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Old 13th Jun 2020, 1:44 pm   #21
sexton_mallard
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Default Re: Murphy V2023

Quote:
Originally Posted by slidertogrid View Post
It's difficult to say how long the original line output valves would have lasted often valves were replaced unnecessarily. If a set had an intermittent fault a busy field engineer would take an educated guess and pop in a couple of "bottles" with the remark "see how it goes..." many valves were prematurely replaced I think.

When I worked for a rental company one of the engineers would replace valves that looked old and heat stained as he thought it would save breakdowns in the future however I have found that many old heat marked "old looking" valves are still fine.
To replace say a Mullard with a far eastern made cheapo probably would do nothing to improve reliability. My service manager always shopped around for the best prices. Some Solus branded valves were not very good but they were cheap...

A way to see how much use the set has had would be to look at the original valves a well used PCL805 or PCL86 will usually show signs of heat marking. Also the general appearance of the set inside, heat staining around valve bases. The condition of the solder on the high wattage "dropper" resistors (I think there are two white ceramic block types on this chassis) are all signs of use.
My mate had a new old stock Thorn 1500 some time back it absolutely sparkled inside ! I had never seen a new one inside as they were all a few years old by the time I saw them.
Thanks for this insight. The PCL86 looks to be a newish unbranded valve with 'Foreign' just as the branding. Also are images of one of the droppers and the speckled corrosion on the multisection can electrolytic showing the set was stored in a damp location. I took a picture of the CRT gun. Youtuber Shango066 demonstrated the heat discolouration of the anode rings at the back of the CRT gun as guide to the hours on the CRT. Here we can see little discolouration suggesting low hours.

As for the fuzzy picture. I'll run the set for a few more hours as I have done today with the set facing outwards into the garden playing Youtube videos while doing some DIY outside. The set may 'wake up' a bit more and that acrid pong of mouldy chipboard should hopefully disperse!
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Last edited by sexton_mallard; 13th Jun 2020 at 1:49 pm. Reason: Correction
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Old 20th Jun 2020, 12:28 am   #22
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Default Re: Murphy V2023

I replaced the two other grey high voltage caps in the frame and line stages. I then had a cramped picture at the bottom with a 3 inch band... I replaced the .015uf cap with a 0.1uf by mistake . Once this was replaced with a 1kv 0.01 cap, order was restored on the screen and after more twiddling a nice even picture was produced. A display wobble slowly went away over the next couple of hours of use and the set 'wakes up' and moisture gets driven out of the set. The odd changes in brightness and contrast with changing scenes seem to get less and less noticeable with more use...or I'm getting used to it!

I found a quite acceptable picture can be obtained by backing off the brightness a little more than usual with reasonable contrast. The set has been on for about 4 hours and the LOPT does not feel warmer than a warm cup of coffee which bodes well. On screen text is now quite legible and even if the definition is not great, it is acceptable. The damp smell is taking longer to disperse but with the back on again the insides are getting warmer than with the chassis hanging out in the service position as before. Now to tart up the cabinet a bit, mend some scratches and find some nice little feet to stand the cabinet on and to ensure the ventilation slots in the plywood base are not blocked.

I guess that there is some sort of limit on video bandwith compromising the picture quality? Lack of black level clamping on a set of this size is also a limiting factor. Was this a cheap and cheerful set in its day? I estimate this set dates from 1972-73 from the date stamps on the original electrolytics so this example is fairly early production?
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Last edited by sexton_mallard; 20th Jun 2020 at 12:35 am.
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Old 24th Jun 2020, 12:07 am   #23
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Default Re: Murphy V2023

This set must now have clocked up about 12 hours of use. I put the back on and brought it indoors and run it for a few hours tonight watching Youtube via the PS4. Apart from the volume (SWMBO told me to turn it down) I did not have to touch any other controls. The picture was stable with reasonable contrast and brightness and the 'breathing' of the picture is now hardly noticeable. Maybe someone has slipped another chassis type in there and I did not notice...

I'll keep an eye on temperatures and might add a cooling fan to the LOPT area as suggested.
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Old 24th Jun 2020, 11:07 pm   #24
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Default Re: Murphy V2023

Hi Dom,
Glad to see you have got everything working as well as it can- it certainly looks a perfectly reasonable picture!

