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TDA3562 in a TX10?...

 
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TDA3562 in a TX10?...

Post by Rebel Rafter » Thu Nov 05, 2015 6:20 pm

Hi from RR. Here's an interesting idea. I was rummaging through my collection of "television" magazines the other day looking for something but didn't find it, but I did find an article about the TDA3562 colour decoder chip. And I looked at the internal block diagram and it looks like it allows brightness and contrast control over the RGB inputs, ( I don't know about the colour ) so I thought I wonder if it's really practical to try and fit one in a Thorn TX9 or TX10. I know this chip has some extra features like auto greyscale to compensate for ageing tubes, and needs some special feedback from the RGB output stages and it needs a 3 step sandcastle pulse which according to the article it can get from a TDA2578 which is also used in the TX10, I can't remember which sync chip is used in the TX9 but I know it's different one. Does anyone have any idea if this is a realistic idea or would it be a bit too impractical? RR.

 
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Re: TDA3562 in a TX10?...

Post by jayceebee » Thu Nov 05, 2015 9:36 pm

Hi RR,

A good idea but not straightforward. The TDA9503 timebase generator used in the TX9 I believe produces the required sandcastle pulse, the problem is the class A RGB output stages are incapable of providing the necessary feedback for the TDA3562A and its auto greyscale function. It may be possible to adapt the TX10 RGB output but would be an awful lot of work.

To do this would be best achieved by using the CRT base board from a TX100. Some thought would also needed to be adapt the beam limiter operation which ISTR acts on the contrast pin of the TDA3562A.

Good luck if you try it.

John.

 
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Re: TDA3562 in a TX10?...

Post by Rebel Rafter » Thu Nov 05, 2015 10:49 pm

Hi, Jayceebee, RR here. I've never seen the TX100, does it use a TDA3562? What tube does it use, is it one of those that uses different types in different versions like the Rediffusion Mk4. The only set I ever had with a TDA3562A was a Panasonic U5 chassis which had no colour due to a knackered chroma filter and I couldn't find another one anywhere so it got scrapped and I haven't got the circuit anymore. I could probably find an application circuit diagram for the TDA3562A online somewhere, I've done that before. RR.

 
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Re: TDA3562 in a TX10?...

Post by Mark Hennessy » Thu Nov 05, 2015 11:12 pm

I once built a PAL decoder based on the TDA3562 into an RGB computer monitor. It worked pretty well, considering how inexperienced I was at the time... The circuit was based on the TX100. I'll still have it (just the decoder board) somewhere in the attic.

As for the TX10, forget it. What does it bring to the party? Auto grey-scale and contrast adjustment to the RGB inputs. You've already got brightness control with the TX10, of course. No, you won't get saturation adjustment - and nor should you want it. One of the nice things about going digital is that finally, people aren't able to get the colour control wildly wrong - it's easy to forget how bad things were in the PAL-only days!

To adjust the saturation of an RGB signal, you need to transcode from RGB to component. Then, you can alter the gain of the B-Y and R-Y signals (these must be altered simultaneously, and track together correctly). Having done that, you then have to transcode back to RGB for application to the CRT. None of that is especially difficult, but in a consumer set where you are desperately trying to save every penny, it's not going to make it past the "cost-benefit analysis" stage. Imagine the meeting: "You want to do that to allow the customer to make their pictures worse?". But having said that, I have seen a few high-end Sony sets that provide the facility - there's no-doubt others as well. These days, with digital signal processing, it's a bit easier to implement.

Years ago, I did fit a TDA3561 to my TX10 as an upgrade. It's a pin-compatible swap. I can't remember what improved, if anything - this was 25 years ago - but I believed I saw an improvement, and the chip hadn't cost me anything...

If you desperately need to adjust the contrast of an RGB signal, there are chips out there that will do that for you. You could even use a ganged pot - if people still make them. I did it once using a triple 1k job (need a low value to minimise HF loss). In practice, most RGB sources have a contrast adjustment hidden away in the menus. What I do - which is cheating, I admit! - is to borrow our Philips colour analyser from work and set the RGB gain controls to give me the "studio standard" 80 Candelas light output for 700mV in.

 
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Re: TDA3562 in a TX10?...

