Captions causing picture to dim. - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

Arnold69 · 6th Sep 2012, 12:52 pm

I wonder if this is a problem endemic to 'old' sets. I have posted elsewhere my BRC2000 woe of beam limiter seeming to cut in too early. The only conlcusion I have come to is that my idea of brightness and contrast settings are 'too high'.

I have correctly set up voltages, and beam limiter as described in manual. I have a recently regunned tube. I have tried being 'generous' with the beam limiter, but then despite the supoosed good eht regulation of these sets, the picture size varies. Is is just that a recent CRT set outstrips a 40 yr old one in terms of brighntness/contrast achievable? I remember when I was a kid that no TV during day wasn't really a problem as it was a struggle to see it when sun was out.

Arnold

ukcol · 6th Sep 2012, 4:31 pm

Check R565 which is connected between the 32 volt rail and pin 7 of the LOPTx.

I believe this resistor provides the off-set current for the beam limiter threshold.

It may be 20k or 27k; the value is chosen on screen size.

K35 circuit

Colin M

McMurdo · 6th Sep 2012, 7:35 pm

ISTR there's no overall A1 adjustment on the K35, it's set when adjusting the grey scale. (?) I had a similar problem with one once, milky pic with contrast down and picture pumping / flyback lines at normal settings. I ended up changing all the blue philips electrolytics on the base panel as most read 50% or less capacitance.

harvestgold · 7th Sep 2012, 5:36 pm

Thank you for those suggestions. I'll have a look over the weekend and report back!

Neil.

AndrewM · 8th Sep 2012, 12:18 am

I've attached a scan from the KT3 circuit description manual that describes the operation of the beam limiter. The circuit is much the same for all the KT3, K30, K35 chassis family with only minor variations.

The beam limiter is not adjustable and relies on components being within specification for correct operation. There is only around a dozen or so components so it should not take to long to track down.

Andrew

Heatercathodeshort · 8th Sep 2012, 9:31 am

I have watched this thread with interest. My guess is that the CRT is slightly soft. Some receivers display this fault under certain conditions. In all other respects the picture may appear to be 100% with regard to picture quality. As suggested, worth checking the beam limiter circuit just to be certain. 30AX tubes do tend to run at the high end of beam current. John.

Welsh Anorak · 8th Sep 2012, 2:16 pm

Hi there
I'm tending to side with John here - K30s did tend to do this if there was a slight interelectrode leak.
However one thing to check is the aquadag earthing as these sets almost always had a plastic lug break off leading to loss of tension on the earthing strap with consequent beam limiting problems.
Glyn

harvestgold · 10th Sep 2012, 2:41 pm

Thank you again - especially AndrewM for that information! I've managed to get some help to shift the set back onto the table to work on, so I'll take a look at the beam limiting circuit now. If it turns out to be a slightly soft tube, well that's sad, but I can live with it like this for a while until another nice set turns up!

Neil.

harvestgold · 10th Sep 2012, 5:07 pm

Well, I've had a fun afternoon checking out all the components in the beam limiter (the tiny C566 was especially fun to re-insert, good old blu-tac on a slim screwdriver did it in the end!). There are some slight variations from the KT3 circuit, but it's very similar.

Sadly, all components measure fine, and I've checked the aquadag earthing and that's okay too. so it looks very much like as per John's and Glyn's suggestion, the tube is showing its age.

I've achieved a compromise setting with the contrast reduced to a point where it's still watchable and the dimming isn't too irritating. so I should be able to enjoy using the set for a while!

Thanks again to all who have helped - it really is appreciated.

Neil.

pichacker · 11th Sep 2012, 8:09 am

Can you not carefully override the beam limiter tempoararily to rule it out?

flyingtech55 · 16th Sep 2012, 8:51 am

I'm with Pichacker on that one. That's what I used to do way back when. The trick is to eliminate bits of circuit before getting bogged down in the nitty gritty. You need to prove (or disprove) your initial diagnosis on a sort of macro basis. Then once you've proven the rogue section of circuitry then you can get stuck in to diagnosing it to component level.

HTH

TimR

Heatercathodeshort · 18th Sep 2012, 8:29 am

I would not advise disconnecting the beam limiter as a permanant cure. The 30AX tubes give a brilliant picture and draw a fair amount of beam current when working correctly. A few molecules of gas will increase this current over the maximum and will bring the limiter into action as it does with this receiver. If the current is not limited you are in danger of rupturing the final anode connection inside the tube thus reducing the tube to scrap. At least it is working 'OK'. 30AX tubes are around and if you wait long enough, one will turn up. They are as simple as a light bulb [or easier] to fit. Take care, John.

