.7uF on a TV22? - UK Vintage Radio Repair and Restoration Discussion Forum

Boom · 20th May 2009, 6:00 pm

Can anyone tell me the purpose of the large .7uF cap' (250VAC) on the TV22? I thought it was something to do with spot supression but I've had a closer look today and it seems to be connected from the HT rail to the heater chain so this is unlikely.

I was trying to stuff it with a modern X2 .68uF but really cannot see any reason for this cap' at all and a .47uF would fit far easier.

The set works fine with it in or out.

Any ideas what it's for? Thanks

Steve_P · 20th May 2009, 6:28 pm

Our mains now is (supposedly) free from variations at different times of day, but it wasn't the case then. The capacitor in question was to keep Brighness steady as the mans went up, down and round about.

I think...

Cheers,

Steve P.

Boom · 20th May 2009, 6:32 pm

Hmmm that sounds plausible Steve but still doesn't make sense. How can a cap in that position affect the brightness at all?

A quick glance had me convinced that it was part of the brightness circuit but it obviously isn't.

kalee20 · 20th May 2009, 6:33 pm

That's brilliant! A type of feed-forward system.

brianc · 20th May 2009, 6:42 pm

I think that it is to kill beam current on switch-off so that the non-scanned beam does not burn the phosphor as the EHT dies - I'm sure that I read that somewhere!
Cheers
Brian

Ed_Dinning · 20th May 2009, 7:54 pm

Hi Gents, I had also heard that it was for turn off spot suppression, it could also be anti-phase hum injection into the HT rail to reduce hum.

Ed

Boom · 20th May 2009, 8:09 pm

Thanks for input Ed. It definitely has nothing to do with spot supression. The set is also AC/DC which has me thinking what on earth is it for.

I can think of no reason why a cap should join the HT line to the heater supply so your second idea may well be on the right track but how it works is beyond me.

The set is working fine with it out or in. If it was spot supression I would have thought there would be no raster without it

AlanBeckett · 20th May 2009, 8:18 pm

David,
Could you hang a 'scope on there and see what's happening?
If it's for spot suppression it will only be a transient effect as you switch off, so it wouldn't affect 'normal' operation.
Alan

dazzlevision · 20th May 2009, 8:21 pm

Hello,

Bush used this technique right up to their TV115 (405/625 line timebase) range. The service manual states "hum cancellation" against this component.

Regards,

Dazzlevision

Boom · 20th May 2009, 8:47 pm

Bingo. Many thanks Dazzlevision

murphyv310 · 20th May 2009, 9:46 pm

Hi.
the effect of the cap is to be out of phase with the mains (its a critical value) and reduce the effect of hum on the HT line, this is the theory any way.
The effect is hard to see on a picture and it does draw more current.
I snip it out on all TV22/24's I do
Cheers
Trevor

murphyv310 · 20th May 2009, 10:24 pm

Hi.
Just to add to my previous posting, please dont get confused how the circuit is drawn, this cap is not for spot supression or anything to do with mains stabilisation.
Its just an old hum bucking circuit and not that effective, it also can damp spikes that occur on DC mains from noisy Dynamo's used in the 50's

Trevor

Boom · 20th May 2009, 10:34 pm

Thanks. I think the circuit showing the cap' connected to the brightness control threw me. I'd assumed that the cap'' was affecting the brightness because of this.

I've stuffed both TV22s .7uF that I have here but if there is a next time I think that there would be little point in doing anything other than making a discrete snip of the wire under the covering to isolate it at both ends.

20th May 2009, 6:00 pm	#1
Boom Retired Dormant Member Join Date: Jan 2007 Location: Westbury, Wiltshire, UK. Posts: 2,451	.7uF on a TV22? Can anyone tell me the purpose of the large .7uF cap' (250VAC) on the TV22? I thought it was something to do with spot supression but I've had a closer look today and it seems to be connected from the HT rail to the heater chain so this is unlikely. I was trying to stuff it with a modern X2 .68uF but really cannot see any reason for this cap' at all and a .47uF would fit far easier. The set works fine with it in or out. Any ideas what it's for? Thanks Attached Thumbnails

20th May 2009, 6:28 pm	#2
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: .7uF on a TV22? Our mains now is (supposedly) free from variations at different times of day, but it wasn't the case then. The capacitor in question was to keep Brighness steady as the mans went up, down and round about. I think... Cheers, Steve P. __________________ If we've always had it, why is the Car Boot open? You're not sneaking another Old TV in are you...?

