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Old 23rd Mar 2018, 1:15 pm   #1
Murphy Maniac
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Default EKCO TC267/1 going negative

I am lucky enough to own an Ekco TC267/1 in fantastic original condition however, after approximately 1 hour of use the picture suddenly turns negative. If I turn it off and leave it to cool down, it will come on again as normal with a very clear, well focussed and bright picture which is easily viewable in daylight. Could anyone please help with suggestions as to the cause and possible remedy?
I have enclosed a picture which, owing to my very poor photographic skills, does not do it justice at all.
Thanks for looking,
Geoff.
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Old 23rd Mar 2018, 1:47 pm   #2
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Default Re: EKCO TC267/1 going negative

Didn't early 405 sets have a "vision interference limiter". When advanced too far, the picture would turn negative. Maybe an intermittent fault in the limiter could be your problem, possibly the potentiometer?
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Old 23rd Mar 2018, 2:02 pm   #3
Al (astral highway)
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Default Re: EKCO TC267/1 going negative

A generic reason, not specific to your set: this would happen in a black and white set iif the positive video signal on the control grid of the CRT —-with respect to the cathode— goes -ve, instead of positive. So anything that could affect this balance would do it . I don’t know more than that.
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Old 23rd Mar 2018, 3:55 pm   #4
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Default Re: EKCO TC267/1 going negative

Try substituting the 6/30L2, I have had this before on a T293 which is the same chassis. In my case one half of the triode was very low emission whilst the other was normal. I also replaced the 30FL1 as this too was very low emission but not the cause of going negative.
This suggestion based only if all the paper capacitors have been changed.
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Old 23rd Mar 2018, 4:03 pm   #5
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Default Re: EKCO TC267/1 going negative

I seem to recall that, in these sets, if the contrast control was advanced too far, and the aerial signal was high enough, the picture would turn negative, so you could try attenuating the aerial input signal to see if that affects the picture.

If that does clear the problem, then maybe the vision AGC circuit is playing up.

Has this set been restored? If not all the wax capacitors (and those hiding in metal cans) have not been replaced with modern plastic film types, they may be causing problems. Then there are the Hunts brown or black Moldseals.....and the Dubilier "bumble bee" types (which were also originally fitted in this set).

These are nice sets. I had one many years ago (when I was at school).
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Old 23rd Mar 2018, 4:12 pm   #6
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Default Re: EKCO TC267/1 going negative

Then there is a cap in the tuner that gives this I think.
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Old 23rd Mar 2018, 6:08 pm   #7
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Default Re: EKCO TC267/1 going negative

Hamish Boxer wrote:
"Then there is a cap in the tuner that gives this I think."
When investigating any vision AGC faults in these Ekco TV sets the prime suspect is always C7 inside the tuner unit.

DFWB.
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Old 23rd Mar 2018, 8:13 pm   #8
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Default Re: EKCO TC267/1 going negative

Thanks to everyone for the 'food for thought'. As soon as I get the time I will start work eliminating all these possibles one by one.
Geoff.
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Old 29th Mar 2018, 7:00 pm   #9
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Default Re: EKCO TC267/1 going negative

Many thanks for all the help following my request regarding the picture going negative on my Ekco TC267/1. I have now had a look inside and find that it has been extensively re-capped some time in its history. Following the advice from Stephen (Freya), I investigated the area around V6 (Trader Sheet 1278) and found it to indeed be very low emission in both halves; this was replaced. C67 was reconnected to chassis as the lead had clearly become detached, and R68, which was crumbling, was replaced. I was rewarded by a very clear, bright and well focussed picture, to which my photo does not do justice. I am now left with sound on vision problems which were not in evidence before, any help would be gratefully received.
Geoff.
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Old 29th Mar 2018, 10:58 pm   #10
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Default Re: EKCO TC267/1 going negative

Does the sound on vision become worse with higher setting of the volume control, if so check decoupling and poor earthing, microphonic valves.
Sound on vision with volume control turned down, check the if the sound rejector trap has been got at.
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Old 29th Mar 2018, 11:38 pm   #11
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Default Re: EKCO TC267/1 going negative

The ideal way to find the trouble and fix it is to scope the video detector output, with the scope timebase set to see horizontal rate or a few lines of the video signal, then follow the signal along to the video pre amp stage (input and output- grid - plate circuit usually) and the output drive to the crt and document that.

When everything is normal, the polarity of the video signal presented to the crt will be roughly around 30vpp with a test pattern, it will have a positive going sync pulse if it is fed to the crt cathode and a negative going one if fed to the grid. As noted by Astral Highway, to get a negative picture the signal polarity is reversing here.

So when the image goes negative it will be very easy to see, along the signal chain from the detector towards the crt, where the signal is inverting/reversing.

Practically any scope would do to see this effect, even a cheapo 5MHz to 10Mhz bandwidth one would be fine. If you don't have a scope yet, this is the ideal time/reason to get one. Without one, it is flying blind when trying to diagnose TV faults and relying on trial and error & parts substitution which is not as good.

If the output from the video detector is flipping, then you will know to look in the RF & IF sections of the set.
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Old 30th Mar 2018, 4:20 am   #12
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Default Re: EKCO TC267/1 going negative

.....actually, if its not losing sync when the picture inverts, it is unlikely there is anything wrong with the output from the video detector (meaning the tuner & RF sections are ok) or one would expect it to lose sync as well. I don't have the circuit, but likely the sync take off is at the detector or maybe after some video amplification and the fault is probably further along the video amp chain.
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Old 30th Mar 2018, 7:12 am   #13
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Default Re: EKCO TC267/1 going negative

You may find the output from the Aurora is to high which causes overloading, an 18db attenuator should be used.
There is also a control to lower the volune which is done with a small flatblade screwdriver.
.
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Old 30th Mar 2018, 10:54 am   #14
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Default Re: EKCO TC267/1 going negative

Thanks again. I will have a try with attenuators first before I start investigating with a scope, which will be a very steep learning curve for me. Having worked on radio for years, I have all the necessary kit , but TV is a relatively new adventure for me. I need the wisdom of someone more experienced to recognise and distinguish a good trace from an incorrect one; books are no substitute for experience!
Geoff.
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Old 30th Mar 2018, 9:08 pm   #15
Murphy Maniac
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Default Re: EKCO TC267/1 going negative

Semi success, the only attenuator I had was a 10dB which has improved matters somewhat, not completely sorted but much better. The sound on vision is not affected by the position of the volume control. So far, no further problem with the picture going negative although I have not soak tested it for any considerable length of time yet.
Thanks again everyone for your help and support.
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