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Old 23rd Apr 2016, 11:23 am   #1
1100 man
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Default Pye CT205

Morning all,
I'm new here but have been reading posts on and off for years. I've posted a bit of an intro in the New members section.
My current project is a CT 205 with the (I believe 697 chassis). I bought it for £1.00 about 10 years ago from the Wooton Bassett auction and it has sat doing nothing ever since! Since the demise of analogue broadcasting, it seemed only right that I dusted off my memories of hybrid tv's and got it working.
This was the first colour tv I aquired in my youth and I remember spending many hours sorting out all its faults. I never was happy with the picture and replaced the entire chassis with a Philips G8!! So this is a bit of a nostalgia trip.
However, I digress (as I often do).

The set is not in bad condition and the cabinet is good. Internally, it does not look like it has had a hard life. The CDA board is not cooked and even the line scan board only has one track repair! All the solder joints look undisturbed so I guess it has not been repaired very often.
I've never been a fan of powering things up on a variac so I removed the cap across the mains and switched on.
A shower of sparks shot out of the HT thermistor but it calmed down and the valve heaters started to warm up. There was hiss coming from the speaker.
Soon a very bright green raster appeared and shortly after some big cracks from the EHT cap.
I removed the cap and cleaned it up with IPA. The area round the connection to the tube was de- gunged as was the tripler and focus unit.
The bright green screen was traced to a broken pin in the PCL84 valve holder. Bending what was left of the pin effected a remarkably stable cure!
Switch on again and all was quiet, valve heaters glowing, gentle hiss from the speaker. I was waiting to hear a line whistle and assumed there was a fault. Looking round the front I saw a picture!! Seems I can't hear a 625 line whistle any more- oh well (20 years from now I probably won't be able to hear a 405 one!!)
Tuned in one of the few working buttons to a colour bar pattern and the picture wasn't too bad. For a few minutes that was then the colour disappeared, never to return.
Switched to cross hatch pattern and the convergence wasn't too bad. Adjusted the line shift and the pot fell apart!!
Have just started investigating the decoder fault and am re- learning how a decoder works.
Hopefully will have more to report soon
Cheers
Nick
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Old 23rd Apr 2016, 11:54 am   #2
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Default Re: Pye CT205

The HT thermistor was a stock fault, TH7 comes to mind, about the size and shape of a two pence piece.
Frank
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Old 23rd Apr 2016, 2:12 pm   #3
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Default Re: Pye CT205

Hi Frank
It was indeed once the size of a 2p; sadly no longer as several bits have fallen off it and it looks very sorry for itself. It seems to have arced itself back together for the moment.
It drops 3V when the set is running wheras the 3R3 in series drops 1V. My plan therefore was to replace it with a 10R wirewound. I can probably live with a change in picture size as everything else warms up- I'll just have to try it and see!
Nick
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Old 23rd Apr 2016, 3:06 pm   #4
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Default Re: Pye CT205

They are still available, RS have TH7 thermistors or I found NOS on line for around £2:50.
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Old 23rd Apr 2016, 8:58 pm   #5
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Default Re: Pye CT205

Hi Nick.

The lack of colour is almost certain to be one of those pesky BC148s - It shouldn't be too difficult to find out which one but if you're feeling in the mood then I should replace the whole lot of them with BC108s - those old lockfit transistors are astonishingly unreliable and fail at the drop of a hat. The BF194s are just as bad and they can be replaced with something like a BF199 or BF255.

Once it's working with a nice new set of trannies you should have a tip-top picture and a nice reliable decoder.

Kind regards.

From Mike.
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Old 24th Apr 2016, 10:08 am   #6
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Default Re: Pye CT205

The colour is back!!
The culprit was VT21, delay line driver. The circuit says BC148 but there was a BF595 in place. Judging by the solder, it was original- in fact the underside of the decoder and CDA boards look like new. Non of the components look to have been replaced!
Ironically, this was the only non-lockfit transistor in the decoder!!
Replaced with BC548, adjusted delay line balance to reduce Hanover blinds, tweaked T5 (AFC coil) to get optimum tuning and a nice set of colour bars was displayed.
My attention will now focus on the line scan board. Remarkably, all the components, except R203, look clean and tidy. R203 has disintegrated as has the line shift pot. I will also try and source the correct thermistor (VA1104) in the HT feed.
If I can figure out how to get pictures into a post, I will include some.
Cheers
Nick
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Old 24th Apr 2016, 12:10 pm   #7
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Default Re: Pye CT205

Use the advanced button and then the manage attachments button.
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Old 24th Apr 2016, 3:44 pm   #8
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Default Re: Pye CT205

Hi
I have a NOS VA1104 if you would like it please send me a pm and I will post FOC
Regards Derrick

Last edited by 1955APREN; 24th Apr 2016 at 4:12 pm. Reason: word missing
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Old 24th Apr 2016, 5:03 pm   #9
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Default Re: Pye CT205

Ooo! thanks very much, I will send you a pm:
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Old 7th May 2016, 10:04 am   #10
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Default Re: Pye CT205

