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Blog posts of '2013' 'June'

MULLARD APPRENTICE AT MITCHAM

During the 1950s, Mullard had an apprentice system, following your application and an interview with the Plant Training Officer you could perhaps secure a position - in 1953 from a candidate pool of 12 'boys,' three were selected to become Mullard apprentices - good odds eh?      During the 5 year apprenticeship, day release was given which could culminate in either the HNC or BSc in Radio Engineering.  After qualification, and 9 years of employment, the successful 'boy' having earned his spurs could become one of the monthly paid staff.

In the picture below you see a nice lady who was an 'apprentice nanny' - whether or not she clasped any of them to her bosom and fed them sulphur and treacle or syrup of figs whilst soothingly murmuring there, there, the Mullard apprentice dossier does not unfortunately record: - 

The world was a quite different place in 1953, Stalin had just died, the Royal yacht Britannia had just been launched, rationing of sugar in the UK ended and the first colour television went on sale in the USA for $1200 and here you see a newly apprenticed young chap hard at work at Mullard Mitcham.   He certainly flourished and then went on to become a director of a property management company in Chichester.    As an interesting aside, it may be that TPA Smith, the Mitcham training officer, seemed to like the name Michael for the 1951, 1952 and 1953 apprentices  all had the same christian name - MIchael.    Having said that, at the time, Michael was the 4th most popular male name for newborn children even though this wasn't the case when these chaps were born some 16-17 years earlier.     

CATHODE RAY TUBE AGEING AND TESTING (iii) - FINAL TEST

A number of tests were undertaken on the finished CRT under actual operating conditions, below, we shall discuss each of them in turn: -

Cathode emission -  insulation resistance between all electrodes was checked, anode current, Ia was checked as was "cut off" which is the negative grid potential required to nullify emission

Vacuum integrity - the degree of vacuum was determined using an ionisation gauge.

Final visual check - the focus was checked, the raster was checked, the faceplate was checked for any blemishes, unevenness of colour or stray emission, the whole tube structure was checked for mechanical defects.  Only then was the tube given a final polish clean, marked with a type number and then packed in it's sale carton for storage in the stockroom.

Above, we have described the Quality Control (QC) tests that were carried out on each and every CRT that Mullard manufactured but in addition, Mullard were early exponents of Quality Assurance (QA) and from each production batch of CRT a sample was taken (√n+1 sampling plan) then independently checked for the same range of tests as the on sale devices. Each test was logged using Shewart charts such that trends in performance could be seen and corrective action taken to ensure product consistency.  Pretty impressive and all implemented long before our American colleagues reinvented this form of quality management and renamed it Six Sigma!

Of the QA samples taken, a portion were stored as batch retained samples whilst the remainder were subject to Accelerated Life Testing where operating conditons were chosen such that a 1000 hour continuous run would mimic the typical domestic useage experienced in approximately 6 years of operation.   This data was used again to assess product consistency but also to give valuable data on expected product Life Cycle and End of Life product performance.

 

 

 

 

 

CATHODE RAY TUBE AGEING AND TESTING (ii) - FIRING

After exhausting and capping, the tubes were fitted with their bases and still in their linen bags, were transferred onto a conveyor belt where each tube was plugged into an adaptor which supplied the appropriate voltages to the heater, cathode, grid and first anode as well as a heat from an embedded element which baked the cement inside the base cap thus hardening and attaching the base to the bulb neck: -

After heating and as power was applied to the tube, the heater fired and cathode emission commenced and this initial 'firing' reduced a part of the cathode material from the oxide form back to metallic Strontium and Barium which increased the cathode's emissive properties.    Once the fired tube reached the end of the conveyer belt, it was transferred to one of the test benches where it received an intensive check under true operating conditions as you can see from the photo below: -

CATHODE RAY TUBE AGEING AND TESTING (i) - INTRODUCTION

Well, it's been a long journey, having inserted the electron gun, withdrawn the air from the bulb and sealed it off, the only remaining thing to do is to perform that final process of "ageing" and a Mullard Cathode Ray Tube was available for sale.   More deatil on this final process in my next blog entry but for now, I just wanted to share with you this evocative photo of tubes being aged - don't they look cute in their Moygashel linen bags.   As we all know, linen from Maigh gCaisil (the place where the stone fort is)  is much prized for it's softness and hard wearing characteristics and was just the thing for 1950s Mullard  CRT manufacture for whilst the valve assembly girls wore nylon, the CRTs wore only linen sourced from Ireland: - 

