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Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc.

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Old 11th Sep 2006, 11:29 pm   #1
Valve'n'vinyl
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Default Decca Bradford - EHT but no raster

http://i108.photobucket.com/albums/n...l/tellybox.jpg

Hiya Folks
I'm a new user on here and in need of help.
I have recently bought a Decca television set a Bradford 30 I believe ( see pic).
I bought it off an ex-TV engineer who had brought it up slowly on an iso-transformer / variac so no sudden mains jolt would knacker it; it has worked fine for the last week or so but I put it on today and after 20-30 mins use the picture went off but was still left with perfect sound, nothing at all on the screen, all valves light up and I am getting EHT static.

I don't know much about TV sets , I'm more of a Hi-Fi buff, but am able to use a soldering iron (does help doesn't it?)
Can anyone help??

Thanks from Dom
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Old 12th Sep 2006, 12:05 am   #2
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Default Re: Decca Bradford NEED help PLZ!!

Hi
These are great sets and well worth saving. An excellant picture by the way.

First of all a warning, all TVs use high operational voltages and a potentially live chassis, so unless you are familiar with these high voltages I would advise extreme caution before live faulting on this set.
(That's the all important safety bit done)

I used to earn my living repairing these and still run an original 1910 mk1.
I'll assume you have got a test meter of some kind and access to a service manual.
First check is to see if the tube heater is glowing ( it could be bad news if it isn't) as you have EHT static and sound we can safely say that HT is present and the line output stage is working OK. What you need to check for is voltages around the tube base.

If all three cathode voltages are high it could be a beam limiting fault or something around the MC1327 IC. If you have lost first anode volts it could be a feed resistor from either the HT line or boost line.

A final thought; as the tube can hold its EHT charge for some time, leave the set switched off for, say a day, then when you switch it back on listen out for the EHT as the set warms up. If this does not happen then we are dealing with a line output fault.
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Old 12th Sep 2006, 12:21 am   #3
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Default Re: Decca Bradford NEED help PLZ!!

Hi Simon! Thanks for your advice.
I'm going to collect a schematic for it tomorrow ( 56 mile round trip but worth it!)
I know about the HT involved and live chassis but thanks for the warning!

I'm hopefully going to get an iso-transformer/ variac as well; I do have a multimeter but I dont have a clue about droppers, boost caps and rasters.
Is there a book or a site I could go to that could teach an idiot like me (the complete novice) about vintage TV repair?

I find visual guides the best , then I know what I'm dealing with.
I have waited so long to get sets like these and I would love to keep them going; I bought 3 at once!

Incidentally this one cost me a paltry 10- I spent 2 days re-doing the veneer to a perfect match and re Letrasetting and lacquering the front panel ( yes I did mask the back!)
Many thanks if you or anyone else can help.

PS Simon, yes the CRT heater lights up and I can hear the line whistle and hear the static on the back of the CRT.
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Old 12th Sep 2006, 12:27 am   #4
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Default Re: Decca Bradford NEED help PLZ!!

That's good to hear. For a novice, the best site is mine....

www.oldtellys.co.uk

Regards,

Steve P
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Old 12th Sep 2006, 2:10 am   #5
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Default Re: Decca Bradford NEED help PLZ!!

Hi

You do not need to leave your computer keyboard to get a schematic.for a bradford 30. At the very top of this listing on the right hand side is Paul Stenning's (the owner of this website) "advert" for service information by email. This is an automated service that takes about 15 minutes. Pay by Paypal.

Set looks very nice.

You MUST have a reasonable meter to check the tube base voltages, but AVOID the thck insulated lead which has about 5KV potential on it; this is the focus connection. Give us the voltage on the other pins.

The chassis can be rolled back to its service position by loosening a couple of screws and lifting the chassis an inch, then pull the chassis along on its rollers.
This should be done with the mains disconnected! Beware, this chassis has some very sharp edges.Try not to cut yourself.

REGARDS


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Old 12th Sep 2006, 9:06 am   #6
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Default Re: Decca Bradford NEED help PLZ!!

Hello Dom,
The Decca Bradford chassis was one of the best colour TV chassis of its period.
Like Hybrid, I also repaired huge numbers of these sets and never experienced a tricky fault. My guess is that you are lacking the CRT first anode potentials probably due to a short circuit decoupling capacitor or feed resistor. The blue and white Dubilier caps were prone to this.

Common 'stock' faults are as follows, Mains switch, mains filter capacitor, the large 3.9ohm surge limiter resistor, boost capacitor, height, frame lin presets, the line 'boost' preset [replace with fixed resistors] the resistors feeding the height control from the boost line, the EHT tripler and the odd LOPT.
Pictures show my own 2230. Hope you get it sorted.
Regards John.
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Old 12th Sep 2006, 7:58 pm   #7
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Default Re: Decca Bradford NEED help PLZ!!

Hi folks

Thanks to everyone that has helped me through my darkest hours with this set.
I would particularly like to thank SIMON (Hybrid tellies) as he was right about the MC1327AP IC - this was the fault!
It took my pal Rick an hour to locate it but it turned out to be PIN 3 luminance out that was at fault.
Luckily with him being a mass hoarder, he had a second hand IC that he kindly put in and wound it up on his iso -trans.
WHOOOPEE - it came up with a picture!
After slight re-alignment of the beam limiter and the contrast it was done.
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Old 12th Sep 2006, 8:06 pm   #8
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Default Re: Decca Bradford NEED help PLZ!!

Quote:
Originally Posted by Heatercathodeshort View Post
Hello Dom,
The Decca Bradford chassis was one of the best colour t.v.chassis of its period. Like Hybrid, I also repaired huge numbers of these sets and never experienced a tricky fault. My guess is that you are lacking the CRT first anode potentials probably due to a short circuit decoupling capacitor or feed resistor. The blue and white Dubilier caps were prone to this.
Common 'stock' faults are as follows, Mains switch, mains filter capacitor, the large 3.9ohm surge limiter resisitor, boost capacitor, height, frame lin presets, the line 'boost' preset [replace with fixed resisitors] the resisitors feeding the height control from the boost line, the eht tripler and the odd lopt. Pictures show my own 2230. Hope you get it sorted. Regards John.
Hi John , Nice set !!! Yeah its all sorted now thanks for the info, apreciated!
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Old 12th Sep 2006, 11:48 pm   #9
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Default Re: Decca Bradford NEED help PLZ!!

Great news.
Glad the fault has now been found and cleared.
These really were the top CTVs from this era, together of course with the ITT/KB CVC5-9 sets: great looking, excellent results and superb to work on.
You can do no better than visiting Steve's excellent old tellies site, it's an education even to ex telly engineers.

Enjoyed seeing John's pictures of his 2230. Perhaps when I have time to learn more about my digital camera I will post some pictures of my Decca 1910. I have some good pictures but unable to downsize them to a suitable size for this site.
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