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

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Brian Cuff
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11/09/2016 11:43 pm  

The saga continues!! from Part 1 here \r\n\r\nThe EHT guard is now in place on the PSU where the 5kV backed by 0.2mfd, was readily touchable. So dangerous!\r\n\r\nThe picture shows the acrylic sheet guard placed over the tagboard. The acrylic sheet came from Autoglass when they made a temporary side window for my car which blew up (the side window, not the car!).\r\n\r\nI notice that the tagboard appears to be very dirty. I had not noticed that before so into the ultrasonic cleaner it will go.702-PSU-EHT-Protection.jpg\r\n\r\nI have started on the TRF chassis (the vision receiver) and I will post some pictures etc. in the next few days.


Chris
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12/09/2016 12:51 am  

 Hi Brian,\r\n\r\nGood to see part two of the Marconi 702.\r\n\r\nBeing a total coward thinking of that panel and what lies within, gives me the raging heebie-jeebies  eek_gif\r\n\r\nLooking forward as no doubt others are too, for your continued progress on this set.


Marconi_MPT4
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12/09/2016 6:39 pm  

\r\nBrian Cuff said \n [...]The EHT guard is now in place on the PSU where the 5kV backed by 0.2mfd, was readily touchable[...] \r\n\r\n

\r\nHi Brian,\r\n\r\nIt is surprising how little \'re-guard \' manufacturers placed on individual safety back then. Will it stay in place after refurbishment or be removed to keep original layout? In the past when working on oscilloscopes with mains derived EHT, I have used two neon indicators, in case one fails, together with appropriate resistor networks to show if the caps are still charged.\r\n\r\nBTW was the VCR511 with its 15 second phosphor intended this set? wink\r\n\r\nCheers\r\n\r\nRich


Brian Cuff
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12/09/2016 6:45 pm  

The TRF is a strait 68 valve r45MHz receiver using MSP4 pentodes with a selected version, MSP41, in the final stage. A D42 single low capacitance diode as the detector which feeds the CRT grid directly with 15  - 20pp of video.\r\n\r\n702-TRF-Chassis-showing-valves-web-2.jpg\r\n\r\nThe 702 chassis is quite clean but will be painted to match the other chassis and that’s going to need a lot of masking etc.\r\n\r\nI went through the chassis measuring all resistors and replacing those more than 12-15% high – I just measure across the components because the effect of any circuitry in parallel with them will lower the R, not increase it. Should one measure low, I check it out of cct.\r\n\r\nThis TRF chassis came up trumps and gave the requisite 15-20pp video at the feed to the TRF grid so no problems there.\r\n\r\nNow to dismantling: Again lots of pictures during the procedure – ‘aint digital cameras wonderful. All the removable bracketry went into the cleaner and were painted with the silver wheel spray I have used on this project. I must say that I like the finish but it does take a few days to harden – not too good for me with my impatience. One thing that surprised me was the way that the ultrasonic cleaner I have cleaned up the drawn alloy valve screening cans. They have come up like new with no further treatment required – they look great which is more than I can say about the other bits including the seven copper screening “cups” under the chassis. These, too, have been painted. Three bars hold the screening cans in position by clamping down onto circular foam rubber pads. These I punched out using a large wad punch left over from my mechanical music (pianolas etc.) days. One thing about changing direction in hobbies, is that one assembles differing sets of tools, all of which overlap in many directions.\r\n\r\n702-TRF-Chassis-Assorted-Metalwork-web-2.jpg\r\n\r\nThe multi-capacitors used in the TRF are interesting (see pic). They are the sqiddy looking things – 4 off 500pf in each package and they wire directly to the valve pins – very good design (specifically for the TVs or a standard package?).\r\n\r\n 702-TRF-detail-inside-lower-cans-web-2.jpg\r\n\r\nThe EHT bleeder board has been cleaned and is much better. They say every cloud has a silver lining – when re-installing the board after cleaning, I found that I had missed a connection. I would have had a very soft blob on the CRT without it – it’s the red wire at the top and is the CRT focus electrode feed!\r\n\r\n702-PSU-EHT-Protection-web-1.jpg


Brian Cuff
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12/09/2016 6:49 pm  

 

 

Marconi_MPT4 said

Hi Brian,

It is surprising how little \'re-guard \' manufacturers placed on individual safety back then. Will it stay in place after refurbishment or be removed to keep original layout? In the past when working on oscilloscopes with mains derived EHT, I have used two neon indicators, in case one fails, together with appropriate resistor networks to show if the caps are still charged.

