Cossor 494AC Tone correction capacitor. - UK Vintage Radio Repair and Restoration Discussion Forum

gary_crutchley · 6th Jan 2016, 9:45 am

Hi all,

I have a Cossor 494AC that I am hoping to get up and running. I plan first off to change the AF coupling capacitor, then also the tone correction capacitor which is listed as C20 on trader sheet 924. My question relates to the latter.

I have three versions of the circuit diagram in total which show that the required capacitance is 0.005uF, however, whilst two don't specify the voltage rating one says it should be 1kV.

If I order 0.0047uF 630v capacitors will these be suitable please?

Regards,

Gary

ukcol · 6th Jan 2016, 9:54 am

630v will probably be OK but 1kV are available so why not get the right one.

http://uk.farnell.com/lcr-components...ial/dp/9520309

Sideband · 6th Jan 2016, 9:56 am

630V should be fine. Most capacitors in tone correction circuits were rated at 500V. It's only rated that high because there is a chance of high level AF signals appearing across it superimposed on any DC voltage. Modern 630V polyesters should be fine.

gary_crutchley · 6th Jan 2016, 9:58 am

Quote:

Originally Posted by ukcol

630v will probably be OK but 1kV are available so why not get the right one.

Thanks for the pointer, I will order some of those. Why do two of the trader sheets not specify a voltage rating?

Cobaltblue · 6th Jan 2016, 10:00 am

Trader expected people in the business to know what was acceptable rating in a given location would be my guess.

Cheers

Mike T

gary_crutchley · 6th Jan 2016, 10:07 am

Hi Mike,

That's what I thought, but why are the voltage ratings specified for some caps and not others? Presumably because something unusual was being used for a given location?

Do you have a Cossor 494 in your collection Mike?

Regards,

Gary

Cobaltblue · 6th Jan 2016, 10:48 am

Hi Gary

Yes I have a few of the these no time to do much with them however

Yes it does seem hit or miss if there is a voltage rating or not on the Trader sheets.

I have a pretty good collection of post war Cossors (And Pre-War Cossors as well

) 464's seem to breed by themselves I must have 5 or 6 of the little devils.

Cheers

Mike T

gary_crutchley · 6th Jan 2016, 10:58 am

Thanks Mike, very interesting.

Gary

HamishBoxer · 6th Jan 2016, 11:38 am

There are not many if any domestic radios that 630Volt caps won't suffice or 400 Volts for that matter. Possibly excluding electrolytics in certain cases.

gary_crutchley · 6th Jan 2016, 11:49 am

Thanks for the advice, I appreciate it.

paulsherwin · 6th Jan 2016, 11:51 am

Tone correction caps were normally 1kV devices because if they failed following a voltage spike they would fail short circuit, with possibly disastrous results. Modern plastic film caps will usually self heal after a spike so don't need to be rated so conservatively.

That said, it's good practice for any replacement cap to at least equal the voltage rating of the original.

gary_crutchley · 6th Jan 2016, 12:00 pm

Hi Paul,

Many thanks for the advice, I have ordered some 1kV caps, so will fit one of those.

Gary

HamishBoxer · 6th Jan 2016, 12:11 pm

I stand corrected Paul.

gary_crutchley · 6th Jan 2016, 12:23 pm

I think that was sort of semi-corrected actually, if I interpreted the message correctly

Looks like the consensus is 630V is probably okay, but fit the 1kV anyway as it's available. Sounds like good advice to me, so that's what I intend to do.

Cheers ...

6th Jan 2016, 9:45 am	#1
gary_crutchley Heptode Join Date: Apr 2013 Location: Bloxwich, West Midlands, UK Posts: 554	Cossor 494AC Tone correction capacitor. Hi all, I have a Cossor 494AC that I am hoping to get up and running. I plan first off to change the AF coupling capacitor, then also the tone correction capacitor which is listed as C20 on trader sheet 924. My question relates to the latter. I have three versions of the circuit diagram in total which show that the required capacitance is 0.005uF, however, whilst two don't specify the voltage rating one says it should be 1kV. If I order 0.0047uF 630v capacitors will these be suitable please? Regards, Gary

6th Jan 2016, 9:54 am	#2
ukcol Rest in Peace Join Date: May 2003 Location: Harlaxton, Lincolnshire, UK. Posts: 3,944	Re: Cossor 494AC Tone correction capacitor. 630v will probably be OK but 1kV are available so why not get the right one. http://uk.farnell.com/lcr-components...ial/dp/9520309

6th Jan 2016, 9:56 am	#3
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,580	Re: Cossor 494AC Tone correction capacitor. 630V should be fine. Most capacitors in tone correction circuits were rated at 500V. It's only rated that high because there is a chance of high level AF signals appearing across it superimposed on any DC voltage. Modern 630V polyesters should be fine. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....

