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Old 28th Aug 2016, 12:55 pm   #121
beery
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Default Re: HMV 905 Project

Hi All,

I had got to the point with the set where I had to tackle the problem of the missing scan coils.

I used a set of coils borrowed from Brian Cuff to copy. Without dismantling the coils I was able to measure the outer and inner circumference of the line coils by carefully passing a piece of string through the assembly to measure the length of one of the windings and then add to that the width of the winding which is the visible part at the end of the scan coils.
By working out the shape the coils would be if they were flattened, I was able to design a coil former.

I was also able to measure the diameter of the wire used for the scan coils which is 32SWG or 0.28mm.

0.28 mm wire has a resistance of 0.278 ohms/metre. By using the average of the inner outer circumferences, I was able to estimate that 91 turns would give the correct resistance of 5 ohms for each line coil (the figure quoted in the manual). I wound a prototype coil and it was spot on 5 ohms, however as this is an early set, I figured that a round figure would have been used. The resistance of the coils from Brian's set was actually about 5.7 ohms, which actually equated to 100 turns and this is the number of turns I used on the two final line coils that I have wound for the set.

The first photo shows a line coil being wound.
In the second photo cords have been tied around the former to allaow the coil to keep its shape whilst the coil former is removed.
The third photo shows one half of the coil former removed and the fourth photo shows the line coil ready to be bent into shape.

So that is the line coils. The frame coils are a whole world of pain though...

Cheers
Andy
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Old 28th Aug 2016, 1:29 pm   #122
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Default Re: HMV 905 Project

Hi all,

Here is an initial attempt at a frame coil...

As described in my last post, it was quite easy to measure the circumference of the line coils and for the frame coils it was just the same. However, the frame coils are bound up in varnished tape with spliced on leadout wires. So I can't measure the diameter of the wire used and I'm not about to dismantle a working set of coils from someone else's set.

So I went with a hunch. I know that EMI used 45 SGW (0.071mm) wire for the EHT windings in the transformers of their pre-war sets, so I figured that it would be a good place to start.
To get the required 2500 ohms, I worked out that 3022 turns would be needed. I rounded it down to 3000 turns and attempted to wind a prototype coil. I found that I had to disable most of the tensioning gear on the coil winder to prevent the wire from snapping.
After winding the coil I measured it as being 2470 ohms, which is almost identical to the original set of coils.
I found though that I could not easily remove the winding from the coil former. I've now modified the former so that the inner part can be collapsed to enable the winding to be removed without damage.

I tried the prototype line and frame coils in the new scan coil assembly and they seem to fit together ok.

I will have to try winding another coil with the modified former and this time lace the string instead of tying individual knots to see how it comes out.

The first photo shows the rather messy prototype frame coil.
The second photo shows the test fitting of line and frame coils.
The third photo shows the modified coil former, with a gap in the middle.

Cheers
Andy
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Old 29th Aug 2016, 8:30 am   #123
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Default Re: HMV 905 Project

This is fascinating, Andy. I can see similarities between the methods you have used and the articles on the Wireless World TV construction. I can only imagine the problems with winding 42SWG wire with such a complicated former and the work that needs to be done afterwards to form the coils to the correct shape and fit. Many, many congratulations on the job so far. - I must get the package off to you with the eyelet tags and wrapper material.
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Old 29th Aug 2016, 11:29 am   #124
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Default Re: HMV 905 Project

This must rank as one of the most difficult projects ever described on this forum. Really impressive craftsmanship.

Peter
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Old 29th Aug 2016, 2:39 pm   #125
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Default Re: HMV 905 Project

With your second frame attempt. I would try tying it together with cotton thread as thin as you can get and as soon as you get it out of the former, lace it with the same thread. with the knots close together but not too tight or you'll loose flexibility. With the lacing, the coil should stay stable as it can be imagined that if the windings are too loose, then the geometry etc. could change with vibration, heat etc. A fantastic job so far!
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Old 23rd Sep 2016, 10:46 pm   #126
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Default Re: HMV 905 Project

Hi all,

I have now wound two pairs of frame coils and have laced them as Brian suggested. Linen seems to be the best cord for the job.
I need to make more line coils before I get round to folding them to shape.

