HMV 900 update.

[Heatercathodeshort]

I obtained my HMV 900 in the early 1970's from it's original owner who purchased it from a large department store in Kingston-upon-Thames.

I have posted its history in the past on this forum thus I don't want to repeat myself only to say that it is completely original, clean with its original polished cabinet finish.

It was returned to EMI in 1946 for a check over and replacement of the electrolytics. It appears a new condenser block was fitted replacing the can containing C13 C15 C17 C19 C21 with a very well constructed skeleton panel with individual tubular electrolytics. [Excuse the out of focus pic]

The 900 worked to some degree when first switched on all those years ago. The EHT transformer was arcing causing a mass of white spots across the screen as if it was being operated next to the Silverstone race track and the frame was severely cramped.

At the time just three capacitors were replaced, C16 .5uf osc to output coupling to the N41 valve, C4 feedback .1uf and C5 .01uf flyback suppression. The EHT transformer was rewound and I had a working HMV 900. That was over 40 years ago and to date nothing has needed replacement, the instrument, as EMI liked to call it has burst into life whenever it has been demonstrated.

Recently I noticed that the frame hold control needed to be on its end stop to secure the correct frequency and I decided to do something about it.

The side inspection cover was removed and the circuit from the omnibus edition of the service manual showed a .05uf capacitor C2 in the grid circuit of the frame blocking oscillator, a MS4B. It was original and yes after 80 years it had a slight leak! Replacement of this component returned the frame hold to center.

It was decided to restuff the original component and while I was about it I did the same thing with the three capacitors I had replaced with standard Radiospares components back in the 70's.

All the remaining components were checked in the frame circuit and found to be up to spec but there is a slight frame cramp at the bottom of the scan. EMI state in the manual that this is normal but in bad cases a frame linearity circuit can be added but after 80 years I think I will leave it well alone. To be honest I feel like I'm walking on a friend's grave when I unsolder a connection!

The 6/6 CRT is slow to brighten up taking around 10 mins to give a fair picture. I have recently been presented with a 6/6 that appears to have some emission. I will attempt to connect it up with extension leads externally to see how it performs. Fortunately it is complete with scan coils and push about coil so it will involve little disturbance. A job for the New Year.

Considering 90% of the components are now 80 years old it's quite incredible that this example of an early television receiver is still in full working order.

While adjusting the push about coil [an addition probably added in 1946] I noticed that there was a complete lack of sound, just a faint hum from the mains energised speaker. It belted it out when last tested but I have a hunch that the sound output transformer has gone O/C primary. If this is the case I'm sure Mike Barker can help by way of a rewind. I will remove the inspection panel on the radio side after Christmas and make a few tests. It may be an O/C supply resistor but the devil is sitting on my shoulder..

A truly remarkable 'instrument' in design so far ahead of its time. You may think the receiver is incredible but what about the EMItron camera? You are talking 1936. Amazing.

I have attached some pictures of the timebase [sync] chassis with the components I fitted in the 70's and the restuffed ones fitted yeasterday. The dust is original and it is staying as it is. The bright chassis metalwork and screening cans can be seen. Regards, John.

[Heatercathodeshort]

A few more pictures including one showing the picture from the dusty rear of the 6/6.

[peter_scott]

It's great to see your 900 working and I really like your "keep the dust" approach. I'm looking forward to seeing the effect of your spare 6/6 too. I think the focus/astigmatism in mine definitely took many hours of use to improve.

I'd love to add your 900 to my list if you can find the chassis numbers.

http://www.nostalgiatech.co.uk/new_page_31.htm

Peter

[Freya]

The metalizing on that valve looks like its about to morph into something else

[Argus25]

What a wonderful set .

I'm of two minds about the idea of leaving the dust.

It won't do anything to the crt, but on other surfaces, in the long run it will encourage corrosion & rusting because the dust particles absorb water and may also contain salts depending on where they came from. So it is better to clean the dust off where you see it and that won't detract from the originality of the set. After all it wasn't part of the original design.

If you want the metalwork to stay good in the long run, a wipe over with cloth sprayed with WD40 every year or two will help keep it looking good and corrosion free.

I suppose the dust certainly makes it look authentic though, like it was just discovered in somebody's attic.

