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Steve
(@steve)
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Hi I am new here hello all

Please look at my photo..can anyone help tell me what the red circle componant is  mine is missing on my tv..I have sound  but no activity on screen thanks 

sgap
Topic starter Posted : 20/06/2022 5:06 pm
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crustytv
(@crustytv)
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Posts: 10842

Hi Steve and welcome to Vrat. 👍 

You uploaded the photo, but forgot to attach it to the post. Once uploaded, you need to tick the box next to the photo, and then chose insert. No problem, I have attached it for you. To save on duplication of photos, I have removed your copy.

As to the missing part, that's what is known as a spark gap, simply an air gap  between the two wires. A voltage surge causes a spark which jumps from lead wire to lead wire across the gap. These were used to prevent damaging arcs between the elements of the electron gun(s) within the CRT.

This will not be the cause of your no picture fault. The cause could be a number of things. First, you need to tell us what your level of knowledge is with regard to vintage electronics. Second, do you own and know how to use a multimeter. If the answer to the first is none or limited, You need to be aware of extremely high voltages present on these sets.

Posted : 20/06/2022 5:16 pm
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19Seventie
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Hi, welcome to the forum!

No photo has been attached unfortunately.

I'm assuming this is using the Decca Bradford chassis? (see attached) If so there's a good chance the IC on the decoder board is faulty. It should be an MC1327 or a TDA1327A. 

1

I'm sure there's a guide on this forum on how to upload photos but unfortunately I can't find it, im sure someone will be along soon with how to upload photos to posts

EDIT: Posts crossed with crustytv

Posted : 20/06/2022 5:19 pm
Steve
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@crustytv 

Topic starter Posted : 20/06/2022 5:26 pm
crustytv
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Erm, the post above is just a mention, it sends me an e-mail stating you posted something, but you didn't if you see what I mean.

Anyway.......

The reason I ask about your experience and if you have a meter, is you will need to do voltage and resistance checks. When doing so, there is always a risk to the less experienced, to get a whacking great belt from the high voltage circuits. The fault could be simple or complex, The brightness could be turned down (have you checked), or inoperative, you may not have EHT.

I'm guessing you don't have and EHT (25kV) meter (don't use a normal meter).  There is a simple way to know if EHT is there. Do you hear a rustle when the TV is turned on, it will be brief, this is the 25,000Volts rustling up. A simple way of telling if there is EHT, is to go around to the front of the TV, place the back of your hand about 2-3mm from the screen, do you feel the hairs on your hand tickling? If you heard the rustle and the hairs tickled, there is almost certainly EHT.

Note: Do Not Touch The CRT Glass From The Rear.

If there is, then there are some stock faults covering the "No Picture but Sound" scenario.

First of all, check the HT feed resistor R298 (39R) which feeds first video amplifier and emitter follower as it does the whole colour section; the effect would be to remove the voltage from the colour output drivers resulting in a high collector voltage in the colour outputs and thus a high cathode voltage in the CRT and a blank screen.

Check for HT short of the HT line which may account for component being faulty, otherwise check A1 voltages, feed resistor R475(220K) disconnect and check resistance.

Check operation of brightness control circuit, which comprises D603(BA155) and associated decoupler C605(32uF)

Check also preset brightness control VR601 (470K on PSU/sound PCB)

R612 (47K) and R600 (270K, disconnect and check resistance. Check also R476 (390K), R328 (22k), R 326 (18K), VR327 (10K brightness control), trace voltage from its slider.

Note: Resistance checks must be done with the set powered off.

Check all plug connections

Recommended reading, elsewhere on this site

https://www.radios-tv.co.uk/decca-bradford-1030/

https://www.radios-tv.co.uk/decca-bradford/

https://www.radios-tv.co.uk/decca-bradford-30-series/

Posted : 20/06/2022 5:55 pm
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Steve
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@crustytv hi may thanks  for explaining all of that  I shall work my way through it  but  there is no hairs standing on end  I know this from previous crt,s   thankyou kindly 

Topic starter Posted : 20/06/2022 6:37 pm
crustytv
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If there is no EHT, read the first link I provided, it's an extract on servicing the Decca Bradford chassis. Page 492/493 has some checks you can do for that. Good luck and let us know how you get on 👍 

Posted : 20/06/2022 7:04 pm
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PYE625
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Hi Steve, as a matter of interest, did the set work ok previously and the fault suddenly develop?

