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Forum 141

[Closed] A Marconi 702 Mirror Lid Part 1

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Brian Cuff
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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?

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Posted : 27/10/2015 2:18 pm
Jac Janssen
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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

 
Posted : 27/10/2015 2:38 pm
Brian Cuff
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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:

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

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Posted : 27/10/2015 2:57 pm
Jac Janssen
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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

 
Posted : 27/10/2015 3:05 pm
Brian Cuff
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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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Posted : 27/10/2015 3:12 pm
peterscott
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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

www.nostalgiatech.co.uk

 
Posted : 03/11/2015 8:39 pm
Till Eulenspiegel
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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.

 
Posted : 03/11/2015 9:05 pm
peterscott
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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

www.nostalgiatech.co.uk

 
Posted : 03/11/2015 11:02 pm
Brian Cuff
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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!!

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Posted : 04/11/2015 7:16 pm
peterscott
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That looks to be in very good shape Brian! Am looking forward to the hearing of the restoration.

Peter

www.nostalgiatech.co.uk

 
Posted : 04/11/2015 7:39 pm
Panrock
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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

 
Posted : 06/11/2015 6:18 pm
Brian Cuff
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I already noticed that. Steve. I've also read your restoration "blog" so I know what I am in for!

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Posted : 06/11/2015 6:28 pm
peterscott
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Hi Steve,

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

Peter

www.nostalgiatech.co.uk

 
Posted : 07/11/2015 9:06 am
Brian Cuff
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My 600 had a switch like that for a lamp inside the cabinet for servicing - very swanky!

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Posted : 07/11/2015 12:57 pm
Panrock
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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

 
Posted : 07/11/2015 1:17 pm
Brian Cuff
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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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Posted : 29/11/2015 2:00 pm
peterscott
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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 :)

www.nostalgiatech.co.uk

 
Posted : 29/11/2015 8:51 pm
Cathovisor
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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.

 
Posted : 29/11/2015 9:03 pm
Till Eulenspiegel
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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.

 
Posted : 29/11/2015 9:47 pm
Brian Cuff
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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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Posted : 30/11/2015 11:02 am
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