Problem with Thorn 1500 chassis - UK Vintage Radio Repair and Restoration Discussion Forum

line sync · 31st Dec 2015, 1:04 pm

Does anyone know how to cure caption buzz on the thorn 1500 mono chassis ?

Robin

Welsh Anorak · 31st Dec 2015, 1:35 pm

Hmm...they don't do that!
Now if you'd said the 1400 that would have made sense. I'd make absolutely sure you're not overloading the tuner first - check various sources. The printed coils on the 1500 were a big improvement but there's not much to twiddle, so I'd look at attenuating the signal and see how you go from there.
Glyn

jayceebee · 31st Dec 2015, 7:02 pm

Possible causes are the small value electrolytic in the ratio detector circuit or the small preset pot. Sorry no circuit diagram any longer but I'm sure someone will be along with the circuit references soon, 4uF rings a bell for the capacitor.

John.

line sync · 31st Dec 2015, 9:45 pm

Thanks Glyn and John for your help.
I`ll try an attenuator tomorrow and if that doesn`t work I`ll look for the electrolytic.

line sync · 1st Jan 2016, 1:45 pm

Had a go at the 1500 this morning.
I tried a few attenuators and they did not really solve the problem.
I fitted a new 4.7uf cap C65 which made no difference and tried adjusting pot R84 which didn`t improve things much.
I also tried a new PCL82 but again the problem is still there.

Robin

Hybrid tellies · 1st Jan 2016, 3:34 pm

Though not as bad as the 1400's the 1500's were a bit prone to caption buzz. I seem to remember we use to tweek the 6Mhz inter carrier sound coil. I'll dig the manual out later and get back to you.

Hybrid tellies · 2nd Jan 2016, 4:19 pm

If you have an earlier version of the 1500 check resistor R31 in the collector circuit of VT8 it was changed on later production sets from 1.5K to 2.7K to improve inter sound carrier performance. Apart from that check C65 which is 4uF electrolytic cap in the FM ratio detector. It might just be a careful tweak of the 6Mhz intercarrier coils L14/L15 or the ratio detector balance control R84 that is required.
Just a word of caution which applies to all 1500's. To reduce dissipation in the width control circuit R130 was changed from 330K to 680K and the width preset was changed from 1M to 2.2M. I think most of the 1500's have these values fitted just some of the earliar models slipped through. This of course was a big safety issue on the 1400 chassis.

Focus Diode · 2nd Jan 2016, 5:20 pm

Sound advice from Simon and others.
Bear in mind the Freeview etc box could be set to a low volume and DVDs are often very quiet even if the player's volume is set to maximum.
With the latter some DVDs are so quiet even the modern flat screen LED set struggles to cope with an audible buzz in the background.
Cheers
Brian

line sync · 3rd Jan 2016, 11:32 am

Thanks for all the advice guys.
Simon I adjusted the coil L14/15 and this made a big difference , there is still a little buzzing every now and again but it is now acceptable.
I do like these 1500`s as they are so easy to work on and I bet the rental boys liked them too.

Robin

Hybrid tellies · 4th Jan 2016, 1:33 pm

Thats great news.
They never gave the best of pictures, or sound, but they were good enough. And yes the rental companies loved them because they were the easiest set to work on with its swing out chassis and component marking on both sides of the board.
As well as being very popular with the big rental chains they were in abundance across the board with the smaller companies putting many thousands out on rental as well.

31st Dec 2015, 1:04 pm	#1
line sync Heptode Join Date: Sep 2009 Location: Kinver, Staffordshire, UK. Posts: 634	Problem with Thorn 1500 chassis Does anyone know how to cure caption buzz on the thorn 1500 mono chassis ? Robin

31st Dec 2015, 1:35 pm	#2
Welsh Anorak Dekatron Join Date: Apr 2005 Location: North Wales, UK. Posts: 6,921	Re: Problem with thorn 1500 chassis Hmm...they don't do that! Now if you'd said the 1400 that would have made sense. I'd make absolutely sure you're not overloading the tuner first - check various sources. The printed coils on the 1500 were a big improvement but there's not much to twiddle, so I'd look at attenuating the signal and see how you go from there. Glyn

31st Dec 2015, 7:02 pm	#3
jayceebee Heptode Join Date: Jun 2007 Location: Durham, County Durham, UK. Posts: 640	Re: Problem with thorn 1500 chassis Possible causes are the small value electrolytic in the ratio detector circuit or the small preset pot. Sorry no circuit diagram any longer but I'm sure someone will be along with the circuit references soon, 4uF rings a bell for the capacitor. John.

