[Closed] A Marconi 702 Mirror-lid TV restoration: Part 2

I thought that I had had a real breakthrough this afternoon when I "found" three valve boxes, each with a brand-new looking MSP4 valves in them. Unfortunately, my joy was replaced soon afterwards by despair as these valves had 7 pin bases and not the then, more common, 5 pin bases. The only recourse open to me was to change the bases from 7s to 5s and see if I can keep the VHF performance up to the levels require.

One can clearly see the difference between the 7 pin and 5 pin valves.

One of the difficulties in working on valves is glass and how to hold it! I eventually came up with the solution to the problem by holding it in bits of that strange white plastic stuff which is quite unusual in that it seems to have quite a high coefficient of friction and so will grip glass. The photos show how I used this to quite good effect by using several thicknesses of the plastic, each with a hole cut into it to grip the valve. Using several thicknesses, I could make quite a deep "cave" in which to bury it which gave a very good grip to the envelope and enabled a very useful "vice" with which to hold the glass.

An ex-seven pin MSP4 valve in its "cave" to enable the soldering of the wires to the pins.

I haven't quite got round to that part of the procedure yet but never fear, that time will come.

Brian Cuff said
 The only recourse open to me was to change the bases from 7s to 5s and see if I can keep the VHF performance up to the levels require.

 For a moment, I thought you meant the bases in the actual set, not the valves themselves.   

PYE625 said

Brian Cuff said
 The only recourse open to me was to change the bases from 7s to 5s and see if I can keep the VHF performance up to the levels require.

 For a moment, I thought you meant the bases in the actual set, not the valves themselves.     

Me too, and is probably the approach I'd have taken - it would not occur to me to 'rebuild' the valves themselves....

Katie_Bush said

Me too, and is probably the approach I'd have taken - it would not occur to me to 'rebuild' the valves themselves....  

Same here, but in this case I think Brian has the best solution. It will preserve originality of the chassis and make use of valves that would otherwise lie idle.

PYE625 said

Same here, but in this case I think Brian has the best solution. It will preserve originality of the chassis and make use of valves that would otherwise lie idle.  

Agreed 100%, and just goes to show Brian's unending resourcefulness. Absolutely top rank stuff!

Don’t make me blush too much Marion!

I have, at last, managed to convert one of the valves. The readings are exactly the same as they were before the operation – cue Marion for a name for the op – but that’s at base-band, not VHF. Extra pins were added to early VHF valves to lower the inductance from the pin to the electrode lowering the impedance at the higher frequencies so whether this will affect anything, I have no idea. With this sort of construction, 45MHz is considered a higher frequency!

Anyway, back to practicals:

The base, in this case, was already loose so it was easy to loosen it a bit further. How to grip the glass envelope had been solved the other day (see post #142) so in order operate, I needed to set up a surgery for the three valves. The Guinea pig was the poor blighter sticking out of the valve cave in an earlier post so he was first in.

I had broken the base cement seal and it was just a case of making it worse and sticking it in the cave. Each pin in turn was heated and my trusty Solderpult (I never use braid) applied to remove the solder. After a very short while, I had a 5pin base in my hand and a glass envelope with 6 wires sticking up out of it.

The start of the mechanical joint can be seen on the wire with the red sleeving.

Firstly clean it up with a penknife and a spot of flux making sure that the wires are tinned up to the ends of the sleeving which I personally add for insulation between the pins. A long “Guide Wire” is added to the bunch – this will be of great help later – using a mechanical joint so that it cannot come undone during the operation.

I don't think that any comment is necessary for the guide wire!

Should any of the wires appear too short to pass through the new valvebase pins, then an extension wire must be added, again mechanically, to the offending wire – not so long this time!

Should several need this sort of attention, make sure that the extension wires are of differing lengths as this makes it much easier to thread them through the base pins when the time comes.

It’s too late for me to carry on now so I will close and reassemble tomorrow!!!

Brian,

I like your valve support/operating table.

Once again a great job and getting over a problem using "open base surgery"

I hope this patient and the others awaiting surgery make a full recovery.

Thanks Jon. One thing that a forum like this does is to make you realise how quickly time passes - my last post in this thread was one month ago!

I thought that members may be interested in the way that the scan coils are constructed in this very earliest of TVs. I must admit, the method of construction surprised me as it seemed quite cumbersome but I shouldn’t think that these sets were made in their hundreds but may be in batches of ten or twenty!

Anyway, In order to remove the coils, they needed to be dismantled so it was a perfect chance to see how they were made. As in “modern” coils, both the line scan and field scan coils are made in two identical parts, each fitting on opposite sides of the CRT neck with the line coils closest to the neck (lower power requirements for scanning). The parts are potted (in a form of early epoxy, I would think?) so that bringing them together and connecting them in series forms a complete scanning coil. The line coils are closest to the neck and the similar, but larger field coils come together round the line coils with their upturned ends fitting closely inside the upturned ends of the line coils – here's some pictures pictures:

The coils are firmly clamped by the pole pieces to prevent any movement. 

These are the six bits required to complete a scan coil assembly.  The coil components fit together like this to form a coil assembly, the larger diameter field coils fitting round and inside smaller diameter line scan coils to produce the complete coil assembly as shown in the first picture.

Mods: I seem to have lost a picture and can't get it back!

The fields from the coils are concentrated by use of a laminated iron core assembly which clamps the coils tightly together preventing any movement.

