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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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Re: A Marconi 702 Mirrir Lid

Post by Brianc » Mon Aug 08, 2016 7:32 pm

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:
P8080002 (Medium).JPG
Home-made 0.5mfd cap and cup showing!
P8080003 (Medium).JPG
P8080006 (Medium).JPG
Pots unscrewed from the front panel
P8080011 (Medium).JPG
The complete set of electronics except for the transformers
P8080012 (Medium).JPG
A nice bare chassis!

 
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Re: A Marconi 702 Mirrir Lid

Post by peter scott » Mon Aug 08, 2016 7:47 pm

Great to see you back in action again Brian.

Peter.

 
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Re: A Marconi 702 Mirrir Lid

Post by Till Eulenspiegel » Mon Aug 08, 2016 8:05 pm

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.

 
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Re: A Marconi 702 Mirrir Lid

Post by ianm » Tue Aug 09, 2016 8:06 am

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.

 
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Re: A Marconi 702 Mirror Lid

Post by Brianc » Tue Aug 09, 2016 8:47 am

Thanks guys. Let's just hope it lasts - radio-therapy to my lower skull next week :ccf .

 
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Re: A Marconi 702 Mirror Lid

Post by Katie Bush » Thu Aug 11, 2016 12:27 am

Brianc wrote: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

 
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Re: A Marconi 702 Mirror Lid

Post by Brianc » Thu Aug 11, 2016 9:38 am

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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Re: A Marconi 702 Mirror Lid

Post by Katie Bush » Thu Aug 11, 2016 9:36 pm

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

 
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Re: A Marconi 702 Mirror Lid

Post by Brianc » Fri Aug 12, 2016 9:55 am

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.
Burnt-Tagboard-Web.jpg
Not a very nice sight.
Sync-Chassis-New-Tagboard-Web.jpg
It doesn't look too bad.
Sync-Chassis-Interim-Assembly-Web.jpg
A general view.
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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Re: A Marconi 702 Mirror Lid

Post by Cathovisor » Fri Aug 12, 2016 11:07 am

I thought you were having the chassis re-plated, Brian? As for lacing, I hope you've dug out your copy of ED122 :qq1

 
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Re: A Marconi 702 Mirror Lid

Post by Brianc » Fri Aug 12, 2016 12:15 pm

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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Re: A Marconi 702 Mirror Lid

Post by Brianc » Thu Aug 18, 2016 6:29 pm

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:
Sync-Chassis-Finished-Top-Web.jpg
[attachment=2]Sync-Chassis-finished-RHS-web.jpg[/attachment
Attachments
Sync-Chassis-finished-RHS-web.jpg
Sync-Chassis-under-Web.jpg

 
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Re: A Marconi 702 Mirror Lid

Post by peter scott » Thu Aug 18, 2016 7:06 pm

Even when hindered by health problems your productivity puts most of us to shame Brian. Top quality work!

Peter (put to shame) :ccg

 
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Re: A Marconi 702 Mirror Lid

Post by Brianc » Fri Aug 19, 2016 3:45 pm

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!
702-Sound-Chassis-Front-Web.jpg
702-Sound-Chassis-Back-Web.jpg
702-Sound-Chassis-Under-Web.jpg

 
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Re: A Marconi 702 Mirror Lid

Post by Jac Janssen » Fri Aug 19, 2016 4:01 pm

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

 
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Re: A Marconi 702 Mirror Lid

Post by Brianc » Fri Aug 19, 2016 6:40 pm

Thanks for your thoughts, Jac. Unfortunately, as I am in what is effectively Palliative Care, I won't be "getting better" but you can bet your boots that I will be fighting to last as long as possible!!
I have now stripped the sound chassis of it's wiring and will be spraying it tomorrow. Stripping this chassis was more difficult than the Sync chassis as the IF transformers are mounted on the top with the connections passing through he chassis. More tomorrow:
Empty-Chassis-&-Wiring-Web.jpg

 
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Re: A Marconi 702 Mirror Lid

Post by Jac Janssen » Fri Aug 19, 2016 6:55 pm

Hi Brian,

I hope you certainly can be with us for a very long time!

