The 1000microfarad 12volt reversible electrolytic capacitor which is in series with the frame scanning coils was replaced today. Also the PL82 frame output has been substituted with the PL84. The PL84 was introduced in 1956 the same year the Pye 405 line TV set was made. Although PL84 is well known as a frame output valve in TV receivers having 110 degree CRTs I believe the valve was originally intended to be an audio output valve in TV receivers.
Also replaced are the DY86 EHT rectifier valves with the later DY802.
Focus control insulated shaft replaced with a longer one so now it is possible to adjust it with back cover in place.
Luminance delay line replaced with one from a GEC C2110.
The results of all these efforts. No improvement in frame linearity but there is more spare picture height.
The EHT regulation is much better with the DY802 EHT rectifier valves but a beam current limiter would still be a desirable addition in the circuit.
This circuit was developed in the middle 60’s by Amperex to use its new series of valves for the line output stage.
The valves 6KG6 output, 6EC4 boost diode (damper) and 3BH2 high voltage rectifier using 9 pin glass bases. The circuit is interesting due to the use of a VDR to stabilise the stage and hence the EHT. A similar idea was used in UK BW sets from around 1963/64.
The specified regulation of the EHT was 24KV at 0 beam current to 21.5KV at 0.85ma. Although Amperex state that this was adequate it appears much worse that the shunt regulation circuit designed to hold the high voltage steady from 0 to 1ma. The new circuit would I believe use less power and allow reductions in components and cost, I wonder how well it performed.
The UK valve design circuits using a tripler never seemed to have the same stiff regulation as the shunt circuit.
Further reductions in cost the article states would be to use a voltage divider from the EHT instead of the focus rectifier circuit and with tight tolerances will require no horizontal shift control.
That was a mechanical sequential colour system I believe similar to the one developed by CBS in New York. It was used to relay the 1953 coronation procession to the children's ward but probably had a more serious use as a teaching aid in live surgery.
That frame linearity issue is a real mystery.... on the face of it, you would think an easy thing to correct.
Hi Andrew,
the sawtooth waveform present at the grid of the frame output valve is perfectly linear. Without doubt the cause of the non-linearity is in the output transformer secondary circuits. Various frame output transformer have been tried.
Hi Frank,
those valves employed in the 1966 stabilised colour TV line output stage are in fact re-designated Philips sourced types:
Suggested circuit for a beam limiter. The values of the three resistors Rlim can be determined by experiment. The voltage developed across the summing resistor can be used to control the vision AGC or alternatively a contrast control device in the luminance circuits.
That raises the question in my mind, did Amperex just import the valves rebadged from Philips or have them made in the USA with a licensing deal, if they hoped to sell lots they probably had or would have had a license.
Thanks for the information David, I didn’t look further than the article.
Seems that Amperex was acquired by Philips in 1955. Those CTV valves we discussed earlier in this topic were imported from Holland. From Wikipedia: https://en.wikipedia.org/wiki/Amperex_Electronic
Over the past two days I've been trying various frame output transformers with the hope that a better one can be found. Even one from a Decca Battersea chassis. No success, so it'll have to be more modifications to the frame output circuit.
Results from todays work done to the frame timebase.
The 0.1mf coupling capacitor between the oscillator and height control replaced by one of 0.5mfd. Also, the linearity feedback capacitor returned to the original value of 0.1mfd and connected to direct to the anode of the PL82. The two 18Kohm resistors fitted across the primary of the output transformer now removed from the circuit.
The linearity is still not perfect but it's considerably better than it has been.
Thanks for your comments. I reckon the linearity can be further improved by changing the 1000microfarad series capacitor between the transformer secondary and scan coils with one of 2000mfd. The purpose of the capacitor is to allow for the injection of a controlled DC in the scanning coils for vertical picture shift.
The redrawn circuit of the frame output stage. The 1000 microfarad reversible electrolytic capacitor has been replaced with a series connected 2200 + 2200 microfarad capacitor. This is not a nice solution and in due course it will be replaced with the correct type of capacitor having the correct properties. Picture shift choke apart the circuit isn't all that different from the frame timebases in colour TV sets made eleven years later.
Hi Andrew, it is not a common practise to return the screen grid decoupling capacitor to the cathode of the valve. No doubt there is a reason for doing this. The potential between the screen grid and cathode will stable. My take on this arrangement is that it forms a cancellation circuit to correct the shape of the waveform. Something similar can be found in some audio amplifiers.
In the KB PV70 and PV100 large screen TV sets the frame timebase employs a variation on the screen grid decoupling arrangement in the Pye CTV.
In the KB the HT supply decoupling capacitor C92 is connected to the cathode of the output valve and the screen grid decoupling capacitor C94 to chassis.
It's about time this rare set received some attention. This set hasn't been switched for ages, let's find out if it still works.
The last time it displayed a colour picture was eight five years ago.
I still have the special Aurora 405 line colour converter. The 405 line colour bars generator will have to be reconstructed.
Till Eulenspiegel.
Mod note: No need to start yet another topic when you already had a sticky running for this saga since 2016. Therefore, the new topic has been merged with the old. Also, unnecessary duplicated photos removed from this post as server space is finite. Members can navigate the thread to see the various photos already attached as to the various states since this set was last tackled.