When I first connected up the scan coils, I had the picture that I was using as a reference, upside down! The C in the test card was, therefore, also inverted as I copied the upside down picture

Brian Cuff said

Mods: I seem to have lost a picture and can't get it back!

Hi Brian, you've not lost it just unfamiliar with the process of retrieval. As you can see above I've sorted it for you.

To explain what you need to do. All members photo uploads can be personally managed from their own personal image upload folder. To do this click on the attachments button, this expands the field as shown below.

Now click on "view existing image uploads". This will then load a popup applet to allow you to navigate your personal uploads folder structure. Navigate to the year and month, in your case 2017, 06. You will see the filenames, hover your mouse over the filename and it will provide an image preview icon. Once happy you have the file you wish to use, click the file name and the image will be inserted into your post.

This is quite handy if you want to reuse a photo you've already uploaded and saves creating duplicates in the filesystem.

Hope this helps you and others.

Brian Cuff said

When I first connected up the scan coils, I had the picture that I was using as a reference, upside down! The C in the test card was, therefore, also inverted as I copied the upside down picture  

Hi Brian,

It's great that you are feeling well enough to tackle the 702 again.

Just to check I hope you are not looking at the direct CRT face when assessing that Test Card C is left/right swapped.

It only looks correct when viewed in the mirror.

Peter

Hello Peter.

The mirror in the lid reverses the vertical positioning of the picture but not the horizontal position. Thus there is no east-west swap, only North-south.

I have come across another anormality on my 702 - they're all creeping out of the woodwork now! According to the service information, the focus control pot and the width control pot seem to be swapped over in position. In the photographs taken by me during the dismantling part of the restoration, the pot is mounted on a Paxolin sub-panel and an insulating shaft coupler fitted between it and the knob position. As you know, this is to insulate the pot as it is connected to the EHT supply and, should the bleed chain and pot go open circuit, there will be 5kV at the knob .

I have now rebuilt the sync chassis according to the service information and photographs I took during the restoration  process and everything seems to be working. However, I have found the problem. The first iteration of the escutcheon was wrongly engraved and and width and focus interchanged. I proved this to myself by checking the escutcheon on my 703 which is a much later model. This has the engraving in the correct place and the disk which covers the TV standard selection has now become an integral part of the escutcheon as no standards switching is necessary.

Now I have never heard of this "drop off" before and wonder why not! It can't be that nobody has noticed before as dare I say it but "Elf & Safty" sits in the middle of this as one of the pots is associated with the EHT supply.

So what is it? Why has this been so quietly ignored?

Here is a drawing showing the two versions of the Bakelite escutcheon:

It seems obvious to me that at some time soon after the dropping of the Baird system, there must have been changes made to the manufacturing of all dual standard TVs and the escutcheon changes were some of them.

Has anyone heard this saga before??

Hello Brian,

It seems that there have been several versions of the bakeliite panel.
My 702 has the 240/25 and 405/50 the other way around. The reason is not clear to me.
Mounting the system switch (with bracket) the other way around handles the other version, so there was no need to change the layout for the system switch text. There are escutchions that have the 240 / 405 text engraved a second time over the old one.

Jac

Brian Cuff said
Hello Peter.

The mirror in the lid reverses the vertical positioning of the picture but not the horizontal position. Thus there is no east-west swap, only North-south.

So what is it? Why has this been so quietly ignored?

Here is a drawing showing the two versions of the Bakelite escutcheon:

It seems obvious to me that at some time soon after the dropping of the Baird system, there must have been changes made to the manufacturing of all dual standard TVs and the escutcheon changes were some of them.

Has anyone heard this saga before??  

Hi Brian,

Here's my small adjustment panel and the controls are correctly marked. Somehow I've put the photos in your quoted box! 

Note that when seen directly from the screen the correct way up the C is the wrong way around.

Peter

The attachments show the pre-set controls panel of my Marconi 702.  This set must be a later production model because the Bakelite controls escutcheon does not have any provision for a 240/405 switch.  Note the position of the focus control.

Till Eulenspiegel.

Thank you for the pictures, Till. It does show that there are at least two forms of ESCUTCHEON.

However, it is very easy to change that so it is no indication of the age of the set.

My real question is why on earth was the engraving changed? The error can only have been in the drafting stages of the manufacturing process as an error in the electronic manufacturing stages would have been much more serious! We are in a position where we can either just ignore the anomaly, perpetuate any errors that are out there in the web or to try to clear up any false information that is out there.

What we could do is to add another column to Peter's spreadsheet for EMI MkI sets with a link to a photo of the escutcheon fitted to the set - or maybe no-one is very interested in pursuing the question.

Hi Brian,  Later today I'll take pictures of the pre-set controls panel of my HMV 901, the receiver that is electrically identical to the 702 except for the cabinet presentation.

As for my "late production" 702 it will be interesting to find out if any of the wiring for the Baird/EMI switch exists in the cableforms.

Till Eulenspiegel.

Hi Brian,

I suspect that all the TV only sets use the same minor control configuration. I don't know whether all the TV/radios and more share your 703 configuration.

On the picture reversal did you see what I meant about the Test card C being reversed left to right?

Kind regards,

Peter

The focus control in my HMV 901 is in the same position as the Marconi 702 shown in post #155.  The only significant difference is the HMV has the blanking disc to cover the hole for the 240/405 switch.

Till Eulenspiegel.