6th Jan 2016, 10:00 am	#5
Cobaltblue Moderator Join Date: Dec 2010 Location: Exeter, Devon and Poole, Dorset UK. Posts: 6,880	Re: Cossor 494AC Tone correction capacitor. Trader expected people in the business to know what was acceptable rating in a given location would be my guess. Cheers Mike T __________________ Invisible airwaves crackle with life or at least they used to Mike T BVWS member. www.cossor.co.uk

6th Jan 2016, 10:07 am	#6
gary_crutchley Heptode Join Date: Apr 2013 Location: Bloxwich, West Midlands, UK Posts: 554	Re: Cossor 494AC Tone correction capacitor. Hi Mike, That's what I thought, but why are the voltage ratings specified for some caps and not others? Presumably because something unusual was being used for a given location? Do you have a Cossor 494 in your collection Mike? Regards, Gary

6th Jan 2016, 10:48 am	#7
Cobaltblue Moderator Join Date: Dec 2010 Location: Exeter, Devon and Poole, Dorset UK. Posts: 6,880	Re: Cossor 494AC Tone correction capacitor. Hi Gary Yes I have a few of the these no time to do much with them however Yes it does seem hit or miss if there is a voltage rating or not on the Trader sheets. I have a pretty good collection of post war Cossors (And Pre-War Cossors as well ) 464's seem to breed by themselves I must have 5 or 6 of the little devils. Cheers Mike T __________________ Invisible airwaves crackle with life or at least they used to Mike T BVWS member. www.cossor.co.uk

6th Jan 2016, 10:58 am	#8
gary_crutchley Heptode Join Date: Apr 2013 Location: Bloxwich, West Midlands, UK Posts: 554	Re: Cossor 494AC Tone correction capacitor. Thanks Mike, very interesting. Gary

6th Jan 2016, 11:38 am	#9
HamishBoxer Dekatron Join Date: Aug 2007 Location: W.Butterwick, near Doncaster UK. Posts: 8,935	Re: Cossor 494AC Tone correction capacitor. There are not many if any domestic radios that 630Volt caps won't suffice or 400 Volts for that matter. Possibly excluding electrolytics in certain cases. __________________ G8JET BVWS Archivist and Member V.M.A.R.S

6th Jan 2016, 11:49 am	#10
gary_crutchley Heptode Join Date: Apr 2013 Location: Bloxwich, West Midlands, UK Posts: 554	Re: Cossor 494AC Tone correction capacitor. Thanks for the advice, I appreciate it.

6th Jan 2016, 11:51 am	#11
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,977	Re: Cossor 494AC Tone correction capacitor. Tone correction caps were normally 1kV devices because if they failed following a voltage spike they would fail short circuit, with possibly disastrous results. Modern plastic film caps will usually self heal after a spike so don't need to be rated so conservatively. That said, it's good practice for any replacement cap to at least equal the voltage rating of the original.

6th Jan 2016, 12:00 pm	#12
gary_crutchley Heptode Join Date: Apr 2013 Location: Bloxwich, West Midlands, UK Posts: 554	Re: Cossor 494AC Tone correction capacitor. Hi Paul, Many thanks for the advice, I have ordered some 1kV caps, so will fit one of those. Gary

6th Jan 2016, 12:11 pm	#13
HamishBoxer Dekatron Join Date: Aug 2007 Location: W.Butterwick, near Doncaster UK. Posts: 8,935	Re: Cossor 494AC Tone correction capacitor. I stand corrected Paul. __________________ G8JET BVWS Archivist and Member V.M.A.R.S

6th Jan 2016, 12:23 pm	#14
gary_crutchley Heptode Join Date: Apr 2013 Location: Bloxwich, West Midlands, UK Posts: 554	Re: Cossor 494AC Tone correction capacitor. I think that was sort of semi-corrected actually, if I interpreted the message correctly Looks like the consensus is 630V is probably okay, but fit the 1kV anyway as it's available. Sounds like good advice to me, so that's what I intend to do. Cheers ...