Whilst waiting for extra winding wire I have been working on the speaker grill.

When I obtained the set it came with two matching expanded aluminium speaker grills (see pictures on page one). I endeavoured to obtain the correct HMV grill, but had seemingly drawn a blank.
However in the course of obtaining parts I had broken up a very poor Marconi 857 radio (it actually fell to bits). Despite the set being very water damaged, the grill still looked ok.

The grill is woven together from brass strips and lacquered to give a bronze effect. The Marconi and HMV grills have different vertical stripes.
I dismantled the grill completely and each brass strip cleaned with T cut to remove dirt and corrosion whilst trying to preserve the bronze finish.

I rebuilt the grill to the HMV pattern with the worst parts at the top and bottom edges where they would not be seen. I then had to cut 4 rows off it to make it fit and so further rearranged the worst parts to give the best appearance.

The grill was now complete and although it was just wider than the aperture, it was very close, so it would need more felt to hold it securely.

The old 1/4" felt on the baffle board was removed and replaced. The horizontal strips were 3/8" wide as before, but the vertical pieces were enlarged to provide more support at the grill edges.
The grill was then secured with four 16mm cut tacks. The 1/8" felt around the inside of the speaker aperture in the cabinet was also replaced.

Before re-assembly, new wires were attached and the missing speaker selector socket was replaced with one donated by Brian Cuff.

It looks much better now and the speaker works perfectly.
Now back to the scan coils...

Cheers
Andy Beer
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Old 24th Sep 2016, 9:20 am   #127
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Default Re: HMV 905 Project

It really is coming along nicely, Andy. I do like the trouble you took with the speaker mesh! Lesser mortals wouldn't have noticed the difference between the two EMI camps and would have accepted the grille as it was. I've never tried unpicking one of those - was it a pain or did it go reasonably well?
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Old 24th Sep 2016, 9:30 am   #128
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Default Re: HMV 905 Project

Quote:
Originally Posted by brianc View Post
I do like the trouble you took with the speaker mesh!
Me too!

Peter
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Old 24th Sep 2016, 10:42 am   #129
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Hi Peter and Brian,

Thanks for your kind comments.

I forgot to take a photo of the grill going back together, but I have one here of it coming apart which is the reverse of reassembly.

The horizontals have wavy bends presseed into them and the verticals have even finer wavy bends. This means that the whole lot lock together and stay square.
It was not possible to slide the verticals along to get the correct pattern, so the whole thing had to be taken apart by removing each horizontal strip one by one.
A strip would be removed by prising it from its neighbour by using a plastic pry tool (the sort used to open remote controls and mobile phones etc.) Then the strip could be walked off by pushing it to the top in a zig-zag fashion.

There are two variations of vertical strips, being laquered on one side or the other depending on where in the weave it fits.
The horizontal strips are identical and can simply be turned round to fit in odd or even positions.

Reasemmbly was very easy. I started with two horizontals at the bottom to hold all the verticals in the right place. Then horizontal strips were slid in from the top, one at a time. When a strip reached its place next to its neighbour, it went into place with a reassuring click.

The hardest part was cleaning it without totally destroying the laquer.
When I initially tried reassembling the grill there were tell tale marks from where the original verticals used to be.
By stroking each strip 4 times through a cloth soaked in T cut I was able to clean them up nicely. On some of the strips the laquer did come off and on the most corroded ones it came off completely, leaving me with shiney brass However about 95% of strips turned out perfectly with shiney laquer and no tell tale marks, so I had enough good parts in the end to use for the visible part of the grill

Cheers
Andy
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Old 24th Sep 2016, 10:49 am   #130
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That differentiates the bodgers (like me) from the true restorers!
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Old 24th Sep 2016, 11:20 am   #131
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Brilliant work Andy!

As far as the grille is concerned, I salute your efforts to keep the finish original. An alternative method might have been to strip the finish off them, reassemble, then lightly respray with a walnut toner followed by a clear lacquer (keeping one of the originals unstripped for colour density matching). As far as I can tell, this is what was done originally.

