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Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc.

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Old 25th Oct 2020, 2:00 am   #1
Nipkowdisc
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Default Green L.E.D. Question.

Does anyone remember a Colour T.V. Set design which had a green L.E.D. used not as an Indicator Lamp for something but in the way one would use a Zener Diode (although this was forward biased and did light up)?
It's purpose was to provide a common stabilised potential for the R.G.B. Output Transistors Emitters and I think it was located on the Tube Base Panel.
I think this was a Thorn Ferguson Set.
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Old 25th Oct 2020, 5:07 am   #2
toshiba tony
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Default Re: Green L.E.D. Question.

Yes, think it was either the TX10 or the TX100. It was on the CRT base. Can't remember it's function but there is mention of it in the manual.
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Old 25th Oct 2020, 8:11 am   #3
Electronpusher0
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Default Re: Green L.E.D. Question.

This was covered in another thread recently but I cannot find it now.
Peter


Found it
https://www.vintage-radio.net/forum/...ight=led+zener

Peter

Last edited by Electronpusher0; 25th Oct 2020 at 8:23 am. Reason: found the link
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Old 25th Oct 2020, 10:35 am   #4
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Default Re: Green L.E.D. Question.

They started to appear in regulators once they got cheaper than zenas.
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Old 25th Oct 2020, 11:19 am   #5
barrymagrec
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Default Re: Green L.E.D. Question.

Yes, it`s a bright idea.
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Old 25th Oct 2020, 11:39 am   #6
neil29
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Default Re: Green L.E.D. Question.

I've got 2 TX10's and they certainly do have the LED on the tube base panel.

Cheers
Neil.
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Old 26th Oct 2020, 1:30 am   #7
Nipkowdisc
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Default Re: Green L.E.D. Question.

My thanks to everyone who responded.
So it was the Thorn TX10 Chassis which had this L.E.D. as a "Zener Diode".
I had heard about it from a College Lecturer back in the early 1990's and I wondered if it was true as I have never seen inside one of these Sets.
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Old 26th Oct 2020, 6:29 am   #8
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Default Re: Green L.E.D. Question.

Presumably these components are long lasting and not very likely to need replacement ?
If however replacement IS needed, then you might have to search around a bit for the correct replacement. The earlier green LEDs had a forward voltage of just under 3 volts, or only slightly higher than an orange LED.
Then arrived the newer and brighter green LEDs with a rather higher forward voltage of nearer 4 volts.
The very latest types are in fact a violet LED with a green phosphour. these have a slightly higher forward voltage.

For use as indicator lamps, these differences are seldom important, but would be significant when used as a voltage regulator.
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Old 26th Oct 2020, 10:28 am   #9
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Default Re: Green L.E.D. Question.

Yes, the TX10 had one on the CRT base, but I don't think it was carried over to any other Thorn chassis - the TX9 and TX100 or any subsequent chassis didn't have one. They were pretty reliable, giving rise to black level or greyscale problems when they failed - but handily you could tell when things were working by the green glow!
On a similar theme, ISTR small bulbs were used in stabilising circuits many years earlier because of their warm-up characteristics. Were they used in TVs?
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Old 26th Oct 2020, 5:10 pm   #10
Philips210
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Default Re: Green L.E.D. Question.

Hi

The TELEVISION magazine "Colour Portable" project from 1981 used a 2mm red LED as a zener in the RGB output stages. The signal panel used in that project was a slight development on the "New CTV Signals Panel" to replace the earlier panel used in the 1978 project. A picture of the signals panel can be seen on the front cover of the September 1980 issue of TELEVISION.

The odd thing is, in the September 1980 issue, a normal zener diode is shown in the circuit diagram despite an LED being present in the pic of the panel. The components list does actually list an LED in the October issue. In the 1981 project (see June 1981 issue), the same LED is specified but I can't find any mention of it in the circuit description.

I built the 1981 "Colour Portable" project and used an Hitachi 20" CRT (510VSB22) and it all worked very well. I had one failure of the line output stage gate controlled switch device and a fault with the Plessey SL470 tuning voltage control IC. This was upgraded to a SL471. Later, I added teletext. I purchased a Mullard VM6101 teletext decoder from Sendz Components and a SAA1272C remote control interface IC , built a text power supply and carried out the conversion. The Hitachi CRT gave a splendid picture. The set was built in a Thorn 9000 cabinet courtesy of Rediffusion. Shame that I dismantled the set as I needed the CRT for an ITT TV.

Regards,
Symon

Last edited by Philips210; 26th Oct 2020 at 5:28 pm. Reason: additional text and correction
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