LOPT focus control substitute/fix? - UK Vintage Radio Repair and Restoration Discussion Forum

gridrunner · 8th Dec 2020, 3:12 pm

Hi everyone,

I've just replaced a failing LOPT in a Commodore 1081 monitor with a substitute unit from Donberg.

However I dropped a clanger with the focus control. I had to remove it to fit the LOPT but when I powered up I couldn't focus the screen. I put the control on the meter and it was open circuit. It looks like a small carbon pad has fallen out of the wiper and there's no contact now, lost forever on the floor.

The service manual says its 59 Mega Ohms, but a quick cast about and I can't find ready spares. Does anyone have advice on material I can use to fit in the wiper to make contact?

Thanks

Stu

Freya · 8th Dec 2020, 3:52 pm

It may have been a graphite section, you could try using some pencil lead, it sands easily so could be fashioned to fit. See what the others say.

G6ONEDave · 8th Dec 2020, 4:13 pm

Perhaps a piece from a motor carbon brush. Will need to be fairly soft, so as not to wear away the high resistance track whilst adjusting focus.
Dave

slidertogrid · 8th Dec 2020, 4:13 pm

That type of focus control was used by various television manufacturers The Pye / Philips KT3 / K30 for instance. I don't know if the resistance is the same so the whole control may not be a direct replacement but one should at least provide the wiper to repair yours .
Maybe someone has a control or a scrap chassis with one on it?
Rich

Maarten · 8th Dec 2020, 5:11 pm

Don't use pencil as it will either not work or arc.

These units were made by Philips (possibly also at Mullard Blackburn) and are very common. You should be able to obtain the missing part from a similar potmeter unit.

Heatercathodeshort · 8th Dec 2020, 8:41 pm

Just a tip to find tiny irreplaceable items that have gone 'ping'.
Empty your vacuum cleaner bag or fit a new replacement paper type. Clean the entire area with the vacuum making sure you get in the corners and all the hidden areas. It's amazing where these items land!

Empty the bag into a bucket of water and stir gently. Any dense items will sink to the bottom of the bucket. gently pour off the liquid and soft sludge. The item can be easily found in the grit and bits at the bottom.

I've done this with small items from VCRs, pick ups etc with 90% success. John.

Maarten · 9th Dec 2020, 3:50 am

Also, I just realised that if memory serves me right, you should probably be looking for a tight wound metal spring, not for a rubber or carbon part.

gridrunner · 10th Dec 2020, 2:45 pm

Hi everyone,

Thanks for the advice. In the end I put a piece of soft thick metal wire in the groove, with enough clearance to make good contact and set the focus to the middle of the range. It works fine. Not ideal, but I doubt I'll ever need to adjust it again.

8th Dec 2020, 3:12 pm	#1
gridrunner Pentode Join Date: Jun 2008 Location: Manchester, UK. Posts: 227	LOPT focus control substitute/fix? Hi everyone, I've just replaced a failing LOPT in a Commodore 1081 monitor with a substitute unit from Donberg. However I dropped a clanger with the focus control. I had to remove it to fit the LOPT but when I powered up I couldn't focus the screen. I put the control on the meter and it was open circuit. It looks like a small carbon pad has fallen out of the wiper and there's no contact now, lost forever on the floor. The service manual says its 59 Mega Ohms, but a quick cast about and I can't find ready spares. Does anyone have advice on material I can use to fit in the wiper to make contact? Thanks Stu Attached Thumbnails

8th Dec 2020, 3:52 pm	#2
Freya Octode Join Date: Jan 2009 Location: Falmouth, Cornwall, UK. Posts: 1,990	Re: LOPT focus control substitute/fix? It may have been a graphite section, you could try using some pencil lead, it sands easily so could be fashioned to fit. See what the others say. __________________ Stephen _________"It`s only an old telly" ___

8th Dec 2020, 4:13 pm	#3
G6ONEDave Octode Join Date: Dec 2019 Location: Owston Ferry, North Lincolnshire, UK. Posts: 1,704	Re: LOPT focus control substitute/fix? Perhaps a piece from a motor carbon brush. Will need to be fairly soft, so as not to wear away the high resistance track whilst adjusting focus. Dave

8th Dec 2020, 4:13 pm	#4
slidertogrid Octode Join Date: Apr 2005 Location: Peterborough, Cambridgeshire, UK. Posts: 1,898	Re: LOPT focus control substitute/fix? That type of focus control was used by various television manufacturers The Pye / Philips KT3 / K30 for instance. I don't know if the resistance is the same so the whole control may not be a direct replacement but one should at least provide the wiper to repair yours . Maybe someone has a control or a scrap chassis with one on it? Rich

8th Dec 2020, 5:11 pm	#5
Maarten Dekatron Join Date: Dec 2007 Location: Haarlem, Netherlands Posts: 4,203	Re: LOPT focus control substitute/fix? Don't use pencil as it will either not work or arc. These units were made by Philips (possibly also at Mullard Blackburn) and are very common. You should be able to obtain the missing part from a similar potmeter unit.

8th Dec 2020, 8:41 pm	#6
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: LOPT focus control substitute/fix? Just a tip to find tiny irreplaceable items that have gone 'ping'. Empty your vacuum cleaner bag or fit a new replacement paper type. Clean the entire area with the vacuum making sure you get in the corners and all the hidden areas. It's amazing where these items land! Empty the bag into a bucket of water and stir gently. Any dense items will sink to the bottom of the bucket. gently pour off the liquid and soft sludge. The item can be easily found in the grit and bits at the bottom. I've done this with small items from VCRs, pick ups etc with 90% success. John.

9th Dec 2020, 3:50 am	#7
Maarten Dekatron Join Date: Dec 2007 Location: Haarlem, Netherlands Posts: 4,203	Re: LOPT focus control substitute/fix? Also, I just realised that if memory serves me right, you should probably be looking for a tight wound metal spring, not for a rubber or carbon part.

10th Dec 2020, 2:45 pm	#8
gridrunner Pentode Join Date: Jun 2008 Location: Manchester, UK. Posts: 227	Re: LOPT focus control substitute/fix? Hi everyone, Thanks for the advice. In the end I put a piece of soft thick metal wire in the groove, with enough clearance to make good contact and set the focus to the middle of the range. It works fine. Not ideal, but I doubt I'll ever need to adjust it again.