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Old 15th Nov 2016, 4:13 pm   #141
brianc
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Default Re: HMV 905 Project

Excellent work, Andy. A real labour of love.
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Old 15th Nov 2016, 4:19 pm   #142
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Default Re: HMV 905 Project

Quote:
Originally Posted by beery View Post
this is the first of a two part update about tube mounting.
I think the scan coils need realignment on the third and 4th pics

I'll get my coat......
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Old 16th Nov 2016, 12:24 am   #143
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Default Re: HMV 905 Project

Hi Courtney,
I see what you mean, the cross hatch is wonky


cheers.
Andy
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Old 15th Dec 2016, 12:12 am   #144
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Hi all,

I'm still working on the set as and when I get time.

I spent some time trying to solve a problem with one of the sound IF coils that would not quite peak. This seemed to be due to a compression trimmer. Actually there are two in parallel, giving a range of 100 to 470pF. I took the trimmers apart and cleaned them, which not easy as there are several plates joined at the solder tags. I replaced damaged mica insulators where nessesary.
This did not really make any difference, so in the end I decided to add a 75pF mica cap in parallel. I could have saved myself a lot of time if I had opted for that straight away!

I also had to make up the terminal board arrangement for the sound output transformer. The original arrangement was simply made by soldering brackets for the terminal board on to a standard output transformer U clamp.
I had a rusty example from a 709. I cleaned up the terminals and re-soldered one of the brackets that had come off. However I decided not to use this transformer, partly because another forum member wants it for their 709 and partly because the rusty frame looks a bit out of place on my set.

Using an output transformer from an HMV radio, I first scratched off the plaiting on one side ready for soldering. I added solder and then applied some flux on top.
I then clamped the transformer in a work bench with a new tin plated bracket sandwiched between a couple of spacers and a block of wood.
I heated the bracket up with a 100W iron until the solder under the bracket melted and the bracket could be pushed home. After cooling I repeated the process on the other side.
I then added the terminal board which I had copied using new Tufnol and terminals from an HMV mains transformer. Only the centre two terminals used in the 905.
It all looks good except for the lack of terminal numbers. Some white letraset would have been good, but it turns out to be unavailable these days.

That's all for now.
Has anyone else had trouble with the sound IF coils on this set? One of the other sound IF coils only just goes past the peak with its trimmers fully tightened...

Cheers
Andy
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Old 15th Dec 2016, 10:18 am   #145
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Default Re: HMV 905 Project

Quote:
Originally Posted by beery View Post
Has anyone else had trouble with the sound IF coils on this set? One of the other sound IF coils only just goes past the peak with its trimmers fully tightened...
Hi Andy,

I'm not sure if it's relevant but both Panrock and I had sound IFs in 901/702s that had shorted turns and needed to be rewound.

Peter
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Old 15th Dec 2016, 12:47 pm   #146
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Default Re: HMV 905 Project

I had similar problems with the IF transformers in a Ekco AD65 which use similar trimmers. Dismantling and cleaning them made no difference, so like you I added a fixed parallel capacitor. That did the trick.
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Old 15th Dec 2016, 1:19 pm   #147
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Default Re: HMV 905 Project

Andy,

I had a similar problem in my 904. After I restored the set all was working well, then suddenly the sound went dead. On investigation I found that TC18 (across coil 25) of the sound takeoff coil was shorted out. So I investigated whether it was the trimmer itself or the coil that had shorted. After unscrewing the compression trimmer the short vanished, so I disassembled the trimmer just like you did, and replaced some of the more dodgy looking mica insulators with those from a donor trimmer, but not all of them. After that it failed again after a while, leaving me wondering about the coil. So I replaced all the mica and its been ok since. But I still wonder about the coil. By the way when TC18 was in correct adjustment, I disconnected it and measured its capacitance at 489pF.
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Old 23rd Jan 2017, 1:13 am   #148
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Default Re: HMV 905 Project

Hi all,

I've been so busy with the little one that I've made very slow progress on the 905...

Thanks for your comments about the trimmer capacitors. Thanks to Argus25 for the capacitance figure, it would seem about the same as the figure I need to bring the sound IF coil in tune, so that should mean the coil is ok.