BTW was the VCR511 with its 15 second phosphor intended this set? wink

Cheers

Rich  

Hi Rich

No, the set was the GEC BT8161 which Mike has taken over from me. I had no idea that the VCR115 had a 15 second phosphor. It was, of course, discounted immediately. It Mike has now an original GEC 12\" B&W CRT for the set!

Edited: 3 weeks  ago

Jac Janssen
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19/09/2016 9:03 pm  

Hi Brian,

Those valve screening cans look amazing!
Your 702 certainly must have been stored under favourable conditions!

You're making good progress.
Looking forward to the results of the complete set.

Best wishes,
Jac


Brian Cuff
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19/09/2016 11:53 pm  

I was surprised how well the cans looked after treatment. The treatment was simply about 1 minute in the ultrasonic cleaner with a mild detergent used normally for cleaning jewelry. Obviously, the detergent was very good at removing aluminium oxide!


Brian Cuff
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25/09/2016 11:04 pm  

The TRF chassis has now been masked and sprayed. The masking was very fiddly, with many spray angles to be covered. The pictures show it in the final stages where the beautifully shiny valve screening cans have been completely covered in 3M Pressure Sensitive Tape which I got from my old company. It\'s rather expensive (or would have been) but perfect for the job at hand. I am leaving the chassis to harden for a couple of days and will be getting back to it on Wednesday and reassembling all the brackets and side cheeks onto the main lump. Then, after cleaning out the cabinet and touching up the paint on the copper earth bussing, it will be time to re-install the 4 chassis into the cabinet. I will need help with that, especially moving the PSU from the workshop to the cabinet in the \"Museum\".\r\n\r\n702-TRF-Chassis-during-spraying-Web.jpg


Brian Cuff
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26/09/2016 2:00 pm  

Just as an aside, I decided to make some replica paper caps as used in the EMI MKI sets and in the small 5\" and 7\" sets. Fairly simple to make - draw a paper label and glue it round a suitable SRBP tube. The leadout wires pass through very small eyelets and soldered. I used white glue but I\'m not happy with it. I\'ve also tried Copydex but that was worse. Has anyone got an idea which is the best adhesive to use?\r\n\r\nOne of the things that is difficult to get is tar or pitch to fill the caps as it has been deemed carcinogenic. I may have solved this problem - black hot glue sticks!\r\n\r\nEMI-Green-Paper-caps-Web-1.jpg


Cathovisor
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26/09/2016 2:48 pm  

 Those caps really are superb, Brian. The first time I saw them was not in an EMI set, but in a Pye P/B 'Rising Sun' set. I seem to recall that BI (British Insulated) caps are of this pattern.

Can you still get Gloy Gum?


Katie Bush
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26/09/2016 4:35 pm  

Brian Cuff said \n\n One of the things that is difficult to get is tar or pitch to fill the caps as it has been deemed carcinogenic. I may have solved this problem - black hot glue sticks!\n \n EMI-Green-Paper-caps-Web-1.jpg  