20th May 2009, 6:32 pm	#3
Boom Retired Dormant Member Join Date: Jan 2007 Location: Westbury, Wiltshire, UK. Posts: 2,451	Re: .7uF on a TV22? Hmmm that sounds plausible Steve but still doesn't make sense. How can a cap in that position affect the brightness at all? A quick glance had me convinced that it was part of the brightness circuit but it obviously isn't.

20th May 2009, 6:33 pm	#4
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,088	Re: .7uF on a TV22? That's brilliant! A type of feed-forward system.

20th May 2009, 6:42 pm	#5
brianc Rest in Peace Join Date: Jun 2004 Location: Burghfield, Reading, Berkshire, UK, Posts: 1,055	Re: .7uF on a TV22? I think that it is to kill beam current on switch-off so that the non-scanned beam does not burn the phosphor as the EHT dies - I'm sure that I read that somewhere! Cheers Brian

20th May 2009, 7:54 pm	#6
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,195	Re: .7uF on a TV22? Hi Gents, I had also heard that it was for turn off spot suppression, it could also be anti-phase hum injection into the HT rail to reduce hum. Ed

20th May 2009, 8:09 pm	#7
Boom Retired Dormant Member Join Date: Jan 2007 Location: Westbury, Wiltshire, UK. Posts: 2,451	Re: .7uF on a TV22? Thanks for input Ed. It definitely has nothing to do with spot supression. The set is also AC/DC which has me thinking what on earth is it for. I can think of no reason why a cap should join the HT line to the heater supply so your second idea may well be on the right track but how it works is beyond me. The set is working fine with it out or in. If it was spot supression I would have thought there would be no raster without it

20th May 2009, 8:18 pm	#8
AlanBeckett Rest in Peace Join Date: Dec 2008 Location: Burton upon Trent, East Staffordshire, UK. Posts: 1,686	Re: .7uF on a TV22? David, Could you hang a 'scope on there and see what's happening? If it's for spot suppression it will only be a transient effect as you switch off, so it wouldn't affect 'normal' operation. Alan

20th May 2009, 8:21 pm	#9
dazzlevision Dekatron Join Date: Sep 2003 Location: Near Swindon, North Wiltshire, UK. Posts: 3,623	Re: .7uF on a TV22? Hello, Bush used this technique right up to their TV115 (405/625 line timebase) range. The service manual states "hum cancellation" against this component. Regards, Dazzlevision

20th May 2009, 8:47 pm	#10
Boom Retired Dormant Member Join Date: Jan 2007 Location: Westbury, Wiltshire, UK. Posts: 2,451	Re: .7uF on a TV22? Bingo. Many thanks Dazzlevision

20th May 2009, 9:46 pm	#11
murphyv310 Dekatron Join Date: Sep 2006 Location: Kilmarnock, Ayrshire, UK. Posts: 5,422	Re: .7uF on a TV22? Hi. the effect of the cap is to be out of phase with the mains (its a critical value) and reduce the effect of hum on the HT line, this is the theory any way. The effect is hard to see on a picture and it does draw more current. I snip it out on all TV22/24's I do Cheers Trevor __________________ Cheers, Trevor. MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member

20th May 2009, 10:24 pm	#12
murphyv310 Dekatron Join Date: Sep 2006 Location: Kilmarnock, Ayrshire, UK. Posts: 5,422	Re: .7uF on a TV22? Hi. Just to add to my previous posting, please dont get confused how the circuit is drawn, this cap is not for spot supression or anything to do with mains stabilisation. Its just an old hum bucking circuit and not that effective, it also can damp spikes that occur on DC mains from noisy Dynamo's used in the 50's Trevor __________________ Cheers, Trevor. MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member

20th May 2009, 10:34 pm	#13
Boom Retired Dormant Member Join Date: Jan 2007 Location: Westbury, Wiltshire, UK. Posts: 2,451	Re: .7uF on a TV22? Thanks. I think the circuit showing the cap' connected to the brightness control threw me. I'd assumed that the cap'' was affecting the brightness because of this. I've stuffed both TV22s .7uF that I have here but if there is a next time I think that there would be little point in doing anything other than making a discrete snip of the wire under the covering to isolate it at both ends.