Steve
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Old 24th Sep 2016, 11:25 pm   #132
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Default Re: HMV 905 Project

Hi Andy,
You do realise that this is a totally bonkers restoration?? Good grief, winding your own scan coils- I wouldn't have been surprised if you had just made a new CRT using some sand and a small blast furnace fashioned from the kitchen cooker
If I had one, I would take my hat off to you for sheer determination to overcome every new hurdle in the most authentic way. Such an amazing amount of effort for a tiny 5" picture!! Truly well done and many thanks for the entertaining write up
cheers
Nick
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Old 23rd Oct 2016, 2:49 am   #133
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Dear Andy,

I have only recently found this thread, great to read it and see someone like yourself go down this road, that I have traveled down myself. I previously fully restored an HMV904 down here in Australia. I fitted it with a 5FP4, there is a brief article on it here:

http://worldphaco.com/uploads/HMV__904_ARTICLE.pdf

On that website is also an article on how to fit a 5FP4 to the 904.

I see you got that great Russian CRT for yours, that was a very good move. Again interesting to see and be reminded of the technical challenges we were both faced with on a project like this and how we solved them, terrific work on your set, well done.

Best Regards,
Hugo.
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Old 23rd Oct 2016, 10:21 pm   #134
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Also there was a thing I forgot to mention (and it's not in the restoration article or anywhere in the HMV literature either) about the brass tuning slugs that decrease the inductance of the coils they tune. One feature of them is that they result in a significant capacitance to ground and it is important that the the slug is properly earthed, or the tuning can jump around with with an intermittent connection in their threads to the bush that mounts them to the chassis. Unfortunately, hampering a good connection here, the originals were sealed with a wax like substance which tends to insulate the threads. To get around this problem I cleaned off every trace of the old wax, polished the threads and when I'd finished tuning the set up, I sealed them with electrically conductive silver based paint, like the sort used to repair PCB's. It is a tiny detail, but if an intermittent fault developed due to this issue it could be extremely difficult to find and could present as intermittently degraded picture resolution.
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Old 25th Oct 2016, 1:03 pm   #135
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Default Re: HMV 905 Project

Excellent progress Andy,
you have spurred me on to make a new deflection yoke for my replica HMV 904. What are you going to use as the outer magnetic core for the final assy or will it be air cored? In the past I have employed soft iron tape or S.I. 22SWG wire. I have always assembled my deflection yokes so that I could adjust their quadrature spot on. Perhaps if one is as careful as you are in assembly it will not be required. At present I am using a transformer to match low Z coils, not original but it meant I could complete the project and correct the error later. Good luck with a wonderful restoration.
Victor.
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Old 30th Oct 2016, 11:40 pm   #136
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Hi Hugo and Victor.
Thanks for your encouraging posts.
I am still working on the set, but only very occasionally at the moment.
This is partly due to me starting a new job with a long commute, but mainly because little Beer is due in only a few days and I am rushing to finish building the baby changing station as well as renovating my original cot...

I've installed the replacement screen glass in the cabinet and had used Rustins scratch remover on the cabinet front and sides. The scratches on the top are however too big to cover with wax, so I will leave them until I work out what to do with them.
I hope to post photos in a few days.

Hugo. Thanks for the info on the coils. The actual slug is insulated from the brass thread to try to minimise the capacitve effect, but clearly then it did not work that well!

Victor. I need to check the original Iron wire diameter. Basically the core was 4 layers of soft black iron wire. It was only a 1" wide core winding.
The coil alignment is referenced by using four pieces of paxolin glued to a former made of varnished paper. This is the same as in the original coils, but I'm actually using a scan coil former obtained from a scrap GEC BT2147.
The frame coils are very inefficient, with most of the parts of the winding being outside of the core area. I figured that they were not worried about the efficiency since the beam to be deflected was only at 2.5KV.

Anyway that's all for now. I hope to update soon.

