Pye 405 line CTV.

[FERNSEH]

It's getting on for almost eighteen years since this rare colour TV set came into my possession and in all that time I haven't really done much to it.
The set always had bad frame linearity and it's been my belief that Pye would never make a TV set irrespective whatever model it is with such a bad fault condition.
So this week I set about to do something to improve the performance of the frame timebase. Over the past three days I tried various frame output transformers with the hope that a better one can be found. Even one from a Decca Battersea chassis. None of the transformers tried out improved matters so the original transformer was wired back into the circuit. Perhaps a change of component values might bring about an improvement?
The coupling capacitor between the oscillator and output valve grid raised in value from 0.1uF to 0.5uF. Components in the linearity feedback loop changed in value or removed.
These component changes have effected a worthwhile improvement to the frame linearity, but it's still not perfect. See the attached pictures. Still more work to be done.

DFWB.

[Argus25]

One thing initially to try is to lift the ground legs of the 270R cathode resistor and the 2.2uF capacitor and place a small current sensing resistor in there, maybe 4.7R to ground. Then you can put a scope across that and see if the waveform shape during V scan on a scope matches the linearity you are seeing on the raster. The difference there will indicate how much of the linearity problem is due to the properties of the output transformer and yoke, vs the drive current (anode current shape) and it will help you see what you are dealing with if there is an issue there.

As you can see they modify the shape of the output valve's drive voltage by a signal fed back from the output transformer's primary. If you cannot attain good linearity by altering the values of their existing circuit (have you tried adjusting the 2.2uF cap ?), there are three things you could try.

The first thing that is wrong with the design there should ideally be a resistor (peaking resistor) in series with the sawtooth integration capacitor (the one from the plate of the ECC82 to the cathode of the output valve) This is essential to create the proper trapezoidal grid drive waveform, when its not there the output valve doesn't cut off well at retrace and the linearity at the top can be off. So its worth a try adding a little resistance there, possibly a few k.

(One thing trying to fix a circuit like this by altering values, it is sometimes not obvious that a component might need to be added in series with another)

Place a resistor in series with the valve's cathode to create some negative feedback, increase it until the scan amplitude drops to about 75 to 80%. Then increase the height control and check for any improvement.

Or possibly if the output transformer's secondary and yoke connections are isolated, wire those into the cathode circuit (or a part of the voltage generated there) much as is seen sometimes in audio output stages for a small amount of negative feedback.

You may be up for some experimentation.

[Argus25]

Your problem here dragged up a very old memory. I looked through my old papers and found a diagram I made when I was about 16 playing around in my bedroom with vintage TV chassis when I should have been out socializing and going to discos like a normal teenager.

I had a vintage set that I struggled with for weeks, I could not get control over or satisfactory vertical scan linearity. When one part of the scan was right, another would be wrong. Also I didn't own a scope, but I had the raster to look at.

I had been playing around with some valves for audio and experimenting with triode connected pentodes to reduce distortion. I got the idea to apply this to the vertical output valve, to run it as a triode and I found it was also a way to control the linearity on part of the scan. I have attached the circuit. As I recall this solved all the issues and I had a scan on a 12" CRT with perfect scan linearity. Looking at this circuit now, I think I might have made a mistake documenting it, not sure.

You could always start from scratch and re-design the stage you have to try something like this as a method to solve the problem.

[System A]

Hi David.

That looks a lot better, it's the best linearity so far. Keep up the good work.

Regards, Gary.

[FERNSEH]

Hi Gary,
The attachments show just how bad the frame linearity was.

Hi Argus,
I'll insert a 4.7 ohm resistor in series with the PL82 cathode bias components and then connect up a 'scope to monitor the performance of the output stage.

DFWB.

[Heatercathodeshort]

I love it David! That lin looks within acceptance but it should be 100% with a colour receiver of this standard. What a rare instrument! John.

[FERNSEH]

Hi John,
It's certainly an impressive looking beast and it has created a lot of interest in the shop. Folks can't believe that they are looking at a sixty-two year old colour TV set. And it's British!
Considering the high standard of construction we can't believe this was a one-off receiver. Just how many of these receivers did Pye make?

DFWB.

[Argus25]

One thing about the circuit I put in post 3, when I arrived at that it was from many hours of experimentation and it did solve the problem by providing excellent control of the linearity of the middle section of the scan too.

But at that time I was completely unaware of the the nature of the load in magnetic deflection circuits and the fact that the combination of inductance and resistance there required a trapezoidal, not sawtooth drive wave for the output valve.

So I don't think or recall ever trying the small peaking resistor in series with the sawtooth integration capacitor.

Of note; the more inductive the yoke, the more of a peaking resistor is required, the more resistive the load the less its required and in the end with a purely resistive load, if it was one, you require the sawtooth drive only and not a trapezoid.

In general, if a peaking resistance is not there, the linearity at the start of scan ends up getting compromised , the scanning lines compressed together. Then if the usual linearity controls are used to help correct that, the linearity is off in other places. Its always a problem that happens at higher drive levels too. So over the years I have learnt the importance of this resistor.

Regardless of what voltage waveform you apply to the output valve's grid though, some shaping of the drive waveform with feedback is often required due to the transfer characteristics of the output valve. In many American sets, to get around that, they just ran the output very degenerated with an un-bypassed cathode resistor, of course this lowers the gain and requires a higher amplitude drive voltage.

[Heatercathodeshort]

I have seen a few 405 line colour receivers and I must admit the picture was so good that it was difficult to ascertain if they were actually 405!

We could have gone down the 405 path with colour but it was probably a journey too far. It just proves how good the World's first regular television service was starting off in 1936 with a break for WW2 and a short period in 1947 due to one of the worst winters ever, causing a power emergency.

A very rare receiver David that must be preserved, especially as it was produced at Cambridge England. All the leading manufacturers produced experimental 405 line colour receivers. Regards, John.

[FERNSEH]

HKS wrote: "A very rare receiver David that must be preserved, especially as it was produced at Cambridge England. All the leading manufacturers produced experimental 405 line colour receivers. Regards, John."

Including Murphy Radio Ltd.
Attachments show the 405 line colour TV receiver manufactured by Murphy during the mid nineteen-fifties.

Argus25 wrote:
"Regardless of what voltage waveform you apply to the output valve's grid though, some shaping of the drive waveform with feedback is often required due to the transfer characteristics of the output valve. In many American sets, to get around that, they just ran the output very degenerated with an un-bypassed cathode resistor, of course this lowers the gain and requires a higher amplitude drive voltage."

The frame output valve's cathode bypass capacitor has the very small value of 2.2microfarads and this will favour the higher frequency components of the waveform. Therefore the reason why the linearity is so degraded is because the 50Hz fundamental part of the waveform is of a lower amplitude than it should be. The selective feedback of the linearity controls do correct matters but more work needs to be done.

I remember reading an article in the Wireless World in which the contributor created a sawtooth waveform by adding together a series of sinewaves in the correct values of amplitude and frequency.

DFWB.

[FRANK.C]

I like the sentence on the brochure "The cabinet is small enough to go through an average living room door."
Today you will find the same phrase on the brochure of an American style fridge freezer.

Frank

[Heatercathodeshort]

I discovered this article in the 'OSRAM BULLETIN' Published by the GEC for October 1958. The control knobs are from the monochrome model BT1748 from two years earlier suggesting development of this receiver pre dates the article by at least two years. John.

[Dave Moll]

I notice that one of the men pictured with the G.E.C. set is Peter Carnt, who was one of the authors of an excellent book about the NTSC System of colour television and the British 405-line version in particular.