Old Thorn T.V. Question. - UK Vintage Radio Repair and Restoration Discussion Forum

Nipkowdisc · 20th Jan 2021, 8:50 am

Does anyone know if the Tone Control on the deluxe 24" version of the Thorn 1615 Monochrome T.V. Set actually worked?
The basic model (3850) had a Shorting Plug fitted which kept a Treble Cut arrangement always fully in circuit while the deluxe model (3852) had a Potentiometer of a value I don't know wired as a Variable Resistor plugged into this Socket instead.
I have tried removing the Shorting Plug on a basic model Set but I can't hear any difference in the Sound Reproduction apart from when the Set has no signal (just listening to noise).
It could be that my Ears are too old and tired but I'm wondering whether or not Engineers in the past used to link out R66?

Heatercathodeshort · 20th Jan 2021, 12:36 pm

I sold quite a few of these 1615 series and must be honest I was completely unaware that this link [ 3/4 3/6] was fitted! There is no mention in the service manual. The 24" version was a very rare bird and I can't remember actually having one in for service.

When you say deluxe, do you mean the 3652 or did Thorn make an upmarket version of the 1615? If so I have never seen one. Colour was rolling out at a fast pace at that time and there was little if any demand for up market mono receivers.

I suspect it may have been more for cutting down possible sibilance than a tone control.

Regards. John.

Welsh Anorak · 20th Jan 2021, 2:30 pm

Like John, I didn't know there was a deluxe 24" version. The only variant I've seen was a six-button 20" version. However, Google does show one with the extra slider on the front and a CRT with an odd label (TMT61-101), probably a Unitra.
Here's the link: https://www.radios-tv.co.uk/1982-ferguson-3852/

G6ONEDave · 20th Jan 2021, 4:17 pm

The Thorn service manual for the 1615 chassis shows parts for the 3852 and 6852, both being 24" versions with the same cabinet and controls. The 20" versions were 3850 and 6850. The tone control is not listed as an individual item, it came as part of an assy with the other slider controls. I would try a 22K ohm resistor upto a 500K ohm resistor to find out which value works best tonal range wise. Then try to obtain a slider pot of that value.
Dave

Heatercathodeshort · 20th Jan 2021, 5:08 pm

Reminds me of one of those 'spot the differences' competitions there used to be in the Sunday Peephole, sorry People.

I really should have noticed this but the front cover of the 1615 service manual does show a 24" model fitted with the additional slider. Odd but I can't remember this at the time. It must have been one of the most pointless customer user controls of all time.

The chassis itself was not too bad but suffered from LOPT arc over and a nasty dry joint on the width/lin coil. The later models fitted with the Thorn sourced tube in place of the Mullard displayed pictures that appeared to be viewed through blotting paper!

They enjoyed a very short run compared with the 1500 but were mostly replaced early in their life by a colour receiver. John.

Richard_FM · 20th Jan 2021, 11:32 pm

I read that some of the later ones had Eastern European CRTs that weren't as good as the Mullard ones.

Supposedly they were made until as late as 1984, but in small numbers.

I think it was on Facebook someone mentioned they knew a family who had one as their main set until 1995 as the woman of the house refused to allow a colour set.

Nipkowdisc · 21st Jan 2021, 5:53 am

Thanks for your thoughts folks!
I guess it was a bit like the Line Flyback Blanking circuit used in these Sets (people wouldn't have been able to tell if that was operating or not either).
Given a good Mullard Picture Tube I think the 1615 would have shown a better picture than the 1500 because it had a Synchronous Video Demodulator and (almost) full D.C. Coupling from this to the Picture Tube and the 1500 didn't.

Welsh Anorak · 21st Jan 2021, 12:01 pm

The tubes fitted to the 1615s were, I think, made by Unitra Zatra with a generic Thorn white label and a type number unlike any Mullard or Mazda. The phosphor was granular with a sort of greenish tint to it. Added to that they didn't last long before defocusing and generally losing their emission.
Remember to resolder the line scan circuit on your TV - that the LOPT pins, the line linearity coil and the line scan plug, possibly strapping it with wire if there's been any sign of overheating. These sets were quite reliable but had a justified reputation for catching fire due to this problem, so much so that Radio Rentals recalled and scrapped them, replacing them with the old 1500s. However once this is seen to there shouldn't be any problem.

slidertogrid · 21st Jan 2021, 12:22 pm

Not Thorn's finest hour...Apart from the burn up mentioned wasn't this the set that a dropper section went O/C causing a L.T supply line to go high? There were no symptoms until the set failed with multiple transistor failures.

