Murphy A242 Deluxe help. - UK Vintage Radio Repair and Restoration Discussion Forum

recoil325 · 11th Jun 2018, 6:59 pm

Looking for some help on a Murphy A242.

Could some one tell me if C71 5uF is a polarised or non polarised electrolytic capacitor? The one that's in it is in a bit of a state and I can't see any markings for + or -. The poor schematic I have shows it as a polarised one but I just wanted to check .

I mainly work on ham/CB radio equipment, so this kind of stuff is a bit different to me, not really played with something this old.

Thanks in advance, Rob.

Station X · 11th Jun 2018, 7:25 pm

Hello and welcome to the forums.

In vintage sets any capacitor over 1uF will be a polarized electrolytic type.

That is the case here. C71 (Manufacturer's sheet) is a 5uF electrolytic capacitor.

recoil325 · 11th Jun 2018, 7:35 pm

thanks graham

im new to these old valve sets so just wanted confirmation .I picked this up cheap at a radio rally I wanted something different as a project . I like the sound of these old radios so it may be the start of another collection of equipment .

thanks for letting me join this forum

rob

Graham G3ZVT · 12th Jun 2018, 12:23 am

I am sure Graham is correct, but I can understand why you thought it might have been non-polarized.

Is the purpose of R45 to keep C71 happy by introducing a DC bias?

marconi_pete · 12th Jun 2018, 1:49 am

As Graham has mentioned, almost all caps above 1uf in vintage sets will be polarised electrolytics. If the markings on the cap are obscured and the polarity is unknown. The general rule of thumb is that if one side of the cap goes to chassis it's almost always going to be the negative end.

Thanks,
Peter

Chris55000 · 12th Jun 2018, 4:43 am

Hi!

C71 is the Ratio Detector Stabilising Capacitor and is an essential part of the (complex!) operation of the ratio discriminator!

The use of a separate 6D2 double–diode means it's not necessary to use an unbalanced (DC load–wise!) circuit as is necessary with the EABC 80 T.D.T. valve, where the common valve cathode to two of the valve diodes means the positive side of the load has to be earthed!

R45 is deliberately added to unbalance the circuit slightly to improve the a.m. impulse–noise rejection, e.g., from thunderstorms., etc. (in intercarrier–sound 6 Mc/s TV sound ratio discriminators this component was almost always added, usually preset, as it can be set to minimise "inter-carrier" buzz).

You can use a 4u7 plastic–film capacitor to replace C71 but a modern axial electrolytic (105°C!!!) is perfectly adequate for this use!

Chris Williams

recoil325 · 12th Jun 2018, 7:21 pm

Thanks all.

Could anyone point me in the direction of a service manual for the radio as all I have is the schematic?

Rob

AC/HL · 12th Jun 2018, 7:58 pm

Manufacturer's manual: https://www.service-data.com/product...05/5128/m13905

Tim · 14th Jun 2018, 1:16 pm

I'm sure Murphy considered any polarity issues in the original design. If you don't have one handy, you could try two 10ufs "back to back" (each positive to each cathode and the two negative ends connected together). There is probably not much DC across it in reality, as each end seems to be taken to chassis via an equal value resistor.

Manual available on the service data page at the top right.

Ta-da!
https://www.service-data.com/product...05/5128/m13905

Very reasonable prices and money earned supports this forum.

Nuvistor · 14th Jun 2018, 1:35 pm

I suppose it depends on what you mean by not much voltage across it, on strong signals in valve VHF FM sets I have seen 30-40 volts across the stabilising capacitor, the manual states 50Vdc working so I would fit a 4.7ufd 63vdc working electrolytic or something near that size that you may have in your spares.

recoil325 · 14th Jun 2018, 5:48 pm

Thanks again everyone.
I've used a 4.7uf 63v electrolytic and it seems ok, will sort out getting the service manual soon.
Also need to track down a magic eye tube. I believe the EM34 is a replacement for the 6M2, and probably easier to get hold of.

11th Jun 2018, 6:59 pm	#1
recoil325 Retired Dormant Member Join Date: Jun 2018 Location: Chesterfield, Derbyshire, UK. Posts: 4	Murphy A242 Deluxe help. Looking for some help on a Murphy A242. Could some one tell me if C71 5uF is a polarised or non polarised electrolytic capacitor? The one that's in it is in a bit of a state and I can't see any markings for + or -. The poor schematic I have shows it as a polarised one but I just wanted to check . I mainly work on ham/CB radio equipment, so this kind of stuff is a bit different to me, not really played with something this old. Thanks in advance, Rob.

