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Old 22nd Oct 2017, 4:30 pm   #21
Panrock
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Default Re: The pre-war picture on the screen

The 5FP4 has been fitted and various modifications carried out as per Argus25's advice in his online article. In my case I also found it necessary to grind out some of the brass from the centre of the focus coil to accommodate the slightly fatter neck of the new tube.

This will never be a sharp picture, but that's "what you got for your 29 guineas". The 1.5 MHz bars are present but the 2 MHz are semi-imaginary. Still, the saving grace here is the tiny picture and it actually looks quite watchable on a programme.
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Old 23rd Oct 2017, 9:22 am   #22
Argus25
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Default Re: The pre-war picture on the screen

Steve,

I found a photo of the C testcard from my 904 of the 5FP4. The 2MHz bars are readily visible at least on the photo, but when I attach things here the photo resolution drops drastically, I think I labelled the 2MHz bar correctly ?

(This result was with the system with the final adjustment after I swept the frequency response and adjusted to get a 2Mhz bandwidth, it would have not been like this, with the manufacturer's method of adjustment).

I agree though, you don't need a lot more than 2 to 2.5MHz video bandwidth for these small CRT's for a good looking picture.

(The vertical deflection linearity looks practically identical on the two images, both would have been more crushed at the top without the added peaking resistor)

The interlace is definitely better on my image than yours, but I wonder why that would be so, maybe check the vertical integrator capacitor C54, but it might be replaced already. If that or R45,R48 were the wrong value, H pulses might get through and upset the interlace.The integrator time constant here looks roughly around 100uS to 200uS.

In American sets like the 1939 KTE-5 they grossly over integrated the V sync with a time constant of about 1000uS that was 10 times longer than the post war values. They never got interlace issues with this but the V sync was very weak.

If the V lock was solid on yours you could try increasing the value of C54, to see if it helps. And check for any way that H sync interference could be getting to the V osc.

Also by the post war period when the time constant for the integrator was about 50uS to 100uS overall, they had gone to a 2 stage RC filter.

(the time constant of a two stage RC filter is R1(C1+C2) + R2C2)

Later to improve interlace, some sets went to a 3 stage filter which does wonders to the smoothness of the separated V sync especially when there are equalizing pulses too.
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Old 23rd Oct 2017, 8:38 pm   #23
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Default Re: The pre-war picture on the screen

I tried increasing C54. This rectified the interlace but upset the frame period mostly outside the range of the vertical hold control. Yes, I know it's possible to compensate for this, but... Not Tonight Claude! Deeper analysis needed.

On top of that, accessibility is the pits... this is a terrible set to work on. The chassis is ultra-heavy, cumbersome, hard to prop up safely and you must never take your eye off the ball with the ever-present threat of bare terminals bearing mains EHT. No wonder they were scrapped in the '50s with a sigh of relief...

There is no gain in hand for use with an Aurora now I have spread the IFs. Any more on the lower sideband and I can see the beginnings of sound-on-vision. Yes, the trap is correctly set up.

On top of that the picture is becoming dim. Tube failing already? Or is this due to the mains voltage, which I can only get up to 230v this evening even with all the boost of my workshop variac?

Nine months of slaving over this set (part time) and all I now get is a piddling little dim picture.

Sometimes it's good to have a rant.

Anyway I give up!

At least until tomorrow.

Steve
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Old 24th Oct 2017, 3:18 pm   #24
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Default Re: The pre-war picture on the screen

Tomorrow has come, and the world looks brighter. So is the picture.

C54 has been increased by .001uF, which has fixed the interlace. The focus coil had moved back along the tube and the mains voltage was low. These caused the dimmer picture. The focus coil needs to be as far forward and close to the scan coils as possible with this different tube. Of course the original harness was only designed to suit the 3/1.

It all now goes back in the cabinet.
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Old 25th Oct 2017, 2:30 pm   #25
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Default Re: The pre-war picture on the screen

Quote:
Originally Posted by Panrock View Post
Nine months of slaving over this set (part time) and all I now get is a piddling little dim picture
Hi Steve, sometimes it's like that. The 7" tubes probably look a bit better, but then again the EHT is still only 2.5Kv. The 6" Cossor 54 is even worse as the electrostatic CRTs tend to be gassy and so have poor focus.
The best small screen pre-war pictures I have seen were on an Ekco.

I've been putting off getting the chassis of my HMV 905 on the bench since I moved house 6 months ago. Trying not to break those tag boards that hang below the chassis is annoying. I made a couple of wooden skids so that I could get the chassis on the bench.

Anyway, I'm sure it will be working better than it ever did when it was new with the 5FP4 in it.

Can you take some pictures of the focus coil? You mentioned having to file out the big brass eyelet in the centre to fit the CRT neck. I would guess it is an early focus coil with a 9K resistance. Later sets were fitted with the coil from the 907 which had a 6K winding and quite a large hole in the centre. I've not seen the earlier type.

