filter. These are most effective when fitted inside the equipment housing very close to the mains inlet point. Not easy to do with ready built equipment. Filters are available built into the mais input connector. If your equipment uses an input connector of the `Euro` type, a filtered version is available from Maplin as catalogue number FT36P. Otherwise fiters are available for wirng into equipment (several types are available fro RS Components out you will need someone with an account as RS are strictly trade only), fitted into a housing that plugs into a standard 13A outlet e.g. RS catalogue number 238-671 or 238-700 or fitted into a mains lead with `Euro` connectors as RS number 238-558. Other firms such as STC Electronic Services (phone No. 0279 626777) have a bewildering variety including one which fits into an existing mains lead. This is their stock number 001002B in black or 001003X in white. You can probably find a source by scanning the ads in electronic magazines. Spend half an hour in your local Smiths or wherever, don`t buy the magazine, just read the ads!. If you are really stuck I can give you one but it would need fitting inside your equipmnt. Note that if your set up involves several items of connected equipment with seperate mains connections, all need to be filtered. You could achieve this most easily using a multi-outlet extension lead fed from the filter. It would help if you could find convenient local conditions under which the problem occurs otherwise it`s filter and hope."
Well there you have what I consider to be the nearest to a definitive answer to my problem that I could hope for. Thank you John. I am purchasing the Maplins filter plug and when I`ve worked out how to fit it into my `Electrosonic` box I shall feel much happier.