Your chassis already looks very presentable to me.
But having seen your high standards, it will become even better!

What is this?
Under component.jpg
Under component.jpg (20.06 KiB) Viewed 1336 times


Best wishes,
Jac

 
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Re: A Marconi 702 Mirror Lid

Post by Brianc » Fri Aug 19, 2016 7:10 pm

Cheers Jac.
Those are 4 x 0.5mfd radial caps replacing the large wax C12 and C16 1.0mfd caps which, fortunately, are still present so that I can re-stuff them.

 
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Re: A Marconi 702 Mirror Lid

Post by Brianc » Sat Aug 20, 2016 7:48 am

There is problem on the sound chassis with the greasy systoflex sleeving used for insulation on the wiring. It seems to have amalgamated into a sticky, gooey mess and any attempt to separate the different circuits results if one or both the sleeves tearing with the resultant loss in insulation so it looks as though I will have to re-wire - it's not a complicated chassis and I would like it to last another 70 years!

 
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Re: A Marconi 702 Mirror Lid

Post by Cathovisor » Sat Aug 20, 2016 11:14 am

I've often wondered if heat will help it separate, you know...

 
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Re: A Marconi 702 Mirror Lid

Post by Jac Janssen » Sat Aug 20, 2016 11:40 am

Cathovisor wrote:I've often wondered if heat will help it separate, you know...


Yes, it did in my 702. It appears to be sticky hardened oil which was used to impregnate the sleeves.
But heating a sleeve with hot air certainly softenes the stuff and makes the sleeve more pliable as well.

Jac

 
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Re: A Marconi 702 Mirror Lid

Post by Cathovisor » Sat Aug 20, 2016 12:20 pm

A friend e-mailed, having read the last couple of posts to say:

Heat will help separate systoflex but in all honesty you wouldn't want to re-use it. Plus, EMI used something with a white (plastic?) insulation in their wiring looms that doesn't like heat. You either leave well alone or re-wire.

 
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Re: A Marconi 702 Mirror Lid

Post by Brianc » Sat Aug 20, 2016 1:50 pm

One of the things I'm worried about is the stuff becomming leaky. I had an early HMV superhet and it was as awful performer. I traced it down eventually to leaking insulation in the looms - a real bu**er to fix. I will re-wire with braided solid but this time I will see what it looks like after a touch of varnish to give it a bit of a shine.
I have sprayed the chassis and bracketry and almost finished the ACAD drawing of the wiring so that I can strip the existing tagboard and start again using as many of the resistors as possible.

 
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Re: A Marconi 702 Mirror Lid

Post by Brianc » Wed Aug 24, 2016 7:56 pm

Well, I bit the bullet and decided to replace most of the decaying Systoflex with the push-back wire I bought from the 'States using heat-shrink to stop it fraying. I stripped the tagboard of all its faulty components and cleaned it well, replacing one tag which had fatigued and broken. The assembly of the chassis started with drilling out the valve-holder rivet holes (chassis and holders) to 6BA and bolting them in using new screws etc. The IF cans, SOPT and new decoupling caps - I had two originals which I restuffed to replace the wax components which had been fitted some time in the past. Next came the volume control and the frequency changer components, coil and tuning cap etc. Unwittingly, I have lost the small brass bar which fixes the oscillator coil to the chassis - I didn't realise it was there and it dropped out somewhere on the premises so I'll have to make a new one.
The pictures show the progress so far and I'm sorry that they are a bit soft but there wasn't too much light around:
702-Sound-Chassis-top.jpg
702-Sound-Chassis-Bottom-Mid-Rebuild.jpg
I now have to install the tagboard and do all the linking to connect up the HT+ and 0V chains. Then comes the reconnects to the valves, but that is another day! At least this chassis will be easy to test on the bench as all that is required is a 45MHz Aurora output and a loudspeaker.

 
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Re: A Marconi 702 Mirror Lid

Post by Focus 2 » Wed Aug 24, 2016 10:25 pm

Hi Brian,
That's fantastic work so far. Look forward to hearing more.

Cheers
Brian R

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