Marconiphone 4714 (Thorn 3500) - Page 3 - UK Vintage Radio Repair and Restoration Discussion Forum

toshiba tony · 28th Aug 2008, 2:12 pm

well, i planned to leave this site and retire, but i cant, its soooo good, btw anyone on this thread know why 3000 and 3500? tubes were mounted upside down, once heard they were a defective tube with purity issues, but i dont know, anyone? thanks

Steve_P · 28th Aug 2008, 2:35 pm

In those days, there was a convergence problem at the edge of the screen, mostly at the top. Somebody at BRC felt that this would be less annoying at the bottom of the screen, so the tube was inverted.

When the 3500 came along, it stayed like that!

Cheers,

Steve P.

Tazman1966 · 28th Aug 2008, 7:17 pm

Hello Tony.

Steve's right. As well as convergence, the upside down mounting of the tube was beneficial to the pincushion correction. As the larger error is now at the bottom.

Cheers,

hans · 28th Aug 2008, 9:16 pm

I suppose you didn't have many programs with subtitles.

Were these convergece/purity/geometry problems great?

MALC SCOTT · 28th Aug 2008, 9:59 pm

Hi Hans, i own a very early (1969) 25 inch BRC 3000 chassis Rediffusion model. There is slight pincusion distortion if you compare it with the 3500 chassis which has pincusion correction. There is much less adjustment for the blue convergence, but the geometry remains good. I remember that the line timebase panels in the 3000 chassis gave much less trouble than the 3500. This could have been due to the higher scan current required for the raster correction circuit. Sets with the CRT fitted upside down were more prone to dampness around the EHT cavity connector. I spent 10 years repairing 3000/3500 panels on a special test rig when i worked at Rediffusion. I still get the odd nightmare even now!! Cheers, Malc.

toshiba tony · 29th Aug 2008, 1:46 pm

thanks all, its all coming back, i used to do a bit for radio rentals on a saturday morning, was it really so long ago?

28th Aug 2008, 2:12 pm	#41
toshiba tony Heptode Join Date: May 2006 Location: Accrington, Lancashire, UK. Posts: 978	Re: Marconiphone 4714 (Thorn 3500) well, i planned to leave this site and retire, but i cant, its soooo good, btw anyone on this thread know why 3000 and 3500? tubes were mounted upside down, once heard they were a defective tube with purity issues, but i dont know, anyone? thanks

28th Aug 2008, 2:35 pm	#42
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: Marconiphone 4714 (Thorn 3500) In those days, there was a convergence problem at the edge of the screen, mostly at the top. Somebody at BRC felt that this would be less annoying at the bottom of the screen, so the tube was inverted. When the 3500 came along, it stayed like that! Cheers, Steve P. __________________ If we've always had it, why is the Car Boot open? You're not sneaking another Old TV in are you...?

28th Aug 2008, 7:17 pm	#43
Tazman1966 Nonode Join Date: May 2006 Location: St Albans, Herts, UK. Posts: 2,193	Re: Marconiphone 4714 (Thorn 3500) Hello Tony. Steve's right. As well as convergence, the upside down mounting of the tube was beneficial to the pincushion correction. As the larger error is now at the bottom. Cheers, __________________ All the very best, Tas

28th Aug 2008, 9:16 pm	#44
hans Heptode Join Date: Jan 2008 Location: Oslo, Norway. Posts: 632	Re: Marconiphone 4714 (Thorn 3500) I suppose you didn't have many programs with subtitles. Were these convergece/purity/geometry problems great?

28th Aug 2008, 9:59 pm	#45
MALC SCOTT Octode Join Date: Aug 2006 Location: Willington, County Durham, UK. Posts: 1,499	Re: Marconiphone 4714 (Thorn 3500) Hi Hans, i own a very early (1969) 25 inch BRC 3000 chassis Rediffusion model. There is slight pincusion distortion if you compare it with the 3500 chassis which has pincusion correction. There is much less adjustment for the blue convergence, but the geometry remains good. I remember that the line timebase panels in the 3000 chassis gave much less trouble than the 3500. This could have been due to the higher scan current required for the raster correction circuit. Sets with the CRT fitted upside down were more prone to dampness around the EHT cavity connector. I spent 10 years repairing 3000/3500 panels on a special test rig when i worked at Rediffusion. I still get the odd nightmare even now!! Cheers, Malc. __________________ Malc Scott Last edited by MALC SCOTT; 28th Aug 2008 at 10:08 pm.

29th Aug 2008, 1:46 pm	#46
toshiba tony Heptode Join Date: May 2006 Location: Accrington, Lancashire, UK. Posts: 978	Re: Marconiphone 4714 (Thorn 3500) thanks all, its all coming back, i used to do a bit for radio rentals on a saturday morning, was it really so long ago?