11th Jun 2018, 7:25 pm	#2
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: Murphy A242 Deluxe help. Hello and welcome to the forums. In vintage sets any capacitor over 1uF will be a polarized electrolytic type. That is the case here. C71 (Manufacturer's sheet) is a 5uF electrolytic capacitor. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

11th Jun 2018, 7:35 pm	#3
recoil325 Retired Dormant Member Join Date: Jun 2018 Location: Chesterfield, Derbyshire, UK. Posts: 4	Re: Murphy A242 Deluxe help. thanks graham im new to these old valve sets so just wanted confirmation .I picked this up cheap at a radio rally I wanted something different as a project . I like the sound of these old radios so it may be the start of another collection of equipment . thanks for letting me join this forum rob

12th Jun 2018, 12:23 am	#4
Graham G3ZVT Dekatron Join Date: May 2010 Location: Greater Manchester, UK. Posts: 18,715	Re: Murphy A242 Deluxe help. I am sure Graham is correct, but I can understand why you thought it might have been non-polarized. Is the purpose of R45 to keep C71 happy by introducing a DC bias? Attached Thumbnails __________________ -- Graham. G3ZVT

12th Jun 2018, 1:49 am	#5
marconi_pete Hexode Join Date: Mar 2012 Location: London, UK. Posts: 347	Re: Murphy A242 Deluxe help. As Graham has mentioned, almost all caps above 1uf in vintage sets will be polarised electrolytics. If the markings on the cap are obscured and the polarity is unknown. The general rule of thumb is that if one side of the cap goes to chassis it's almost always going to be the negative end. Thanks, Peter __________________ BVWS and BVWTM member

12th Jun 2018, 4:43 am	#6
Chris55000 Nonode Join Date: Aug 2007 Location: Walsall Wood, Aldridge, Walsall, UK. Posts: 2,870	Re: Murphy A242 Deluxe help. Hi! C71 is the Ratio Detector Stabilising Capacitor and is an essential part of the (complex!) operation of the ratio discriminator! The use of a separate 6D2 double–diode means it's not necessary to use an unbalanced (DC load–wise!) circuit as is necessary with the EABC 80 T.D.T. valve, where the common valve cathode to two of the valve diodes means the positive side of the load has to be earthed! R45 is deliberately added to unbalance the circuit slightly to improve the a.m. impulse–noise rejection, e.g., from thunderstorms., etc. (in intercarrier–sound 6 Mc/s TV sound ratio discriminators this component was almost always added, usually preset, as it can be set to minimise "inter-carrier" buzz). You can use a 4u7 plastic–film capacitor to replace C71 but a modern axial electrolytic (105°C!!!) is perfectly adequate for this use! Chris Williams __________________ It's an enigma, that's what it is! This thing's not fixed because it doesn't want to be fixed!

12th Jun 2018, 7:21 pm	#7
recoil325 Retired Dormant Member Join Date: Jun 2018 Location: Chesterfield, Derbyshire, UK. Posts: 4	Re: Murphy A242 Deluxe help. Thanks all. Could anyone point me in the direction of a service manual for the radio as all I have is the schematic? Rob

12th Jun 2018, 7:58 pm	#8
AC/HL Dekatron Join Date: Nov 2003 Location: Heckmondwike, West Yorkshire, UK. Posts: 9,642	Re: Murphy A242 Deluxe help. Manufacturer's manual: https://www.service-data.com/product...05/5128/m13905

14th Jun 2018, 1:16 pm	#9
Tim Dekatron Join Date: Jan 2003 Location: Bradford on Avon, Wiltshire, UK. Posts: 3,310	Re: Murphy A242 Deluxe help. I'm sure Murphy considered any polarity issues in the original design. If you don't have one handy, you could try two 10ufs "back to back" (each positive to each cathode and the two negative ends connected together). There is probably not much DC across it in reality, as each end seems to be taken to chassis via an equal value resistor. Manual available on the service data page at the top right. Ta-da! https://www.service-data.com/product...05/5128/m13905 Very reasonable prices and money earned supports this forum. __________________ "Nothing is as dangerous as being too modern;one is apt to grow old fashioned quite suddenly."

14th Jun 2018, 1:35 pm	#10
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,433	Re: Murphy A242 Deluxe help. I suppose it depends on what you mean by not much voltage across it, on strong signals in valve VHF FM sets I have seen 30-40 volts across the stabilising capacitor, the manual states 50Vdc working so I would fit a 4.7ufd 63vdc working electrolytic or something near that size that you may have in your spares. __________________ Frank

14th Jun 2018, 5:48 pm	#11
recoil325 Retired Dormant Member Join Date: Jun 2018 Location: Chesterfield, Derbyshire, UK. Posts: 4	Re: Murphy A242 Deluxe help. Thanks again everyone. I've used a 4.7uf 63v electrolytic and it seems ok, will sort out getting the service manual soon. Also need to track down a magic eye tube. I believe the EM34 is a replacement for the 6M2, and probably easier to get hold of.