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

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.

Brian Cuff said
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.   

Good for you Brian, that's the spirit.

Our 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. 

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.

I had a visit from a physiotherapist today to try and get me a bit motivated and she suggested a routine which may help!

I 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!

Well. 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!

Next 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) .

I think you are doing fantastic Brian, I enjoy reading updates to the project, no rush you will get it sorted.

Frank

Thanks Frank. Encouragement really helps one get down to it!

Here is the ACAD drawing (via Windows Metafile so not such good quality) for the fillet for the castors. It should be very easy to make using my relatively new vertical bandsaw. I bought it about 1 year ago and I must say, it is a most useful tool. I have a decent circular saw but its uses are limited. I could use it for this project but even with a fine-toothed TCT blade, the band saw has the edge on cleaness of cut.

That should be quite easy to make!

Hi Brian,   I'm going to make some sort of MSP4 substitute, possibly employing the Mazda SP41.  The SP41 differs considerably from the MSP4, for starters the control grid is the top cap in the Mazda whereas it's one of the base pins in the MSP4. Also the SP41 has a much high slope, a remarkable 8.4mAV compared with 3.3mA/V in the MSP4.  When it comes to replacing the final RF amplifier valve which is the MSP41, will the SP41 have a sufficient grid base to handle the signal present at G1?

The SP41 is still plentiful and cheap, the MSP4 is now difficult to find and commands high prices.

Till Eulenspiegel.

Brian Cuff said
Thanks Frank. Encouragement really helps one get down to it!

Here is the ACAD drawing (via Windows Metafile so not such good quality) for the fillet for the castors.

I don't use AutoCAD, but use DraftSight 2017 instead which is alarmingly like AutoCAD (but free!) and is published by Dassault Systemes, the makers of SolidWorks: the CAD tool used by a certain group of people building a large Gresley locomotive at the moment

That has the ability to export a drawing directly as a PNG file?

Till Eulenspiegel said
Hi Brian,   I'm going to make some sort of MSP4 substitute, possibly employing the Mazda SP41.  The SP41 differs considerably from the MSP4, for starters the control grid is the top cap in the Mazda whereas it's one of the base pins in the MSP4. Also the SP41 has a much high slope, a remarkable 8.4mAV compared with 3.3mA/V in the MSP4.  When it comes to replacing the final RF amplifier valve which is the MSP41, will the SP41 have a sufficient grid base to handle the signal present at G1?

The SP41 is still plentiful and cheap, the MSP4 is now difficult to find and commands high prices.

Till Eulenspiegel.

                                   

I'm not sure what relevance your post has to do with Brian's fitting castors to his 702, but...I think without major modifications, you will be fighting a rearguard action against instability everywhere.

Some time ago now, a former colleague modified his guitar amplifier by dumping the original op-amps and fitting modern, hi-spec ones. It then came to me with a comment that there was a funny distortion on sustained notes. That was the amplifier oscillating wildly at about 60+ kHz. The PCB layout and circuit was fine with low gain-bandwidth amplifiers; not so with the newer ones. Similar things occurred when people tried subbing out EF80s with EF184s and the like.

I might consider fitting castors to my Marconi 702 and the HMV 901.  It's not so much the weight of the sets for me but it can't be doing cabinets any good by dragging them about.

The MSP4 to SP41 conversion will be a plug in affair, perish the thought of permanently fitting a Mazda valve in an EMI product.

Till Eulenspiegel. 

Cathovisor said

I don't use AutoCAD, but use DraftSight 2017 instead which is alarmingly like AutoCAD (but free!) and is published by Dassault Systemes, the makers of SolidWorks: the CAD tool used by a certain group of people building a large Gresley locomotive at the moment
That has the ability to export a drawing directly as a PNG file?  

Hi Cathy. I use ACAD as I inherited it from my company when I sold it. I've been using it in the 2D mode all from about 1972 so it is a case of old dogs and new tricks. However, I have bought a beginners' 3D printer and need to get into 3D drawing but my ACAD is the "light" version" so I will need to learn something else as the scanner and editing software with the printer is hopeless!

Till Eulenspiegel said
The MSP4 to SP41 conversion will be a plug in affair, perish the thought of permanently fitting a Mazda valve in an EMI product.  

I suppose it would be possible to add damping Rs in the module to reduce the gain of the SP41s. Perhaps a deeper look into the available, not appreciated, valves with the same orientation would yield some more suitable types!

Brian Cuff said

Hi Cathy. I use ACAD as I inherited it from my company when I sold it. I've been using it in the 2D mode all from about 1972 so it is a case of old dogs and new tricks. However, I have bought a beginners' 3D printer and need to get into 3D drawing but my ACAD is the "light" version" so I will need to learn something else as the scanner and editing software with the printer is hopeless!  

Well....

http://www.solidworks.co.uk/sw/products/3d-cad/solidworks-standard.htm

Thanks for the info, Cathy. I will look into that pretty soon. Meanwhile, I have made the four fillets for the castors to bolt onto - pictures to follow - and the job has turned out simpler than I thought! I now have to cut some 22mm glue blocks, bolt the castors onto the fillets and fix into place. The PSU chassis looks as though it will not cause a problem - I hope.

Once the castors are in position, then I will clean and spray the inside of the cabinet which is, at the moment, pretty dirty at the bottom - the information labels have been printed and will replace the damaged ones.

A bit more progress today! The four castor fillets have been completed except for spraying them black. I took a few pictures during the process which are reproduced here.

In the middle of last year, I bought my first vertical band-saw. I have a horizontal one for cutting metal stock but never ventured into wood-working examples - WHY NOT??? It is so useful for cutting shapes as well as straight edges. Needless to say, the band-saw was used to cut the fillets. I do recommend  the purchase of such a beast.

Here are the pictures:

     The blanks, one marked out for the castor plate.

     The castor holes drilled and a transfer punch used to transfer the hole positions.

    The four fillets ready for spraying black.

I pictured the transfer punch as it is really a metalworking tool but is very useful in woodwork and I'm not sure how many forum members would know about them. They are very cheap in sets of about 20 different sizes, imperial and metric and can save a huge amount of time. However, it is very easy to ruin a piece of work by allowing relative movement between the pieces os make sure that they are clamped - the picture was stages, honest .

All fine work, Mr. C

Band saws, to me, fall into the same category as routers; superb power tools that are a fast track to A&E with interesting injuries Oddly enough, a router is on my list of power tools to buy in order to sort out a motor-tuned Ekco radiogram I own.

Well done Brian

I'm afraid woodwork is not my forte (Neither is metalwork).

Thanks Andrew.

At last the castors are on the cabinet. I haven't tried them yet as my supper arrived too early - not toooo early!! - so I couldn't get Sarah to helpme invert the cabined. Notice I have used slotted screws, not posidrives because I hate to see them on early sets - I know that Pye used them for their self-tappers but not too many manufacturers did and certainly not EMI. One idea that has crossed my mind is to add a small rectangle of 5mm ply between the truncated corner of the fillet and the base of the cabinet. This, in effect, changes the main force from a tension between the glue blocks and the fillet to a compression force and as the weight of the set produces compression, this would be less stress for the cabinet - would it??

Here's how it looks. I will go over it with a small brush and some matt black paint to mask the slight damage imparted by me during assembly.

A splendid job Brian, they are out of view when upright and provide a valuable method of moving the set around.

Yes Andrew, these pre-war sets are quite heavy (this is probably the lightest of the mirror lids) and the simple chrome carpet glides are no good on carpet with any pile at all so castors is the way to go, especially when one is a weak as I am (always have been ).