CRT PC Monitor Focus - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

IanBland · 29th May 2015, 6:49 pm

Just as an update, it's now been running for over three hours with no problem at all. I'm planning to strip it down tomorrow; maybe it just needs cleaning out internally. If anything looks interesting, I'll take a picture or two

Nicklyons2 · 29th May 2015, 7:59 pm

I've got a feeling that the tube base (which may have integral spark gaps) or the external spark gaps (if they're not integral to the tube base) are the problem. Is the tube base/ tube base pcb covered it fluff, very dusty, covered in sticky deposit etc? If so classic cause of 'bother' with spark gaps. With the set OFF unplug the tube base panel (not eht cavity connector); vac it off well with a clean paint brush and vac's crevice tool. If it has got sticky deposits on it 'swish' base/panel around in a dish (old margarine tub etc) of meths/ propanol etc and dry thoroughly with hair dryer. Reassemble and try again. Any better?

IanBland · 29th May 2015, 8:16 pm

I've only about half an hour ago switched the monitor off and moved it to a table to work on, Murphy's Law having come into operation: it worked for about 4 hours with no problems at all! I'm resisting the urge to get the cover off until tomorrow (not least so I've daylight to work in) but "sticky deposits" are quite likely since I'm a smoker

I'm still totally frit at the thought of the discharging the HT part of the operation...

Nicklyons2 · 30th May 2015, 9:37 am

If the set has been off for an hour or so you don't really need to discharge the e.h.t. to remove the tube base. The cavity connector (sucker looking connector on the tube bowl) is a different matter.

I would always caution those not fully familiar with TVs/monitors to unplug the equipment from the supply when working on it - don't just rely on switching it off.

dseymo1 · 30th May 2015, 12:15 pm

Quote:

Originally Posted by Nicklyons2

If it has got sticky deposits on it 'swish' base/panel around in a dish (old margarine tub etc) of meths/ propanol etc and dry thoroughly with hair dryer. Reassemble and try again. Any better?

I wouldn't recommend a hairdryer for this - you'll get away with it most times, but one day the vapour will be ignited by sparks from the brushgear. Been there, done it!

IanBland · 30th May 2015, 4:20 pm

Well, latest update. Got the case off and had a look. After steeling myself for discharging the tube, there was no snap, crackle or pop whatsoever, disappointingly. Maybe there's a bleeder resistor?

The insides of the monitor were rather thoroughly coated in tendrils of tobacco-y gunk, particularly around the HT stuff (including the flyback transformer). I've cleaned all that off with a toothbrush and vacuum cleaner. Removed the board at the neck which focus etc attaches to, but hard to inspect as it's soldered into a shield. The focus wires from the FBT attach to that board and visually and wiggly-wise they seem to be in good order.

I'm a bit out of practise at reading circuit diagrams (boo!) but when I finally found that part on the diagrams in the service manual, it indicates spark gaps on the H1, G2 and RGB lines but not on the focus lines, which seem to just connect straight onto the tube via PCB tracks. There is really very little in the focus circuit- the FBT, two wires and the tube, so I'm wondering if the connector onto the tube neck itself might have been arcing, though I can't see any evidence of that visually.

I can't really think of anything else to look at before putting it back together and lighting the blue touch paper, unless anyone here has any suggestions?

McMurdo · 30th May 2015, 5:06 pm

The focus spark gap is often built into the tube socket as I described before, or sometimes takes the form of an earthed ring moulded-in that passes 'close' to the various pin sockets. Those tin shields are a nuisance to the serviceman, and need alot of heat to unsolder them quickly without tearing tracks. Most TV's dont have them as the frequencies are much lower so they aren't so susceptible to radiating or receiving interference.
Did you check for dry joints around the LOPT as others have suggested while the back's off? Also- given you're a smoker - checking the dag earthing braids or springs are making good contact and the bowl isn't saturated in cigarette tar.

IanBland · 30th May 2015, 5:19 pm

I couldn't see anything like that in the tube socket, just the pin sockets.

I haven't had the board out that the LOPT is mounted on, but I don't think that can be the problem as the focus secondary winding is in series with the other HT secondaries, isn't it? So the PCB connections must be sound, or else both HT and focus would be affected, wouldn't they?

IanBland · 1st Jun 2015, 6:42 pm

Just to update or complete the thread, having reassembled the monitor it all appears to be working fine, so I suspect it was the tube socket, which might have been making poor contact on one of the pins.

