U502 first cap. change - UK Vintage Radio Repair and Restoration Discussion Forum

colebrooker · 27th Dec 2016, 5:40 pm

Hello, before I start on the change of C44 in my Murphy U502 (the cap. to change) there is a query on the voltage rating. The parts list specifies 500v but I have to hand a red Wima mks 4 rated at 400v. Would this be ok to use? Thanks for any replies. David.

marconi_pete · 27th Dec 2016, 5:45 pm

Hi, I don't have the circuit to hand but can't imagine there being any problems with using a 400v rated cap in this set as a decoupling cap. If it needs to be connected across the mains then you'd ideally need a X2 rated cap. Hope that helps.

Thanks,
Peter

Cobaltblue · 27th Dec 2016, 6:07 pm

Hi David

C44 is the grid coupling capacitor for the output valve according to the Murphy service information.

As the HT on this set is less than 200V I would be happy to use a 400V cap in this position.

Cheers

Mike T

colebrooker · 27th Dec 2016, 6:43 pm

Thanks Mike T for the prompt reply. I can see what you mean, it's something that I hadn't thought to look at. david

Leon Crampin · 27th Dec 2016, 6:57 pm

I think you will find that on the U502 Murphy have used a ceramic "dog bone" capacitor in this position. If so, it will be very unlikely to need replacement.

Leon.

colebrooker · 28th Dec 2016, 1:57 pm

it is tubular with a clear sleeve and TCC type cp335 on it.
David.

colebrooker · 28th Dec 2016, 6:18 pm

Leon, could you describe the 'dog bone' cap a bit better. Would it be basically tubular?
If so that might be what I have.
David

Cobaltblue · 28th Dec 2016, 7:08 pm

What your describing sounds more like a Metalmite capacitor like the first picture.
The Dogbone ceramic is more likely to look similar to the second picture,

A lot of Metalmites are getting tired now but still worth checking,

Cheers

Mike T

ukcol · 29th Dec 2016, 8:49 am

i was worried when you said you were going to fit a Wima, but the worries went away when I saw that it was a polyester type,

colebrooker · 29th Dec 2016, 8:27 pm

Thanks for the information on the Metalmite and the Wima catalogue. It is a Metalmite and I will check it further before deciding whether to change it.
David.

colebrooker · 1st Jan 2017, 4:54 pm

Hello, I have now changed 5 Hunt caps., 2 of which showed some cracking in the sides. Also the Plessey can with the 50 + 50 has been disconnected and 2 seperate 50's mounted under the chassis. As an ex Plessey man I wanted to leave the original, I know that is best anyway. My next question is about c41. Should I change it just because it is an electrolytic or leave it to see if it is a problem?
David

ms660 · 1st Jan 2017, 5:03 pm

It's across the FM discriminator, those electrolytics can be problematic in receivers of that age, I would replace it.

Lawrence.

colebrooker · 2nd Jan 2017, 9:49 pm

OK, I have now replaced c41 with a new electrolytic so that's finished my planned replacements. I did notice the polarity of the capacitor and copied it.
David

27th Dec 2016, 5:40 pm	#1
colebrooker Retired Dormant Member Join Date: Feb 2016 Location: Swindon, Wiltshire, UK. Posts: 51	U502 first cap. change Hello, before I start on the change of C44 in my Murphy U502 (the cap. to change) there is a query on the voltage rating. The parts list specifies 500v but I have to hand a red Wima mks 4 rated at 400v. Would this be ok to use? Thanks for any replies. David.

27th Dec 2016, 5:45 pm	#2
marconi_pete Hexode Join Date: Mar 2012 Location: London, UK. Posts: 347	Re: U502 first cap. change Hi, I don't have the circuit to hand but can't imagine there being any problems with using a 400v rated cap in this set as a decoupling cap. If it needs to be connected across the mains then you'd ideally need a X2 rated cap. Hope that helps. Thanks, Peter

27th Dec 2016, 6:07 pm	#3
Cobaltblue Moderator Join Date: Dec 2010 Location: Exeter, Devon and Poole, Dorset UK. Posts: 6,879	Re: U502 first cap. change Hi David C44 is the grid coupling capacitor for the output valve according to the Murphy service information. As the HT on this set is less than 200V I would be happy to use a 400V cap in this position. Cheers Mike T __________________ Invisible airwaves crackle with life or at least they used to Mike T BVWS member. www.cossor.co.uk

27th Dec 2016, 6:43 pm	#4
colebrooker Retired Dormant Member Join Date: Feb 2016 Location: Swindon, Wiltshire, UK. Posts: 51	Re: U502 first cap. change Thanks Mike T for the prompt reply. I can see what you mean, it's something that I hadn't thought to look at. david

27th Dec 2016, 6:57 pm	#5
Leon Crampin Octode Join Date: Nov 2005 Location: Surrey, UK. Posts: 1,874	Re: U502 first cap. change I think you will find that on the U502 Murphy have used a ceramic "dog bone" capacitor in this position. If so, it will be very unlikely to need replacement. Leon.

28th Dec 2016, 1:57 pm	#6
colebrooker Retired Dormant Member Join Date: Feb 2016 Location: Swindon, Wiltshire, UK. Posts: 51	Re: U502 first cap. change it is tubular with a clear sleeve and TCC type cp335 on it. David.

28th Dec 2016, 6:18 pm	#7
colebrooker Retired Dormant Member Join Date: Feb 2016 Location: Swindon, Wiltshire, UK. Posts: 51	Re: U502 first cap. change Leon, could you describe the 'dog bone' cap a bit better. Would it be basically tubular? If so that might be what I have. David

28th Dec 2016, 7:08 pm	#8
Cobaltblue Moderator Join Date: Dec 2010 Location: Exeter, Devon and Poole, Dorset UK. Posts: 6,879	Re: U502 first cap. change What your describing sounds more like a Metalmite capacitor like the first picture. The Dogbone ceramic is more likely to look similar to the second picture, A lot of Metalmites are getting tired now but still worth checking, Cheers Mike T Attached Thumbnails __________________ Invisible airwaves crackle with life or at least they used to Mike T BVWS member. www.cossor.co.uk

29th Dec 2016, 8:27 pm	#10
colebrooker Retired Dormant Member Join Date: Feb 2016 Location: Swindon, Wiltshire, UK. Posts: 51	Re: U502 first cap. change Thanks for the information on the Metalmite and the Wima catalogue. It is a Metalmite and I will check it further before deciding whether to change it. David.

1st Jan 2017, 4:54 pm	#11
colebrooker Retired Dormant Member Join Date: Feb 2016 Location: Swindon, Wiltshire, UK. Posts: 51	Re: U502 first cap. change Hello, I have now changed 5 Hunt caps., 2 of which showed some cracking in the sides. Also the Plessey can with the 50 + 50 has been disconnected and 2 seperate 50's mounted under the chassis. As an ex Plessey man I wanted to leave the original, I know that is best anyway. My next question is about c41. Should I change it just because it is an electrolytic or leave it to see if it is a problem? David

1st Jan 2017, 5:03 pm	#12
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: U502 first cap. change It's across the FM discriminator, those electrolytics can be problematic in receivers of that age, I would replace it. Lawrence.

2nd Jan 2017, 9:49 pm	#13
colebrooker Retired Dormant Member Join Date: Feb 2016 Location: Swindon, Wiltshire, UK. Posts: 51	Re: U502 first cap. change OK, I have now replaced c41 with a new electrolytic so that's finished my planned replacements. I did notice the polarity of the capacitor and copied it. David