Decca 1000 update!

[murphyv310]

Hi.
I've had my Decca 1000 for quite a while now, it doesn't really get a lot of running two or three times a year, in saying that it always is fine never any problems.
A few months ago I was given a brand new optical unit with CRT and today I thought I would fit it.
Its really an easy job, all plugable, the EHT connector is easy to remove and the optical unit bolts to the mounting plate with three 2BA nuts and bolts with one self tapper at the lower part of the optic.
The job only took about three quarters of an hour.
The results were well worth it, my optics were suffering with the effects of damp with pitting on the mirror surfaces and general tarnish. The overall results are excellent. The new CRT along with perfect mirrors give an excellent picture and really quite bright.
The set has been running for about five hours and to be honest the only gripe is Decca's rotten focus arrangement, which I think will be modified in due course.
I'll post some photos tomorrow.
The results are quite amazing, with this set giving pictures no doubt as good as when delivered to its first owner.
Cheers
Trevor

[Peter.N.]

I sold one of those for a fiver just before we moved to Dorset, mind you that was nearly fourty years ago. We used to service quite a number of Philips sets with the same optical unit but obviously different focal length as they were back projection, you could use the brightness control to focus it - the EHT regulation was that bad! Does the Decca use the same EHT unit?

[murphyv310]

Hi Peter.
Yes indeed it does use the same optical unit though the corrector lens is different. The EHT unit is a standard Mullard one.
I don't think they were that popular and most that survive now are really tatty, my one is quite reasonable though.
Cheers
Trevor

[Ghostuser]

Hi Trevor,
I remember seeing the coronation on one of these. Before the broadcast, there was a Test Card C transmission and I remember the picture being sharp,linear and bright enough in a darkened room, with the 3Mc/s bars clearly resolved at nearly full contrast.
I have a running Decca 1000, but the mechanical focus is poor and the scanning lines can not be seen on the screen. Focus is OK on the tube itself, but I suspect the corrector lens. I understand that these were made by coating gelatine onto a glass plate. Maybe the gelatine suffers with time. I have no reason to believe that mine was made for a back projection set, as it seems worse at shorter projection distances. I'd be very interested to try another corrector. I did have one from a back projection set once, and that gave a really awful result as you would expect. Any idea where I might find another corrector lens or complete optical unit?
regards
Nigel

[murphyv310]

Hello Nigel.
I've got three full assemblies but the corrector lens is different on all three, the number on the lens is unique to the set its fitted to.
They rarely are faulty but if its been off it may not be fitted correctly, I would start by running the set at 8ft 6in away from the screen ensure the screen is at the same angle as the corrector lens, loosen the clips that hold it and turn it as it could be at the wrong point you will see the lins structure in focus at certain parts and this will change aim for best focus in the central area.
Normal adjustment should then be possible with the three knobs.
Do you have the Mullard sheet if not I am sure I have one I can send to you.
Cheers
Trevor

[Ghostuser]

Dear Trevor
Thanks for the advice. I will certainly give it another try. Fortunately, I have the Mullard data sheets and the Decca data on setting-up, alignment etc, so I am well placed. There were two main problems with the set; Firstly, the EHT was low and that was down to the blocking oscillator running at half frequency. I was able to find a swap for the oscillator transformer and also a brand new EHT transformer, so that issue is solved. The second lay with the focus coil, which had been rewound badly off centre and electrical focus and picture centring was impossible. I rewound the focus coil and that is also now sorted, so if I can persuade the corrector to work, I will be well pleased. I do not seem to have any problems with the mechanical focus knobs. The service sheet makes it sound worse than it really is!

Thanks again
Nigel

[murphyv310]

Hi.
I have had the Decca now for nearly three years, after doing the necessary then the set has been ever so reliable since although there has been a couple of issues recently.

I have noticed a little line drift and the tiny spot on the tube, although hard to see does annoy me.
So I spent a couple of hours on it the other day, I fitted a NOS MW6-2 tube firstly, the spot obviously had gone, and while in the optics replaced the rather pitted mirror. I was rather surprised that the picture wasn't really any brighter so the old CRT must be good, but what I did find was the video amp load resistors (R19a & R19b) were both quite high they are 4.4k 5% body,tip,spot types and overall were up to over 11k, I fitted 4, 2.2k 2watt MOS resistors, this has changed the DC conditions on the tube and upped the contrast level and improved overall brightness, so well worth doing.

The line drift fault was the 50uf electrolyitic on the T41 thyratron line osc, I also checked and replaced some other 50uf electrolyitics that were way off.

The Decca although I was really pleased with the results up to now has been improved so much that it now really amazes me, these sets must have been far better than we all are led to believe, and considering the set was found on a tip in London thanks to HK Short.
Cheers again
Trevor

[murphyv310]

Hi.
You may notice that I have fitted a optic unit previously, which was the case. The one I fitted last year was from a Decca for the 8x6ft screen and I found it hard to get it to focus down to 4x3ft. So I had fitted the original back in, this time I fitted another NOS MW6-2 I had and used the mirror from the 8x6ft optic unit this time I have excellent focus at the correct positioning of the set.
This is just to clarify what may look like an error on my part.
Cheers
Trevor

[peter_scott]

Trevor,

Having been brought up watching my parents' Philips 1400A and also later playing with it to display pictures on the ceiling I was absolutely amazed at how good your Decca was in comparison when you demonstrated it at your "Up West" open day. This was not just because it had decent dc restoration but principally because of the excellent brightness and contrast and sharp focus over the whole screen area. I certainly look forward to further amazement.

Well Done!

Peter