\r\nHi Brian,\r\n\r\nJust thinking aloud here, but I was wondering about kids\' black wax crayons? could they be used to make a filler for these caps? Another thought was black underbody sealant, not WaxOyl, which would melt at fairly low temperatures, but the more conventional stuff that sets semi-hard?\r\n\r\nAs to glue, would PVA craft adhesive not do the job? http://www.wilko.com/glue-tape+corrector-fluid/wilko-pva-glue-500ml/invt/0300256 which can be thinly applied.. Otherwise, there is still an old fashioned \'paper gum\' of the type we had as kids, if only I could remember what it was called (It wasn\'t Copydex, nor Gloy - though Gloy was also on my mind as a possibility).. Otherwise, I have used this stuff to very good effect,  http://www.wilko.com/glue-tape+corrector-fluid/wilko-liquid-glue-pen/invt/0433908 and it becomes almost non-existent when it dries - I have actually reattached metal (brass at least) decals to machinery with this stuff! Best one was a brass instruction plate reattached to and old arc welding transformer! Impressed my dad with that one....\r\n\r\nMarion

If it ain't fixed, don't break it!............


Brian Cuff
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26/09/2016 9:49 pm  

I assume, Marion, that the PVA to which you refer is the same stuff, in a different bottle, as Resin W, the woodworking adhesive. I have tried that and I think that it containstoo much water and softens the paper too much. I suppose the other thing to look at is the type of paper to use. I am using just ordinary (90gm) copier paper which is not designed to meet much moisture - is ink-jet paper different in that way - a bit more tolerant to the wet! I'll do some research.


Katie Bush
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26/09/2016 10:26 pm  

Hi Brian,

In all honesty, it years since I last had to use the PVA craft glue, but if memory serves me, it's less viscous than the commercial stuff.

Now as for paper, there certainly are differences between makes of paper, copier paper is probably the most likely to turn soggy - it certainly doesn't like high density ink.. In that sense, it behaves more like blotting paper.. For my own use, I grudgingly started using HP ink-jet paper, nothing special, just the basic stuff but HP branded.. What I can say is that it doesn't soak up the ink and turn soggy, so would hope that the same would be true in the presence of a water based glue.

Those glue-pens I linked to are very good in that respect because the glue is very thin, and if spread uniformly with a plastic spreader, doesn't 'soggify' the paper.. When it dries, it almost completely disappears, leaving just a trace of the adhesive to do the gluing.. To me, it looks like somewhat like a thin and clear PVA.

Marion

If it ain't fixed, don't break it!............


Brian Cuff
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22/12/2016 9:45 pm  

Hello all - After a three month delay due to illness and lack of energy and drive. I spent about an hour in my workshop today, the first time spent in either workshop for over two months. Cancer surely does knock it out of you but I\'m determined to get going again, under palliative care, with the four projects that I have either on the go or scheduled: The Argus, this 702, a simplex from a couple of years ago and I have now bought a completed Premier kit (new thread).\r\n\r\nThanks for the advice, Marion and I will be returning to the capacitors soon so I suppose that makes five projects!! RS sell SRBP tube which makes a very good wax capacitor body so no need now, to have it custom made.\r\n\r\nThe next two jobs on the 702 are to reassemble the TRF chassis after its restoration and the cleaning and spraying black (where necessary) of the cabinet internal walls. Firstly the TRF chassis:\r\n\r\nThe picture shows most of the bits that go to make up the TRF chassis which is the Vision receiver. The sound receiver is tapped off after the second vision RF stage via a rather thick co-axial cable which I have mimicked by using woven sleeving over a normal coax as the original is crumbling. As can be seen, the chassis comprises quite a few metal components, most of which are fitted together using self tapping screws which I will de-rust using my trusty wooden stick with holes in it and a wire brush. The screws are pushed into the holes and the heads scrubbed with the brush. The result is lots of clean screw heads.\r\n\r\nMetalwork-ready-for-assembly-1.jpg


Nuvistor
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22/12/2016 9:55 pm  

Glad your are feeling up to doing a bit more, the chassis in the photo looks really good.

You can only do what you feel up to, the treatment is debilitating, luckily I have not suffered it but know some who have, take care.

Frank

Frank


Cathovisor
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22/12/2016 10:31 pm  

Brian Cuff said

[...] with the four projects that I have either on the go or scheduled: The Argus, this 702, a simplex from a couple of years ago and I have now bought a completed Premier kit (new thread).