Niechcial,Steve · 21st Jan 2021, 1:51 pm

I have seen one if these recently with a good tube giving the best picture you'll see on any thorn in terms of definition and black level. However the tubes rarely stayed healthy to maintain that situation for long. The LOPT reliability was a huge backward step on the earlier jelly pots. The cheap and tacky slider pots and tuner push buttons were dreadful. Overall durability and longevity nowhere near that of the 1500.

Welsh Anorak · 21st Jan 2021, 3:01 pm

Agreed. Going back to a conventional LOPT was a mistake, and the buttons and sliders looked as though they'd snap off in a moment - and they did! The late 1500s with varicap tuning were much more robust, and even the cabinets were made of thicker plastic. A budget set in all ways.
Mind you, as John says, these were only ever meant as a stopgap while the customer saved up for a colour TV, so a quality mono chassis such as the Rank A816 would have been a bit pointless.
Regarding the dropper section, wasn't that actually the 317 ohm section on the 1500? Might be wrong.

Heatercathodeshort · 25th Jan 2021, 4:16 pm

I think you are right Glyn. It was a very rare failure but I can't remember it doing any damage other than baffling symptoms. I used to change the mains dropper on most 1500s that came in for service especially if they had a little white ring around the heater section.

They were a pleasure to replace and very cheap from the fantastic BRC service dept.

I have a 1615 that has had very little use. The Mullard A50-120W/R takes a little time to reach full emission. The getter is tiny in this last range of Mullard mono tubes. John.

20th Jan 2021, 8:50 am	#1
Nipkowdisc Triode Join Date: May 2010 Location: Burntwood, Staffordshire, UK. Posts: 33	Old Thorn T.V. Question. Does anyone know if the Tone Control on the deluxe 24" version of the Thorn 1615 Monochrome T.V. Set actually worked? The basic model (3850) had a Shorting Plug fitted which kept a Treble Cut arrangement always fully in circuit while the deluxe model (3852) had a Potentiometer of a value I don't know wired as a Variable Resistor plugged into this Socket instead. I have tried removing the Shorting Plug on a basic model Set but I can't hear any difference in the Sound Reproduction apart from when the Set has no signal (just listening to noise). It could be that my Ears are too old and tired but I'm wondering whether or not Engineers in the past used to link out R66?

20th Jan 2021, 12:36 pm	#2
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: Old Thorn T.V. Question. I sold quite a few of these 1615 series and must be honest I was completely unaware that this link [ 3/4 3/6] was fitted! There is no mention in the service manual. The 24" version was a very rare bird and I can't remember actually having one in for service. When you say deluxe, do you mean the 3652 or did Thorn make an upmarket version of the 1615? If so I have never seen one. Colour was rolling out at a fast pace at that time and there was little if any demand for up market mono receivers. I suspect it may have been more for cutting down possible sibilance than a tone control. Regards. John.

20th Jan 2021, 2:30 pm	#3
Welsh Anorak Dekatron Join Date: Apr 2005 Location: North Wales, UK. Posts: 6,918	Re: Old Thorn T.V. Question. Like John, I didn't know there was a deluxe 24" version. The only variant I've seen was a six-button 20" version. However, Google does show one with the extra slider on the front and a CRT with an odd label (TMT61-101), probably a Unitra. Here's the link: https://www.radios-tv.co.uk/1982-ferguson-3852/ __________________ Glyn www.gdelectronics.wales

20th Jan 2021, 4:17 pm	#4
G6ONEDave Octode Join Date: Dec 2019 Location: Owston Ferry, North Lincolnshire, UK. Posts: 1,701	Re: Old Thorn T.V. Question. The Thorn service manual for the 1615 chassis shows parts for the 3852 and 6852, both being 24" versions with the same cabinet and controls. The 20" versions were 3850 and 6850. The tone control is not listed as an individual item, it came as part of an assy with the other slider controls. I would try a 22K ohm resistor upto a 500K ohm resistor to find out which value works best tonal range wise. Then try to obtain a slider pot of that value. Dave

20th Jan 2021, 5:08 pm	#5
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: Old Thorn T.V. Question. Reminds me of one of those 'spot the differences' competitions there used to be in the Sunday Peephole, sorry People. I really should have noticed this but the front cover of the 1615 service manual does show a 24" model fitted with the additional slider. Odd but I can't remember this at the time. It must have been one of the most pointless customer user controls of all time. The chassis itself was not too bad but suffered from LOPT arc over and a nasty dry joint on the width/lin coil. The later models fitted with the Thorn sourced tube in place of the Mullard displayed pictures that appeared to be viewed through blotting paper! They enjoyed a very short run compared with the 1500 but were mostly replaced early in their life by a colour receiver. John.

