CDA drive

[Dr-Watts]

I've heard a lot of nasty things about the CDA Drive on the pye.
On one of my recent threads someone kindly let me know that my GEC had a CDA panel.
Here's my question...
What does CDA stand for (I've got an idea) and why was it so terribly bad ?

When I started working in a telly shop in luton in 88-89, I never saw any hybrid sets, looking back I suppost that's slightly strange (or lucky), all I ever got was an abundance of G11's and Dillip saying "G11 David fixit" all pronounced as one word
Happy days

Answers on a postcard please...

Thanks

Dr. Dave

[plaka78]

From what I can remember the CD part is Colour differences but cant remember what the A stands for. With the PYE CT205 It used a PL802T for luminance which drove the cathode of the tube and three PL84s (Colour Difference) that drove each grid of the tube. This board was very troublesome due to the board being cooked by the valves causing leakage and also the high value resistors. I used to remove the old valve bases and replace them with stand off types raising the valves from the board

[Chris55000]

Hi!

It simply meant Colour Difference Amplifier! The principle was simply to amplify the (R-Y), the (G-Y) obtained from the G-Y matrix, and the (B-Y) Colour difference signals to a suitable level for driving the CRT grids, which were then AC-coupled to the grids with Clamp-triodes (triodes of PCL84s) restoring the Grid DC level removed by the AC coupling. The actual matrixing is done in the CRT itself, hence R-Y - (-Y) = R and similarly for the other two colours.

A very good explanation of CDA drive can be found in "The Mazda Book of PAL Receiver Servicing" by Mr J. Neal, and I recommend any vintage CTV collectors to get a copy - they're still to be had! This beautifully produced book has colour photos of setting-up and fault location in early CTVs, with block diagrams and colour oscillograms to help with the explanations. The book is based on the Decca Hybrid CTV25 D/S and the BRC 2000 S/S design, and covers both RGB and colour-difference drive. It also introduces the chopper PSU (BRC 3000) and the thyristor-regulated PSU (Philips G8 and BRC 8000)!

Chris Williams

[Chris55000]

Hi!

As a PS to the above I remember Manor Supplies actually selling at one time replacement fibreglass-epoxy PCBs made to the original circuit, (although not identical print patterns so as not to infringe the copyright!), and these were made for the Pye CDA panel, the Bush A823 Power supply and the G8 550 convergence panel! Although the physical layout differed from the original they were designed to take the original components and pluggery!

I suspect the time building them was a reason why they weren't more popular - I only saw 'em in Manor Supplies window for a few months!

Chris Williams

[Steve_P]

The CDA is not a bad system! It was the way Pye designed it on those boards that was the problem!! There were two types of board, and eash one was evil in it's own way. The system is not complicated, but that board had resistors which would cook and kill other things.

If you get a good one, fine. If you get a bad one.... oh dear....

I notice that you have talk on here of the PL802/T. These were solid state versions of the PL802. In some sets they didn't fit!

The PYE CDA board. Shudder...

Cheers,

Steve P

[ALANS ANITAS]

Hi

The main trouble with the Pye CDA panel was heat. Rare are the panels that are not wavey. A later manufacturing mod was to use standoff valve holders, to limit heat transfer from the valves. An earth connection is made by the panel being in good contact with the chassis, retained by the spring clips. The 2.2 Megohm resistors should be replaced by 680 Kohms.

I have used the PL802T, but the valve gave a better performance. Mine came from Ampmace in Coventry.

I am a bit puzzled by Colin55000's comments. Tha PL802 was not used in the A823 or the G8. The problems I encountered was with the A823 power supply was with BT106 thyristor and the surge limiting varistor (VA1004?). I am not so familliar with the G8.


alan

[Mike Phelan]

When I did the grid leak mod for the CDA panel (years ago now!) that Alan mentioned, that prevented the PCL84s developing positive grid volts, the valves ran cooler.
At that point, they were not too bad for reliability. Older ones without the printed LTB panel are better.

Chroma was always noisy, though, unless you had a really good signal.
Never came across one smaller than 22" - you live and learn!

The BRC 2000 that Jeffrey mentions is a great set - I was somehow thinking "hybrids"!

[flyingtech55]

I am of the opinion that colour difference drive gave more subtle and pastel colours (dunks head below parapet). The Pye 205 (yuk) was not a good ambassador of the principle. Did the G6 not use colour difference drive? Really nice subtle colours HMHO.

TimR

[Dr-Watts]

Thanks chaps
Some interesting reading there.

Kind Regards

Dr. Dave

[Mikey405]

Hi All.

As the others have said, it was always heat which was the biggest problem on these panels, although, to be fair, I've never come across a panel which has been beyond redemption. And, despite what people say, these sets can produce stunningly good pictures if they are set up correctly and have a good tube. If anyone doubts this claim, I can post a picture of an Invicta set which probably has the best picture of any of my old tellies.

I remember the fully-transistorised CDA boards for the Pye sets, but I never actually saw one in action.

Thanks everyone.

From Mike.

[Mike Phelan]

Originally Posted by flyingtech55

I am of the opinion that colour difference drive gave more subtle and pastel colours (dunks head below parapet). The Pye 205 (yuk) was not a good ambassador of the principle. Did the G6 not use colour difference drive? Really nice subtle colours HMHO.

TimR

It did, Tim, as did all the Philips hybrid CTVs - K70, K8 et al.