[FERNSEH]

Hi John
when I return to the shop, possibly tomorrow, I'll drag out that 900 like TV receiver. The is the set we believe is a very early production model.
It's all to evident it was a dual standard receiver so I reckon it should be in order to return it back to it's original state, that is if the correct three pole two way switch can be found. Like your HMV 900 it's time to rid the set of those 1980s servicing solutions. So I'm searching for all those old EMI capacitors.

DFWB.

[Heatercathodeshort]

I will definitely keep WD40 well away from its chassis. It just forms a sticky mess and would probably end up looking like the Gardner diesel engine in my old Atkinson!
This 900 has been in my possession for over 40 years and is still bright and shiny.

It is damp that ruins electronic equipment, not dry dust!

Ha ha! I knew the condition of the screening on the MS4B would be noticed. We all get a bit wrinkled when old and this one has suffered like this since before I took custody of this great bit of kit. I have a replacement but I'm leaving well alone.

It's probably one of the most unmolested examples still existing. The cabinet has it's original finish and required just a coat of wax and a buff to bring the shine back.

Pictures show the black felt with the words 'Place over scale when viewing' to prevent the radio dial bulbs being reflected in the mirror and the card showing picture faults caused by incorrect control adjustment.

EMI thought of everything! Regards, John.

[Freya]

It may be "just an old telly" but it sure is a thing of beauty

[mark pirate]

I love the build quality of EMI kit of that era, I have two HMV radios of the same age. Built like a tank, and excellent performers with superb cabinets.

I certainly agree with doing only what is needed to keep the old girl running, leave the dust where it lies.
I hope your spare 6/6 turns out to be a good one!

Mark

[Tractionist]

An amazing 'apparatus' !!

[turretslug]

What a lovely "instrument", and a fantastic piece of technical archaeology! I like the capacitor frame- maybe not "as original" but obviously plenty good enough for factory re-work of a very expensive piece of apparatus- and a whole lot easier to work on. Possibly naive question- isn't scan coil adjustment a little, um, exciting with all that EHT-ery lurking nearby?

I've come across a few metallised valves with that hanging grey petticoat effect, it makes one loath to disturb it when it's been hanging precariously for decades. Sometimes it almost floats there, a millimetre or so just above the glass and almost perfect, making eggshells look like concrete! Maybe the coating absorbs water and swells slightly? That long CRT looks even more "19th Century" in the flat-screen age- the sort of thing that cries out for preservation to show future generations how things were done- and just how difficult it was at the time.

[Jac]

Originally Posted by Heatercathodeshort

Pictures show the black felt with the words 'Place over scale when viewing' to prevent the radio dial bulbs being reflected in the mirror.

That's interesting John! The first time I've seen that.
Thank you for the photos.

Jac

[McMurdo]

The knob twiddling instructions must be very rare and no doubt helpful to the owner who had probably never owned a TV before. I'm surprised you aren't being asked to scan the label for the other survivors.

The black felt is a simple British solution. Had it been a German set, the dial bulbs would probably have been made to gradually fade out as the CRT warmed up.

In my line of work the mixed dielectric Dubilier/RS caps are replace-on-site, as they have a habit of going short and splitting open in high energy circuits.

[Nuvistor]

Originally Posted by McMurdo

In my line of work the mixed dielectric Dubilier/RS caps are replace-on-site, as they have a habit of going short and splitting open in high energy circuits.

They did that 40 years ago but we had little else that was any better.

[Argus25]

Originally Posted by Heatercathodeshort

I will definitely keep WD40 well away from its chassis. It just forms a sticky mess
It is damp that ruins electronic equipment, not dry dust!

If you look very carefully at corrosion on many metal surfaces you will see that it is directly under microscopic strands of lint, especially the larger pieces rather than finer dusts. The lint and other microscopic particles absorb water and hold in on the surface of the metal. This is why the corrosion you see say on the tops of aluminium IF cans etc often has a speckled look, where the larger dust and lint particles once were (before someone wiped them off).This is why the corrosion is on the top of the IF cans, where the dust is, and not on their sides, I'm sure you have noticed that.

WD40 doesn't make a sticky mess either if it is just wiped on as a thin coat and wiped again with a dry cloth to remove any excess, it simply helps create a corrosion resistant boundary layer and sits in the microscopic roughness of the metal.

In any case you need to do what you think best.

