Thorn 1500 Contrast issues - UK Vintage Radio Repair and Restoration Discussion Forum

AnalogueMatty · 5th Jul 2018, 11:17 am

Hi all, I've got an issue with the contrast control on my HMV 1500 - when I adjusted it slightly last night in order to watch a very dark film, the picture went very milky, turning the control this way and that made the image flicker but didn't alter the contrast unless held right over to " hard "...
Had the back off it today and checked the contrast pot with an AVO meter and couldn't find any dead spots as I swept across the track , seemed to be operating ok... so I switched it on and checked the picture again. Still milky.... then I gave the height pot a tweak and lo ! That was affecting the contrast on the picture too ! So I gave both pots a clean with Servisol and now the contrast control is working - kinda - but I have a good picture again so I've closed her up for now and I'll leave the contrast alone !
Should the frame output presets interact with the contrast like this ? Any ideas what could be causing my problem ?
Regards, Matt

Restoration73 · 5th Jul 2018, 11:36 am

Check C42 for leakage and C37 in case its dried up. The video is AC coupled so
"soot and whitewash" are to be expected.

jayceebee · 5th Jul 2018, 10:29 pm

Sometimes the pot doesn't respond to cleaning and to eliminate it should be replaced, often it's not he wiper at fault but where it's mounting pins are connected to the resistance track. It wasn't a great design.

The height pot interacting is unusual but unfortunately on very high mileage sets the PCB around the frame stage can become conductive and may be introducing some unwanted voltage into the video output stage. Another possible explanation could be that when you increase the height you increase the black gaps between the line structure giving the false impression of increased contrast, just a thought.

John.

Heatercathodeshort · 6th Jul 2018, 8:15 am

Typical symptoms of a dirty contrast pot in the 1500. Don't delve too deeply with this one. It is a low value pot and usually responds to cleaning as it did on yours. I had some spares in the preset pots boxes but it would take a while to go through them but if yours plays up I will take up the challenge. [I don't think it will]

The 1500 is capable of excellent pictures but this is entirely dependent on the condition of the tube. Typical symptoms are smeary picture, unable to obtain good focus, [check the boost supply resistor 1.5M first] lack lustre picture and out of focus edges. Unfortunately these 20" and 24" tubes have very small getters and were not anything like the quality of the earlier AW47-90/91 etc. John.

AnalogueMatty · 6th Jul 2018, 12:42 pm

Thanks for the advice guys, it does appear as it is an issue with the pot itself as the slightest touch of the control makes the contrast go dead soft/ picture flickery... so mounting pins like you say, just wanted to make sure it wasn't something else affecting it. I have a semi retired engineer with a large cache of vintage spares who can replace the pot.
The rest of the chassis is in very good condition with no scorching round the valves so looks like a low mileage set, it has had a new mains dropper and width pot / cap, it does have a Mullard CRT and the picture is good enough, I have a Mazda Gold Star tube in my back up / donor Ultra set too should I decide to swap.
Kind regards, Matt

5th Jul 2018, 11:17 am	#1
AnalogueMatty Triode Join Date: Jan 2017 Location: Manchester, UK. Posts: 17	Thorn 1500 Contrast issues Hi all, I've got an issue with the contrast control on my HMV 1500 - when I adjusted it slightly last night in order to watch a very dark film, the picture went very milky, turning the control this way and that made the image flicker but didn't alter the contrast unless held right over to " hard "... Had the back off it today and checked the contrast pot with an AVO meter and couldn't find any dead spots as I swept across the track , seemed to be operating ok... so I switched it on and checked the picture again. Still milky.... then I gave the height pot a tweak and lo ! That was affecting the contrast on the picture too ! So I gave both pots a clean with Servisol and now the contrast control is working - kinda - but I have a good picture again so I've closed her up for now and I'll leave the contrast alone ! Should the frame output presets interact with the contrast like this ? Any ideas what could be causing my problem ? Regards, Matt

5th Jul 2018, 11:36 am	#2
Restoration73 Nonode Join Date: Jun 2013 Location: Surbiton, SW London, UK. Posts: 2,801	Re: Thorn 1500 Contrast issues Check C42 for leakage and C37 in case its dried up. The video is AC coupled so "soot and whitewash" are to be expected.

5th Jul 2018, 10:29 pm	#3
jayceebee Heptode Join Date: Jun 2007 Location: Durham, County Durham, UK. Posts: 640	Re: Thorn 1500 Contrast issues Sometimes the pot doesn't respond to cleaning and to eliminate it should be replaced, often it's not he wiper at fault but where it's mounting pins are connected to the resistance track. It wasn't a great design. The height pot interacting is unusual but unfortunately on very high mileage sets the PCB around the frame stage can become conductive and may be introducing some unwanted voltage into the video output stage. Another possible explanation could be that when you increase the height you increase the black gaps between the line structure giving the false impression of increased contrast, just a thought. John.

6th Jul 2018, 8:15 am	#4
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: Thorn 1500 Contrast issues Typical symptoms of a dirty contrast pot in the 1500. Don't delve too deeply with this one. It is a low value pot and usually responds to cleaning as it did on yours. I had some spares in the preset pots boxes but it would take a while to go through them but if yours plays up I will take up the challenge. [I don't think it will] The 1500 is capable of excellent pictures but this is entirely dependent on the condition of the tube. Typical symptoms are smeary picture, unable to obtain good focus, [check the boost supply resistor 1.5M first] lack lustre picture and out of focus edges. Unfortunately these 20" and 24" tubes have very small getters and were not anything like the quality of the earlier AW47-90/91 etc. John.

6th Jul 2018, 12:42 pm	#5
AnalogueMatty Triode Join Date: Jan 2017 Location: Manchester, UK. Posts: 17	Re: Thorn 1500 Contrast issues Thanks for the advice guys, it does appear as it is an issue with the pot itself as the slightest touch of the control makes the contrast go dead soft/ picture flickery... so mounting pins like you say, just wanted to make sure it wasn't something else affecting it. I have a semi retired engineer with a large cache of vintage spares who can replace the pot. The rest of the chassis is in very good condition with no scorching round the valves so looks like a low mileage set, it has had a new mains dropper and width pot / cap, it does have a Mullard CRT and the picture is good enough, I have a Mazda Gold Star tube in my back up / donor Ultra set too should I decide to swap. Kind regards, Matt