Early Decca CTV 25.

[clivechap]

Hi all.

I was at Decca in Ingate Place, Battersea, London working on main chassis repair. The EHT unit was exposed and we had a round lead glass sleeve we had to fit. Sometimes the workload was crazy it was often left off.

[Welsh Anorak]

I was told by a fellow engineer that he had a customer with a Decca CTV25 who, together with his wife, suffered migraine style headaches. On servicing the TV, he noticed the lead glass shield had been left off and the cage was partially dismantled. On replacing these items they reported no more headaches. How true this is I have no idea.

[Peter.N.]

I think that is an anti radiation sleeve fitted round the EHT regulator, PD500? The valve can dissipate quite a lot of power under low screen illumination conditions. I believe the shield is lead glass and is an X ray shield.

Peter

[Hybrid tellies]

Interesting. On some sets removing the screening around the PD500 would operate an interlock switch cutting off the HT supply to the line output stage.
Xray radiation from the PD500 was considered to be quite a health threat for customers and engineers.

[Welsh Anorak]

The Philips G6 had such a device. I thought the Decca did as well, but I might be wrong. The Bush CTV25 used a leaded glass tube over the PD500 and a sceening cage. The only other British set I can think of that used a PD500 was the original Pye dual standard. Again, that had elaborate screening.
The GEC dual, CVC2 and later Pye and Decca did well without the PD500 using a tripler, though the Bush Mk3 still retained it. The good old G6 carried on using it until the G8 came along, and was probably the last British TV to use the PD500.

[Maarten]

Didn't they make the changeover to the PD510 or was that a continental thing?

[toshiba tony]

At tech we were warned about PD500/GY501 output stages and it has always made me concerned when repairing such sets. Yes, the G6 had such a switch and working on line stages always made me cautious. But now we are told about pylons, phones, smoking, alcohol, and to be quite honest are there any proven cases of damage caused. Except fags of course. Wasn't there a set called a Metz based on the ITT CVC5 chassis that used a GY501? Don't know if it used a shunt stabaliser as well. I never saw one but saw loads of the CVC5 right up to CVC9. Excellent tvs. PCL85 Frame stage that rarely failed.

[Welsh Anorak]

The German CVC5-ish (was it a Graetz?) TV did have an overwind and a GY501 as did early Korting chassis, but no shunt stabiliser so no need for X-ray protection here. Perversely Bush went the other way using a tripler and PD500.
B&O of course did usee a GY501 but achieved regulation in a typically elegant way.

[dazzlevision]

Originally Posted by toshiba tony

Wasn't there a set called a Metz based on the ITT CVC5 chassis that used a GY501? Don't know if it used a shunt stabaliser as well. I never saw one but saw loads of the CVC5 right up to CVC9. Excellent tvs. PCL85 Frame stage that rarely failed.

The CVC5-ish colour TV with a GY501 EHT rectifier, that was imported into the UK was a Schaub Lorenz “Combi Colour” (a West German ITT brand at the time - early 1970s). The mods to achieve UK system I operation weren’t particularly elegant.

[Nuvistor]

Originally Posted by Welsh Anorak

The German CVC5-ish (was it a Graetz?) TV did have an overwind and a GY501 as did early Korting chassis, but no shunt stabiliser so no need for X-ray protection here.

Always believed the GY501 required X-ray protection, from The Valve Museum some valves had a warning on the envelope. The CRT also was a source of X rays and the glass faceplate was special glass as far as I recall. It’s the operating voltage of 25KV that gave rise to the soft X-rays.
Digging in the deep past of my memory didn’t the old projection sets with the EY51 tripler have similar problems.

http://www.r-type.org/exhib/aaa0325.htm

Happy to be corrected though, long time ago with both items.

[Welsh Anorak]

The sets I mentioned (thanks for the correction of the ITT make) had only rudimentary screening over the line stage. In the Bush, the PD500 was the one that had the leaded sleeve as when in full conduction (zero beam current) the whole 25kv hit the target giving rise to the soft X-rays. Apparently the valve would glow cherry red under this condition, though I wasn't going to stick my nose in to check!

[Niechcial,Steve]

The Mullard projection unit certainly came with an X ray warning. It wasn't the 25Kv EHT generator unit with the three EY51s that was a problem because that was all inside a sealed oil filled screening can. The problem was X ray emission from the screen surface of the tiny projection tube. Mullard warned that when operated outside of the optical unit you either had to observe the screen through lead glass, or alternatively keep a specified number of feet away from it. I think I'm right in saying that the projection tubes were the very first with aluminised screens. Whether that made the X-ray situation better or worse I don't know.

[Niechcial,Steve]

PS Wasn't the big X-ray problem with the PD 500 or GY501 if the heater wasn't alight for any reason?

[Nuvistor]

Didn’t know about the heater problem,

[Welsh Anorak]

Interesting point. There would be no problem with the PD500 as it's always fed from the heater chain, but the GY501 was invariably fed from a single turn winding on the LOPT, often via a low-value resistor, which could go open circuit. In this case I assume it could become a 'cold' X-ray generator, but the target would be floating ao probably not a concern.

[AC/HL]

The aluminised screen was introduced to increase screen brightness I believe, nothing to do with X rays. It would be especially important in a projection tube.
You need emission to produce X rays, so a cold cathode wouldn't. I think the problem was excessive EHT under fault conditions and zero beam current.

[TonyDuell]

To get significant X ray emission you need a reasonable current through the device (getting on for 1mA) and a reasonable voltage drop (10's of kV) across it

The PD500 (or other shunt stabiliser) has getting on for 25kV across it and will passing 1mA or so when there is little beam current in the CRT (the rough idea is to maintain a constant load -- CRT + shunt stabiliser -- across the EHT supply). The CRT also has getting on for 25kV between cathode and final anode and could have a beam current of 1mA. So both of those could produce X rays. The PD500 is put inside the lead glass screen to reduce the external level of Xrays, I read somewhere that the faceplate glass of a colour CRT contained lead for much the same reason.

It's hard to see how the GY501 rectifier can produce X rays though. When it is reverse biased or the filament is not alight, it's passing very little current. When it is forward biased it is hopefully not dropping 10's of kV. Perhaps a dead short on the EHT output would do it but I would think the line output stage would generate much lower voltages under such a heavy load.

I've never seen a PD500 or similar, I've never worked on a TV set that uses one. I do have a B&O set as mentoned earlier with a GY501 rectifer. It's in an encloed metal box (along with the line output valve, EHT generator valve and their boost diodes) but I don't remember any special shielding, lead glass, or whatever around the GY501.

[Nuvistor]

I don’t remember either the Bush CTV25 or the Pye dual standard CTV having a glass shield around the PD500, if there was I would have replaced it after servicing but I only remember the metal screening assembly.
There was only me fixing them so they would not have been removed by another person.

Perhaps there was a glass shield, just don’t remember it.

[toshiba tony]

Thanks for putting me right re the CVC5 error.

[Welsh Anorak]

The Bush CTV25 MkIII had a redesigned line tower, with good reason, considering the Mk I's reputation. The troublesome pitch overwind, the DY802 focus rectifier and the GY501 were removed and replaced with a skeleton quadrupler. Oddly, the PD500 was retained, but housed vertically in a metal tube at the front right of the unit and surrounded by a leaded glass cylinder. The whole tower had a hinged lid with a metal gauze cover. The earlier designs used a heavier gauge metal but I had little experience with these or the original Pye duals so I can't say whether there was an additional cover over the PD500.