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

 Hi Brian,

Good to see part two of the Marconi 702.

Being a total coward thinking of that panel and what lies within, gives me the raging heebie-jeebies  

Looking forward as no doubt others are too, for your continued progress on this set.

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

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?

Cheers

Rich

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.

The 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.

I 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.

This TRF chassis came up trumps and gave the requisite 15-20pp video at the feed to the TRF grid so no problems there.

Now 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.

The 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?).

The 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!

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?

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!

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

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!

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".

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?

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!

 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?

Brian Cuff said

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!

  

Hi Brian,

Just 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?

As 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....

Marion

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.

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

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).

Thanks 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.

The 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:

The 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.

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

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...

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!  HELP

Twenty quid??!!

The 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.

Hi Brian,

Just wanted to say it's lovely to have you back at the bench again and looking forward to the story continuing.  

Marc.

Good to hear from you Brian !

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

Jac