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[Closed] Cossor 54 Electrostatic TV Re-birth

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Brian Cuff
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At last I’ve done a bit to the Cossor 54 I bought off ebay some months ago.

When I got it, I was surprised by the huge Visconols which had been shoe-horned in, replacing the original cardboard units.

Fortunately, most of the clips had been re-used (re-shaped, of course) so were available if I were to re-make the capacitors. Mike Barker, who has a Cossor 437T in at the moment, was able to give me the dimensions of the originals. He also suggested using rainwater down pipe as the large ones are 68mm in diameter1; the smaller ones are 34mm which is also a standard water pipe size – what luck. To get the cardboard effect, I needed some rough brown paper which I eventually found used as packing in a shipment from CPC. Of course, it was all screwed up so I had to iron it flat before printing the label onto it. Unfortunately, my Brother printed played up a bit and I lost patience with it and it will never be the same again! I then painted the printed sheets with shellac to darken it and give it that “knackered” look. It is important to do this before it is glued to the plastic pipe because the glue will make the effect patchy. I also add a bit of brown pigment to give the dirty look.

All the rubber insulation had perished and was crumbling so all the rubber-covers cables had to be replaced. The paper capacitors had all been replaced the previous “restoration” so I decided the, even though it is a pre-war set, I would not try and re-construct the capacitors as they were hidden underneath the chassis. Most of the resistors I measured were at least 25% high so they all had to be replaced using 2W components.
I decided not to paint the chassis as the rust was mainly surface rust and it left a bit of staining but not too much and I didn’t want it to look brand-new! So off came most of the large components in order to improve access with the Scotchbright pads. The transformer with the mains tappings had rubber lead-out wires but fortunately, these were soldered onto copper strips between the layers. I therefore dismantled the transformer, replaced the lead-outs and re-assembled it. The picture shows the process.

The tagboard across the rear of the set was in a dreadful condition with really grotty components and a lot of botching so I stripped it and completely rebuilt it after straightening out most of the tags – many were already broken off – and cleaning them up. The pictures show it both before and afterwards.

I am waiting for some smallish EHT cable before I can go any further but I feel that I’ve made reasonable progress but I must admit, I’m really looking forward to getting this electrostatic TV going.

I will report later on any further progress made.

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Posted : 26/11/2012 9:04 pm
Anonymous
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Coax inner as EHT cable?

 
Posted : 26/11/2012 9:23 pm
Brian Cuff
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The problem with coax inner, Michael, is its colour. I want black cable to be a near match for the old rubber stuff.
Thanks for the comments, Trevor and, of course, I used to work hard :D . I am really pleased the way that the caps have turned out. For the first attempt, I glued the un-coated brown paper onto the pipes using PVA adhesive. It worked OK until I came to "paint" them. In some places, the adhesive had soaked into the brown paper and sealed it so when the shellac was applied, large light blotches appeared. This problem was solved by applying the shellac before gluing the paper onto the pipe! For the large caps, I have turned the end caps from Tufnel sheet and will set then slightly recessed and fill the recess with black wax (cabinet makers' wax).

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Posted : 26/11/2012 10:27 pm
Till Eulenspiegel
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My Cossor 54 is out of use a the moment. I'm looking for a 4TP valve. This is a special Cossor triode-pentode and is employed in the frame timebase. The pentode operates as a blocking oscillator and drives from the anode one of the CRT X plates. Oscillator action is between the control grid and screen grid. The triode section acts a phase inverter and drives the other X plate.

Till Eulenspiegel.

 
Posted : 26/11/2012 11:16 pm
Brian Cuff
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Didn't you make an adapter using an ECL80, Till? Did that work very well?
I also want to make an adapter for an ECC34 for my Pye D16T. The audiophools have pushed the prices up so much that I refuse to pay the asking prices. Unfortunately, they have an extremely long grid base so make ideal driver valves.
There is an asymmetric double triode, the 6DR7, designed for TV field timebases which I have to hand but that's just another job that I will do sometime!

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Posted : 27/11/2012 12:00 am
Till Eulenspiegel
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Hi Brian,
The ECL80 worked quite well as a substitute for the 4TP. An additional transformer was required for the 6.3 volt heater.
The ECC34: You are qute right about the crazy prices they asking for that valve. There is an American valve, the 6N7 which is an early octal double triode. I think the grid base is favourable for frame timebase work, but the problem with the 6N7 is that it has a common cathode connection. The 6N7 was developed from the 6A6. I'm considering using this valve as the deflector plate drivers in my pre-broadcast TV construcion project.
www.radiomuseum.org/tubes/tube_6n7gt.html
The 1939 RCA model TT5 employs a 6N7GT in the frame timebase.

Till Eulenspiegel.

 
Posted : 27/11/2012 2:02 am
Brian Cuff
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The 6N7 has a much shorter grid base but it may work, Till. The common cathode problem can be got over by generating a negative bias rail and biasing the output triode using that instead of the cathode resistor! It may work. Next time I've got my D16T out, I might make an adapter and try it- if I can find a 6N7!

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Posted : 01/12/2012 8:46 pm
Brian Cuff
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I've spent a little more time over the last few days and here's the result. The "drainpipe" capacitors are now mounted and wired together with the rear tagboard. It is my intention to add a perspex guard over this board as half the tags are at EHT voltages, including the ones at both bottom corners - just where they are likely to get touched. For this, I have bought a sheet of 2mm acrylic and will try to form a cover by bending the two ends so that they will cover the tags. A horizontal top guard will be glued between these two ends, allowing for the cables to the CRT to pass. Here's the first picture:

These two pictures show the manufactured caps which replace those massive Visconol jobbies which had been installed during a previous repair session:

There is also a tagboard under the chassis and this, too, has EHT all over it so I fitted a guard over the hot parts which also include a new tagstrip to anchor the mains wiring junctions which were suspended in mid-air:

I think that I'm going to camouflage those yellow caps after all. They look so out of place! I'll print off some small Cossor labels to wrap round them and put wax in the ends - I wish I hadn't wrapped the wires round the tags now :( .