20th Jan 2021, 11:32 pm	#6
Richard_FM Nonode Join Date: Sep 2017 Location: Stockport, Cheshire, UK. Posts: 2,002	Re: Old Thorn T.V. Question. I read that some of the later ones had Eastern European CRTs that weren't as good as the Mullard ones. Supposedly they were made until as late as 1984, but in small numbers. I think it was on Facebook someone mentioned they knew a family who had one as their main set until 1995 as the woman of the house refused to allow a colour set. __________________ Hello IT: Have you Tried Turning It Off & On Again?

21st Jan 2021, 5:53 am	#7
Nipkowdisc Triode Join Date: May 2010 Location: Burntwood, Staffordshire, UK. Posts: 33	Re: Old Thorn T.V. Question. Thanks for your thoughts folks! I guess it was a bit like the Line Flyback Blanking circuit used in these Sets (people wouldn't have been able to tell if that was operating or not either). Given a good Mullard Picture Tube I think the 1615 would have shown a better picture than the 1500 because it had a Synchronous Video Demodulator and (almost) full D.C. Coupling from this to the Picture Tube and the 1500 didn't.

21st Jan 2021, 12:01 pm	#8
Welsh Anorak Dekatron Join Date: Apr 2005 Location: North Wales, UK. Posts: 6,918	Re: Old Thorn T.V. Question. The tubes fitted to the 1615s were, I think, made by Unitra Zatra with a generic Thorn white label and a type number unlike any Mullard or Mazda. The phosphor was granular with a sort of greenish tint to it. Added to that they didn't last long before defocusing and generally losing their emission. Remember to resolder the line scan circuit on your TV - that the LOPT pins, the line linearity coil and the line scan plug, possibly strapping it with wire if there's been any sign of overheating. These sets were quite reliable but had a justified reputation for catching fire due to this problem, so much so that Radio Rentals recalled and scrapped them, replacing them with the old 1500s. However once this is seen to there shouldn't be any problem. __________________ Glyn www.gdelectronics.wales

21st Jan 2021, 12:22 pm	#9
slidertogrid Octode Join Date: Apr 2005 Location: Peterborough, Cambridgeshire, UK. Posts: 1,898	Re: Old Thorn T.V. Question. Not Thorn's finest hour...Apart from the burn up mentioned wasn't this the set that a dropper section went O/C causing a L.T supply line to go high? There were no symptoms until the set failed with multiple transistor failures.

21st Jan 2021, 1:51 pm	#10
Niechcial,Steve Hexode Join Date: Jan 2007 Location: Beckenham, London, UK. Posts: 373	Re: Old Thorn T.V. Question. I have seen one if these recently with a good tube giving the best picture you'll see on any thorn in terms of definition and black level. However the tubes rarely stayed healthy to maintain that situation for long. The LOPT reliability was a huge backward step on the earlier jelly pots. The cheap and tacky slider pots and tuner push buttons were dreadful. Overall durability and longevity nowhere near that of the 1500.

21st Jan 2021, 3:01 pm	#11
Welsh Anorak Dekatron Join Date: Apr 2005 Location: North Wales, UK. Posts: 6,918	Re: Old Thorn T.V. Question. Agreed. Going back to a conventional LOPT was a mistake, and the buttons and sliders looked as though they'd snap off in a moment - and they did! The late 1500s with varicap tuning were much more robust, and even the cabinets were made of thicker plastic. A budget set in all ways. Mind you, as John says, these were only ever meant as a stopgap while the customer saved up for a colour TV, so a quality mono chassis such as the Rank A816 would have been a bit pointless. Regarding the dropper section, wasn't that actually the 317 ohm section on the 1500? Might be wrong. __________________ Glyn www.gdelectronics.wales

25th Jan 2021, 4:16 pm	#12
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: Old Thorn T.V. Question. I think you are right Glyn. It was a very rare failure but I can't remember it doing any damage other than baffling symptoms. I used to change the mains dropper on most 1500s that came in for service especially if they had a little white ring around the heater section. They were a pleasure to replace and very cheap from the fantastic BRC service dept. I have a 1615 that has had very little use. The Mullard A50-120W/R takes a little time to reach full emission. The getter is tiny in this last range of Mullard mono tubes. John.