CATHODE RAY TUBE PUMPING

Once sealing was completed, the cathode ray tube was passed to the pumping machine which comprised of a rotary carousel having a separate vane pump for each tube.  The tubes were mounted as you can see in the photo below and the tube pumping spigot was connected to a reinforced vaccuum hose and pumping was commenced: -

 

The pumping machine indexed and hence automatically carried the tubes through a tunnel furnace where they were strongly heated for 90 minutes to help drive off air and other gaseous components from the interior as well as to convert the faceplate phosphor from the carbonate salt to the corresponding more luminescent oxide salts. The level of vacuum was checked by measuring the ion current within the tube and once satisfactory, the getter material was flashed  by application of  an RF inductive heating pulse.

The final pumping operation was to seal off the pumping spigot and this was done by the application of a gas air flame as close to the tube neck - pumping spigot join as possible.  Gradually, the glass softened and as the tube was lifted from the pumping machine the glass parted leaving a full seal as a 'pip' which in a finished tube would be concealed under the bakelite base cap.

 

CATHODE RAY TUBE SEALING

The electron gun we described in a previous blog entry was mounted and sealed into the neck of a tube using an automated machine which comprised of a number of fixtures which moved around the carousel and revolved on their own axis.   At the first position, the tube was mounted top downwards in the top part of the fixture and the electron gun  was mounted base downwards in the lower part of the fixture .    In the following photo, you can see the loading arrangement: -

 

Operation of a lever moved the gun such that it entered the neck of the tube and the fixture then moved to a sequence of stations at which the bulb was rotated in a flame from carefully postioned gas blowpipes playing onto the join area such that the neck and gun were gradually fused together this leaving the gun seated into the tube with the pumping tube projecting downwards from the end of the neck - "sealing in" was thus completed.  

At this stage, the glass was allowed to cool slowly to provent heat induced stresses being left in the glass which had the potential to weaken the glass and hence pose an implosion risk at pumping - more on pumping in the next blog entry.

THE MONOSCOPE AND THE BBC TEST CARD C

Now here’s something you don’t see every day – it’s a Monoscope. These were invented in the immediate post war period in the GEC laboratories at Wembley following on from the need for broadcasters to derive a signal with which to test their equipment.

Within the cathode ray tube (CRT) of the monoscope, a solid silver plate, photo etched, curiously using photo sensitive fish glue (yes, really!) and carbon coated depicted the information that had to be produced – in the case of this device, the redoubtable BBC Test Card C.

A signal was generated as a focused beam of electrons scanned the target and a sharp facsimile of the target was produced due to differential secondary emission between the silver plate and the carbon screening where electron impingement onto the silver produced an output and conversely, the carbon screening, no current. In place of the fluorescent screen seen on a typical CRT the faceplate connects to a metal plate inside the tube. This plate has an insulated photographic pattern engraved upon it. The scanning beam produces an output where it strikes the plate and no current where it strikes the insulation.

The monoscope was electrostatically focused and magnetically deflected and had an anode voltage of 5kV to ensure a tight focused beam which was essential to achieve good definition of the picture plate, the maximal value being approx.100 lines per inch with a beam current of 5uA and a signal output of 2.5uA peak to peak.

 

 

This arrangement was an elegant way of producing a stable image and was much more cost effective than having a camera pointed at a printed test card. It was, of course, limited to a single design. Additionally it was a monochrome only technology and its use ceased with the advent of colour television.

Apocryphally, R.A Dixon & D.M. Mackay proudly presented their invention to an excited GEC board of directors – one less enlightened chap stated, “Very nice, but what does this thing actually do that a magic lantern and candle cannot?….”

I am indebted to my pal Donovan C from Chinnor for allowing me to be the custodian of this exquisite piece of thermionic engineering.
 

MULLARD TICONAL MAGNETS

The production of cast Ticonal involves heating the material to above 1500oC then pouring it into moulds and today, I have a nice picture of Frankie Howerd's dad transporting moulds containing the white hot material from the pouring bay to the cool area.

And just in case you wondered what effect a touch of something at 1500oC can do to you well, this should remind you: -