UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Television and Video

Notices

Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc.

Closed Thread
 
Thread Tools
Old 13th Feb 2007, 1:50 am   #1
TNC
Hexode
 
TNC's Avatar
 
Join Date: Apr 2003
Location: Westcliff-on-Sea, Essex, UK.
Posts: 375
Default Baird T18 - swapped Brimar C12B gave focus problem.

I have been replacing the this tv's original tube (completely dead) with a Brimar C12B kindly provided through the kindness of another Forum member.
After fixing the leaky EHT and also a slight re-design of the tube-locating arrangements (basically to stop its tendancy to fall forwards..and out!) I was able to fire it up.
Plenty of contrast/brilliance but a distinct lack of focus. Adjustment of the focus coil to its optimum fails to fully resolve the image. (It's as if the coil could do with a bit more oomph). Lowering the brilliance/contrast/attenuating the input signal (aurora) helps a little but not to a watchable (that is comfortable) standard.
So I would be grateful for any thoughts on the matter. The tube is a simple triode so could it produce plenty of contrast and still be failing - isn't that unlikely??...could it be because its preferred EHT is 10k.ish and the T18 only produces 5kv...but there is no blooming of the picture.
The T18 has a common heater/cathode...would the tube be sensitive about which side of the heater this common connection is made to?
Thanks,
Trevor.
TNC is offline  
Old 13th Feb 2007, 10:19 am   #2
Heatercathodeshort
Dekatron
 
Heatercathodeshort's Avatar
 
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 7,521
Default Re: Baird T18 - focus problem.

Hello Trevor,
The BRIMAR C12B 12" crt is made by CINTEL. It is a very unusual tube of high quality and was used in the KB EV30. It is rated and was operated at 10kv, very high for a 12" tube of its day and will produce a brilliant picture of incredible contrast and brightness.
The neck is rather longer than most tubes of this type and you may find that the focus unit will have to be moved further along the neck.
I replaced a Mazda CRM121B with one of these tubes in a DEFIENT fitted with the Plessey chassis. I had to fit shunts across the focus magnet and move the assembly. [I can't remember if it was back or forward] I also had to extend the 'bowler hat' due to the length of the neck. That was in 1961 and I was 13 at the time.....
This would suggest that the field needs to be reduced but I expect it is operating at half its original eht so it will be very much an experiment.
Hope this may help. Regards John.
Heatercathodeshort is offline  
Old 13th Feb 2007, 9:47 pm   #3
Heatercathodeshort
Dekatron
 
Heatercathodeshort's Avatar
 
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 7,521
Default Re: Baird T18 - focus problem.

Hello again Trevor,
I understand the problem of moving the focus assembly forward. I would suggest you try the effect of shunting the focus coil with a wire wound resistor. I am not familiar with the Baird power supply circuit and presume the focus coil is either wired in series with the smoothed H.T. or connected across the H.T. supply. The resistor will need to be connected across the coil. Start around 1-2k when the coil is in series with the H.T. supply and say around 15k when connected across it. As I mentioned the EHT on this tube needs to be maintained around 9-10kv for correct focus. I doubt if you set produces more than 5kv. This will effect the ability of the electron stream to focus due to it being considerably 'softened' by the reduced EHT and will not focus at the intended position within the tube neck.. It is a flat faced tube and I suspect may well have been designed as a studio monitor tube originally. As far as I am aware it was only used in two K.B. domestic television receivers.
The contrast control action is odd but I wondered how the brilliance works. Is it about centre for a 'normal' brightness picture? As you probably realize, this will not effect raster focus as such.
Frustrating results! Regards John.
Heatercathodeshort is offline  
Old 13th Feb 2007, 11:33 pm   #4
Steve_McVoy
Pentode
 
Steve_McVoy's Avatar
 
Join Date: Feb 2004
Location: Columbus, Ohio, USA
Posts: 105
Default Re: Baird T18 - focus problem.

When I restored my Baird T18C I also had to replace the tube, with a MW 31-18, and I also had to fool with the focus. If I remember correctly, I had to change the value of a series resistor in the focus circuit.
Steve_McVoy is offline  
Old 14th Feb 2007, 11:48 am   #5
TNC
Hexode
 
TNC's Avatar
 
Join Date: Apr 2003
Location: Westcliff-on-Sea, Essex, UK.
Posts: 375
Default Re: Baird T18 - focus problem.

Hi John/Steve

Attached are details of the Baird T18 circuit. The focus coil is shown on the top r/h corner of the time-base diagram...it connects via the socket the HT feed (but not going through the smoothing choke L37 as the other HT feeds do). I assume the adjustment VR3 is a variable power bleed to earth.

