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[Closed] A Marconi 702 Mirror Lid Part 1

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

Peter :aab

www.nostalgiatech.co.uk

 
Posted : 30/11/2015 11:39 am
Cathovisor
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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."

 
Posted : 30/11/2015 12:51 pm
Brian Cuff
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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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Posted : 30/11/2015 1:52 pm
Panrock
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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.

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

 
Posted : 01/12/2015 6:19 pm
Jac Janssen
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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):

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

Jac

 
Posted : 02/12/2015 2:26 pm
Brian Cuff
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Feeling a little bit more like my old self (probably thanks to being prescribed steroids and eating much more) I decided to get on with the 702. The easiest chassis to remove is the sync chassis so after removing the CRT and putting it somewhere very safe, out it came It most certainly did go back to the EMI factory for a re-fit. The cap box was replaced by an open framework with three electrolytics in/on it and that bracket with the pot. The pot was for the push-about coils but they are missing so whoever did the last work on it forgot to put them back and this made me decide to rebuild as originally supplied. In addition, all three special bolt-on electrolytic caps were removed together with ripping off the three receptacles which held the 0.5mF paper decoupling ones. Mike Barker kindly supplied the three electrolytics and I have made the three paper caps and receptacles to hold them.
The chassis is in quite good condition but I wanted to do a special job on this set so I will get the chassis re-finished. Fortunately, all the wiring and components except for the four transformers are on the same side of the chassis so with judicious drilling out of the copper rivets, unscrewing the tag-boards and loosening the pots, the whole of the wiring came off in one piece leaving a completely bare chassis which will go straight off to the platers in Camberley. I have also build a capacitor box and that is waiting for some capacitors that I have ordered from Canada. The LOPT as fitted is the wrong type and there were several circuit mods around that stage so that will go back to as it was originally and Mike Barker is making me a new LOPT. The other 3 Xfmrs and 1 choke have been re-painted using Plasticote Matt Chrome. They are lined up on the AGA to dry out.
Here are some pictures:

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Posted : 08/08/2016 8:32 pm
peterscott
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Great to see you back in action again Brian.

Peter.

www.nostalgiatech.co.uk

 
Posted : 08/08/2016 8:47 pm
Till Eulenspiegel
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Hi Brian.
, that's an ambitious restoration, pays off good rewards in the end though. On not such a large scale I removed all the components and cable forms in a similar manner on an Ekco B86 battery radio receiver, the set called the "Dougal"

Till Eulenspiegel.

 
Posted : 08/08/2016 9:05 pm
ianm
 ianm
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Hi Brian,

As Peter says, it's great to see you back in action again, and to once again enjoy detailed accounts of your epic restoration projects!

I'll be watching this one with great interest.

 
Posted : 09/08/2016 9:06 am
Brian Cuff
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Thanks guys. Let's just hope it lasts - radio-therapy to my lower skull next week :ccf .

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Posted : 09/08/2016 9:47 am
Katie Bush
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Thanks guys. Let's just hope it lasts - radio-therapy to my lower skull next week :ccf .

Hi Brian,

I wish you the very best of luck with that, and hope and pray that it's a truly successful outcome.

Marion

 
Posted : 11/08/2016 1:27 am
Brian Cuff
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Thanks Marion - they're going to bolt my head down using a mask made from a thermo-plastic to make sure I don't move and get shot in the brain. I'm really looking forward to it, as you can imagine :aaq

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Posted : 11/08/2016 10:38 am
Katie Bush
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Hi Brian,

Thanks to modern medical science.. I understand the principal behind the treatment, with multiple targeted 'beams' of radiation and the need to be absolutely immobile during the session - far better than the older 'blunderbuss' approach.

Let's all hope for the best.

Marion

 
Posted : 11/08/2016 10:36 pm
Brian Cuff
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Cheers Marion.
The workshop has beckoned over the last few days so it was back to the sync chassis. I sprayed the chassis with Plasticote Matt Chrome Appliance paint which, I think, gives a good, dull finish. All the rivet holes were drilled out to No33 drill for 6BA clearance and a visit to the web bought me a range of new 4 and 6BA screws, shakeproofs and nuts. The valveholders also had to be drilled out and here, I was most careful as the black bit which is visible is Bakelite so very brittle. Nevertheless, I managed to break the corner off one - it doesn't show!
One of the tagboards had a hole burnt in it. This was obviously due to a 0.5mfd screen decoupler going leaky and toasting the screen resistor. I decided to make a new one using the tags I had made for my 707 project (still very rusty). Please excuse the punching mistake at the left-hand end.

I've just noticed that the heater wiring needs tidying up and unfortunately, the systoflex insulation on the harness going from the long tagboard to the control panel has split in places so I will have to replace the whole loom - I enjoy lacing. :qq1

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Posted : 12/08/2016 10:55 am
Cathovisor
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I thought you were having the chassis re-plated, Brian? As for lacing, I hope you've dug out your copy of ED122 :qq1

 
Posted : 12/08/2016 12:07 pm
Brian Cuff
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I was going to but things got the better of me and I didn't fancy the trip to Camberley - I don't travel too well what with being bony and feeling every bump (no, I don't have bling wheels on my C-RV as I got the salesman to take the 17" wheels off the demonstrator to swap with the 18" supplied with my model!).

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Posted : 12/08/2016 1:15 pm
Brian Cuff
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After a splendid afternoon with Graham and Jeremy who helped load the GEC BT8161 project bits into Mike's hired Van, I took some pictures of the now finished (except for the LOPT which Mike Barker is making for me) 702 Sync Chassis. The chassis had been back to EMI for the post-war refurbishment but I wanted it to show the pre-war condition so I removed the replacement electrolytic capacitor assembly and replaced it with an original type capacitor block. Before the mods, there were two harnesses, one along each tag-board to the control panel. I replaced these with harnesses made using the American-type push-back solid braided wire and avoided the very untidy fraying at the ends by mounting a short heat-shrink sleeve to prevent this. It looks so much better than the untidy white inner cotton wrapping which is exposed by the fraying. Here are some of the pictures:

[attachment=2]Sync-Chassis-finished-RHS-web.jpg[/attachment

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Posted : 18/08/2016 7:29 pm
peterscott
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Even when hindered by health problems your productivity puts most of us to shame Brian. Top quality work!

Peter (put to shame) :ccg

www.nostalgiatech.co.uk

 
Posted : 18/08/2016 8:06 pm
Brian Cuff
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Thanks Peter but being in the workshop is a wonderful way of distracting the mind from other important things :qq1 .
I have now removed the Sound Chassis from the cabinet and it's in quite good condition overall except that all the original caps have been removed and replaced with blue ones which I have not met before. I will replace them with some that are more in keeping. I will also give the chassis the same treatment as I gave the sync unit so that they match. This means, of course, get drilling!

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Posted : 19/08/2016 4:45 pm
Jac Janssen
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Hello Brian,

Good to see that you are enjoying the work on the 702.
Yours looks in much better condition than mine (which was quite rusted).

Hope you have a full recovery and can enjoy many more years working on these very interesting sets!

Best regards,
Jac

 
Posted : 19/08/2016 5:01 pm
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