Thanks for the advice, Marion and I will be returning to the capacitors soon so I suppose that makes five projects!!

Good to see you back on here, mon ami. You forgot a certain radio set in that list...wink

Edited: 3 weeks  ago

Brian Cuff
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22/12/2016 10:43 pm  

Mike is referring to a GEC BC3890 radio I bought a few weeks ago for £20. It uses, and still has them, push-pull PX4s for audio output. I bought it for the PX4s but don\'t have the heart to part them from the set so Mike is right, make that six!  HELPfright


Cathovisor
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23/12/2016 12:01 am  

Twenty quid??!! eek_gif\n\r\n\r\nThe BC3890 will be an interesting project; it might make me want to do mine up or its predecessor, the BC3880 with its push-pull MPT4s. As it stands, I still have a Panzer boot-full of radios from last week - four lovely Shrubs (SW45/PB53/DAC53/PB83), although the first two are really destined for spares. The DAC53 in particular is an excellent example and thus my current one joins the auction/doomed pile.


Marc
 Marc
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23/12/2016 12:19 am  

Hi Brian,\r\n\r\nJust wanted to say it\'s lovely to have you back at the bench again and looking forward to the story continuing. thumb_gif \r\n\r\nMarc.

Marc


Jac Janssen
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23/12/2016 8:22 am  

Good to hear from you Brian !

That 702 TRF-unit looks like new (or even newer..).

Jac


Brian Cuff
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23/12/2016 9:48 am  

Thanks guys. I am determined to get in the workshop for at least two hours per day - let\'s see how it goes as there are always the jigsaws to occupy me inside the house. eek_gif


PYE625
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23/12/2016 10:08 am  

Brian Cuff said \nThanks guys. I am determined to get in the workshop for at least two hours per day - let\'s see how it goes as there are always the jigsaws to occupy me inside the house. eek_gif  

\r\nGood for you Brian, that\'s the spirit.\r\n\r\nOur thoughts are with you and may I wish you a special Happy Christmas and may you fully enjoy every moment you are able to spare in the workshop. 

Andrew


Brian Cuff
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28/12/2016 5:15 pm  

Many thanks for your support. I must admit that over the Christmas period (which was smashing), the workshop played a very second place but I did get some assembly done. The TRF section is now nearly completed (pictures and words later) and after that, all is that is left will be the interior of the cabinet to de-spider and blow over with satin black, replacing the paper labels affixed to the floor of the cabinet. I will also add some castors.

Someone asked me to make an EHT Bleeder tag board earlier on - do you still want it as I have measured my original and can now make an identical one with the proper tags etc. Just let me know.


Brian Cuff
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28/01/2017 9:07 pm  

I had a visit from a physiotherapist today to try and get me a bit motivated and she suggested a routine which may help!\r\n\r\nI get tired after about 20 mins so set a timer (iPhone) to 20 mins and dedicate that time to a project/part project and astop and rest at the end of the allocated time. Seems quite simple!\r\n\r\nWell. this afternoon, I set 20 minutes aside to measure up to fit castors on the 702 that I am restoring. Ex-factory, they are fitted with chrome plated carpet glides which are not really suitable to modern carpets and these pre-war sets are quite heavy so castors are the answer. I just about got the measurements done (it required inverting the cabinet as I can\'t bend down very easily) before my \'phone burst into song!\r\n\r\nNext came a rest followed by an Autocad session to design the method of mounting the casters (ebay) in the corners. One side, RHS, is fine but the LHS has a part of the PSU chassis hanging down into the \"cellar\". I need to do some more checks before proceeding. One pleasing thing is that the distance between the bottom of the castor plate and the long front and back glue blocks is 18mm - now that makes a change as it would more normally be 14 or 15mm and not a standard plywood thickness!!  As can be seen from the pictures, A plywood \"fillet\" can be fixed in the corner, resting of the side-to-side glue-block with an extra, small block added front-to back. The Acad drawing is nearly completed and will be posted tomorrow (according to my work-schedule) laugh.\r\n\r\n1.JPG2.JPG


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