Cheers
Andy
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Old 9th Nov 2016, 9:43 pm   #137
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Andy,

I have been researching yoke alternatives for the 904/905 and have also calculated a wire size for high resistance vertical yokes:

Thinking of the concept of modifying a fixed volume coil (with a fixed cross sectional area and average turn length) to another coil with a different resistance, but same Ampere-turns (A-T):
Since the number of turns and the length possible are proportional to the inverse of the square of the wire diameter, and the resistance of copper is proportional to the length and inversely proportional to the square of the wire diameter then it turns out that the resistance is proportional to the inverse of the diameter to the power of 4.

The implication of this is that if one wants to replace some winding of resistance R1 create a new winding with a different resistance R2 for a fixed volume coil and identical A-T's, then the wire diameter must be altered to be equal to the fourth root of the resistance ratio.

With this in mind I decided to look at the classic American 50 degree deflection yokes, such as those used in American sets such as the RCA 621TS and 630TS. These have line coils (each saddle) practically identical to a 905's yoke which are close to 5 ohms each and around 8mH total, however the frame coils have a resistance of around 31 ohms each or 62 ohms total (and 50mH). For the vertical coils, they are used with a 10:1 matching transformer (Butt stacked to prevent core saturation and with a 590R primary and a 7R secondary and presumably the lowest leakage inductance that the manufacturers could get), and during active scan time at least, this arrangement, presents about a 6200 Ohm dominantly resistive load to the plate of the vertical output tube which was typically a triode connected pentode or a triode in the 621 and 630 TS sets.

The wire on these American yokes on the vertical coils measures around 0.255 mm diameter. The fourth root of 5000/62 is about 3. So this means, to rewind this type of yoke, while preserving the same A-T to a 5K yoke (2.5k per saddle); the new wire size would be 0.255mm/3 = close to 0.085mm, or about 44swg wire.

You found that 45 swg was suitable, a good hunch.

The implication of the fact that the two wire sizes are so similar; it therefore appears very likely that the magnetic field intensities within the early post war American 50 degree yokes are very similar if not nearly identical to the 904/905. This means that one of these American yokes with a 10:1 matching transformer (like the type originally used for it) for the vertical coils would work fine in the 904, and this could be an option to get a 904/905 working again if the original yoke failed and a person was unable to rewind the yoke with the fine wire or create a yoke free from deflection distortion.

Your coils look great, when they are done can you give the exact dimensions/details of the former system you ended up with ?

Hugo
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Old 14th Nov 2016, 2:00 am   #138
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Hi all,
this is the first of a two part update about tube mounting.

Hi to Hugo as well.
Interesting calculations for the frame coils. 45 SWG was a guestimation based on the fact that EMI used it for their EHT transformers. 44 SWG might work, but if the wire is much larger there won't be enough space for it. The coils must be very inefficient since 2/3 of the winding is going round the tube neck. It is because of the amount of wire in the winding that it proved impractical to shape it more accurately, however EMI got away with it, but of course the line coils had to be formed into an efficient shape as there is much less time for the beam to be deflected at line rate.

Anyway, back to tube mounting...

The first picture shows the new mounting block (with captive 2BA nuts), mounted in front of the original one.

The second picture shows how I made absolutely sure I was not going to drill through the top of the cabinet when drilling the pilot holes for the wood screws to hold the new mounting block in place.

The third picture shows the new felt in place for holding the protection glass. The red felt is to help keep the dust out.

The fourth picture shows the tube mask and protection glass in place.

The fifth picture shows a piece of the 'E' profile draught excluded which I fitted inside of the original rubber for mounting the front of the CRT. This was to make up for the fact that the Russian CRT is 1/4" smaller in diameter than the original one.

The next part of the tube mounting exercise is coming right up...

Cheers
Andy
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Old 14th Nov 2016, 2:06 am   #139
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Hi all,

This is part 2 of the tube mounting report...

The first photo shows the tube in place as seen from the rear. The CRT mounting bracket and clamp ar reproductions I made using vice mounted folding blocks.

The second photo shows the front view with the tube in place.

I will have to try fitting the tube again later with all the coils in place.

Cheers
Andy
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Old 14th Nov 2016, 8:12 am   #140
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Default Re: HMV 905 Project

Hi Andy,

Tremendous progress and looking fantastic!

Jac
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