Post by jayceebee » Thu Nov 05, 2015 11:53 pm

Hi RR,

The TX100 was very flexible, in its various flavours it had the possibility of many options and could drive a range of CRTs, 90 degree, 110 degree, FST, an additional small daughter board was fitted if it required E-W correction. Yes, it did use the TDA3562A.

Mark, I believe the TDA3561 was introduced to improve the range of customer brightness adjustment something that the TDA3560 was lacking in. I'm not aware of any other advantages but there may have been improvements in other areas of the ICs spec.

Something I always wanted to do with the TX100 chassis was to add CTI "Colour Transient Improvement" which was fitted to a lot of continental sets such as Grundig etc. The TX100 PCB had provision for an extra 16 pin IC and other components around the TDA3562A, I worked out this was for a TDA4660 CTI IC but it was never fitted in the UK sets and never appeared on any circuit diagrams. Does anyone know if the TX100 was ever exported to Europe and if so was CTI included?

John.

 
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Re: TDA3562 in a TX10?...

Post by Mark Hennessy » Fri Nov 06, 2015 1:54 pm

jayceebee wrote:Mark, I believe the TDA3561 was introduced to improve the range of customer brightness adjustment something that the TDA3560 was lacking in. I'm not aware of any other advantages but there may have been improvements in other areas of the ICs spec.


I didn't have time to investigate further last night, but it was rattling around while I was trying to get to sleep. I would have got my information from a back-issue of Television magazine, which was my only source of data about television and video theory at the time, but no idea which issue that would have been in...

The datasheet says this:

Additional to the TDA3560, the circuit includes the following features:
• The peak white limiter is only active during the time that the 9,3 V level at the output is exceeded. The start of the limiting function is delayed by one line period. This avoids peak white limiting by test patterns which have abrupt transitions from colour to white signals.
• The brightness control is obtained by inserting a variable pulse in the luminance channel. Therefore the ratio of brightness variation and signal amplitude at the three outputs will be identical and independent of the difference in gain of the three channels. Thus discolouring due to adjustment of contrast and brightness is avoided.
• Improved suppression of the internal RGB signals when the device is switched to external signals, and vice versa.
• Non-synchronized external RGB signals do not disturb the black level of the internal signals.
• Improved suppression of the residual 4,4 MHz signal in the RGB output stages.
• Cascoded stages in the demodulators and burst phase detector minimize the radiation of the colour demodulator inputs.
• High current capability of the RGB outputs and the chrominance output.


So, quite a reasonable upgrade. That's my recommendation to RR :thumb

 
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Re: TDA3562 in a TX10?...

Post by Till Eulenspiegel » Fri Nov 06, 2015 9:00 pm

After the TDA3562 Philips developed the TDA3566. I fitted the later chip into a Ferguson 51A3, a 21" FST teletext model which was fitted with the TX100 chassis. The set wasn't very old when it was accepted as a trade-in against another set of another make.
The set turned out to be a butchered heap of junk, the PCB had been got at, particularly around the decoder. The print around the TDA3562 was in bad condition so to facilitate replacement in the future an IC holder was soldered in. This was the early 1990s and the TDA3566 was the current production type to replace the TDA3562. The TDA3566 is a drop in replacement for the earlier chip, well almost. Some minor mods were required.
The later decoder chip did seem to produced a better picture, even better than the original TX100 which was pretty good anyway.
Getting back to the TX10. You'll need to study the circuit diagrams of both the TX10 and TX100 to work out if the modification to the TDA3562 is viable. If it is, why not use the later and better TDA3566?.

Till Eulespiegel.

 
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Re: TDA3562 in a TX10?...

Post by Rebel Rafter » Sat Nov 07, 2015 12:02 pm

Hi, all, RR here. I've been looking at application circuits for both the TDA3562 and the TDA3566 and I've noticed that they both have the composite video input at the standard 1 volt p-p unlike the 2.7 volt input on the TDA3560. So if I was to try and fit one of these later chips all my hard work with video preamps would've been all for nothing, so I think I'll get some TDA3561's and try them and see what if any difference they make, they're available cheap enough from Cricklewood. RR.

 
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Re: TDA3562 in a TX10?...

Post by Till Eulenspiegel » Sat Nov 07, 2015 1:12 pm

Data about the TDA3561: http://www.farnell.com/datasheets/101139.pdf
We are informed that the TDA3561 is an improvement over the TDA3560.

Till Eulenspiegel.


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