Hi everybody,
Have just returned to the Island and hope to do a bit more to the Pye hybrid amongst other projects.
Very many thanks to Derrick (1955apren) for sending me a va1104 thermistor to replace mine which emits a shower of sparks at switch on! Will fit it this afternoon along with a new 47k resistor (R203) which has disintegrated.
Does anybody know the official mods to enable the set to be used with a VCR? They used to publish lists of these in 'Television' magazine every so often. I imagine it will involve reducing the value of various caps around the flywheel sync circuit but it would be nice to know which ones.
At the moment, with a vcr, the picture shakes like a jelly!!
I am also going to attempt to repair the line shift pot which has fallen apart.
Cheers for now,
Nick
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Old 7th May 2016, 11:06 am   #11
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Default Re: Pye CT205

This series of sets are notoriously bad for VCR use. Suggested modifications below taken from an article in "Television" May 1982:

Change R36 to 15K
C202 to 820pF
C203 to 180pF

This gives tolerable results but I found this didn't completely eliminate the problems with some hooking at the top persisting for example.

Brian
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Old 7th May 2016, 2:01 pm   #12
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Default Re: Pye CT205

Thanks Brian, I will try that later and do some experiments to see if I can improve on those mods.
I plan to use it with a 3v29 vcr which I imagine will have extremely jittery sync compared to a modern vcr, so will just have to see how good I can get it.
Cheers
Nick
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Old 7th May 2016, 6:34 pm   #13
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Default Re: Pye CT205

Quote:
Originally Posted by 1100 man View Post
jelly
I never had much success with running VCRs through these sets. You could always cheat though and get one of those tiddly little "CYP CTB-100" Timebase Correctors. They also do a good job of removing any Macrovision-fading-type problems when playing DVDs through the old sets etc.
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Old 8th May 2016, 1:08 am   #14
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Default Re: Pye CT205

Thanks Mikey, I'll bear that in mind but I'll see how good I can get it first.
Leaving VCR issues for a bit, I have a more pressing problem. There is too much width and a slightly stretched raster on the LHS (viewed from front).
The width coil is set to minimum and the LIN coil to an optimum position but the picture is a good inch or more too wide.
The EHT is set to about 22Kv and I've checked the resistors across the width and lin coils.
I,m guessing it will be a capacitor but if anyone has any ideas that would be great.
Cheers
Nick
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Old 8th May 2016, 5:33 am   #15
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Default Re: Pye CT205

Hi Nick.

The EHT sounds a bit low, which could account for the width issue. There is a twiddler to set the EHT and once it's set to 25KV then the width should come in a bit. If not then there's a 10M resistor in the grid of the PL509 - They normally go high, giving a lack of width but (according to the McCourts book anyway) they can go low giving too much width. But my money is on the low EHT.

Thanks Nick.

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Old 8th May 2016, 6:07 am   #16
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Default Re: Pye CT205

Low EHT means the electrons will be accelerated less, which means they will fly more slowly, which means they will spend longer within the influence of the deflection coils, which means they will be deflected further, which means the picture will be bigger.

If the distance through the vertical-deflection field is greater than the distance through the horizontal-deflection field, then the effect of reduced EHT on the picture hieight will be less, and the control may have enough range to compensate.
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Old 8th May 2016, 3:01 pm   #17
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Default Re: Pye CT205

Thanks Mike and Julie for your suggestions- I often wondered why the picture got bigger with low EHT.
With EHT control at max (about 25KV on my meter) and the brightness fairly low, I can just see the edge of the picture. The extreme left is severely cramped.The castellations on my pattern generator are squashed into a couple of vertical lines. The scan is then stretched for the first couple of inches and then it evens out.
I have hooked up the 'scope to display the line drive at the grid of the PL509 and simultaneously the stairstep video waveform.
The drive voltage cuts off just before the sync pulse as I would expect but doesn't get going again until half way through the first white bar of the pattern. So the oscillator would seem to be 'off' for too long.
I have checked all the resistors around the line osc and replaced a couple that were high. I will investigate the caps in this area next.
Any ideas??
I'm off to cut the grass
Cheers
Nick
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Old 9th May 2016, 12:26 pm   #18
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Default Re: Pye CT205

The two polystyrene capacitors in the line oscillator can and do give odd symptoms. Also check the electrolytics in the line oscillator circuit - I have 12 and 4 uF in my book but it's the ancient dual-standard circuit so yours may well be different.
Glyn
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Old 10th May 2016, 10:40 am   #19
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Default Re: Pye CT205

Success!!, well partially.
Changed the electrolytics and polystyrene caps mentioned by Glyn but no difference. I noticed after the set had been on for 10 mins, C219 (180p 8kv) mounted on the LOPT looked a bit hot and bothered. Not having anything suitable, I had to string a load of 1000p caps in series to get anything small enough.
It proved the point, however, and reduced the excess width by a good inch on each side. It also got rid of the foldover on the extreme LHS of picture and enabled the set to run without the eht control at max! Its value seems critical so will get the correct cap.
Sadly it hasn't done anything for the poor linearity on the left hand side so I will carry on searching for the culprit.
However, definite progress
Cheers
Nick
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Old 10th May 2016, 12:48 pm   #20
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Default Re: Pye CT205

Good progress indeed Nick. You'll be able to adjust the width and line linearity satisfactorily with this capacitor changed.
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