Cheers
Andy
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Old 25th Oct 2017, 5:07 pm   #26
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Default Re: The pre-war picture on the screen

Hi Andy,

Thanks for your supporting comments. I also just noticed I've been running the set on the 250v tap, which won't have helped either. But I tend to err on the side of cooler running if I can. Especially with everything crammed in as it is - see pic of the situation today attached. Except for fettling and the new label, the cabinet has been left alone for this job.

I greased the wooden bearers in the cabinet before persuading (you couldn't call it sliding) the chassis in!

Yes the focus coil is the 9K type. You might be able to glimpse it in the pic. Does this mean the set is likely to be 1938 rather than 1939?

Knobs go on next and then I switch her on. The last remaining adjustment will probably be the tilt of the picture, tho' I set this up as best I could on the bench.

Steve
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Old 26th Oct 2017, 3:40 pm   #27
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Default Re: The pre-war picture on the screen

Quote:
Originally Posted by beery View Post
Can you take some pictures of the focus coil? You mentioned having to file out the big brass eyelet in the centre to fit the CRT neck. I would guess it is an early focus coil with a 9K resistance. Later sets were fitted with the coil from the 907 which had a 6K winding and quite a large hole in the centre. I've not seen the earlier type.
Andy,

I'm convinced there are two focus coils, like you say. I looked at mine and the internal diameter of it is at least 10mm bigger than the 5FP4's neck, but on Steve's set he had to file it out for it to fit.

One interesting thing about a small CRT, despite the bandwidth of these sets being around 2MHz and its easy to be critical on a testcard image, an actual image looks quite respectable. I have attached a screen photo from my set which originated from a camcorder on still frame, passed via David Grant's 625/405 converter and my RF modulators to the set.

(The darkish grey wide vertical bar about 1/4 the height at the top is an exposure artifact of the photo, which is an un-touched up photo from the screen with an Olympus 3000 digital camera)
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Old 26th Oct 2017, 4:54 pm   #28
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Default Re: The pre-war picture on the screen

Here's a better look at the early-type focus coil. Photo taken prior to restoration during the dismantling phase.
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Old 27th Oct 2017, 12:06 am   #29
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Default Re: The pre-war picture on the screen

Certainly different to the focus coil in my 904. That focus coil looks like it has a lower profile (front to rear) and maybe slightly larger diameter too as well as the much smaller hole for the tube neck.

Is there any mention in the 904 documentation that there were two types ? I couldn't see it in any of the documents I have. Its very interesting, I wonder why they changed it.
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Old 27th Oct 2017, 7:37 am   #30
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Default Re: The pre-war picture on the screen

Quote:
Originally Posted by Argus25 View Post
I wonder why they changed it.
Probably to have more play to make it possible to center the image by positioning the focus coil.

Jac
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Old 27th Oct 2017, 2:45 pm   #31
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Default Re: The pre-war picture on the screen

Today is something of a moving occasion for me. The set is finished!

Other pre-war television restorers will understand the colossal amount of work involved in doing one of these. After months of wrestling with what can seem an uncooperative 'pig', one develops a close bond with the thing. It's not to exaggerate to describe it as a form of love.

I have watched the first programme and the viewing experience actually is quite good! And of course it has that pre-war 'magic' quality of a technology somehow ahead of its time. The picture looks sharp and crisp with the 5FP4 tube, unlike the original soft 3/1.

I've wittered on long enough. My Parkinson's means this will have to be my last ever restoration. Even wiring the plug today was becoming difficult.

I'll try to get round to doing a You-tube of it in operation. Lupino Lane "Doing the Lambeth Walk" was very popular in 1938/9 so maybe it will have to be that on the screen.

Thanks to those who have offered help and encouragement here.

Steve
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Old 27th Oct 2017, 3:08 pm   #32
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Default Re: The pre-war picture on the screen

Hi Steve,

I'm very sorry to hear that Parkinsons is restricting your very impressive capabilities. The write-ups of your restorations have always been interesting and very helpful for me and I'm sure for many other old set owners too. You set standards that many of us can only hope to aspire to.

Thanks and kind regards,

Peter
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Old 27th Oct 2017, 4:15 pm   #33
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Default Re: The pre-war picture on the screen

Hi Steve,
Sorry to hear that the Parkinsons means and end to your repair work. I suppose it is a nice one to end on in a way, but yes they are a right pain to work on at the best of times. I think we have all been wowed by the quality of the restorations you have completed over the years.

For your interest, I've included a bit of info about early and late chassis.

Early chassis had an extra hole drilled on the tag strip bracket to the right of the EHT bleeder to locate the line feedback capacitor C85 (first photo).
Later chassis have this extra hole drilled on all the tag strip brackets (see second photo).

The change of focus coil is mentioned briefly in the parts list section of the manual for the 904 etc. (third photo).
Notice that only the part number for the later coil is given. This is the same part found in the 907 parts list (fourth photo).

Going back to the poor vision response of these sets, it is worth remembering how it works without a video amp (EMI seemed to have an aversion to video amps!).
The fifth picture shows the CRT drive. V9 is an anode bend dectector. L29 working in conjunction with the CRT capacitance has the almost impossible task of filtering out the 8MHz vision IF fundemental whilst passing video frequncies. It is amazing it works at all really!

Cheers
Andy Beer
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