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Pye 405 line CTV.

 
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Re: Pye 405 line CTV.

Post by Michael Watterson » Tue Jan 26, 2016 1:03 pm

Like 1970s :) Proves it's got a colour CRT :D

Excellent progress. I doubt I'd have the patience and nerve to work on such a set nowadays.

 
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Re: Pye 405 line CTV.

Post by Studio263 » Tue Jan 26, 2016 3:17 pm

Niall wrote:That's crying out for a caption :-)

Despite a large investment in equipment, the cover for "Introspective" still wasn't quite right.

 
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Re: Pye 405 line CTV.

Post by Briancuff » Tue Jan 26, 2016 8:27 pm

Excellent work, Till. You're soon going to prove that that BBC engineer at the CCU was telling the truth!! :) Is the frame linearity better too?

 
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Re: Pye 405 line CTV.

Post by Till Eulenspiegel » Tue Jan 26, 2016 9:36 pm

Hi Brian,
I'm sorry to say that although there is more spare height available the linearity is as bad as ever. Later today or more likely tomorrow I'll scope the waveform present at the grid of the PL82 frame output valve. I'm thinking about introducing pre-distortion to the drive waveform to improve the linearity. The inverse parabolic waveform at the cathode of the frame output valve has an amplitude of 10 volts sitting on 10 volts DC bias.
Removing the twin chokes which isolate the frame shift from the line shift control has resulted in more picture width and more EHT. I haven't measured the voltage but I reckon it's some scary figure. The 6BD4A shunt stabiliser glows a nice and red under low CRT beam current conditions. X ray tube? The reduced loading on the line output stage has caused this effect.

I just don't believe Pye made a TV with such bad frame linearity. I've just gotta tough this one out.

Till Eulenspiegel.

 
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Re: Pye 405 line CTV.

Post by Till Eulenspiegel » Wed Jan 27, 2016 5:13 pm

The circuit of the frame timebase. The very low value 2.2mfd cathode bypass capacitor was the norm for almost all hybrid CTVs. Connect a 220microfarad electrolytic across the 2.2mfd capacitor and the linearity is almost perfect but of course the parabolic waveform for the convergence circuits is lost.

Till Eulenspiegel.
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Re: Pye 405 line CTV.

Post by nuvistor » Wed Jan 27, 2016 7:52 pm

Does this mean the PL82 is not up to the job? Looking at an early RCA circuit it uses a triode for vertical output and and another valve for vertical convergence amplifier. I Suppose it depends on the deflection angle of the CRT and its neck diameter, the use of a triode did surprise me.
Frank

 
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Re: Pye 405 line CTV.

Post by Bobhowe » Wed Jan 27, 2016 8:50 pm

Hi David been holding my breath since the mishap & i am feeling guilty about holding the poll :ccf no doubt you will get it sorted at least its a real tv & not a lcd rubbish

 
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Re: Pye 405 line CTV.

Post by Till Eulenspiegel » Wed Jan 27, 2016 9:27 pm

Hi Bob,
One thing is sure, I'm certain there is a solution to the timebase problem. Could it be that the designers of this set were happy to tolerate the bad frame linearity? Somehow, I don't think so.

LCD TVs? Just a waste of time. I've called it a day with those awful things. Folks just don't want to spend any money on repairs these days. I've no idea what the hourly rate is for a TV engineer these days, £10 , £15 and hour. anyone here got a clue?

So I'm looking for something else to do.

Hi Frank,
Some American colour sets certainly do get away with a triode frame output valve. Motorola model TS902A-03 employs a 6BL7 double triode. Not sure if this is the special double triode in which the two triodes has different characteristics. A low power triode for the blocking oscillator and a power triode for the output. A separate convergence amplifier is best solution but that increases the valve count.
The 6BL7 is a big beefy thing. 10watts pa.
http://www.radiomuseum.org/tubes/tube_6bl7gt.html
Till Eulenspiegel.

 
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Re: Pye 405 line CTV.

Post by Bobhowe » Wed Jan 27, 2016 10:15 pm

Hi David i agree with you there you will get it sorted could it have been a batch not properly finished during development ? ( will start a thread on cars about a Nissan Micra Diesel that was not put into production )

 
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Re: Pye 405 line CTV.

Post by Katie Bush » Wed Jan 27, 2016 10:44 pm

Okay.... A bit blurred and convergence a bit off, but it's looking promising once more.. Geometry looks good, and the overall grey looks right, so it looks like you've mastered the foundations at last, now to get the colours and decoration sorted?

Marion

 
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Re: Pye 405 line CTV.

Post by Till Eulenspiegel » Wed Jan 27, 2016 11:08 pm

I'm going to add an interlock switch to the line output stage. Either that or simply a toggle switch to disconnect the line output stage while tests on other parts of the set are being carried out. A dummy load can be introduced into the circuit while the line OP stage is out of the circuit.
The efficacy of the X ray shielding is unknown.

Till Eulenspiegel.

 
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Re: Pye 405 line CTV.

Post by Till Eulenspiegel » Thu Jan 28, 2016 12:17 am

I think you can now guess how this linearity problem is going to be solved. A separate convergence amplifier. Another valve, a PL82 for example if one can be introduced into one of two the heater chains could be employed. Certainly it will have to be a powerful enough valve for the job. Convergence circuits do require considerable current drive. The trouble is when you abide by the rule, "only use the tech of those times" it's not so easy do such modifications..
Looking the circuits of various American sets, valve convergence driver circuits were used in many early sets. For example the CBS 205.

Till Eulenspiegel.

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