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A Marconi 702 Mirror Lid

Pre War 240/405, Post War 405 Line, B&W Dual Standard, Colour Dual Standard, B&W S/S 625, Colour S/S 625 line, Hybrids. Standards converters & modulators, video recorders.
 
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A Marconi 702 Mirror Lid

Post by Brianc » Tue Oct 27, 2015 2:18 pm

I've been after one of these for ages and at last I have one here at home. The deal to buy it was struck about 18 months ago but my son and I picked it up on Sunday last. I hadn't seen it before but I am delighted with it's condition and it makes a super addition to my Pre-war Marconi collection 702, 703, 706, 707, 709!
I will post a selection of pics. later but I wonder if anyone has seen this modification before. I'm assuming that it's the recommended frame linearity mod. but I'm not sure. Any ideas?
Sync-Chassis-web.jpg
The bits in question are mounted in place of the usual multi-cap. can in the centre of the chassis

 
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Re: A Marconi 702 Mirrir Lid

Post by Jac Janssen » Tue Oct 27, 2015 2:38 pm

Fantastic looking condition Brian!

The pot most likely is for controlling the position of the picture by means of the "push-about" coil.
That was a later addition, and the potentiometer usually is mounted on the back side of the synch unit.
It gets the power via a series resistor out of the 280V and the p-a-coils is connected between the slider of the pot and gnd.
Schematic is in the Omnibus.

The "modification" probably is just a replacement of the original capacitors in the box.

Jac

 
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Re: A Marconi 702 Mirrir Lid

Post by Brianc » Tue Oct 27, 2015 2:57 pm

I originaly thought that, Jac, but there are no Push About coils fitted and there are only 4 connections to the scan coil area. Here's a picture:
CRT-Cage-Web.jpg
No Push About coils.

That really only leaves the Linearity mod which also involves a pot.

 
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Re: A Marconi 702 Mirrir Lid

Post by Jac Janssen » Tue Oct 27, 2015 3:05 pm

Then indeed the frame linearity mod is the most likely Brian.
I tried it and decided not to include it after all. The effect was minimal. It does control the linearity a bit, but the picture became too cramped together in the top section and it displaced the complete picture downwards.
The fact that the picture got a lot larger (in height), could easily be compensated with the height control.

It will not be too difficult to trace the wires.

Jac

 
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Re: A Marconi 702 Mirrir Lid

Post by Brianc » Tue Oct 27, 2015 3:12 pm

The absence of shift controls on these early TVs must mean that EMI had tremendous faith in the accuracy of the mechanical alignment of their CRTs. OK, they brought out the "Push About" coil mod. (don't you just love that name!) but it is fairly rare to find them on the sets existing today!

 
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Re: A Marconi 702 Mirrir Lid

Post by peter scott » Tue Nov 03, 2015 8:39 pm

Hi Brian,

The early sets with the 45 degree corners are perhaps a bit more problematic but with the later rubber masks you can achieve a significant amount of centring simply by pushing the CRT around within the mask. Perhaps this is where the title of the coil came from.

Peter

 
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Re: A Marconi 702 Mirrir Lid

Post by Till Eulenspiegel » Tue Nov 03, 2015 9:05 pm

Read about the fun I'm having with my Marconi 702:

http://www.vintage-radio.net/forum/show ... p?t=115876

and the HMV version, the model 901:

http://www.vintage-radio.net/forum/show ... p?t=115910

The 702 is having problems with the RF amplifier, low video drive to the grid of the CRT.
The 901 was all sorted out then the 5KV EHT transformer burnt out.

Till Eulenspiegel.

 
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Re: A Marconi 702 Mirrir Lid

Post by peter scott » Tue Nov 03, 2015 11:02 pm

I've been without web access for a week or so but it's great coming back to read all about these old sets that you guys have been restoring.

Peter :aad

 
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Re: A Marconi 702 Mirrir Lid

Post by Brianc » Wed Nov 04, 2015 7:16 pm

I just realised that I didn't post an external piccy of the 702 so here is one (two). I am trying to clear some of my back-log so that I can start on the set!!
702-Corrected-Full-Frontal_web.jpg
The perspective is a bit wrong - I corrected it as best as I could in Photoshop but the lid didn't work out too well.
Ivorine-Label-web.jpg
Type and serial number. A Fairly early one with the "Baird" valve hole blanked on the top of the chassis.

 
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Re: A Marconi 702 Mirrir Lid

Post by peter scott » Wed Nov 04, 2015 7:39 pm

That looks to be in very good shape Brian! Am looking forward to the hearing of the restoration.

Peter

 
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Re: A Marconi 702 Mirrir Lid

Post by Panrock » Fri Nov 06, 2015 6:18 pm

Type and serial number. A Fairly early one with the "Baird" valve hole blanked on the top of the chassis.
This No. 223 is the very next one to that of my previously owned set... No. 224 !

As is well known, the story of that set's restoration is at http://www.radiocraft.co.uk/702.htm

Steve
Attachments
702_21.jpg
702_21.jpg (24.33 KiB) Viewed 2966 times

 
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Re: A Marconi 702 Mirrir Lid

Post by Brianc » Fri Nov 06, 2015 6:28 pm

I already noticed that. Steve. I've also read your restoration "blog" so I know what I am in for!

 
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Re: A Marconi 702 Mirrir Lid

Post by peter scott » Sat Nov 07, 2015 9:06 am

Hi Steve,

Was the switch on the rear of your 702 for switching off the EHT to avoid switch-off ion burn?

