Thorn 8500 19in with Mechanical Tuner - UK Vintage Radio Repair and Restoration Discussion Forum

its ur aerial · 11th Feb 2019, 2:27 pm

It`s probably almost 38 years since I last looked at a 19 inch Thorn 8500, but I have been asked for some advice on one, its dead except for the CRT Heaters, no HT on the line O/P Transistor and the regulator on the audio out put/ Power supply PCB appears correct.
Can any one jog my memory with a possible cause.
No circuit diagram to hand at the moment.
All comments appreciated.

slidertogrid · 11th Feb 2019, 5:21 pm

Hi Its a few years since I worked on one regularly. Have you checked the Mains dropper ? I can't remember if the cut out cuts the mains to the whole set or if it interrupts just the supply to the line stage. I don't have a circuit to hand and I can't find one on line. The other thing I would check is for dry joints or O/C print on the power panel they used to get a bit cooked after a few years.
Rich.

jayceebee · 11th Feb 2019, 5:29 pm

First checks I would make would be on the anode of the thyristor W703, no volts here would point to either X701 which is the thermistor mounted on the large mains filter choke below the PSU or W704 on the PSU itself being o/c. If you have volts on W703 anode check to see if the 25V rail is present on the PSU test point. No 25V rail was often caused by a dry joint on R704.

John.

its ur aerial · 12th Feb 2019, 8:58 am

A big thanks for the informed replies, it all now starts to ring bell`s, the X701 thermistor, W704 and the DJ`s on R704.
I will relay the info to the person who asked me.
I never stop being amazed, at the amount of information and knowledge people have on this Forum.
Thanks again.
PS. I have found a TV Mag article of 1976 that includes a circuit diagram.

its ur aerial · 14th Feb 2019, 10:20 am

All sorted, W704 was O/C.
It was repaired for some one who is trying to get a collection together for a small TV Museum.
A big thanks to the members who replied.

11th Feb 2019, 2:27 pm	#1
its ur aerial Hexode Join Date: May 2009 Location: Ryde, Isle of Wight, UK. Posts: 419	Thorn 8500 19in with Mechanical Tuner It`s probably almost 38 years since I last looked at a 19 inch Thorn 8500, but I have been asked for some advice on one, its dead except for the CRT Heaters, no HT on the line O/P Transistor and the regulator on the audio out put/ Power supply PCB appears correct. Can any one jog my memory with a possible cause. No circuit diagram to hand at the moment. All comments appreciated. __________________ Life is not Hollywood, life is Cricklewood.

11th Feb 2019, 5:21 pm	#2
slidertogrid Octode Join Date: Apr 2005 Location: Peterborough, Cambridgeshire, UK. Posts: 1,898	Re: Thorn 8500 19in with Mechanical Tuner Hi Its a few years since I worked on one regularly. Have you checked the Mains dropper ? I can't remember if the cut out cuts the mains to the whole set or if it interrupts just the supply to the line stage. I don't have a circuit to hand and I can't find one on line. The other thing I would check is for dry joints or O/C print on the power panel they used to get a bit cooked after a few years. Rich.

11th Feb 2019, 5:29 pm	#3
jayceebee Heptode Join Date: Jun 2007 Location: Durham, County Durham, UK. Posts: 640	Re: Thorn 8500 19in with Mechanical Tuner First checks I would make would be on the anode of the thyristor W703, no volts here would point to either X701 which is the thermistor mounted on the large mains filter choke below the PSU or W704 on the PSU itself being o/c. If you have volts on W703 anode check to see if the 25V rail is present on the PSU test point. No 25V rail was often caused by a dry joint on R704. John.

12th Feb 2019, 8:58 am	#4
its ur aerial Hexode Join Date: May 2009 Location: Ryde, Isle of Wight, UK. Posts: 419	Re: Thorn 8500 19in with Mechanical Tuner A big thanks for the informed replies, it all now starts to ring bell`s, the X701 thermistor, W704 and the DJ`s on R704. I will relay the info to the person who asked me. I never stop being amazed, at the amount of information and knowledge people have on this Forum. Thanks again. PS. I have found a TV Mag article of 1976 that includes a circuit diagram. __________________ Life is not Hollywood, life is Cricklewood.

14th Feb 2019, 10:20 am	#5
its ur aerial Hexode Join Date: May 2009 Location: Ryde, Isle of Wight, UK. Posts: 419	Re: Thorn 8500 19in with Mechanical Tuner All sorted, W704 was O/C. It was repaired for some one who is trying to get a collection together for a small TV Museum. A big thanks to the members who replied. __________________ Life is not Hollywood, life is Cricklewood.