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Old 9th Feb 2018, 10:27 am   #1
RestorationRod
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Default Ferranti 20T6 Projection TV

I am in the midst of restoring a Ferranti 20T6 Projection TV from 1957.

Nearly all of the Electrolytic and Wax/Paper capacitors have now been replaced.

A lot of the rubber/cloth wiring was perished and had to be redone.

Lots of corrosion has had to be dealt with.

Just the bad resistors to replace and then it's test the PSU time. CRT needs 25kV.

And before anybody starts jumping up and down. I spent four years in a high voltage test lab and have a Variac, an isolation transformer, inline filament lamps and one hand in my pocket.
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Old 9th Feb 2018, 6:28 pm   #2
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Default Re: Ferranti 20T6 Projection TV

Hi Rod.

Welcome to the forum.

Coincidentally, I've just finished one of these - or rather the console version, the 24K6. I think it's the same chassis just a slightly different layout of all the separate sections.

Mine needed similar things to yours (wax caps of course, and a few odd-value resistors). The main "can" variety of electrolytics I found eminently re-formable and none of them complained at all. The other smaller electrolytics in the cathodes of a few of the valves were found to measure fine but I did replace them for reliability's sake.

This particular chassis doesn't hold too many mysteries apart from a few interesting bits and pieces. It has a kind of push-pull video output stage, feeding both the cathode and grid of the MW6/2 CRT in antiphase. Also, as I'm sure you know, there is a separate extra picture-muting circuit which means that the small amount of bias applied to the bottom of the EHT ringing oscillator circuit is not employed until an EB91 starts conducting - and the heaters for the EB91 are fed from the line output transformer. If the line stage fails then the heaters of the muting circuit are turned off and the bias to the EHT oscillator is removed. This is all very clever (if possibly rather unnecessary) but it does mean that the warm up period is very long compared with other sets (not helped by the fact that the self-oscillating line stage seems to take ages to get going anyway).

Quote:
I spent four years in a high voltage test lab and have a Variac, an isolation transformer, inline filament lamps and one hand in my pocket.
Wow - that's very safety conscious. I just make sure I'm wearing my rubber socks and avoid dropping my can of Carlsberg Special-Brew into the works. (Joke)

Are you going to post some pictures up of the old girl?

Thanks Rod.

Kind regards.

From Mike.

*Additional to the standard protection circuit which biases the tube off in the event of a scan failure of the line or frame.
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Last edited by Mikey405; 9th Feb 2018 at 6:47 pm.
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Old 11th Feb 2018, 12:07 am   #3
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Default Re: Ferranti 20T6 Projection TV

The 20T6 is one of the last TV receivers to be made by the old Ferranti company. The standard of construction is very high. These projection sets were made as late as 1956/7 at a time when the 21" CRTs were available and in fact Ferranti made many large screen direct view sets, the models 21K6 and 21SK6.
I still have the Philips 600A small screen projection TV. Someday I'll get around to restoring it.

DFWB.
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Old 11th Feb 2018, 7:45 am   #4
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Default Re: Ferranti 20T6 Projection TV

Thank you .. I am not alone..
I replaced all of the wax/paper and electrolytic capacitors in both chassis.
Now I am working on the crispy resistors.
I'm off to the Harwell rally to-day with a list of parts.
Pictures yes, but I have no idea how to post them to this fancy forum.
It looks hyper controlled and ruled by rules so I would not want to get it wrong.
I had to cope with a lot of corrosion but it's OK now.
How did you go about bringing the TV to life?
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Old 11th Feb 2018, 9:42 am   #5
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Default Re: Ferranti 20T6 Projection TV

Quote:
Originally Posted by RestorationRod View Post
Pictures yes, but I have no idea how to post them to this fancy forum.
See here:-

https://www.vintage-radio.net/forum/...ad.php?t=77650
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Old 11th Feb 2018, 11:09 am   #6
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Default Re: Ferranti 20T6 Projection TV

Hi RR,
Welcome to the Forum! That looks to be an interesting project- I rather fancy doing a projection TV myself but I do try and avoid console sets these days as they are a pain to move about.
Don't worry about the forum rules too much: they don't get in the way and are there to keep things pleasant and civilised.
Pictures are always interesting to see and are easy to post: once you know how of course! My computer skills are dire and even I managed it!
Best of luck
Cheers
Nick
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Old 12th Feb 2018, 10:27 am   #7
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Default Re: Ferranti 20T6 Projection TV

Hi Rod.

Regarding bringing your projection set to life, I can offer a few bits of general advice which I've learned through experience of restoring a selection of decrepit projection sets over the years. I suspect others may have much better views and ideas but these are what I try and stick to.