A forum member pointed me to some white number transfers on ebay. These were old stock, indeed slightly used. The font style and size mached the HMV markings, so I was able to reproduce the terminal numbers on the sound output transformer

So back to the scan coils...
The frame coils were fitted with leadout wires and bound up with flat cotton hem tape. The cotton tape is non-adhesive so it allows for the coils to be formed into shape.
The frame coils were then bound to a curved former and electrical insulating varninsh was applied to help them keep their shape (the former had been wrapped with polythene to stop the varnish sticking to it). After drying out, the coils were removed from the former and given a liberal coating of varnish.

The line coils were marked up and folded upwards at a point which was 20mm from the centre line.
They were then fitted to the scan coil former which is currently attached to a wooden mandril.

That's all for now. I can't wait to get the frame coils on and wind the iron wire core on top. Once I have done that I can compare the inductance figures with the original coils.

Cheers
Andy
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Old 23rd Jan 2017, 8:43 am   #149
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Default Re: HMV 905 Project

Those coils are starting to look really great. Excellent work.
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Old 14th Feb 2017, 1:07 am   #150
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Hi folks,
Time for a scan coil update.
I have had partial success and I'm not quite sure what to do to get things working correctly...

The first photo shows the start of the core winding of iron wire.

The second photo shows the end of the core winding.

The third photo shows the new coils together with an original set.

The fourth photo shows the test wiring in place.

The fifth photo shows the resulting picture.

As you can see the result is a fairly linear picture of the right dimentions (ok, its the wrong way round at the moment). However, the edges are out of focus and I'm not sure why.
What I do know that the core on the original was wound in a slightly different way which made the core start further out from the coils, but it did make it more circular.
Rewinding the core may be the answer, but I'm not sure now how to fix the coils back on to the wooden former as the original fixing tape has been cut (the ends of the tape are buried under the windings).

Any thoughts on the results?

Cheers
Andy
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Old 15th Feb 2017, 8:22 am   #151
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Default Re: HMV 905 Project

Quote:
Originally Posted by beery View Post
As you can see the result is a fairly linear picture of the right dimensions (ok, its the wrong way round at the moment). However, the edges are out of focus and I'm not sure why.
Hi Andy, looking at the frame linearity it is good. On the line scan there is an initial stretch of the scan then a sudden compression of the linearity at one point after the start of scan (before the circle is generated) and then a stretching at the last 1/4 or 1/5 of the scan. Even with that though, there is no gross barrel or pincushion or trapezium error.

With these CRT's the beam is often not in focus simultaneously at the edges and screen center and the focus is a compromise because of the longer path length of the beam to the face plate edge (even though its curvature helps) With a smaller raster size it would probably be much better.

It would be interesting to look at the yoke current with a very small current sense resistor in series with the H coils (less than one ohm or so) and an isolated scope.

It is not obvious to me right away either how the H (or V) scan currents and magnetic fields, or their linearity or quadrature could dynamically upset the beam focus. But if this effect is not there on the standard yoke, then it must be the case. Is this new yoke closer to the focus coil than the original yoke ?

Can you take a photo with the standard yoke (all other settings width/height equal and focused for best at screen center) for a comparison.

Despite any issues to be resolved, I think you have done a fantastic job on this replica yoke.

......Ok, I've just thought of how this might happen. If the iron wire you used was magnetized, this could cause trouble with beam deflection affecting the focus because of the field inside the "wire cylinder". Suggest remove the yoke and treat the iron wire as it is on the yoke with a color TV degausing wand & try again.
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Old 15th Feb 2017, 12:26 pm   #152
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Default Re: HMV 905 Project

What a perfect job!

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Old 16th Feb 2017, 11:35 pm   #153
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Hi all,
Just a quick update.

I wanted to do a quick check of the range of the line linearity control. However it is very difficult to adjust from the front of the chassis, so I have fitted a small plastic knob on it to for the moment.

The linearity control was able to remove the resonance at the start of the line scan. It has a range that at one end shows unwanted resonance and at the other end causes a fold over. So I am at least happy with that.

Argus25. The scan coil position the same as with the original coils, however the mounting brackets have not been fitted yet, so actually I can move the focus coil relative to the scan coils and it does not make much difference to the focus at the edge of the scan.
You might notice that the picture is now sharper on the right, in fact this was acheived by reversing the connections to the focus coil.
I think you might be on to something with regard to unwanted magnetism in the core. I don't have a deguaser, so I might have to make something up.

