Here is my original 7a91
And here is the new set
Remarkably it's badged Philips. I find it difficult to believe that Philips would buy from Plessy, but the badging looks professional. The set is full of Mazda valves too, which would have been sacrilege to Phillips. What is the explanation?
Well it seems complete. I've slowly powered it up and there's EHT, frame buzz on audio and a (bright) spot when I switch off. All good. Now think I'll order some caps before delving further, I know all the paper caps and 'lytics will have failed. I have stock of all resistor values, though 1/4 of the size.
I find it difficult to believe that Philips would buy from Plessey
So do I.
My guess is that the original badge became detached or got lost and someone just stuck a Philips one there for whatever reason best known to themselves.
Nice set anyway 👍
Think it is actually a Defiant 7109, but could be wrong?
I repaired one about 3 years ago....
https://www.radios-tv.co.uk/community/black-white-tvs/defiant-7109/
To understand the black art of electronics is to understand witchcraft. Andrew.
@pye625 Hi, that's certainly the most likely explanation, but the badge just fits so well! When I take it out of the cabinet I'll check the rear fixings.
The 7109 was electrically identical but had a rounded plain wood cabinet, whilst the 7a91 had a more squared, lacquered cabinet. I've read your thread where it is described as a 7109 but it's only a cabinet difference. My set looks in very similar condition to yours.
How do I know the model? In the mid 60s I was an apprentice Radio & TV engineer at the Co-op in London. I worked my first year in the stock repair workshop, mainly on these.
To make sure my memory wasn't deceiving me, there are adverts for the 7a91.. I haven't found any for the 7109 though. I'd certainly rent one for 7/6 a week.
Anyway it's good to be back on the forum. My previous moniker was lost when the forum upgraded and I was absent - probably for the best.
I've been digesting some of the nostalgia and hope to contribute some.
Thanks,
John
Excellent stuff John and welcome back. Looking forward very much to reading how you get along with the set. 👍
To understand the black art of electronics is to understand witchcraft. Andrew.
Unlikely Philips would have badged a Plessey TV set although it has to said in the mid-sixties TV sets with the Cossor and Peto-Scott brand names were made and sold with the Plessey dual-standard chassis. Doubt Philips would permit Mazda valves and CRT in anything with their trade mark on it.
Does your 7A91 employ the very last version of the Plessey chassis first introduced in 1956? Final production chassis employed the PCL84 as the video amplifier and PCL82 for the audio amplifier and output. Receiver had the "synchronguide" flywheel line sync system. Details of the Defiant model 719 can be found in the 1961/62 Radio and Television servicing book which would indicate the set was introduced in 1960. Model type 7A91 is the new model coding introduced in 1960/61.
Note the old CWS wheatsheaf logo in the advert.
Till Eulenspiegel.
It's now clear that the badge was a retrofit - thank goodness, it puts my mind at rest.
Both sets I have are electrically identical to the 7109 with an EF80 as the video amplifier, ECL80 as audio amp and certainly no flywheel sync!
You are correct about the new model coding, sets for the next few years used that nomenclature eg 9a51 3a61.There were b models too but I can't remember any just now. Following that, they switched to RBM chassis with another nomenclature eg 901.
My second 7a91 is now working but the CRT is nothing like as good as the first.
Can someone remind me the purpose of the thin metal ring on the CRT neck. I only have a vague memory of focus uniformity but it makes no discernible difference to the focus on this one.
Also it has a loose socket base. Are there any recommended adhesives?
Thanks,
The ring on mine made no difference, but the CRT was completely worn out. I believe it is to aid even focus.
Araldite is what I use, the standard stuff is fine as it allows time for application and any adjustment of position.
Don't be tempted to use super-glue. It is useless for this application.
To understand the black art of electronics is to understand witchcraft. Andrew.
Thank you! I applied the Araldite, moving the base about to 'suck' the adhesive in. I did this while the neck was cold with a view to switching on and using the heat generated by the gun to accelerate hardening.
I switched on and waited, and waited for some display - check EHT - no display - short grid to cathode - no display. Twisted the base slightly and the display returned! It's now propped in the good position while the adhesive fully hardens.
