Thorn 1400 height width issues - UK Vintage Radio Repair and Restoration Discussion Forum

rbm1973 · 14th Jan 2018, 10:07 pm

I am in the process of restoring a 20 inch hmv DS set,
i have replaced some faulty aged capacitors and have the set running now,

The picture is very good and the set is in very good condition and apart from what i have replaced as a matter of course it is totally untouched,
and in excellent condition.

the problem i have which has me head scratching at the moment is reduced height on both standards,and slightly reduced width on 405.

voltage checks on the ht rails show them not far out except the voltage across c123 is only 183v,
I have checked all sections of the mains dropper all appear to be correct.

Any suggestions before i delve deeper into the frame o/p stage.

PS
boost voltage is correct both lop valves replaced along with the pcl805 fop valve.

Philips210 · 14th Jan 2018, 11:10 pm

Hi.

I would first check the high value resistors in the field timebase, particularly R103 (330k) which takes it's feed from the boost rail to feed the height control R104 (2M5). Any Callins electrolytics still present ought to be hurled out.

For the width problem it's worth reading the servicing article (Television, Dec 1969 to Jan 1970) which again points to high value resistors going high as well as C113 playing up. The articles can be found here: http://www.americanradiohistory.com/...n_Magazine.htm

ISTR there was a more recent article as well.

Regards
Symon.

rbm1973 · 14th Jan 2018, 11:47 pm

whilst awaiting a reply i did check r103 which had risen from 330k to over 580k have replaced r103 and height correct now

.

now onto the width issue but on looking at the diagram and with what you have said that will be my next stop

thanks for your reply.

jayceebee · 15th Jan 2018, 12:44 am

Lack of width on one standard can be due to a s/c S correction capacitor, C108 for 405 and C107 for 625. If they are duo-moulds, the white/blue types it would be a good idea to change them both along with the boost capacitor C105.

The width circuit was modified in later production, if it hasn't already been done change the width pot R142 to 2.2M and R143 to 680K.

John.

Heatercathodeshort · 15th Jan 2018, 10:21 am

You may find this list useful.

https://www.vintage-radio.net/forum/...chassis&page=2

14th Jan 2018, 10:07 pm	#1
rbm1973 Pentode Join Date: Jan 2016 Location: Crewkerne, Somerset, UK. Posts: 119	Thorn 1400 height width issues I am in the process of restoring a 20 inch hmv DS set, i have replaced some faulty aged capacitors and have the set running now, The picture is very good and the set is in very good condition and apart from what i have replaced as a matter of course it is totally untouched, and in excellent condition. the problem i have which has me head scratching at the moment is reduced height on both standards,and slightly reduced width on 405. voltage checks on the ht rails show them not far out except the voltage across c123 is only 183v, I have checked all sections of the mains dropper all appear to be correct. Any suggestions before i delve deeper into the frame o/p stage. PS boost voltage is correct both lop valves replaced along with the pcl805 fop valve. __________________ Keep the soldering iron warm and never give up or you will kick yourself. BVWS MEMBER

14th Jan 2018, 11:10 pm	#2
Philips210 Nonode Join Date: May 2007 Location: Redruth, Cornwall, UK. Posts: 2,573	Re: Thorn 1400 height width issues Hi. I would first check the high value resistors in the field timebase, particularly R103 (330k) which takes it's feed from the boost rail to feed the height control R104 (2M5). Any Callins electrolytics still present ought to be hurled out. For the width problem it's worth reading the servicing article (Television, Dec 1969 to Jan 1970) which again points to high value resistors going high as well as C113 playing up. The articles can be found here: http://www.americanradiohistory.com/...n_Magazine.htm ISTR there was a more recent article as well. Regards Symon.

14th Jan 2018, 11:47 pm	#3
rbm1973 Pentode Join Date: Jan 2016 Location: Crewkerne, Somerset, UK. Posts: 119	Re: Thorn 1400 height width issues whilst awaiting a reply i did check r103 which had risen from 330k to over 580k have replaced r103 and height correct now . now onto the width issue but on looking at the diagram and with what you have said that will be my next stop thanks for your reply. __________________ Keep the soldering iron warm and never give up or you will kick yourself. BVWS MEMBER

15th Jan 2018, 12:44 am	#4
jayceebee Heptode Join Date: Jun 2007 Location: Durham, County Durham, UK. Posts: 640	Re: Thorn 1400 height width issues Lack of width on one standard can be due to a s/c S correction capacitor, C108 for 405 and C107 for 625. If they are duo-moulds, the white/blue types it would be a good idea to change them both along with the boost capacitor C105. The width circuit was modified in later production, if it hasn't already been done change the width pot R142 to 2.2M and R143 to 680K. John.

15th Jan 2018, 10:21 am	#5
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: Thorn 1400 height width issues You may find this list useful. https://www.vintage-radio.net/forum/...chassis&page=2