|
Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
|
Thread Tools |
16th Mar 2023, 1:41 pm | #1 |
Heptode
Join Date: Apr 2013
Location: Bloxwich, West Midlands, UK
Posts: 554
|
Cossor 494AC Electrolytics.
Hi all,
My Cossor 494AC has been running on the original (made 1949) smoothing/reservoir capacitor (16/32uF) since I partially recapped the set a while back. However, I recently noticed a bulge in the base, so will be replacing it. In the circuit diagram I noticed another electrolytic capacitor, C21 on trader sheet 924. I am wondering if I should replace this at the same time? C21 is rated at 25v, how critical is the voltage please? Would a higher voltage (eg: 100v) modern capacitor of the capacitance be suitable? I look forward to your comments. Best regards, Gary
__________________
http://www.crutchleyhistory.co.uk/ |
16th Mar 2023, 1:54 pm | #2 |
Octode
Join Date: Oct 2019
Location: Stockport, Greater Manchester, UK.
Posts: 1,642
|
Re: Cossor 494AC Electrolytics.
Was the set humming?
I would change them both, just for peace of mind. I've read that using too high a voltage rating can cause an electrolytic capacitor to un-form, so it's no longer able to perform its duties to spec. Whether this is likely to cause an issue if you use a 100v cap and expose it to 12v is probably academic. Having said this, since a 25v electrolytic cap costs pennies, I would try to change it like for like |
16th Mar 2023, 1:55 pm | #3 |
Moderator
Join Date: Dec 2010
Location: Exeter, Devon and Poole, Dorset UK.
Posts: 6,867
|
Re: Cossor 494AC Electrolytics.
Hi Gary
You would not normally use such a high voltage capacitor as there is only around 9V on the cathode of the valve. However if its what you have to hand, I would use it. Cheers Mike T
__________________
Invisible airwaves crackle with life or at least they used to Mike T BVWS member. www.cossor.co.uk |
16th Mar 2023, 2:02 pm | #4 |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,535
|
Re: Cossor 494AC Electrolytics.
The 25V is not that critical. A 100V part will be fine, and probably nearer the size of the original. You'll likely not find 25uF either but 22, 33 or 47uF will be fine.
It does only have 9V across it though so 63V ould be a bit better choice. (Electrolytics prefer a bias voltage reasonably close to rated voltage.) Edit: Posts crossed!
__________________
....__________ ....|____||__|__\_____ .=.| _---\__|__|_---_|. .........O..Chris....O |
16th Mar 2023, 2:45 pm | #5 | |
Heptode
Join Date: Apr 2013
Location: Bloxwich, West Midlands, UK
Posts: 554
|
Re: Cossor 494AC Electrolytics.
Quote:
Regards, Gary
__________________
http://www.crutchleyhistory.co.uk/ |
|
16th Mar 2023, 9:48 pm | #6 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,384
|
Re: Cossor 494AC Electrolytics.
Hi Gary, does the hum disappear when the volume control is turned to minimum, or not? Jerry
|
16th Mar 2023, 11:20 pm | #7 | |
Heptode
Join Date: Apr 2013
Location: Bloxwich, West Midlands, UK
Posts: 554
|
Re: Cossor 494AC Electrolytics.
Quote:
The hum is still there with the volume at minimum. Regards, Gary
__________________
http://www.crutchleyhistory.co.uk/ |
|
17th Mar 2023, 10:51 pm | #8 |
Nonode
Join Date: Jun 2007
Location: Coulsdon, London, UK.
Posts: 2,163
|
Re: Cossor 494AC Electrolytics.
If the hum disappears when you ground the control grid of the first AF valve with a capacitor, it could indicate that shielded cable is required between the volume control and valve.
|
18th Mar 2023, 10:03 am | #9 |
Octode
Join Date: Sep 2020
Location: Thetford, Norfolk, UK.
Posts: 1,741
|
Re: Cossor 494AC Electrolytics.
If the mains smoothing capacitor has started to bulge, then that is the most likely cause of the hum.
I too would change the cathode caps, but they don't tend to cause the loud bangs that the reservoir ones do! If the cathode cap is particularly leaky though, then it can upset the biasing of the valve as well as introducing hum. |
19th Mar 2023, 5:34 pm | #10 |
Heptode
Join Date: Apr 2013
Location: Bloxwich, West Midlands, UK
Posts: 554
|
Re: Cossor 494AC Electrolytics.
Hi all, thanks for the advice. I ordered some capacitors of the same value and ratings, so as soon as they arrive I will get it done. Thanks all.
__________________
http://www.crutchleyhistory.co.uk/ |
19th Mar 2023, 5:58 pm | #11 |
Dekatron
Join Date: Sep 2011
Location: Rugeley, Staffordshire, UK.
Posts: 8,831
|
Re: Cossor 494AC Electrolytics.
That depends if the OP wants to go down the route of modifying the set to achieve a performance improvement, or is happy to restore it to spec and achieve the resultant performance that could be expected of it. Personally I wouldn't start using shielded cable in such a vintage set unless I was replacing some already there. It is what it is.
__________________
A digital radio is the latest thing, but a vintage wireless is forever.. |
19th Mar 2023, 10:08 pm | #12 |
Nonode
Join Date: Jun 2007
Location: Coulsdon, London, UK.
Posts: 2,163
|
Re: Cossor 494AC Electrolytics.
I have a Cossor 500 that apparently left the factory with a high level of hum.
It appears to have had very little use. A screened cable improved it. |