I think I'll give this CRT a second chance since I just noticed the A2 voltage is a little low. Maybe there'll be some effect with the focus ring once that is fixed.
You were lucky there then, good thing the short cleared. Sometimes the solder joints in the base pins will need to be remade as the lead-out wires can become loose with movement of the base.
The CRT emission may improve with use as the cathode re-awakens.
To understand the black art of electronics is to understand witchcraft. Andrew.
good thing the short cleared.
I think Sundog is trying to say he shorted the grid to the cathode to encourage a bit of beam current and prove the tube wasn't in cut-off.
Quite right, I mis-read.
To understand the black art of electronics is to understand witchcraft. Andrew.
Replacing the boost cap bought up the A2 voltage and now I can see some effect from moving the focus ring.
Unfortunately there's something intermittent inside the base socket, tapping it affects the brightness. I've tried cleaning and re-soldering all the pins and cleaned the socket contacts. I'm drawn towards it being something inside the base socket (rather than the CRT) as this problem only started when I rocked it to get the adhesive in.
I'm tempted to break open the base socket but only if I can get a replacement perhaps from a defunct CRT. The CRT in this set is a CME1702, neck diameter 38mm.
If you de-solder the pins, you might be able to get a scalpel blade between the glass and base to cut through the araldite and then gently remove it completely. This will enable all the old glue to be removed and the base cleaned and dried. Then refitted, glued and the connections resoldered.
The use of a needle can be made to ensure the wires are free to move in the base pin ends when de-soldered.
To understand the black art of electronics is to understand witchcraft. Andrew.
I have the same problem with the very rare Brimar C24KM CRT in the KB Regina TV set I'm readying for the Queen's Speech topic on the UKVRRR. A position for the tube base has been found and so far no problems. It's likely the insulation sleeves have broken down at the point where two wires cross over each other.
A picture of the the set and the special service table which was made for it so that the chassis can be removed with a degree of safety. The tube is about the same size as the A63-11X but unlike the colour tube it has no implosion protection.
Till Eulenspiegel.
Thank you for the suggestion. It seems like a possible job over the Xmas break as it'll require a little more concentration that can be achieved after dinner with a glass of whisky in my hand.
@Till Eulenspiegel
The focus on your TV is lovely! Are you sure you are running the CRT within it's allowed parameters?
Mod note: Your mention @xxxxxx above for David will not work. You can see it has not turned into a link, and as such it would not have sent an e-mail notification to David. The first mention you supplied in the post to Andrew (PYE625), as you can see, was valid. You MUST not make up mentions, assuming what they might be. If you are unsure of what it is, just look under the members' avatar, you will see their valid mention in brackets. Hope this helps. Davids mention was shortened a very long time ago, due to it being a handful to type in.
@Till Eulenspiegel
The focus on your TV is lovely! Are you sure you are running the CRT within it's allowed parameters?
Hi Sundog, the CRT in the big KB is operating with 16KV on the final anode when displaying a well contrasted picture, The EHT rises to 17.5KV at zero beam current.
EHT is derived from a tripler unit which is connected direct to the anode of the 50CD6G line output valve. Although the line output transformer windings were warmed up over an extended period of time, in use the EHT winding eventually failed so the decision was taken to remove the EHT winding and and find out out if a tripler would supply sufficient EHT. It certainly did so, supplying a rather high EHT of 19KV at zero beam current! The flyback tuning capacitor C60 (120pF) was replaced by a 180pF component. This action reduced the EHT and increased the picture width.
Till Eulenspiegel.
Mod: Thank you for the link info.
Thank you for the suggestion. It seems like a possible job over the Xmas break as it'll require a little more concentration that can be achieved after dinner with a glass of whisky in my hand.
Well, you never know, the whisky might help soften the araldite. 😉
But before going to that extreme, check the base socket again and any components mounted on it. It could be something as simple as a dry joint or faulty inductor (mine has L21 feeding the cathode fitted on the lower edge of the base socket).
To understand the black art of electronics is to understand witchcraft. Andrew.