Page 13 (492 in the article) of the first link that crustytv kindly provided above is of special interest here.

It is quite important not to have the set operating for long under the fault condition that you have. If you notice either of the two valves within the cage upper right of the chassis begin to glow a cherry red on the internal visible grey metal of their anode structure, SWITCH OFF immediately. This will avoid further damage. As you may well know, the small heater part in the middle of each valve is meant to glow, but NOT the outer part closest to the glass.

Of several possible causes, one such item can be the boost HT capacitor becoming short circuit. Here below is a photo of the loaction of a replacement in my set, and with reference to your first photo, it is the large red capacitor extreme lower right. With the set COMPLETELY DISCONNECTED from the mains, if you have use of a multimeter, you could check if it is short circuit.

rsz img 3207

I restored a Decca Bradford a while back, below link is my story if you are interested.

https://www.radios-tv.co.uk/community/colour-television/a-colourful-decca-arrives/

Posted : 20/06/2022 7:39 pm
PYE625
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I really must stress the vital importance of NOT TO TOUCH ANYTHING INSIDE with the set connected to the mains. There are several thousand volts that are produced with enough current THAT WILL KILL YOU if you are not super careful.

Please do forgive me for stating the obvious if you are perfectly familiar with electronics of this nature, but SAFETY MUST be the prime concern.

Posted : 20/06/2022 8:01 pm
Steve
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@pye625 see photo please

bradchas
Topic starter Posted : 20/06/2022 8:36 pm
Steve
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@pye625 valves

plpy
Topic starter Posted : 20/06/2022 8:41 pm
crustytv
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Steve, you are uploading the photos to the forum, but are not inserting them into the post. I mentioned how to do this in post number 2.

Posted : 20/06/2022 8:44 pm
Steve
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@crustytv

Sorry


Topic starter Posted : 20/06/2022 8:55 pm
crustytv
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As I can see the photo on the server, I highly recommend you do not switch it on again, both those valves (PL & PY) appear to have cherry red anodes.

Posted : 20/06/2022 9:27 pm
Steve
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@crustytv are the available  to buy  please

Topic starter Posted : 20/06/2022 9:29 pm
crustytv
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I'll sort out the photos for your posts again so they appear, but it means I have to delete them from your media list and add them to mine. Please read the FAQ on how to upload a photo.

https://www.radios-tv.co.uk/community/faq-members-only/attaching-images-to-posts/

Others give it a couple of mins and then you will be able to see the pictures.

 

Posted : 20/06/2022 9:31 pm
crustytv
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Posted by: @steve

@crustytv are the available  to buy  please

What  do you want to buy? If you mean replacement valves, then yes, they are on e-bay (e-bay links are not permitted). But you don't need to buy any yet, you need to fix the problem first. Then, if the valves have been ruined, buy more. Chances are if you've not run them too long they will be fine. If you buy new ones and fit them without fixing the underlying fault.... well.

Posted : 20/06/2022 9:34 pm
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PYE625
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Just a quick look at the photo would suggest that they are displaying normal heater glow in the centre of the valves.

It is at the sides and edges of the internal structure that any cherry red glow is of concern.

Try to reduce the ambient lighting and look again at the sides of the valves through the metal cage.

Obviously, don't be tempted to touch them whilst switched on.

Posted : 20/06/2022 9:40 pm
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crustytv
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It's been a while since I last saw valves glowing in a colour TV, mostly doing transistor TVs now. My "red plating anodes" due to the line osc not running on PYE (697) CT203/1, at a glance of those photos, it looked the same to me. This is one of the pitfalls of remote diagnosis, varying opinions, happy to stand corrected, though. 👍 

Posted : 20/06/2022 9:43 pm
Steve
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@crustytv many thanks 

Topic starter Posted : 20/06/2022 9:53 pm
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