Bush TV24 trapezium distortion - UK Vintage Radio Repair and Restoration Discussion Forum

Hunts smoothing bomb · 17th Jul 2007, 9:47 pm

Hi all,

I have the above set that has been previously restored, not by me.
Every capacitor has been changed and the set works very well apart from a lazy ECL80 frame output valve (5 minutes to fill screen vertically) and also my gripe is that the picture slants sideways down to the right but not linear!

What I mean is that if you wind the height down, the raster is level with the mask at the bottom of the screen but not at the top, is this shorted scan coil turns?

I have attached a poor illustration to try and show you what I mean!

Thanks
Lee

Tazman1966 · 17th Jul 2007, 9:51 pm

Hello Lee.

It is possible that that is the answer. It's a shame that it's not a later set otherwise there would've been some pincushion correction magnets on bendy brass strips that could've been fudged to correct the fault.

All the best,

Hunts smoothing bomb · 17th Jul 2007, 9:57 pm

Hi Tas,

Wow! thanks for the quick reply!
I thought this might be the case, So... does anyone have a spare bush TV24 scan coil assembly they would like to send me? It is a shame as this set is in such good condition that the raster lets it down!

Many thanks
Lee

jim_beacon · 17th Jul 2007, 10:10 pm

Change the frame output bottle, check the value of all caps and resistors in the frame ouput stage. If you have a 'scope, check the frame scan waveform.

Jim.

Mike Phelan · 18th Jul 2007, 7:58 am

Lee
It sure looks like something to do with one of the frame scan coils; have a close look at the yoke to see if there is any sort of physical damage.

Might be worth swapping the connections to reverse the raster right-to-left - if the fault reverses than it's not the coils, but something weird like crosstalk between line and frame - unlikely, methinks.

plumbweiss · 19th Jul 2007, 9:17 pm

I agree with changing the frame valve especially if it's slow warming up, normally the scan on these is well formed before the picture appears as it were.

Heatercathodeshort · 19th Jul 2007, 9:31 pm

As Mike says, take a physical look at the scan coils. You may find they have become loose and the frame section pair have become out of line. Its very rare to have a coil fault on this chassis. Could be a man-made fault due to attempting to twist scan coils that have become stuck to the tube neck. Regards, John.

17th Jul 2007, 9:47 pm	#1
Hunts smoothing bomb Octode Join Date: Oct 2005 Location: Wimborne, Dorset, UK. Posts: 1,407	Bush TV24 trapezium distortion Hi all, I have the above set that has been previously restored, not by me. Every capacitor has been changed and the set works very well apart from a lazy ECL80 frame output valve (5 minutes to fill screen vertically) and also my gripe is that the picture slants sideways down to the right but not linear! What I mean is that if you wind the height down, the raster is level with the mask at the bottom of the screen but not at the top, is this shorted scan coil turns? I have attached a poor illustration to try and show you what I mean! Thanks Lee Attached Thumbnails __________________ Lee

17th Jul 2007, 9:51 pm	#2
Tazman1966 Nonode Join Date: May 2006 Location: St Albans, Herts, UK. Posts: 2,193	Re: Bush TV24 squewiff picture Hello Lee. It is possible that that is the answer. It's a shame that it's not a later set otherwise there would've been some pincushion correction magnets on bendy brass strips that could've been fudged to correct the fault. All the best, __________________ All the very best, Tas

17th Jul 2007, 9:57 pm	#3
Hunts smoothing bomb Octode Join Date: Oct 2005 Location: Wimborne, Dorset, UK. Posts: 1,407	Re: Bush TV24 squewiff picture Hi Tas, Wow! thanks for the quick reply! I thought this might be the case, So... does anyone have a spare bush TV24 scan coil assembly they would like to send me? It is a shame as this set is in such good condition that the raster lets it down! Many thanks Lee __________________ Lee

17th Jul 2007, 10:10 pm	#4
jim_beacon Retired Dormant Member Join Date: Mar 2003 Location: Bishop's Waltham, Hants, UK. Posts: 939	Re: Bush TV24 squewiff picture Change the frame output bottle, check the value of all caps and resistors in the frame ouput stage. If you have a 'scope, check the frame scan waveform. Jim.

18th Jul 2007, 7:58 am	#5
Mike Phelan Dekatron Join Date: Nov 2003 Location: Near Leeds, West Yorkshire, UK Posts: 4,609	Re: Bush TV24 squewiff picture Lee It sure looks like something to do with one of the frame scan coils; have a close look at the yoke to see if there is any sort of physical damage. Might be worth swapping the connections to reverse the raster right-to-left - if the fault reverses than it's not the coils, but something weird like crosstalk between line and frame - unlikely, methinks. __________________ Mike.

19th Jul 2007, 9:17 pm	#6
plumbweiss Retired Dormant Member Join Date: Nov 2006 Location: Manchester, UK. Posts: 157	Re: Bush TV24 squewiff picture I agree with changing the frame valve especially if it's slow warming up, normally the scan on these is well formed before the picture appears as it were.

19th Jul 2007, 9:31 pm	#7
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: Bush TV24 squewiff picture As Mike says, take a physical look at the scan coils. You may find they have become loose and the frame section pair have become out of line. Its very rare to have a coil fault on this chassis. Could be a man-made fault due to attempting to twist scan coils that have become stuck to the tube neck. Regards, John.