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Old 24th May 2021, 8:58 pm   #1
beltinge bore
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Default Decca 1000 Projection Set.

Hello,

Have spent quite a bit of time on this. Now have a nice bright well focused little picture on the tube. Before I put it back in the cabinet could anyone advise me with regard to cleaning the optical unit please?

It has what I would describe as white Aluminium corrosion type spots inside. I have the Decca and Mullard manuals which suggest that opening it up is not for the faint hearted! I have read some previous posts but would be grateful for some further advice - 24KV EHT is interesting!

I do have a RBM EHT tester but no probe - Could anyone suggest where I might be able to obtain one for it?

Thanks & Regards Steve.
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Old 25th May 2021, 11:44 am   #2
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Default Re: Decca 1000 Projection Set.

Hi Steve. If the picture is reasonable I would strongly advice against trying to clean the optical unit aluminium. The blemishes might just peel away to leave bigger patches. The best you could do would be to use an aerosol duster but it might well make things worse. Mikey has previously posted on here about a place that re-silvers the mirrors at a reasonable price if you need that doing
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Old 25th May 2021, 12:11 pm   #3
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Default Re: Decca 1000 Projection Set.

Hi Steve,
I have a spare optical unit that came from a Decca 1000. I can't say if it is any better though.

Cheers
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Old 25th May 2021, 5:20 pm   #4
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Default Re: Decca 1000 Projection Set.

If Jac spots your post he'd probably know what to do. (Perhaps PM him?)

Peter
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Old 26th May 2021, 9:24 am   #5
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Default Re: Decca 1000 Projection Set.

Hello Steve, do not try to measure the EHT on a projection TV. The glass EHT connector cup is very easily damaged and I can't see any method of attaching the lead from the EHT probe. You cannot measure it on the end of the EHT lead as it requires the graphite coating on the CRT to form the smoothing capacitor. Under no circumstances attempt to remove the lead where it enters the EHT can. Do not operate the CRT outside the optical box unless you keep the brightness/contrast at very low levels. If all else fails, like Andy I too have a number of optical units from various models. You are welcome to one FOC but it will have to be collected and may need the part number on the unit itself. Do not remove the corrector plate as it is VERY difficult to get it back in the right place! John.
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Old 26th May 2021, 11:33 am   #6
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Default Re: Decca 1000 Projection Set.

Good Morning,

Thanks for all your replies I am very grateful.

This set was rescued from a basement in Hertford (very close to Mc Mullens brewery) my late grandfathers favourite beer. The cabinet was badly water damaged the optical and EHT unit were just about bolted down. The main chassis was loose as was the very heavy PSU. I wanted if possible to learn as I went along with this one.

There was evidence of some ancient attempts to repair it and it survived the journey to me in the back of my brothers car.

All valves tested OK - I have a Taylor valve tester on "extended" free loan from my friend Stephen - Freya of Hedghog fame. I would normally not bother testing valves but wanted an idea of their condition as some are not readily available. All pre-set controls were very corroded but careful cleaning meant only 2 needed replacing. The EHT unit EL38 needed a new valve holder as did a couple on the main chassis. Every one of Mr Hunts finest needed changing. Most resistors apart from the large dogbones were OK.

With regards to the 24kv EHT I had a senior moment when I got so excited at the sound of line whistle starting up I forgot that the tube connector had lost its temporary insulator resulting in a truly spectacular spark to the leg of my workbench. I'm not an expert in these matters but it looked about 24kv to me! Methodically working my way through the tube protection circuits etc. resulted in a nice bright linear test card. Sound was OK but a slight smell of arcing turned out to be from the SOPT which is winging its way to young Ed Dinning for a rewind. The frame sync was a little touchy but previous experience with gas thyratrons in EKCO sets helped. A turret tuner has been fitted which looks to be a Cyldon but may be Plessey as it has Decca DM series knobs. This is fed from a valve holder (Rimlock) with a plug carrying all connections 2 sensitivity pots have also been nicely fitted. The tuner does not show any signs of use and was left lying on its side on the top deck. No sign of it ever being fitted to the cabinet exist. I wonder if the owner had it converted but thought Hughie Green and Michael Miles were not worth the bother!

My work shed is not large enough to bench test the final result so it is now back in its renovated cabinet. I will report back later.

With regard to your kind replies I have not operated the tube outside the optical unit as advised in the Philips/Mullard manual. I did give the interior of the optical unit a gentle "blow" with an air duster which seems to have helped a little. Andy and John thanks for your kind offers (typical of the generosity which seems abound in the world of old telly's. Steve and Peter thank you.

Hope I haven't bored you with all this.

Regards Steve.
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Old 26th May 2021, 12:20 pm   #7
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Default Re: Decca 1000 Projection Set.

It is about 24KV!
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Old 26th May 2021, 4:16 pm   #8
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Default Re: Decca 1000 Projection Set.