[Heatercathodeshort]

Originally Posted by turretslug

Possibly naive question- isn't scan coil adjustment a little, um, exciting with all that EHT-ery lurking nearby?

The service manual suggests that you wear 'rubber gloves' when adjusting the 'Push About coil' and presumably the scan coils.
It's difficult to see what is going on screen wise and it would be easy to touch that EHT connection.

The CRT is of truly amazing construction. The gun is a Hexode with electrostatic focus. The section of the neck where the scan coils fit is of a smaller diameter than the main neck. This is difficult to see in the pictures requiring dismantling of the scan coils to remove them. In practice this is a lot easier than it sounds.

This is done to place the magnetic field from the coils closer to the actual electron stream thus increasing sensitivity, the original 'thin neck' common 23 years later.

Nothing is new.

It also has a accelerator electrode placed between the cathode and grid to concentrate and direct the electron cloud. It is constructed of Pyrex glass with the Pyrex trade mark etched on the glass together with the EMISCOPE label.

It really is a work of art on it's own account and the 6/6 fitted in the 900 series appears to have enjoyed a good life.

Surprisingly they were still available from EMI in the early 1960s as replacements but the 3/1 3/2 and 3/3 were completely obsolete.

It is easy to watch and you soon get engrossed in a good program entirely forgetting you are viewing a 12" picture reflected in a mirror.

Imagine what it would have been like in 1936?

Thanks for your comments and well meaning suggestions. Regards, John.

[Argus25]

Originally Posted by Heatercathodeshort

The CRT is of truly amazing construction. The gun is a Hexode with electrostatic focus.
It also has a accelerator electrode placed between the cathode and grid to concentrate and direct the electron cloud.

The gun is super advanced for 1936, it even looks that way in the photo, looking typically more like a post WW2 gun structure. Also an electrode between the cathode an grid is fairly unique. Some later guns had an accelerator on either side of the focus electrode to prevent changes in grid voltage upsetting the focus.

One thing about electrostatic focused tubes is that the focus is more affected by alterations or increases grid voltage or beam current, requiring a slightly higher focus voltage as the brightness increases, but electromagnetically focused CRT's are better in this respect.

[line sync]

A lovely example of an HMV 900 john.
I love that huge eht smoothing capacitor next to the crt.

Robin

[Heatercathodeshort]

It sure is a work of art!

I have taken this picture of the 6/6 recently obtained. The Marconi valve data for 1949 states that the 'accelerator' electrode between the cathode and grid requires 250V at between .25-1.5m/a.

The electrode must be placed further away from the cathode as you might expect. I tested the emission using this electrode as the grid and obtained exactly 1.3m/a current flow. It looks good but of course there is more to a CRT than the gun assembly.

The focus electrode as we now call it is called the1st anode on the 6/6. It requires around 900V at a current between 0-200u/a to focus.

As A25 stated it is a lot higher than later electrostatic tubes from the mid 50's. At a guess I would say that this is probably due to the length of the gun and the distance between the gun electrodes. The nearer the focus electrode is to the final anode, the proportion of required voltage for focus I guess is higher. The tube is 28" long.

The price of the 6/6 in 1949 was £12 plus the purchase tax £3-2s-5d. Quite a sum back then but you got a lot of incredible engineering for your money.

When you examine the circuits employed in the 900 you realize how little it changed from 1936-76, the end of the valve era.

OK, tubes became larger and slimmer, valves smaller, the receiver unit changed from TRF to superhet and the mains derived EHT was obtained from the flyback but any repair guy from the 70's would have been completely at ease with the circuits.

Regards, John.

[beery]

Hi John,

That is a nice 900.
I'm ok of course with very rusty sets being re-plated or painted, but when the rare opportunity comes to get a set working with the minimum of disturbance, I think it is nice to keep even the dust in place. I do like it when a set looks its age, not like new, but as if it were just found in some old boy's front room. Then there is that lovely musty smell...

The dust cover over the tuning scale is a new one on me, I've never heard of such a thing before.

One thing that does amaze me with the EMI sets is the quality of the cabinet lacquer. It does not craze or scratch easily, not like the PYE B18T for instance that seems to get scratched just by looking at it!

I presume you used the same time machine, that you used to get the 900, to go back and get the spare 6/6

Looking forward to see how you get on with the new CRT.

Cheers
Andy