Decca 1000 restoration

[murphyv310]

Hi.
I have now started to work on the Decca 1000 from heatercathodeshort via fernseh. So far I have fully cleaned out the inside of the cabinet, the PSU and receiver/timebase chassis. I have replaced the 16/16uf smoothing block an o/c ht feed and done most of the caps in the receiver/timebase chassis although i need to order some more 0.047uf caps as I have run out! I also repaired an o/c on the anode connection from the line timebase o/p transformer in side the can.
I now have a very bright picture on the MW6-2 although I still have couple of faults, no sync and also the line frequency is way out so perhaps I have a u/s valve or ne of those 0.05uf caps are away.
Sorry no pictures yet as my digi camera packed up.
Trevor

[Ghostuser]

Hi Trevor,
I am also restoring a Decca 1000, but so far do not have a bright enough picture to fill a 4X3 silver screen. I find that the focus control has a massive effect on everything else, including focus, as it seems to cause major variations in HT. The blocking oscillator on my EHT supply seems to be runing at exactly half the correct frequency and I have acquired the parts for a second complete EHT supply to see if it is any better. Currently, EHT regulation is awful.
Finally, try as I may, I cannot get a sharply focused projected picture, although the picture on the tube face is sharp. I can get uniform focus by using the various adjustments on the projector, it is just not as sharp as I remember when I watched the Coronation on a new one in 1953. I am wondering whether the Schmidt corrector lenses deteriorate. They are not solid glass, but some form of gelatine coated glass. Maybe the gelatine has changed shape over the years.
Anyway, at their best, these are great sets, so I wish you good luck. Please let me know how you progress and if you have any comments on my problems, I will be delighted to have them.
Regards
Nigel

[murphyv310]

Originally Posted by Ghostuser

Hi Trevor,
I am also restoring a Decca 1000, but so far do not have a bright enough picture to fill a 4X3 silver screen. I find that the focus control has a massive effect on everything else, including focus, as it seems to cause major variations in HT. The blocking oscillator on my EHT supply seems to be runing at exactly half the correct frequency and I have acquired the parts for a second complete EHT supply to see if it is any better. Currently, EHT regulation is awful.
Nigel

Yes I find that the focus sems to effect everything else so a close study of the circuit may turn up something, My picture is very bright and is good on an old sheet I am using on the shed door!! I would suggest you replace the feedback caps in your EHT unit mine runs at exacly 1khz with 24.9kv and is very stable not only on beam current but between 220v & 240v mains.
I am going to look at my sync fault tonight so watch this space.
Where will I get a better diagram other than the one that was in the Newnes books?
Trevor

[murphyv310]

Hi.
Just a quick update both the sync fault and the line rate faults are fixed the set gives a reasonable picture considering the dirt in the optical unit.
Next job to construct the necessary jigs to dismantle the optics and do a careful cleaning job.
Trevor

[Ghostuser]

Hi Trevor,
I assume you have accessed http://www.thevalvepage.com./teletec...art3/part3.htm. This gives a better story on the optical and EHT units than the trader sheets 1118/T45 etc.
Decca appear only to have published preliminary service notes for the 1000. These include setup and alignment instructions plus circuits for the double and single sideband receivers. If you want these, I could photocopy them for you. Just send me a PM with your address.
Regards
Nigel

[Duke_Nukem]

I have a set of the Decca notes that someone copied for me and I really ought to crack on with scanning them (they kept going to the bottom of the pile because of the awkward paper size). I'll see if I can't sort this tonight.

By coincidence I picked up a 1947 Wireless World at Shifnal that included a bit about a Schmidt projection system being developed (using both 2.5 and 3.5-inch CRT's) and mentioned that part of the lense system used a new type of plastic.

TTFN,
Jon

[murphyv310]

Hi.
Just a few pictures taken with my old Sanyo VGC210 (330k x 3 interplolation).
I will need to get a proper screen!
Trevor

[murphyv310]

and some more

[Duke_Nukem]

Jon,
I have already scanned the pages. They were foolscap, but I got all the necessary text and diagrams on A4.

Too late ! Gone and PDF'd it which can be downloaded here (bottom of page) :-

http://www.thevalvepage.com/tvmanu/decca/decca.htm

TTFN,
Jon

PS : I'm aware there is a broken pic on the page, haveing a few techie probs

[Ghostuser]

Hi Trevor and Jon,
No problem. The pages you PDFd are identical to those I have, so that seems to be the sum of human knowledge.
Nigel

[Dave Moll]

Originally Posted by Ghostuser

that seems to be the sum of human knowledge.

I also have seven and a half pages of info. in R&TVS. I can scan these if they are likely to be of interest.

[FERNSEH]

The Decca 1000 is a very fine set, but there is one part of the design I do not like, it is the focus control. The focus coil is wired in series with one of the HT supply lines and if the focus control is adjusted some alteration of the height will be observed.
A much better solution to the focus control problem is to adopt the arrangement employed in the Ferranti T1625 projection receiver. In the Ferranti an EL41 power pentode controls the current through the coil. A 5Kohm pot in the grid circuit controls the focus coil current. The advantage of this system is that there is some compensation of coil resistance and HT voltage variations by the negative feedback in the EL41.