Before ever plugging the set in:

1. Reform the smoothing capacitors separately (I.e. disconnect them from the circuit and reform them individually.) It doesn't take long to do and a set such as this can have all its smoothers gently reformed in a day. If they won't reform then at least you'll know about it without the rest of the circuit connected up to confuse things.

2. Use an otherwise scrap MW6/2 CRT. If you have a tube which has a burn in it then use it to get the set going. Only replace it with the original tube once everything else is sorted.

3. Make sure you check all the components in the EHT generator. You'll probably want to replace all the wax caps in the EHT generator anyway, but if you're tempted to try the set with the original caps then be prepared for the EHT to be sky-high and nasty (possibly terminal) arcing from the EHT transformer and associated valves.

4. Make sure you check all the components in the CRT protection circuit. This is specially important if you're using the original (undamaged) CRT. Leaky caps here can bias on the protection circuit enough to make it look like you have line and frame scan - even if you don't. A single bright line in either the vertical or horizontal direction can leave a line permanently burned in to the tube in much less than a second.

Once you've done all that then plug the set in and away you go.

You'll have fun with the focus on this particular set Rod as the optical assembly is at such a weird angle that the "band of focus" you need to work with when refocussing the set is at roughly 80 degrees to the horizontal. Fun and games there I think.

I hope that's of some help Rod.

Kind regards.

From Mike.
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Old 14th Feb 2018, 3:50 pm   #8
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Default Re: Ferranti 20T6 Projection TV

Hi Thanks

1. All electrolytic caps will be replaced.
All wax/paper caps will replaced.
All Hunts met/paper will be changed.
All power resistors will replaced

2. Thanks Yes I have a tube with a burn and will use.

3. EHT gen has been a complete rebuild.
Everything changed except the valves in the can and the oscillator transformer.
Yes I know about the close tolerance frequency determining components.

4. Yes protection circuits will need a full check. Thanks for tip.

5. Main problem has been rust.

6. Will upload pictures when I have shrunk them to fit on the forum.

Regards Rod G8DGR
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Old 14th Feb 2018, 9:04 pm   #9
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Default Re: Ferranti 20T6 Projection TV

Hi Rod,
If you just upload the photo's in whatever size they are, the forum software will reduce them down. It just takes a bit longer to upload them. I guess if the files are huge that might cause problems- I don't know!
Will be interested to see pictures of the chassis. Does the EHT circuit use a multiplier or does it have a 25Kv overwind?
All the best
Nick
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Old 15th Feb 2018, 11:25 am   #10
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Default Re: Ferranti 20T6 Projection TV

Well not really either. The EHT is produced by a 1kHz power oscillator driving a three valve tripler in an oil filled can.
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Old 16th Feb 2018, 12:28 am   #11
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Default Re: Ferranti 20T6 Projection TV

Quote:
Originally Posted by RestorationRod View Post
Well not really either. The EHT is produced by a 1kHz power oscillator driving a three valve tripler in an oil filled can.
That sounds exciting! Do you have a circuit (or a link to one) you could post, as I would be interested in having a look?
All the best
Nick
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Old 16th Feb 2018, 8:14 am   #12
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Default Re: Ferranti 20T6 Projection TV

Read all about the three versions here.

TTFN,
Jon
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Old 16th Feb 2018, 10:48 am   #13
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Default Re: Ferranti 20T6 Projection TV

This may help illustrate the EHT unit. They are basically all the same unit manufactured by Philips/Mullard. J.

https://www.vintage-radio.net/forum/...ight=Decca+333
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Old 16th Feb 2018, 6:17 pm   #14
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Default Re: Ferranti 20T6 Projection TV

Thanks Jon & John! Just what I wanted, I'll have a read of that a bit later.
Cheers
Nick
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Old 16th Feb 2018, 7:11 pm   #15
RestorationRod
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Default Re: Ferranti 20T6 Projection TV

Sheesh! trying to upload pictures is much worse than doing a recap.
Where did they go?
RR
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Old 17th Feb 2018, 12:41 am   #16
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Default Re: Ferranti 20T6 Projection TV

This should help: https://www.vintage-radio.net/forum/...ad.php?t=77650
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Old 17th Feb 2018, 12:24 pm   #17
RestorationRod
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Default Re: Ferranti 20T6 Projection TV

That's what I was following. It gets to 98% upload and falls over. Says site cant be reached.
RR
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Old 18th Feb 2018, 8:09 pm   #18
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Default Re: Ferranti 20T6 Projection TV

Hi all.

Rod asked me to upload this photo as a starter.

This is the start of the re-capping of Rod's 20T6.

Thanks all.

Kind regards.

From Mike
Attached Thumbnails
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