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Posted : 01/12/2012 11:14 pm
Till Eulenspiegel
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Hi Brian,
My Cossor 54 is languishing in a corner of the workshop. Bad form, I must do something about that.
The line timebase: A single MS-PEN valve is employed. The sawtooth at the anode is 500 volts P-P. However, this RF amplifier valve seems quite OK operating under such conditions. No flashover between electrodes.
Oscillation is achieved between G1 and G2 in a blocking osc. circuit.

Till Eulenspiegel.

 
Posted : 02/12/2012 12:43 am
IJK2008
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Hi Brian

I second Trevor's comments - it is looking good!

Hope we are to see a picture on screen soon.

Cheers

Ian

 
Posted : 02/12/2012 1:49 pm
Brian Cuff
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After a lovely long weekend in South Devon, I thought that I would get on with the EHT guard for the Cossor. After looking on the web for acrylic bending, I decided that it is very easy to do! The guard is to prevent accidental contact with EHT. Almost all the tags along the top of the rear tagboard are at EHT which is about 4kV - enough to kill! The picture shows how the guard has turned out. It is spaced off the tagboard by 15mm threaded nylon pillars. As can be seen, it would take a really carless person to get at the dangerous tags now!

Tomorrow, I hope to start to apply power 8)) .

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Posted : 05/12/2012 11:02 pm
Brian Cuff
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This afternoon, I decided to bite the bullet and apply power to the set. First of all, though, I realised that I had left something quite important out in my efforts to prevent (my) electrocution when working on the set - the connections to the CRT. These are totally exposed and, of course, most of them are at EHT potential so I cut down a plastic soup container and taped it to the CRT base. I will eventually find a suitable round container (75mm in diameter) and hold it onto the base using a couple of light springs attached to the wooden rear tube support.
I didn't do a HKS and slap full mains on because this set has mains-derived EHT and the EHT transformer is very susceptible to damp - a problem here could lead to an expensive re-wind. So I connected an AVO to the secondary and wound up the Variac slowly and Hurrah!!! I was rewarded with a decent reading which meant the transformer is OK :D .
With the mains input at 110Vac I was rewarded with 200V of line sawtooth and 300V of frame but no light! Then, at 200Vac I decided to check the bias on the CRT and found that the grid was about 40Vdc negative wrt cathode with brightness at max. A quick resistor change brought this up a bit and I was rewarded by a raster of sorts - FIRST LIGHT.

Having proved that the CRT and EHT transformer are both OK, I need now to get the RF side working so that the sync separator and scan generators can be more easily tested. To do this, I will remove the CRT and, with the EHT rectifier anode connection removed and carefully insulated, I will work on the RF side. I have no idea of the IF frequencies, so the first thing will be to find what they are by measurement. This will be tackled tomorrow!

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Posted : 06/12/2012 8:10 pm
Doz
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Superb. :=D

 
Posted : 06/12/2012 8:32 pm
Refugee
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It looks pretty sharp.
Did you adjust the focus at all before taking the photo?

 
Posted : 06/12/2012 9:03 pm
Brian Cuff
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The focus control is in the middle of its travel - I was quite surprised at that because it means that the potential dividers between the plates and anodes are about correct and getting those right can be a problem with electrostatic CRTs! I must admit - I'm very happy with the look of the CRT.

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Posted : 06/12/2012 10:21 pm
Brian Cuff
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A little more progress- but not much! I have got a test card of sorts up now. The scans are all over the place but at least they're locked.

The tube looks pretty good but I've no idea how they were originally. If I stick a 45MHz CW signal in, I get a vision IF of around 4.5-5MHz depending on the local osc. setting so I guess the sound IF will be 1-1.5MHz except that I get zilch when I go to 41.5MHz in at the aerial sockets. It could be that the bandwidth is not wide enough but it requires further testing. More to follow now that the domestic holiday season is nearly over :D .

Have a happy new year everyone.

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Posted : 31/12/2012 3:50 pm
Terrykc
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A little more progress- but not much! I have got a test card of sorts up now. The scans are all over the place but at least they're locked.

It looks as if you've got a nasty dose of unsmoothed HT there, Brian!

When all else fails, read the instructions

 
Posted : 31/12/2012 4:54 pm
Panrock
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... or hum caused by a local magnetic field near the tube. These electrostatic tubes are very sensitive to the mu-metal shielding being tip-top (thinking of the Argus).

Looks a fantastic set otherwise Brian!

Steve

 
Posted : 31/12/2012 5:10 pm
Terrykc
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I don't see how a magnetic field would cause intensity modulation - which is clearly evident here ...

The bright bar isn't due to reduced line spacing - if you compare the strip across the white square on the contrast wedge with the strip across the black square you'll see that the lines in the latter, darker area are actually closer together!

I've taken a stripe from both sides of the screen and overlaid them on the original so that the bright stripe is shifted down. Comparing the distance between the grid lines bears out my original impression.

When all else fails, read the instructions

 
Posted : 31/12/2012 5:28 pm
Panrock
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I'll bow to your in-depth analysis here Terry! :D

Steve

 
Posted : 31/12/2012 5:33 pm
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