Peter

 
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Re: A Marconi 702 Mirrir Lid

Post by Brianc » Sat Nov 07, 2015 12:57 pm

My 600 had a switch like that for a lamp inside the cabinet for servicing - very swanky!

 
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Re: A Marconi 702 Mirrir Lid

Post by Panrock » Sat Nov 07, 2015 1:17 pm

peter scott wrote:Hi Steve,

Was the switch on the rear of your 702 for switching off the EHT to avoid switch-off ion burn?
Peter

Hi Peter
Yes. I understood it originally was for a backlight. I connected it to switch on and off the primary of the EHT transformer.

Cheers Steve

 
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Re: A Marconi 702 Mirrir Lid

Post by Brianc » Sun Nov 29, 2015 2:00 pm

peter scott wrote:Was the switch on the rear of your 702 for switching off the EHT to avoid switch-off ion burn? Peter

Actually Peter, ISTR the ion burn on these CRTs is not a switch-off burn but a burn caused by the constant bombardment by ions, of the phosphor. They are too heavy to be deflected by the scanning fields so are always desensitising the screen. For example, the burn on my 5" 706 is as large as the test card C circle and this, I believe, is not uncommon.

 
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Re: A Marconi 702 Mirrir Lid

Post by peter scott » Sun Nov 29, 2015 8:51 pm

Hi Brian,

Fair point! I wonder if it was to prevent burn from a static electron beam then?
Jeffrey's 702 has a similar switch.

Peter :)

Jeffrey mains.jpg

 
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Re: A Marconi 702 Mirrir Lid

Post by Cathovisor » Sun Nov 29, 2015 9:03 pm

I thought the point of these switches was so you could listen to the high-fidelity sound-only transmissions that were sometimes radiated on 41.5MHz without unnecessarily using the CRT? On the Baird T18/T21 for example, turning the contrast knob to zero switches the EHT off.

 
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Re: A Marconi 702 Mirrir Lid

Post by Till Eulenspiegel » Sun Nov 29, 2015 9:47 pm

Early production versions of the 1946 Marconi VT50 and HMV 1804 also had the facility to switch off the vision circuits and come to think about it the Ekco TSC30 was the same. All these sets were essentially of pre-war design, the VT50 was the 1939 model 710, the HMV the 1800 and the Ekco the TSC902.

Till Eulenspiegel.

 
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Re: A Marconi 702 Mirrir Lid

Post by Brianc » Mon Nov 30, 2015 11:02 am

peter scott wrote:Jeffrey's 702 has a similar switch.

The switch was obviously professionally fitted, perhaps during production or a later service but why no escutcheon as the the main switch has - an afterthought perhaps?

 
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Re: A Marconi 702 Mirrir Lid

Post by peter scott » Mon Nov 30, 2015 11:39 am

Total mystery all round! Would professionals worry about CRT life whilst they listened to TV sound?

Peter :aab

 
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Re: A Marconi 702 Mirrir Lid

Post by Cathovisor » Mon Nov 30, 2015 12:51 pm

I take it by "professionals", you mean the sort of people who could afford to buy a television set? I would say the answer was "yes, they would."

 
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Re: A Marconi 702 Mirrir Lid

Post by Brianc » Mon Nov 30, 2015 1:52 pm

I meant the guys at EMI who were building before the war and servicing the sets just post war - the switch types are the same and the label looks exactly right.
An interesting question in my mind is "What was the reliability of the CRTs during their first years of use? Did EMI suffer from teething troubles with their CRTs or was it plain sailing from the start?". Perhaps there was trouble early on, hence the switch fitted to the early sets.

 
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Re: A Marconi 702 Mirrir Lid

Post by Panrock » Tue Dec 01, 2015 6:19 pm

Brianc wrote:
peter scott wrote:Was the switch on the rear of your 702 for switching off the EHT to avoid switch-off ion burn? Peter

Actually Peter, ISTR the ion burn on these CRTs is not a switch-off burn but a burn caused by the constant bombardment by ions, of the phosphor. They are too heavy to be deflected by the scanning fields so are always desensitising the screen. For example, the burn on my 5" 706 is as large as the test card C circle and this, I believe, is not uncommon.

Could someone kindly explain why the ion burns on the "second-generation" 5, 7, and 9 inch EMI tubes look like an amorphous smudge and the ion burns on the "first-generation" 9 and 12 inch EMI tubes look more like a small 'switch-off' burn? Could it be something to do with the electrostatic focus used on the first-generation tubes?

I recall on my 702 (a first-generation set), there was the usual small central burn, but also a much milder 'fading' covering almost all the picture except around the edge. I assumed at the time this latter was a large, classic, ion burn, but a subsequent visit to RACS taught me that the interior surface of glass itself can discolour from use. I saw this effect - it was very mild - on some re-screened and re-gunned CRM121's that they had done.

Steve

 
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Re: A Marconi 702 Mirrir Lid

Post by Jac Janssen » Wed Dec 02, 2015 2:26 pm

I think the small spot is just a burn-in of the normal (electron-) beam, which is concentrated at too small an area (due to absence of deflection) and thus burns the screen-phosphor.
This can also happen with a modern tube with an aluminized screen (a layer not penetrable for the slower ions):
14SX burnt screen.jpg
14SX burnt screen.jpg (8.54 KiB) Viewed 2495 times

The ion-burn always is a relatively large area, up to half the screen diameter.

Jac

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