A couple of years ago, I investigated the whole issue of black level clamping & DC restoration in UK production TV's. This was prompted by my Pilot TV in which the video is coupled to the CRT cathode by a capacitor. On bright scenes, the background went completely dark and on dark scenes the background went very light grey!

This was very disconcerting to watch so, with extensive help from the Forum, I added black level clamping. I used a double triode valve, but it can equally be done with a diode and suitable transistor.

The result was a major improvement and could be added to any TV. I also did the same to my Thorn 1500.

It seems that almost no B&W TV's had black level clamping or maintained the DC level to the CRT. I only managed to find about five sets that addressed black level after trawling through most of the R&TV servicing books!

What was more surprising was the comments from the chaps in the trade when these sets were new. It seems no one ever moaned about the lack of black level correction but they did seem to mind when the background went black on dark scenes! They thought it was a fault!

So maybe manufacturers realised that sets were better off without it!!

In terms of picture quality, I'm sure sets were much better when they had 90 degree 'fat neck' CRT's. I would guess 110 degree thin neck tubes were a real compromise. Focus at the edges always seems poor and the potential applied to the focus electrode never seems to really do much.

I still think this is a really nice looking set- even better now you have a picture to go with it!

Well done!
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Old 24th Jun 2020, 11:15 pm   #25
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Default Re: Murphy V2023

Customers had got used to sets without the black level clamp or keeping the DC level constant. They expected the screen to to be grey between scenes. When RBM fitted clamping it caused caused problems for some.
Thin neck 110 degree CRT’s could give excellent results, it was later imports when UK manufacturing was running down mono CRT production that were poor from new.
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Old 25th Jun 2020, 10:59 am   #26
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Default Re: Murphy V2023

The A640 had black level clamping as, of course, did the A693 whaich was the single standard version that was around at the same time as your A774 and gave vastly superior pictures. Could have been cost cutting on the 'new' chassis or they had complaints about the effect. However the black level did seem to wander on the A774 far more than on, say, theThorn 1500.
It was a bit confusing model wise. For example the Bush TV183D was the dual 640, the TV183S was the single standard version and the TV183SS was the A774 - at least I think it was that way round.
I agree the original thin necks were good, though perhaps not as long lived as the old soldiers. Mullard as usual led the field, with Mazda equivalents soon becoming defocused at the edges.
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Old 25th Jun 2020, 11:07 am   #27
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Default Re: Murphy V2023

Yes I had forgotten about Mazda CRT’s but Mullard were excellent.
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Old 28th Oct 2020, 12:15 pm   #28
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Default Re: Murphy V2023

This set has now run for many hours and....still works .
The slightly grey muddy picture is not apparent until another set is run side by side. This probably was a low hour set? The attached pic does not show the picture quality which is better in the flesh.
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Old 28th Oct 2020, 8:43 pm   #29
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Default Re: Murphy V2023

The Mullard AW43-88/89 through to but not including the 20" and 24" A50-129WR were probably the best CRTs ever produced. They are still 100% today.
They did not suffer from that frustrating partial heater short that the 38mm types suffered with.
The Mazda equivalents, CME1703 etc gave superb pictures but soon went fuzzy and lost focus especially at the edges. I replaced endless numbers of them but not one single Mullard 110 degree AWxx-xx.
John Brown my mate the rebuilder [Central Tubes] always said the best combination was a Mullard gun and a Mazda screen. I have one of his tubes that he rebuilt in 1965. It is very bright and focused to the corners. Happy days! John.
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Old 28th Oct 2020, 11:24 pm   #30
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Default Re: Murphy V2023

I had A774's for my C&G practical exam.
One of the sets had an OC DY802 heater winding loop. I undid the screw and lifted out the ceramic valve base just to visually confirm how it was done but I had difficulty getting the base reseated and the screw back. I managed it just in time but the guy who I was due to exchange sets with saw me struggling and I'm sure he sabotaged his set to pay me back!
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Old 29th Oct 2020, 2:39 pm   #31
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Default Re: Murphy V2023

They were not the most liked set but its nice to see another A774 saved and working well. With the work you have done this set is looking good and will probably remain working for many years. That Mullard tube does look nice sharp and bright.
At he moment I only have one working single standard 625 black and white TV which also has the original 20" Mullard tube. I try to run it once every few weeks for a few hours to keep it in trim.
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