I am still working on my 702 but this "little" project reared its ugly head! I bought the chassis off ebay some years ago and they have sat on a windowsill ever since so, stumbling over Steve Ostler's site, it reminded me that I had them.
I know that I'm being a bit of a grass-hopper mind but a change is as good as a rest - is it not?
The first thing is "What is an Argus". Described in Practical Television from June 1952 (first article, supplied with a blueprint), it uses typical ex-Radar valves and CRT (EF50, SP61 and VCR97), all of which were available in vast quantities back then. A Type 62 Indicator (Gee) would supply most thermionics for the Argus).
The chassis were not punched so I first had to cut the holes for the valve holders. For this, I bought some step drills which went up to 1 3/8" diameter so were OK for the large B9G bases for the EF50s.
The Set consists of 5 chassis: 2 x RF, 1 x Timebase, 1 PSU and 1 x CRT/EHT chassis. Here they are assembled together as they will be in the finished article.
I started with the time-base chassis as this seemed to be easier than the RF chassis with their inter-stage screens and coils to be wound – never put off today what you can put off tomorrow (at least all the large holes are cut). The chassis are made from 16SWG aluminium and are easy to cut but my scraper has been doing overtime deburring. Most of the components of the TB chassis are carried on a custom tagboard made from 2 strips and 3 brackets. Getting that right caused a lot of gnashing of teeth as I confused myself with views from top and from under! 
I put this down, in part, to my present condition – stupid! This chassis has been tested and the scans run at the correct frequency (first time!) and look reasonably linear but the sync separator has not been as I have not made the vision RX yet and vision of the right level is required. Here’s the time-base chassis:
Next was the CRT/EHT chassis. The PTV blueprint showed a lot of the bits at EHT on the top of the chassis. This I did not like so I made a tagboard which carries most of the “elevated” bit underneath and I will add an acrylic cover for extra protection (2.5kV with 0.2mfd behind it!) – enough to be dangerous.
The last chassis I have worked on is the PSU jobbie. This is fairly nude at the moment as I am waiting for the transformers and choke from Majestic Transformers in Poole who have promised to ship on Monday – so the weekend will be spent on the Marconi 702 project.
There are some rather nasty black lines on the top of some chassis round the fixing holes and these are due to my mistakes in marking out. With countersunk holes, one has to be accurate or the screws just won't fit. This is why there are several markings, proved wrong by offering a template of the tagboard to the markings on the chassis.
Now, back to the 702 
Hi Brian,
Other than reading about its on Steve's site and seeing a few pictures, I've never seen a detailed project so this will be another very interesting thread of yours to follow. For those interested and would like to see the design, here's the Argus build instructions and blueprint at RadioCraft
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A bit of progress over the last few days. The CRT, complete with its new Mumetal screen has been mounted as per instructions. It was quite difficult getting the strap above and the support underneath the CRT right, not too tight nor too loose. The support I made twice and I had to move the side cheeks - originally much too close because extra width now, with the Mumetal screen. The transformers should be arriving middle next week so I can begin testing parts of it. I will be lacing and passing inter-chassis cables above the chassis where possible as this makes for easier testing and error correcting - not so much moving around the fairly large, heavy assembly of 5 chassis. The first to be tested will be the CRT/TB assembly with bench PSU and then I will add the proper PSU. Here are a couple of pictures of the progress:
Next is the building of the two TRF receivers for vision and sound. The valve holder holes have already been done but there are many, many 4BA and 6BA holes to drill (in the right places!). Wiring will then start. The text suggests that the tuning coil earthy ends are taken under the valve holder fixing screws. I will use proper solder tags with shakeproof washers. Again, the receivers will be tested using my bench PSU but I think that that will be a little time away - I hear the 702 calling
Good Heavens!
That's coming along at a pace, Brian.. It's beginning to look like a telly now - How soon do you expect to see anything on the screen?
As an aside, I often wondered where the BBC got some of their 'Doctor Who' props from.. Now I see the Doctor's gizmo for warding off "The Sea Devils".. Just add a few bits of coloured tubing, stick a plastic dial on the CRT face and a few bits of coloured sticky tape here and there, and I swear it's a modified Argus beneath all that.
I look forward to seeing more pictures..
Marion
I can only echo Marion's comment - that's some very quick progress there, Brian!
Apropos the mention of stuff in 'proper' Doctor Who (not the modern incarnation) - I do have a still captured somewhere where a blue-painted DAC90 tuning knob and gang saves the day!
I see what you mean Marion!
The mechanical work would have been quicker to do had the hand punch which I bought to punch small holes in thin metal - dangerous to drill without ruining drills by grinding flats on them - was almost impossible to position properly. Normally, punches have points on them so that the centre pop can be located using it and the hole punched in exactly the right place. Not on this tool. All the punches (8 in all) have a hollow, not a point at the tip and there is little room between the jaws to see the position of either the centre pop or the circle drawn through the mating hole by a scriber. However, I must admit, I enjoy the metalwork as much as the electronics and to have a small project, rather than the all-consuming GEC BT8161 project, is rather refreshing. Thanks again, Mike.
As to first light, this should be during the CRT/TB chassis testing next week - I could use my Bench EHT generator for that if the transformers don't turn up. Real pictures will be quite a time as the two receiver chassis are quite complicated and fiddly to build due to the long, narrow chassis they use.
What you've not said is what CRT you're using...?
I,m using a VCR97 which I had "in stock". What I really want is a CV2810 which is the same CRT but with a different phosphor. It has, in fact, two phosphors - a long persistence orange and a short persistence blue-white. A blue filter should block the orange light so that only the back & white picture is visible. I haven't given up hope of finding one but hens' teeth spring to mind.