31st Dec 2015, 9:45 pm	#4
line sync Heptode Join Date: Sep 2009 Location: Kinver, Staffordshire, UK. Posts: 634	Re: Problem with thorn 1500 chassis Thanks Glyn and John for your help. I`ll try an attenuator tomorrow and if that doesn`t work I`ll look for the electrolytic.

1st Jan 2016, 1:45 pm	#5
line sync Heptode Join Date: Sep 2009 Location: Kinver, Staffordshire, UK. Posts: 634	Re: Problem with Thorn 1500 chassis Had a go at the 1500 this morning. I tried a few attenuators and they did not really solve the problem. I fitted a new 4.7uf cap C65 which made no difference and tried adjusting pot R84 which didn`t improve things much. I also tried a new PCL82 but again the problem is still there. Robin

1st Jan 2016, 3:34 pm	#6
Hybrid tellies Nonode Join Date: Jan 2004 Location: 1966-1976 Coverack in Cornwall and Helston Cornwall. 1976-present Bristol/Bath area. Posts: 2,965	Re: Problem with Thorn 1500 chassis Though not as bad as the 1400's the 1500's were a bit prone to caption buzz. I seem to remember we use to tweek the 6Mhz inter carrier sound coil. I'll dig the manual out later and get back to you. __________________ Simon BVWS member

2nd Jan 2016, 4:19 pm	#7
Hybrid tellies Nonode Join Date: Jan 2004 Location: 1966-1976 Coverack in Cornwall and Helston Cornwall. 1976-present Bristol/Bath area. Posts: 2,965	Re: Problem with Thorn 1500 chassis If you have an earlier version of the 1500 check resistor R31 in the collector circuit of VT8 it was changed on later production sets from 1.5K to 2.7K to improve inter sound carrier performance. Apart from that check C65 which is 4uF electrolytic cap in the FM ratio detector. It might just be a careful tweak of the 6Mhz intercarrier coils L14/L15 or the ratio detector balance control R84 that is required. Just a word of caution which applies to all 1500's. To reduce dissipation in the width control circuit R130 was changed from 330K to 680K and the width preset was changed from 1M to 2.2M. I think most of the 1500's have these values fitted just some of the earliar models slipped through. This of course was a big safety issue on the 1400 chassis. __________________ Simon BVWS member

2nd Jan 2016, 5:20 pm	#8
Focus Diode Octode Join Date: Jan 2006 Location: Borough of Gateshead, UK. Posts: 1,420	Re: Problem with Thorn 1500 chassis Sound advice from Simon and others. Bear in mind the Freeview etc box could be set to a low volume and DVDs are often very quiet even if the player's volume is set to maximum. With the latter some DVDs are so quiet even the modern flat screen LED set struggles to cope with an audible buzz in the background. Cheers Brian

3rd Jan 2016, 11:32 am	#9
line sync Heptode Join Date: Sep 2009 Location: Kinver, Staffordshire, UK. Posts: 634	Re: Problem with Thorn 1500 chassis Thanks for all the advice guys. Simon I adjusted the coil L14/15 and this made a big difference , there is still a little buzzing every now and again but it is now acceptable. I do like these 1500`s as they are so easy to work on and I bet the rental boys liked them too. Robin

4th Jan 2016, 1:33 pm	#10
Hybrid tellies Nonode Join Date: Jan 2004 Location: 1966-1976 Coverack in Cornwall and Helston Cornwall. 1976-present Bristol/Bath area. Posts: 2,965	Re: Problem with Thorn 1500 chassis Thats great news. They never gave the best of pictures, or sound, but they were good enough. And yes the rental companies loved them because they were the easiest set to work on with its swing out chassis and component marking on both sides of the board. As well as being very popular with the big rental chains they were in abundance across the board with the smaller companies putting many thousands out on rental as well. __________________ Simon BVWS member