LOPT cages

[humorist2751]

Hi all.

I am approaching the end of restoration of a set which had received the attention of " The Phantom " at some stage (some components at twice their book value!); one thing which they clearly deemed unnecessary was the LOPT cage.

Since I don't want to send 10.125 kHz in all directions, I need to fabricate a replacement; aluminium is easiest to work with, being quite soft - but might there be some reason why I need to use steel, as the original probably did?

I gather that the cage should enclose the LOPT, the associated EHT rectifier, the line output valve and the boost diode, and it should be well ventilated. Anything else I need to know before I start?

[Aerodyne]

I can't see any objection to the use of aluminium sheet: after all, many coils are screened with it, so it obviously works. It may be virtually non-magnetic but that should not of itself affect any screening properties, as long as it is in electrical contact with the chassis. I've used it very effectively to screen non-metallised valves fitted as replacements to impossible-to-find screened bottles. You might find it even easier to use expanded aluminium alloy mesh and this would provide your ventilation, too. Or, expanded steel mesh. You'd need to find a local metals stockist for the latter, though, or perhaps an agricultural supplier!
Now I'll sit back and wait for someone to tell me I'm wrong
-Tony

[evingar]

Hi Tony

I wouldn't go so far as to say you are wrong, because I don't know for sure that you are.

A question I would ask though, is why would set manufacturers have gone to the trouble of using Mu Metal originally, if aluminium had been sufficient?

Undoubtedly, aluminium is a " boat load " better than nothing though, and D.I.Y shaping and cutting of Mu Metal may well reduce its effectiveness in any case (assuming you could get a small quantity of the stuff in the first place!)

[ppppenguin]

Aluminium should be pretty effective at 10kHz. It will shield the E wave by simply being metallic and the H wave (magnetic) by eddy current effects. Magnetic materials such as steel and mu-metal come into their own at low frequencies. Ali won't give you much screening at 50Hz because it's difficult to emit much E wave these frequencies (need a large antenna for low frequencies!) and the main radiation is the H wave. Low frequencies are more difficult to shield by eddy current effects so you need something with high permeability,

On a slightly related point. If you are trying to shield Xrays from the EHT system of early colour sets (typically GY501 rectifier and PD500 shunt stabiliser) then aluminium is useless. Steel is much better. Lead is better still but it's cheaper and easier to use a thicker sheet of steel than a thinner sheet of lead.

I ought to stress that there is no Xray hazard below about 16kV. The only monochrome sets that had a potential problem were projection sets with 25kV EHT. In these, the CRT was the main emitter and had to be suitably shielded.

[Aerodyne]

Mu-metal is of course high permeability. I wonder how effective such screening was? After all, TV licence detectors could pick up a TV line timebase - as could any nearby AM radio! Also, later sets that created EHT by means of triplers and quadruplers (e.g. Thorn 900 series onward) often used no screening. Could it be that a secondary purpose was protection from shock? prevention of arcing to nearby fingers?
-Tony

[David_Robinson]

I don't think safety would be a consideration - after all the average vintage TV has plenty of other hazards within such as mains droppers carrying 300v DC...

The current detector vans pick up the local oscillator in the tuner. I'm not sure if they ever used the line scan - I guess it would be hard to get an exact direction from it.

As far as AM radio interference is concerned, I think the line timebase is not as guilty as people think. The flyback pulse waveform would not have a lot of energy at MW frequencies. If you listen, the interference often changes with picture content. The culprit is the video output - plenty of energy up to 3MHz, 50v p-p output feeding into a 1 foot long unscreened aerial! Modern TVs have the video output stage on the CRT base panel, which is a big improvement.

Another factor was the use of long wire aerials for AM, which are very prone to local electrical interference. Magnetic aerials - ferrite rod or frame - are much better. I've just done a quick test using a 405-line TV just a few feet from my AM tuner (small frame aerial), on fairly weak stations like Arrow 675, and I can't detect any interference at all!

On X-ray shielding, Thorn used up to 20kV on their later large screen mono sets. I wonder how much X-ray they produced?

[ppppenguin]

20kV could cause very little Xray emission, especially at the low beam currents in monochrome sets. Most of it would come from the CRT and be shielded by the glass of the faceplate. The problem is rather worse at 25kV and 1mA beam current in a shadowmask CRT. It took a lot of care in the materials used for both the shadowmask and faceplate to keep Xrays down to a negligible level.