6th Sep 2012, 12:52 pm	#21
Arnold69 Retired Dormant Member Join Date: Oct 2010 Location: Oxford, Oxon. UK. Posts: 22	Re: Captions causing picture to dim. I wonder if this is a problem endemic to 'old' sets. I have posted elsewhere my BRC2000 woe of beam limiter seeming to cut in too early. The only conlcusion I have come to is that my idea of brightness and contrast settings are 'too high'. I have correctly set up voltages, and beam limiter as described in manual. I have a recently regunned tube. I have tried being 'generous' with the beam limiter, but then despite the supoosed good eht regulation of these sets, the picture size varies. Is is just that a recent CRT set outstrips a 40 yr old one in terms of brighntness/contrast achievable? I remember when I was a kid that no TV during day wasn't really a problem as it was a struggle to see it when sun was out. Arnold

6th Sep 2012, 4:31 pm	#22
ukcol Rest in Peace Join Date: May 2003 Location: Harlaxton, Lincolnshire, UK. Posts: 3,944	Re: Captions causing picture to dim. Check R565 which is connected between the 32 volt rail and pin 7 of the LOPTx. I believe this resistor provides the off-set current for the beam limiter threshold. It may be 20k or 27k; the value is chosen on screen size. K35 circuit Colin M

6th Sep 2012, 7:35 pm	#23
McMurdo Dekatron Join Date: Jun 2009 Location: Staffordshire Moorlands, UK. Posts: 5,274	Re: Captions causing picture to dim. ISTR there's no overall A1 adjustment on the K35, it's set when adjusting the grey scale. (?) I had a similar problem with one once, milky pic with contrast down and picture pumping / flyback lines at normal settings. I ended up changing all the blue philips electrolytics on the base panel as most read 50% or less capacitance. __________________ Kevin

7th Sep 2012, 5:36 pm	#24
harvestgold Hexode Join Date: Jul 2005 Location: Worcestershire, UK. Posts: 282	Re: Captions causing picture to dim. Thank you for those suggestions. I'll have a look over the weekend and report back! Neil.

8th Sep 2012, 9:31 am	#26
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: Captions causing picture to dim. I have watched this thread with interest. My guess is that the CRT is slightly soft. Some receivers display this fault under certain conditions. In all other respects the picture may appear to be 100% with regard to picture quality. As suggested, worth checking the beam limiter circuit just to be certain. 30AX tubes do tend to run at the high end of beam current. John.

8th Sep 2012, 2:16 pm	#27
Welsh Anorak Dekatron Join Date: Apr 2005 Location: North Wales, UK. Posts: 6,927	Re: Captions causing picture to dim. Hi there I'm tending to side with John here - K30s did tend to do this if there was a slight interelectrode leak. However one thing to check is the aquadag earthing as these sets almost always had a plastic lug break off leading to loss of tension on the earthing strap with consequent beam limiting problems. Glyn

10th Sep 2012, 2:41 pm	#28
harvestgold Hexode Join Date: Jul 2005 Location: Worcestershire, UK. Posts: 282	Re: Captions causing picture to dim. Thank you again - especially AndrewM for that information! I've managed to get some help to shift the set back onto the table to work on, so I'll take a look at the beam limiting circuit now. If it turns out to be a slightly soft tube, well that's sad, but I can live with it like this for a while until another nice set turns up! Neil.

10th Sep 2012, 5:07 pm	#29
harvestgold Hexode Join Date: Jul 2005 Location: Worcestershire, UK. Posts: 282	Re: Captions causing picture to dim. Well, I've had a fun afternoon checking out all the components in the beam limiter (the tiny C566 was especially fun to re-insert, good old blu-tac on a slim screwdriver did it in the end!). There are some slight variations from the KT3 circuit, but it's very similar. Sadly, all components measure fine, and I've checked the aquadag earthing and that's okay too. so it looks very much like as per John's and Glyn's suggestion, the tube is showing its age. I've achieved a compromise setting with the contrast reduced to a point where it's still watchable and the dimming isn't too irritating. so I should be able to enjoy using the set for a while! Thanks again to all who have helped - it really is appreciated. Neil.

11th Sep 2012, 8:09 am	#30
pichacker Hexode Join Date: Aug 2008 Location: Essex, UK. Posts: 370	Re: Captions causing picture to dim. Can you not carefully override the beam limiter tempoararily to rule it out? __________________ 6.3v on my heaters is no longer enough to get me going in the morning!!

16th Sep 2012, 8:51 am	#31
flyingtech55 Octode Join Date: Nov 2005 Location: Ayrshire, UK. Posts: 1,096	Re: Captions causing picture to dim. I'm with Pichacker on that one. That's what I used to do way back when. The trick is to eliminate bits of circuit before getting bogged down in the nitty gritty. You need to prove (or disprove) your initial diagnosis on a sort of macro basis. Then once you've proven the rogue section of circuitry then you can get stuck in to diagnosing it to component level. HTH TimR __________________ All those moments will be lost in time, like tears in rain.

18th Sep 2012, 8:29 am	#32
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: Captions causing picture to dim. I would not advise disconnecting the beam limiter as a permanant cure. The 30AX tubes give a brilliant picture and draw a fair amount of beam current when working correctly. A few molecules of gas will increase this current over the maximum and will bring the limiter into action as it does with this receiver. If the current is not limited you are in danger of rupturing the final anode connection inside the tube thus reducing the tube to scrap. At least it is working 'OK'. 30AX tubes are around and if you wait long enough, one will turn up. They are as simple as a light bulb [or easier] to fit. Take care, John.