I am going to add this circuit into my Decca 121.

DFWB.

[murphyv310]

Hi Guys.
Many thanks for the input and very greatly appreciated it is, I am going to download the diagrams tomorrow.
Today I changed the caps in the EHT unit and carried out the focus set ups I cleaned the corrector lens in the week and blew off the dust on the mirrors.
I have borrowed a screen and adjusted the height and width to fill a smaller screen, here are some photos, its really good in the flesh!

[murphyv310]

Hi.
Just a few pictures of the cabinet.
Trevor

[murphyv310]

Hi.
I have now done the final touches to the Decca.
I found R75 2.2k nearly short circuit, not too common to find a carbon low value, also R74 & R88 both 75k up around 90k,
C37 o/c causing frame buzz through the sound.
I have down loaded the manual and its so much better than the Newnes book version.
I have attatched two photos of the end result and am now going to enjoy some DVD,s on a large screen.
Thanks guys.
Trevor

[murphyv310]

Hi.
Yes the MW6-2 is a wonderful piece of 50's engineering and so simple also it's a triode. You can quite clearly see the line structure on the projector screen as well as the bottom gratings (3mhz).Digital cameras dont seem to do the picture justice, the focus is quite stable if you don't set the contrast too high as well.
405 is amazingly good on a 4ft x 3ft screen and the line structure is not too objectional at a reasonable distance, of course the veiwing area mustn't be too bright but you can watch the set even with some subduded lighting, its a bit like being at the pictures.
Altogether I am well pleased with the set and the end result, I must thank all for their help, Fernseh & Heatercathodeshort for the set in the first place.
I have now erected the screen that I can simply fold it up on the roof of my shack so its a 2 second job to have the set on, I have even marked the floor for the correct position for the set.
I think this set might be equal to the Murphy V310 as my favourite 50's set.
Thanks.
Trevor.

[Ghostuser]

Congratulations Trevor. I had suspected low EHT on mine for some time and, as I had a spare new EHT tripler transformer and a spare chassis, I built a new EHT unit. This has effectively doubled the brightness of the picture, but I can't yet get the accurate focus that I used to have on the tube itself - focus control at the zero resistance end of its range. So, more checks to do, but I now know that the end result will be worth while.
Cheers
Nigel
P.S. I see some frame non-linearity on your screen pics. My Test card C shows a perfect circle, so there may still be something astray with your frame oscillator/ output. BTW, the thyratron triode oscillator on my line timebase produced a ragged edge to the picture. I swapped it over with the frame oscillator and now both are fine. I am not sure how long to expect a thyratron triode to last.
NHH

[murphyv310]

Originally Posted by Ghostuser

P.S. I see some frame non-linearity on your screen pics. My Test card C shows a perfect circle, so there may still be something astray with your frame oscillator/ output. BTW, the thyratron triode oscillator on my line timebase produced a ragged edge to the picture. I swapped it over with the frame oscillator and now both are fine. I am not sure how long to expect a thyratron triode to last.
NHH

Yes quite correct, it seems as my height is a bit low this non linearity creeps in, if I extend the height for the correct 3 feet all is ok, both controls are dependent so I am on the lookout for the correct screen..
Have you checked the setting of your focus preset?
regards.
Trevor

[Ghostuser]

Hi Trevor,
The correct screens do appear occasionally on e-bay. The focus preset on my set is also at the end of its range. However, I note that there are a number of resistors marked with an asterisk in the handbook. One is in the focus circuit and I wonder if these were 'Adjust on test' resistors. My set also has the cyldon tuner, for BBC London and, I assume, Channel 9 ITV. I disconnected mine as the rest of the circuit had not apparently been modified and put the right valves back in the two valveholders used for the mod. No joy, so I decided not to bother putting the set back to original as it would be very difficult to fill the tuner hole in the cabinet so as to be invisible. A complete re-veneer of the panel would probably have been required and this would have made the set even more non-original.
Cheers
Nigel

[murphyv310]

Hello.
Although the set is working ok I thought I would go through some more components.
I spent some time on the EHT unit and found R115 2.2m up at 7m, I also replaced R107 & R108 as they were off value. I know "Ghostuser" has had problems with his EHT unit and its worthwhile checking R115 and that its returned to HT and not chassis an easy mistake to make.
The EHT is now 25.2kv and drops to 24.8kv on max brightness so the regulation looks good, overall the picture is a little brighter with better focus on bright scenes.
I have been told that a plain white blackout blind material is an excellent screen. As I often go to Eclipse blinds to repair their pressure washer I am going to see if I can get an off cut to try it out, so watch this space.
I am very well pleased with the set.
One thing that intrigues me is the circuit design and the parts used, it looks so much like a pre-war set with the use of early valves and resistors, does anybody know when the set was designed?
Trevor