I believe that early TV detectors used line scan emission. This was probably more useful in the days when there were relatively few TVs. Later vans certainly used LO emission. Nowadays, TV detector vans are of limited use. The main tool against TV licence dodgers is increasing the threats against unlicenesed households. There are relatively few of these so this is probably economic and effective. You hear occasional horror stories about the pressure put on legitimately unlicensed homes.

[Hybrid tellies]

On my Decca Bradford colour television part of the screening around the Line Output transformer has always been missing. I have got a replacement screen which just screws to the chassis but I never bothered to fit it.
My reasoning is without it you get better ventilation around the transformer and as the transformer is still the original one fitted. I rest my case.
Levels of interference across the AM bands are high but no higher than normal for an early 1970's hybrid colour chassis.
As had been mentioned previously the main source of X rays on this type of colour set comes from the shadow mask tube. So long as the EHT is correct (upto 25Kv) the built in protection of the tube provides the required protection.
It may be interesting to know that some of the last large screen mono sets were using EHT's of between 18-20Kv from either a DY802 valved rectifier or from a TV18 or TV20 stick. There was certainly no X ray warnings on these sets.

[Sideband]

I seem to remember that in the days of the Philips G6 and similar colour sets the line section was completely enclosed to prevent X radiation. I think there was a warning on the assembly that it should not be operated without the cover fitted due to Xrays. In some sets I think there was a micro-switch to kill the HT to the screen grid of the line valve so that it would not operate with the cover removed. These sets used a GY501 and PD500 to provide EHT.

A monochrome set would not have produced harmful x rays so protection from prying fingers and probably TVI screening was a more likely reason for the covers to be fitted.


Rich.

[Duke_Nukem]

ISTR that the EHT of early colour sets had a triode slapped right across the EHT in order to regulate it and this triode is an excellent source of X-rays, almost certainly more than the CRT and with only a thin glass bottle around it, so you'd definately want to screen it.

This arrangement wasn't used in monochrome sets.

TTFN,
Jon

[---later---]

See this :-

http://www.kronjaeger.com/hv-old/xray/tech/PD500/

The guy is deliberately trying to make X-Rays, but does give some figures (admitedly over-running the valve)

" At 60kV and 300uA, I measured up to 5mSv/h in 50cm distance (using a surplus dosemeter). This is extremely high - 200h near the tube would be enough to cause serious radiation syndrom. "

Dunno how that scales to the 25KV ~1mA in a telly.

[ppppenguin]

A PD500 at 25kV, 1mA (what you get with an all black picture on a 1st generation colour TV) generates enough Xrays to be a hazard.

The CRT could generate Xrays but they choose the shadowmask and glass materials to avoid heavy metals which emit Xrays when hit by fast electrons. The faceplate then provides enough shielding. A few early American CRTs got something wrong and did make Xrays and ISTR there was a big fuss.

The GY501 probably does not produce too much Xrays. It's got either volts across it or current through it. You want both together to make Xrays.

[Mike Phelan]

Originally Posted by Richard_Newman

I seem to remember that in the days of the Philips G6 and similar colour sets the line section was completely enclosed to prevent X radiation. I think there was a warning on the assembly that it should not be operated without the cover fitted due to Xrays. In some sets I think there was a micro-switch to kill the HT to the screen grid of the line valve so that it would not operate with the cover removed. These sets used a GY501 and PD500 to provide EHT.
Rich.

I remember them well, Rich. The G6 and K70 had a switch in the cover operated by an insulated pin. One of my field service team used to take out the screw holding the pin to the cover, lose the insulator, and stick a self-tapper in when he put the cover back, so the HT fusible used to spring open. Couldn't get the staff.......
The PD500 was a triode shunted across the EHT so the sum of the current through the GY and PD was constant. The anode glowed red if you turned the brightness right down!

[Sideband]

uhn113x wrote: 'The anode glowed red if you turned the brightness right down!

Don't remember that happening but then I probably didn't get too close to the PD500 to examine it. I understand that these substantial valves had specially coated glass on the inside to reduce the x-rays. I'm told that some service engineers were in danger of becoming sterile if they got too close. Apparently when the set was on the bench, the PD500 was at just the right height if you stood behind it for any length of time.....!


Rich.