I have just discovered that a CV962 is also an identical CRT as the VCR97 but with a blue phosphor. I assume that this is also short persistence so could be usable - another hen's tooth.
I spent too long in the workshop today and am feeling a bit tired! But I finished wiring the sound RF chassis and it's working after a fashion. The audio stages seem fine but the RF section is a bit unstable. There are some silve mica caps which I am waiting for and these will lower the frequency to which the strip is tuned which should help. There were no great difficulties during the wiring except the it's a narrow, deep chassis so getting to some screws and nuts was a bit tricky. I know tuning the chassis will be the most difficult thing as the coils are virtually self supporting (or not, as the case may be) and they will need pulling apart of squashing together to get into the right range for the cores to be useful. I'm surprised how bare the chassis looks - some pictures:
The transformers arrived this morning, very heavy, so the next job, before the mica caps arrive will be checking that the Xfmrs. fit and drilling the mounting holes for them.
Hi Brian,
It is a bit of a 'Hubbard's cupboard' in there isn't it, though I suspect the electrolytics and waxies would have been rather bulkier in their day, even so, am I to believe that that is all there is to go into this strip?
I must say though, to think those valves have been waiting all this time to meet in this chassis - it stirs the imagination.
Marion
Brian Cuff said
I'm surprised how bare the chassis looks - some pictures:
Wow - EF50s that work in Pye B18T frame output stages! (The double black lines)
Cathovisor said
Wow - EF50s that work in Pye B18T frame output stages! (The double black lines)
Eagle eyed Mike
I didn't spot those black stripes, first time round!
Well spotted.
Is that so, Mike? What's the code - two for Frame output and one for RF stages? That would be useful info to have! I want to buy some brand new looking EF50s to make the set look pretty - ahhhhh!
Brian Cuff said
Is that so, Mike? What's the code - two for Frame output and one for RF stages? That would be useful info to have!
From what I recall, that little snippet of information was contained in a Practical Television article on the B18T/LV20 by LL-J and it suggested that the ones with the two black stripes had higher emission and thus were most suitable for the demands of the frame output stage.
As for the origin of the information, I know not; that may need a trawl through early post-war Practical Wireless/Radio Constructor/Wireless World mags.
Most EF50s I've seen have no stripes at all on the top.
You might find these links useful:
http://www.dos4ever.com/EF50/EF50.html
http://www.vintage-radio.net/forum/showthread.php?t=68750 (see post #15)
Cathovisor said
From what I recall, that little snippet of information was contained in a Practical Television article on the B18T/LV20 by LL-J and it suggested that the ones with the two black stripes had higher emission and thus were most suitable for the demands of the frame output stage.
Hi Brian & Catho.
Just checked inside my Pye B18T and that has 'two black stripe' EF50's in the frame output but the rest are unmarked which would seem to back up Catho's point.
Hope that helps.
Marc.
Marc
BVWS member
RSGB call sign 2E0VTN
Hi Steve, I'm not so sure about neatness!
As I mentioned, I bought my unpunched chassis off ebay and they are well made except that there is (was) a huge fillet down the outside of every corner so they had to be filed flush where the chassis are going to be bolted together. A mistake I have also made is to be following the blueprint almost slavishly (except for the EHT) and I should not be doing that, especially in the main PSU chassis as my Xfmr and choke are larger than specified and a different layout would have been beneficial - perhaps I'll have another chassis made (with holes punched this time!).
I notice that there are no sound traps in your Vision RX, Steve. Is this because you found them unnecessary? Perhaps its bandwidth is too narrow to get down to 41.5MHz. I have made one of the two traps but it was very fiddly to do and there doesn't seem to be too much room to install it!
However, I shall soldier on with the project as I am now hooked, even to the cost of my 702 which will follow shortly.
I love the magnifier - it looks really eager to magnify if you know what I means!
Here's the part of the vision receiver with the sound traps - is it different to your cct:
The traps are wound on 1/4" formers, not 3/8" ones as the main tuning coils are and have 2 caps associated with them. These assemblies are supposed to be self-supporting but I couldn't get it to work that way. I had to use a tag ring but it's not ideal. If I can do without them, they will go.
I guess that as I have made one trap assembly, I might as well include it to see if it really works (or needs to). I will be finishing the vision RX today as most of the wiring is now done and I'm into the "more difficult than it looks" task of fitting the 5 inter-valve screens. Fortunately, the company which made them made them very accurately so they fit very well - it's just getting them in the right place, square and getting all 8 holes to line up!
I've been a little quiet of late (Hurray, I hear some cry) but what with tiredness, GP and hospital trips, I have slowed a bit but the last couple of days have seen some progress.
I have now completed and wired the CRT chassis which carries the EHT supply and applied power and, to my amazement, was greeted by an adjustable green spot. Both focus and Brightness seem to work but the shifts are a bit feeble - investigation required.
The timebase chassis was then bolted to the CRT chassis and the deflection plates connected. This time, it half worked - a splendid frame scan with plenty of height but little width - more investigation required!
The pictures show the progress so far:
Next is the PSU chassis. I'm running the TB chassis in the tests using the bench PSU but the Argus PSU chassis is finished so will be tested and connected to the TB chassis. Then comes the RF chassis, always the most troublesome of the lot! I have just received the 5 EF50s which I bought off ebay so I can get on with the Vision chassis. The sound chassis is incredibly deaf so I really do need to look at the tuning coils - I have plenty of Aladin formers so swapping coils around is possible.
All looking good, Brian
Somehow, reading this I really do get transported back to the 1950s in my mind and get a feeling of excitement from all this unfolding; it must have been like this over sixty years ago - your very first television.