29th May 2015, 6:49 pm	#21
IanBland Hexode Join Date: May 2015 Location: Northampton, Northants, UK. Posts: 380	Re: CRT PC Monitor Focus Just as an update, it's now been running for over three hours with no problem at all. I'm planning to strip it down tomorrow; maybe it just needs cleaning out internally. If anything looks interesting, I'll take a picture or two

29th May 2015, 7:59 pm	#22
Nicklyons2 Octode Join Date: Jan 2013 Location: Barnsley, South Yorkshire, UK. Posts: 1,517	Re: CRT PC Monitor Focus I've got a feeling that the tube base (which may have integral spark gaps) or the external spark gaps (if they're not integral to the tube base) are the problem. Is the tube base/ tube base pcb covered it fluff, very dusty, covered in sticky deposit etc? If so classic cause of 'bother' with spark gaps. With the set OFF unplug the tube base panel (not eht cavity connector); vac it off well with a clean paint brush and vac's crevice tool. If it has got sticky deposits on it 'swish' base/panel around in a dish (old margarine tub etc) of meths/ propanol etc and dry thoroughly with hair dryer. Reassemble and try again. Any better?

29th May 2015, 8:16 pm	#23
IanBland Hexode Join Date: May 2015 Location: Northampton, Northants, UK. Posts: 380	Re: CRT PC Monitor Focus I've only about half an hour ago switched the monitor off and moved it to a table to work on, Murphy's Law having come into operation: it worked for about 4 hours with no problems at all! I'm resisting the urge to get the cover off until tomorrow (not least so I've daylight to work in) but "sticky deposits" are quite likely since I'm a smoker I'm still totally frit at the thought of the discharging the HT part of the operation...

30th May 2015, 9:37 am	#24
Nicklyons2 Octode Join Date: Jan 2013 Location: Barnsley, South Yorkshire, UK. Posts: 1,517	Re: CRT PC Monitor Focus If the set has been off for an hour or so you don't really need to discharge the e.h.t. to remove the tube base. The cavity connector (sucker looking connector on the tube bowl) is a different matter. I would always caution those not fully familiar with TVs/monitors to unplug the equipment from the supply when working on it - don't just rely on switching it off.

30th May 2015, 4:20 pm	#26
IanBland Hexode Join Date: May 2015 Location: Northampton, Northants, UK. Posts: 380	Re: CRT PC Monitor Focus Well, latest update. Got the case off and had a look. After steeling myself for discharging the tube, there was no snap, crackle or pop whatsoever, disappointingly. Maybe there's a bleeder resistor? The insides of the monitor were rather thoroughly coated in tendrils of tobacco-y gunk, particularly around the HT stuff (including the flyback transformer). I've cleaned all that off with a toothbrush and vacuum cleaner. Removed the board at the neck which focus etc attaches to, but hard to inspect as it's soldered into a shield. The focus wires from the FBT attach to that board and visually and wiggly-wise they seem to be in good order. I'm a bit out of practise at reading circuit diagrams (boo!) but when I finally found that part on the diagrams in the service manual, it indicates spark gaps on the H1, G2 and RGB lines but not on the focus lines, which seem to just connect straight onto the tube via PCB tracks. There is really very little in the focus circuit- the FBT, two wires and the tube, so I'm wondering if the connector onto the tube neck itself might have been arcing, though I can't see any evidence of that visually. I can't really think of anything else to look at before putting it back together and lighting the blue touch paper, unless anyone here has any suggestions?

30th May 2015, 5:06 pm	#27
McMurdo Dekatron Join Date: Jun 2009 Location: Staffordshire Moorlands, UK. Posts: 5,274	Re: CRT PC Monitor Focus The focus spark gap is often built into the tube socket as I described before, or sometimes takes the form of an earthed ring moulded-in that passes 'close' to the various pin sockets. Those tin shields are a nuisance to the serviceman, and need alot of heat to unsolder them quickly without tearing tracks. Most TV's dont have them as the frequencies are much lower so they aren't so susceptible to radiating or receiving interference. Did you check for dry joints around the LOPT as others have suggested while the back's off? Also- given you're a smoker - checking the dag earthing braids or springs are making good contact and the bowl isn't saturated in cigarette tar. __________________ Kevin

30th May 2015, 5:19 pm	#28
IanBland Hexode Join Date: May 2015 Location: Northampton, Northants, UK. Posts: 380	Re: CRT PC Monitor Focus I couldn't see anything like that in the tube socket, just the pin sockets. I haven't had the board out that the LOPT is mounted on, but I don't think that can be the problem as the focus secondary winding is in series with the other HT secondaries, isn't it? So the PCB connections must be sound, or else both HT and focus would be affected, wouldn't they?

1st Jun 2015, 6:42 pm	#29
IanBland Hexode Join Date: May 2015 Location: Northampton, Northants, UK. Posts: 380	Re: CRT PC Monitor Focus Just to update or complete the thread, having reassembled the monitor it all appears to be working fine, so I suspect it was the tube socket, which might have been making poor contact on one of the pins.