They are awkward to work on but I did a write up a while back on the Decca 131 that uses a very similar circuit. [All Decca projection receivers were manufactured by Plessey]

It was a simple matter to connect the chassis up on the workbench with the CRT removed from the optical unit. It saved a lot of frustration.

The EHT unit is not lethal as such but can give a nasty sting if you come into contact with the EHT connector. [Very difficult to be honest]

I knew a TV guy that worked on projection receivers in the 1950's His right thumb had a weird, what looked like a cut down it's length caused by coming into contact with that spring on the end.. He had eased the connector off the tube but had not fitted it back correctly resulting in it popping off and making contact. The plastic gets very stiff and needs to be warmed before removal and refitting. Keep safe. John.
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Old 26th May 2021, 6:49 pm   #9
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Default Re: Decca 1000 Projection Set.

If you're not going to measure the EHT I would strongly suggest you check every component in the EHT regulator circuit before turning on again. I had faults which generated vastly upped the EHT and destroyed the EHT can in about 20 seconds. After that I measured the EHT but I can't remember how I did it!
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Old 26th May 2021, 7:02 pm   #10
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Default Re: Decca 1000 Projection Set.

What size is the front silvered mirror? I may have one.

Edit, mine was a model 131 back projection set, not the 1000 front projection set. Anyway, I'm pretty sure I have the front 'silvered' mirror for it.
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Old 27th May 2021, 8:43 am   #11
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Default Re: Decca 1000 Projection Set.

The 1000 does not have a mirror. The picture is projected direct from the optical unit to the 3x4ft screen. J.
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Old 27th May 2021, 9:39 am   #12
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Default Re: Decca 1000 Projection Set.

Quote:
Originally Posted by Heatercathodeshort View Post
The 1000 does not have a mirror. The picture is projected direct from the optical unit to the 3x4ft screen. J.
Which is effectively what I said in my edit.
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Old 27th May 2021, 11:22 am   #13
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Default Re: Decca 1000 Projection Set.

Sorry Steve, I got a bit confused. The 131 I restored for the museum was completely devoid of it's mirror surface and there were signs this had been deliberately removed. I have no idea why but incredibly we unearthed a very similar model mirror that was in good condition. It works very well. J.
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Old 27th May 2021, 2:13 pm   #14
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Default Re: Decca 1000 Projection Set.

I once had a projection set mirror re-silvered by a mirror manufacturer. The place who did it obviously didn't know what they were doing because they failed to tell me that the front silvered coating would go black in a matter of weeks. On reflection, obvious really, but having spoken to them about my needs first, I thought they knew what they were doing. In any case, the coating is not silver as we know. You live and learn.
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Old 9th Aug 2021, 3:03 pm   #15
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Default Re: Decca 1000 Projection Set.

Hello.

Sorry this thread has been left for so long.

I took a chance on an optical unit including the tube and scan coils listed on eBay at a very reasonable price which duly arrived very well packed and in much better condition than mine.

I checked the type label before buying (10950) and it looks identical apart from the mounting plate. My original is 10950/23 the replacement is 10950/15.

Would anyone be able to advise me if the /15 or /23 suffix denotes any internal optical differences please?

Changing the mount is not a problem so just trying it may be the simple answer.

Thanks Steve.
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Old 9th Aug 2021, 5:23 pm   #16
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Default Re: Decca 1000 Projection Set.

Hello Steve,

I believe there is a difference in the correction lens.
I expect the /23 to have a much larger "throw" (distance to the screen) than the /15.
You can check this by measuring both units at what distance from the lens the picture is sharply projected on a piece of paper.

Philips/Mullard do not recommend to remove/exchange the lens from the unit. But with very careful noting the precise position, you may get away with it, if nothing else gives a solution.

Hope this helps. Please let us know your findings.

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Old 9th Aug 2021, 5:33 pm   #17
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Default Re: Decca 1000 Projection Set.

Addition:

/15 seems to have a light-way of 82.5 +/- 1.5 cm
/23 ditto 198 cm

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Old 9th Aug 2021, 5:49 pm   #18
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Default Re: Decca 1000 Projection Set.

further addition:

I found a photo of a test I was doing to determine the projection distance of a few of these optical boxes.

Click image for larger version

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On the right you see a strong light almost totally obscured save a thin cross in the middle.
On the left the optical box under test. With a magnifier I was able to see the projection of the cross on the face of the CRT. I varied the distance of the lamp until the projected cross was sharp, and measured the distance.
(It goes without saying that the optical principle works from the CRT to the screen as well as the other way around.)
This way I found out the differences between several optical units.

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Old 9th Aug 2021, 7:08 pm   #19
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Default Re: Decca 1000 Projection Set.

Hello Jac

Thank you for speedy reply. It all makes sense. I will investigate further when my wife allows me back in my shed. (Hernia Surgery recovery).

As an aside we have a connection with your lovely part of the world in that I have an uncle buried in the War Cemetery in Venray. We hope to visit again soon.

Regards Steve.
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