That's all for now.

I hope to look into the problem more at the weekend.

Cheers
Andy
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Old 17th Feb 2017, 3:55 am   #154
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Andy,

The line scan linearity is much better now. Hopefully once the iron wire core is demagnetized the focus problem will be solved too.There are very cheap degaussing wands on ebay.

If you look closely at the vertical scan, you will see that there is very slight compression (crowding) of the lines at the top of the scan. This is actually a design issue with the 904/905. They did not insert a customary peaking resistor in series with the integrating capacitor in the drive to the vertical output stage. This adversely affects the linearity just after vertical flyback.

The fix for this is very simple, just adding a 4k7 resistor in series with C73, see figure 1 of this article:

http://worldphaco.com/uploads/CONVER...A_5FP4_CRT.pdf
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Old 17th Feb 2017, 1:59 pm   #155
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Andy,
Actually ignore my last post above, your vertical scan is good at the top, there is no need to fit the 4k7 peaking resistor. It was needed in my set because it required higher vertical drive to deflect the 5FP4, but obviously it is fine with your CRT wich is more like the emiscope tube.
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Old 31st Mar 2017, 6:12 pm   #156
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Default Re: HMV 905 Project

Pictures of the back cover with particular regard to the CRT neck protection.
The CRT protection "hat" is exactly 3 1/2 inches high and 3 5/8 inches deep.
There is a stiffening bar on the reverse side of the back cover. The dimensions are 16 X 1". Made from 2.6mm bar. It is attached to the card back by four rivets.

DFWB.
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Old 1st Apr 2017, 12:21 am   #157
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Thanks for that David.
I didn't know about the stiffening bar. I think I will have to fit that later, the dome is more urgent...

Cheers
Andy
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Old 2nd Apr 2017, 8:29 am   #158
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Hi Andy,
Great work so far, what happens if you rotate the coils 180deg and reverse the vertical coil connections, just to see what happens to the de-focusing, I imagine if the iron wire core is magnetised that the poor focus will change sides.
Victor
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Old 3rd Apr 2017, 11:04 pm   #159
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Hi Victor,

I did try degausing the scan and focus coils, but it made no difference.
Connecting the focus coil the other way round helped a tiny bit.

I have now removed the core in order to re-wind it slightly differently. However a house move is just a month away, so lets see if I get the chance to try it before I have to pack it all away...

Cheers
Andy
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Old 17th Apr 2017, 1:28 am   #160
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Hi All,

With the house move just two weeks away, I did get to try the other way of winding the core wire.
Focus wise, there was not much difference, but the deflection is now insufficient to fill the mask. The first picture shows the result.
I am considering making a larger centre for the line coil former and winding the line coils again to the coils to be folded away from the CRT more at the ends, but this will have to wait until we are established in our new home.

I have made the scan coil wrapper. Suitable brown 1mm card was found as part of a 'Do Not Bend' envelope. The letters and numbers were punched using a Maplin automatic punch set. The holes for the eyelets were punched with a wad punch. The double solder tags were then riveted in place using 2mm eylets. Strangely, eylet sizes refer to their internal diameter rather than their external diameter, the 2mm type having a 2.5mm external diameter. Some special eylets with a 3mm external diameter were used to rivet the mounting brackets.
The second photo shows the scan coil wrapper, compare it with post 65 on page 4 from Brian Cuff.
The third photo shows the wrapper in place on the coils. The scan coil lead out wires are kept long as I am still experimenting.

As the move is imminent, I have decided to put the chassis back in the cabinet for the first time since I removed it for restoration. The CRT has been left out, partly because the scan coils are not right and partly because I have not yet remade the missing bulge on the cabinet back. I have however blocked up the hole in the back where the CRT neck would normally protrude, with a piece of card held in place with paper fasteners. This is to guard against the temptation to use the hole as a carry handle! Anyway, the chassis with its newly painted scale and waveband pointers looks great. The radio also sounds really good, being tested with the original speaker for the first time. I did cheat when realigning it, so that I could get 'Gold' which would normally be just off the MW scale on this set. I have it set to 'Gold' whilst I am packing for the move.
The fourth photo shows the cabinet with the chassis in it.

That's all for the next few weeks...

Cheers
Andy
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