Decca 121 projection TV

[TopTip]

I bought this Decca 121 a while ago. It sat around until I was able to locate a fresh CRT to replace the burnt original. I finally found one (in the "Merry Christmas" bottle box), salvaged from a German SABA.

A few weekends ago I took the cabinet apart, cleaned out decades of cobwebs and dust. That is also when I discovered, unfortunately, that someone has helped himself to its flyback transformer at some point (upper left, above the projector mirror case). I do not even know how / where to look for one. I suspect there may have been more common sets that used the same transformer?

Until I find one, this 21" Sony LCD TV, which fits well, will have to do temporary duty. I will craft a new bezel to make it look more finished. The screen of the Sony is nearly black as it has a plastic film in the front. I am thinking of peeling it off and replacing it with a translucent white sheet. That will probably ruin the contrast but, again, may help with a more authentic look.

[Heatercathodeshort]

The line output transformer is the cylinder can on the main chassis. The item missing is the 25kv ringing choke/EHT tripler assembly that is mounted in a sealed, oil filled, square can complete with EHT lead.
The unit was used with the optical unit in every British projection receiver. It is a Philips part. John.

[TopTip]

So then in this TV, unlike later TVs, line out and EHT is not combined into one flyback unit? Does that mean that I can use any 25kV supply if I cannot find the original part?

Thank you

[beery]

Yes, any 25KV source will do, however the anode connector would be hard to find on its own.
And yes, I do have a used spare which is untested, but I don't know how easily it can be shipped over as it is oil filled.
I will dig it out over Christmas and send you a PM.

Cheers
Andy

[Heatercathodeshort]

When you get it together I would suggest that you leave the original Mullard MW6-2 in place. It will serve as a good test CRT until you get the chassis working correctly. One single line or frame collapse, even for a fraction of a second will burn the CRT face of your good CRT.
There are protection circuits that cut the CRT off in the event that such a fault occurs but you have to make sure these are working correctly!

From a historical point, these receivers were manufactured by the famous Plessey Co. at Ilford Essex as were all Decca television receivers until 1954/5. John.

[TopTip]

Last night I was reading a Philips site and learned that the same MW6 projector CRTs were sold in the US as the Norelco Protelgram 3NP4. Amazingly, there was one on eBay. It was cheap enough that I bought it. I will see if at least its filament is good when it arrives. Then I will have a spare or one to share!

[TopTip]

Originally Posted by beery

Yes, any 25KV source will do, however the anode connector would be hard to find on its own.
And yes, I do have a used spare which is untested, but I don't know how easily it can be shipped over as it is oil filled.
I will dig it out over Christmas and send you a PM.

Cheers
Andy

That would be amazing. I am in London reasonably often, I can probably arrange to pick it up somehow. I have been reading that to test it, I need 6.3 and 375 volts, the latter at 50mA.

[Heatercathodeshort]

Generally if the screen is a pure white similar to copy paper it will be OK. Tired examples have a light brown square where the picture has been and will give a dull picture. J.

[TopTip]

This seems to have some brown.

[Heatercathodeshort]

That looks OK to me. An aged tube will show a very pale yellow/brown rectangle in the area the picture is formed due to the intense electron beam at 25kv on a 2 1/2" tube.

Burn marks due to circuit faults are small black dots or imperfections that magnify into large blemishes when seen on the receiver screen. Make sure the tube face is clean. Your marks look like they are just dirty smears.
At least you can use this one for test but do not fit a good tube until you are sure everything is operating correctly. John.

[TopTip]

Looks like in the US they sold these as conversion kits for direct view TVs:

https://www.earlytelevision.org/pdf/ee_9-50.pdf

https://www.earlytelevision.org/pdf/duo-view.pdf

[Heatercathodeshort]

These conversion kits were widely advertised in 'Radio and Television News' during the mid to late 1940's. John.
PS Advert from R&TV News Sept 1948. This unit uses the 5" 5TP4. The MW6-2 or 3NP4 arrived just a little later. The American version appears to be the MW6-2 imported and renamed especially as it has the side contact base.
It may be possible to locate the EHT unit used on these models for your Decca. J.

[TopTip]

Yes, I am thinking I need an original unit — someone said, in the Decca, it is part of the main circuit also. I have not checked to see if that is the case yet.

In the meantime, I routed a channel in the plywood of the top of the TV, so it can accommodate the Sony panel. I figure rejuvenating the Decca chassis will be a long term project. This will allow it to be used in the meantime. I can always refill the channel but it also does not in any way interfere with how the Decca screen mounts.