The contrast control (to help, also denoted as VR3!!) is located to the l/h side of the vision socket shown on the r/h side of the larger diagram and via that socket connects to points 4 and 5 which are to be found to the lower centre-left on the vision diagram. 6 on that socket is HT.
It looks to me as if VR3 is some sort of feedback control - but I may well be wrong on that!!

HTH, Trevor.
Attached Thumbnails
Click image for larger version

Name:	Baird T18 (1) a.jpg
Views:	107
Size:	184.2 KB
ID:	8260   Click image for larger version

Name:	Baird T18 (2) b.jpg
Views:	78
Size:	114.0 KB
ID:	8261  
TNC is offline  
Old 14th Feb 2007, 12:29 pm   #6
Ray Cooper
Pentode
 
Ray Cooper's Avatar
 
Join Date: Jan 2007
Location: Porthmadog, Gwynedd, UK.
Posts: 199
Default Re: Baird T18 - focus problem.

To state the fairly obvious: the bulk of the focus coil current is provided by the line and field output stage valves' standing feeds. So if there's any problem here (V3 or V6 taking more current than they should be) this will push the focus control up to and possibly beyond the end of its range. It's well worth checking that these feeds are about correct (assuming that you can find out what they should be, that is) because if they're excessive this indicates a further fault which may cause damage.

Another consideration might be that if your replacement CRT has been standing out of use for a while, it may have become slightly soft. Small amounts of gas within the tube will certainly affect the focus. If so, the focus should improve somewhat as the tube gets further usage.
Ray Cooper is offline  
Old 14th Feb 2007, 4:54 pm   #7
TNC
Hexode
 
TNC's Avatar
 
Join Date: Apr 2003
Location: Westcliff-on-Sea, Essex, UK.
Posts: 375
Default Re: Baird T18 - focus problem.

Well I think I it's sorted.

The focus coil's indicated resistance is 1180 ohm, voltage in was 340 with a 50 volt drop across.
Shunting this coil made the focus even worse.
I then chcked the focus VR (VR3) - this is 50k and to get the best focus (which was awful) it was set to offer zero resistance. No point in shunting!

It seemed to me that the coil needed more umph so I decided to shunt R12 which is nominally 20k but in this case reads about 13.7k. Using a 2w 6k shunt resistance the focus finally resolved with the adjustable V3 set at 9.7k.
The shunt now makes R12 read 4.17k so with the variable resistance at 9.7k there is a total of 13.24k between the coil and the earth rail. Without the shunt, the circuit's lowest resistance was 13.72k.
The shunt runs warm but I make the current to be about 14ma although I may not have worked that out correctly...I am sure someone will correct me!

Anyway the Brimar Cintel C12B is now happily displaying for the first time in many years (despite only at 5kv instead of its preferred10kv)...and I will be have to drop the previous owner a line to let him know the good news! (Incidentally, yes - he did watch the Coronation on it!)

Thanks to all for great inoput/assistance. I will post a couple of .jpgs when I get the chance.
atb
Trevor.
TNC is offline  
Old 14th Feb 2007, 8:41 pm   #8
Ed_Dinning
Dekatron
 
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 6,172
Default Re: Baird T18 - focus problem.

Hi Trevor, take care if you are passing additional current through the coil. The HT rail will probabaly stand it as will the focus coil, but you may notice the focus change as the coil heats up, requiring re-setting of the control.
This is because copper has a high, positive temperature coefficient of resistance.
This has hopefully explained a problem before you have seen it. Some 405 sets had a thernistor (NTC) in series with the frame coils to prevent height variations during warm-up and operation.

Ed
Ed_Dinning is offline  
Old 15th Feb 2007, 1:39 am   #9
TNC
Hexode
 
TNC's Avatar
 
Join Date: Apr 2003
Location: Westcliff-on-Sea, Essex, UK.
Posts: 375
Default Re: Baird T18 - focus problem.

Thanks for your comments Ed - I think the current is higher than I first estimated and that I shall need to change the focus control to a wirewound type.
Other than that it seems to be OK...but time will tell!

I have attached a couple of shots of the set fired-up.

Trevor.
Attached Thumbnails
Click image for larger version

Name:	Baird after focus fix 001a.jpg
Views:	132
Size:	62.3 KB
ID:	8264   Click image for larger version

Name:	Baird after focus fix 004ab.jpg
Views:	132
Size:	42.8 KB
ID:	8265  
TNC is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 2:14 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2020, vBulletin Solutions, Inc.
Copyright ©2002 - 2020, Paul Stenning.