Again, CRT out of focus on the edges! - UK Vintage Radio Repair and Restoration Discussion Forum

Rick62008 · 26th Jan 2016, 4:59 pm

I'm sorry for this age old question, but my googling skills are failling me today and i would like to know the awnser for CRT out of focus on the edges.

What causes this? An aging CRT gun, or some other component like simply capacitors? Or even a unaligned deflection coil (abit too far of too close from the the CRT gun)?
Take note i'm not talking about convergence at all, it's really blurry as out of focus.

Messing around with the focus pot works for most part of the screen but the corners.

Thank you in advance.

toshiba tony · 26th Jan 2016, 5:12 pm

This rings a bell with me, some of our tv's had dynamic focussing, one for the edges and corners and one for the centre, or is yours before this time of dual focussing?

Rick62008 · 26th Jan 2016, 5:35 pm

I forgot to mention this in my original post, but the device in question is not actually a tv, but rather an "old" CRT computer monitor from around January 2002, so most likely is after that.
I'm sorry if by it being a computer monitor makes it somewhat off topic but since it's still crt technology i believe the fix could be similar.

Nuvistor · 26th Jan 2016, 5:53 pm

It could be CRT but try and get a circuit, monitors very often used dynamic focus and it may be faulty.

Monitors required much better display circuits and CRT's than TV's of that time, very wide band video stages perhaps 12Mhz and precise focus and good voltage stability.

Lodgesound · 27th Jan 2016, 10:36 pm

Measure the EHT if you can - low EHT will cause this.

In the 20 inch Sony grade 1 monitors I own the EHT is 32 thousand volts and the focus across the whole CRT face completely uniform.

Rick62008 · 28th Jan 2016, 2:51 pm

I should definitely get a schematic to make things easier, but i guess low EHT voltage could be probably it, unfourtanely i have to way to measure it at the moment.

One thing i noticed, after 1 or 2 hours the crt corners get less and less blurry, never quite perfect but a lot better than before.

Sideband · 28th Jan 2016, 8:37 pm

It's called astigmatism. You'll probably find you can focus the edges and leave the centre blurred or you focus the centre and leave the edges blurred. It could be the sign of a soft (gassy) CRT. Very often in the TV world, an ageing CRT could act like this. Some CRT's were like it from new and focus had to be set to a compromise.

Good quality monitors had, as mentioned a few posts back, two focus controls for 'general' focus and 'dynamic' focus. There was usually a set-up procedure using a test pattern so the focus could be adjusted accurately.

It could also be caused by low EHT.

Maarten · 31st Jan 2016, 12:43 am

Dynamic focus circuits might use a high voltage coupling capacitor. If it goes open, it will give this symptom.

Hybrid tellies · 2nd Feb 2016, 12:35 pm

We found the worst offender for corner de focusing was the 1500 chassis fitted with the standard 24" mono tube, the effect was noticeable even with good tubes. The same 24" tube fitted in other mono sets such as the Decca 2401, 2420, ITT VC200, RBM A640 did not suffer this effect as much or if at all.

26th Jan 2016, 4:59 pm	#1
Rick62008 Retired Dormant Member Join Date: Jan 2011 Location: Braga, Portugal. Posts: 231	Again, CRT out of focus on the edges! I'm sorry for this age old question, but my googling skills are failling me today and i would like to know the awnser for CRT out of focus on the edges. What causes this? An aging CRT gun, or some other component like simply capacitors? Or even a unaligned deflection coil (abit too far of too close from the the CRT gun)? Take note i'm not talking about convergence at all, it's really blurry as out of focus. Messing around with the focus pot works for most part of the screen but the corners. Thank you in advance.

26th Jan 2016, 5:12 pm	#2
toshiba tony Heptode Join Date: May 2006 Location: Accrington, Lancashire, UK. Posts: 978	Re: Again, CRT out of focus on the edges! This rings a bell with me, some of our tv's had dynamic focussing, one for the edges and corners and one for the centre, or is yours before this time of dual focussing?

26th Jan 2016, 5:35 pm	#3
Rick62008 Retired Dormant Member Join Date: Jan 2011 Location: Braga, Portugal. Posts: 231	Re: Again, CRT out of focus on the edges! I forgot to mention this in my original post, but the device in question is not actually a tv, but rather an "old" CRT computer monitor from around January 2002, so most likely is after that. I'm sorry if by it being a computer monitor makes it somewhat off topic but since it's still crt technology i believe the fix could be similar.

26th Jan 2016, 5:53 pm	#4
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,433	Re: Again, CRT out of focus on the edges! It could be CRT but try and get a circuit, monitors very often used dynamic focus and it may be faulty. Monitors required much better display circuits and CRT's than TV's of that time, very wide band video stages perhaps 12Mhz and precise focus and good voltage stability.

27th Jan 2016, 10:36 pm	#5
Lodgesound Retired Dormant Member Join Date: Jun 2009 Location: Surrey, UK. Posts: 66	Re: Again, CRT out of focus on the edges! Measure the EHT if you can - low EHT will cause this. In the 20 inch Sony grade 1 monitors I own the EHT is 32 thousand volts and the focus across the whole CRT face completely uniform.

28th Jan 2016, 2:51 pm	#6
Rick62008 Retired Dormant Member Join Date: Jan 2011 Location: Braga, Portugal. Posts: 231	Re: Again, CRT out of focus on the edges! I should definitely get a schematic to make things easier, but i guess low EHT voltage could be probably it, unfourtanely i have to way to measure it at the moment. One thing i noticed, after 1 or 2 hours the crt corners get less and less blurry, never quite perfect but a lot better than before.

28th Jan 2016, 8:37 pm	#7
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,571	Re: Again, CRT out of focus on the edges! It's called astigmatism. You'll probably find you can focus the edges and leave the centre blurred or you focus the centre and leave the edges blurred. It could be the sign of a soft (gassy) CRT. Very often in the TV world, an ageing CRT could act like this. Some CRT's were like it from new and focus had to be set to a compromise. Good quality monitors had, as mentioned a few posts back, two focus controls for 'general' focus and 'dynamic' focus. There was usually a set-up procedure using a test pattern so the focus could be adjusted accurately. It could also be caused by low EHT. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....

31st Jan 2016, 12:43 am	#8
Maarten Dekatron Join Date: Dec 2007 Location: Haarlem, Netherlands Posts: 4,199	Re: Again, CRT out of focus on the edges! Dynamic focus circuits might use a high voltage coupling capacitor. If it goes open, it will give this symptom.

2nd Feb 2016, 12:35 pm	#9
Hybrid tellies Nonode Join Date: Jan 2004 Location: 1966-1976 Coverack in Cornwall and Helston Cornwall. 1976-present Bristol/Bath area. Posts: 2,965	Re: Again, CRT out of focus on the edges! We found the worst offender for corner de focusing was the 1500 chassis fitted with the standard 24" mono tube, the effect was noticeable even with good tubes. The same 24" tube fitted in other mono sets such as the Decca 2401, 2420, ITT VC200, RBM A640 did not suffer this effect as much or if at all. __________________ Simon BVWS member