Any difference in the actual colour rendition was probably due to the fact that everything could be precisely adjusted (or maladjusted!) especially the G-Y matrixing.

[Studio263]

I think the advantage of the system was that the (single) Y channel amplifier could be made really good with wideband response and substantial freedom from ringing. Overlaying the colours onto a pin-sharp monochrome picture obviously yields superior results, as has been observed. The B&O colour hybrids use colour difference drive, along with a special output valve, the RCA 12HG7 (like a PL802 but better).

The reason the setmakers did it though was for stability. Imagine an RGB drive set with three valve RGB amplifiers DC coupled to the tube (Tandberg actually made one!). Both short-term drift during warm-up and the long-term aging of the valves would alter the bias on the guns and result in a grey-scale shift. Think how annoying that would be! As the colour difference stages in a CDA set only contribute to the coloured content of the picture the drift (which is still there to some extent) is only observed as a change in hue in recognisable coloured objects (e.g. skin tones).

[Heatercathodeshort]

I used to modify all Pye 691/7 CDA boards as a matter of routine when they came in for service, stand off valve holders and resistors etc and that was the end of all the problems. The colour quality was good and never complained of by customers but for some reason the wired versions produced a noise free better quality picture. The dual standard versions were superb but rare. The solid state PL802T never worked as well as the valve often producing a reduced brightness 'odd' picture. Nothing wrong with CDA drive but I expect the purists could lose sleep over it...Regards, John.

[Welsh Anorak]

Hi
I think there were two CDA problems. One was the Pye design as noted above, and the other was that PCL84s were used in most designs that tended to run hot and could leak, causing nasties.
Having said that, GEC, Decca and Bush used similar circuits and didn't cause half as much trouble as the Pye. The G6 used PCF200s and I think that might have helped, because that chassis got away with the PFL200 luminance output valve and gave excellent pictures. As did the B&O but that used a 12HG7 and three thermionic clamps as well....
I think the main advantage of CDA drive is that the mixing is done within the CRT, and therefore is kinder to tolerances within the CRT. I agree with Tim on this - I think the picure using CDA was more subtle than RGB.
As to Chris's comments - I think he meant these were third-party boards made for the troublesome circuits mentioned - the Pye was the only CDA panel among them.
As to the PL802/T - it gave quite poor results I found, especially in brightness range - also it tended to run warmer than a valve due to the thumping great resistor used instead of the heaters!
Mikey mentions the solid state CDA panel. These were made by LEDCO for around £20 and also had a big resistor on the back. I bought a few as I renovated a lot of 205s - they were ex-rental and inevitably had a decomposing CDA panel swapped into the chassis - thanks lads! Unfortunately, there was a resistor in the beam limiting circuit that changed value and gave a very bright picture for a while - that is until the CRT died. If anything they were less reliable (and gave a poorer performance) than the valved panel. For some reason the older panels survived better and often an early 691 would be cannibalised for a 697.
Glyn

[flyingtech55]

Originally Posted by ALANS ANITAS

Hi


I am a bit puzzled by Colin55000's comments. Tha PL802 was not used in the A823 or the G8. The problems I encountered was with the A823 power supply was with BT106 thyristor and the surge limiting varistor (VA1004?). I am not so familliar with the G8.


alan

I think what Chris55000 meant was that unpopulated, replacement PCBs were available from Manor supplies. The PSU PCB in the A823 used to cook in the middle under the VA1104 VDR thingy and a wire-wound resistor. Similarly, the G8's convergence PCB used to cook under some of the convergence pots etc. on the 550 versions. The only cure was to buy a whole, complete PCB assembly as a spare from Philips, Bush. Hence an enterprising manufacturer made pattern part, unpopulated PCBs so that you could build your own replacement assembly for a fraction of the price (but a considerable expenditure in time and 'botheration' factor).

TimR

[Hybrid tellies]

Going back to the cda, I think most of the Hybrid dual standard colour sets used cda rather than RGB drive, and they worked perfectly well, except for Pye that is. Grundig carried on using cda drive in many of there later sets built in the 1980's as did the Korting Hybrid chassis of the 1970's which also used cda rather than RGB drive. Both gave very good pictures.
I tend to agree with previous comments that Pye rather tarnished the cda's reputation with that troublesome cda board.

[Focus Diode]

I believe the only UK produced hybrid sets to employ transistorised RGB drive were the Decca Bradford and ITT-KB CVC5-9 series. There was one imported model that employed RGB drive with THREE PL802 valves!

Most sets with CDA used a PL802 and three PCL84 valves. Been mentioned before but Mike Phelan's suggestion to reduce the PCL84 grid leak resistors from 2M2 to 680K dramatically improves reliability in the Pye Hybrids.

The PL802T was a remarkable development. The resultant pictures were awful! I'm surprised transistorised versions of valves didn't catch on.

Surprisingly the EF184 and 6F28 work well inplace of a PL802 if an 802 is not to hand cut the print to pin 6 of the valve before trying this. It does not have to be reconnected if a PL802 is subsiquently fitted. I couldn't tell the difference when I tried it! (Thanks again Mike P for the tip).

The BRC 2000 was the first UK produced set to employ RGB drive. Some imported solid state CTVs employed CDA drive. Think the Pye hybrids (I had one stamped 10 FEB 1975) were the last in the UK to employ CDA drive.

Brian

[Dr-Watts]

Thanks chaps,
some interesting reading !

Dr. Dave