Poor Focus on Pye D18T - UK Vintage Radio Repair and Restoration Discussion Forum

brianc · 11th Nov 2005, 4:58 pm

Hi All

I have at last got a picture on my D18T but unfortunately, it is impossible to focus the beam! The iron tube which moves in and out of the focus magnet is fully out and moving it towards the front of the CRT makes things worse! Positioning the whole focus assembly along the CRT neck makes little difference.

What effect does the tube have on the magnetic field when it moves into the magnet - weaken or strengthen it? I suppose the magnets can lose their strength? Is this common? Can a gassy tube be the cause - the picture is not too bright and blooms alot but I am assuming that is something to do with spot size? The EHT measures 6kV and tube electrode voltages are reasonably similar to the service sheet.

Advice would be appreciated.

Cheers

brian

Heatercathodeshort · 11th Nov 2005, 7:44 pm

Hi Brian,
As Darius says the focus magnet can lose field strength but its rare to find one that is so weak that it will not focus the crt. Its the 'bloom' that is slightly worrying as such more than the lack of focus. Bloom is normally caused by minute gas particles causing 'halation'. The MW22-14C series tubes generally hold up well emission wise but suffer from ion burn.
If you wish to borrow a B18T to swap parts as a test I have one minus back with a good crt and focus. A couple of the valve holders are suffering from 'verdigris' but when I last powered it up about 10 years ago it gave a bright raster. It may save you some time in diagnosis. The metal tube sliding into the magnet should reduce the field I believe as it will 'shunt' the magnet but I may be wrong with this point.
Regards. JOHN

brianc · 11th Nov 2005, 8:29 pm

Thanks Darius & John

The idea of borrowing your B18T is very tempting John. Meanwhile, I will do a few more experiments with some neodymium (is that how you spell it?) magnets as I have a nearby source of buttons as used in loudspeakers. At least it would prove whether a stronger field improves matters. It could be that "blooming" is the wrong word as the picture is pretty de-focussed anyway, just much worse on the highlights.

I'll do the experiments this weekend and post the results.

Cheers

Brian

brianc · 12th Nov 2005, 12:23 am

Hi Danny
The EHT varies by about 1KV from black to burnt out. I did change the EY51, on spec, for a brand-new one but on switching on, the line frequency halved and no EHT appeared - it turned out that the new valve was soft, so, disillusioned, I put the old one back! Maybe I should have installed another new one. I will do that tomorrow.
I am also thinking of building the PTV tube conditioner that has appeared in another thread. Maybe a cathode strip will help.

Cheers

Brian

brianc · 14th Nov 2005, 8:40 pm

Hi all

Success - a sharp picture (but now I can see some short term streaking!!!).

When I took the focus assembly appart, I found that there was an anular spacer which made the magnet thicker so I replaced it with four neodymium button magnets to, as I hoped, strengthen the magnetic field. Eureka! I now have a focussed testcard which only needs positioning properly. Unfortunately, the rubber springs used to push the assembly against the adjustment screws cannot be called springs anymore so I will get some sort of thick-walled rubber tube to replace them.

Thanks for your advice guys, and Danny, I won't be zapping the tube as it is plenty bright but the EHT regulation is not too hot so I will be changing the EY51 again as you suggested. John, I will not need to borrow your B18T but thanks for the generous offer.

Cheers

Brian

11th Nov 2005, 4:58 pm	#1
brianc Rest in Peace Join Date: Jun 2004 Location: Burghfield, Reading, Berkshire, UK, Posts: 1,055	Poor Focus on Pye D18T Hi All I have at last got a picture on my D18T but unfortunately, it is impossible to focus the beam! The iron tube which moves in and out of the focus magnet is fully out and moving it towards the front of the CRT makes things worse! Positioning the whole focus assembly along the CRT neck makes little difference. What effect does the tube have on the magnetic field when it moves into the magnet - weaken or strengthen it? I suppose the magnets can lose their strength? Is this common? Can a gassy tube be the cause - the picture is not too bright and blooms alot but I am assuming that is something to do with spot size? The EHT measures 6kV and tube electrode voltages are reasonably similar to the service sheet. Advice would be appreciated. Cheers brian Last edited by Mike Phelan; 29th May 2007 at 2:31 pm. Reason: Typo - 'lose' not 'loose'

11th Nov 2005, 7:44 pm	#2
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: Poor Focus on Pye D18T Hi Brian, As Darius says the focus magnet can lose field strength but its rare to find one that is so weak that it will not focus the crt. Its the 'bloom' that is slightly worrying as such more than the lack of focus. Bloom is normally caused by minute gas particles causing 'halation'. The MW22-14C series tubes generally hold up well emission wise but suffer from ion burn. If you wish to borrow a B18T to swap parts as a test I have one minus back with a good crt and focus. A couple of the valve holders are suffering from 'verdigris' but when I last powered it up about 10 years ago it gave a bright raster. It may save you some time in diagnosis. The metal tube sliding into the magnet should reduce the field I believe as it will 'shunt' the magnet but I may be wrong with this point. Regards. JOHN Last edited by Mike Phelan; 29th May 2007 at 2:32 pm. Reason: Typos

11th Nov 2005, 8:29 pm	#3
brianc Rest in Peace Join Date: Jun 2004 Location: Burghfield, Reading, Berkshire, UK, Posts: 1,055	Re: Poor Focus on Pye D18T Thanks Darius & John The idea of borrowing your B18T is very tempting John. Meanwhile, I will do a few more experiments with some neodymium (is that how you spell it?) magnets as I have a nearby source of buttons as used in loudspeakers. At least it would prove whether a stronger field improves matters. It could be that "blooming" is the wrong word as the picture is pretty de-focussed anyway, just much worse on the highlights. I'll do the experiments this weekend and post the results. Cheers Brian

12th Nov 2005, 12:23 am	#4
brianc Rest in Peace Join Date: Jun 2004 Location: Burghfield, Reading, Berkshire, UK, Posts: 1,055	Re: Poor Focus on Pye D18T Hi Danny The EHT varies by about 1KV from black to burnt out. I did change the EY51, on spec, for a brand-new one but on switching on, the line frequency halved and no EHT appeared - it turned out that the new valve was soft, so, disillusioned, I put the old one back! Maybe I should have installed another new one. I will do that tomorrow. I am also thinking of building the PTV tube conditioner that has appeared in another thread. Maybe a cathode strip will help. Cheers Brian

14th Nov 2005, 8:40 pm	#5
brianc Rest in Peace Join Date: Jun 2004 Location: Burghfield, Reading, Berkshire, UK, Posts: 1,055	Re: Poor Focus on Pye D18T Hi all Success - a sharp picture (but now I can see some short term streaking!!!). When I took the focus assembly appart, I found that there was an anular spacer which made the magnet thicker so I replaced it with four neodymium button magnets to, as I hoped, strengthen the magnetic field. Eureka! I now have a focussed testcard which only needs positioning properly. Unfortunately, the rubber springs used to push the assembly against the adjustment screws cannot be called springs anymore so I will get some sort of thick-walled rubber tube to replace them. Thanks for your advice guys, and Danny, I won't be zapping the tube as it is plenty bright but the EHT regulation is not too hot so I will be changing the EY51 again as you suggested. John, I will not need to borrow your B18T but thanks for the generous offer. Cheers Brian