Murphy B40D Osc trim variable capacitor - UK Vintage Radio Repair and Restoration Discussion Forum

Tyso_Bl · 24th Mar 2016, 4:53 pm

Would anyone here be able to post a pic of what the osc trim capacitor fitted to the Murphy B40D receiver is like?

The reason for this slightly strange request is that the one fitted in the B40 I'm working on is responsible for some serious L.O. instability. It may be just worn out, but I'm not sure if it is even the right part.

T

ms660 · 24th Mar 2016, 6:05 pm

I don't have a picture but I'm guessing it's physically small? It's value is given as 1 to 4.2pf in the book of words.

Lawrence.

Tyso_Bl · 24th Mar 2016, 9:52 pm

Ok then, I've got a pic of whats fitted, can anyone confirm if it an original fitment please?

g4aaw pete · 24th Mar 2016, 10:14 pm

On holiday at moment, but will post a picture next Monday if no one else is able to.

Regards
Pete

Sean Williams · 24th Mar 2016, 10:15 pm

Looks very similar to what is fitted in mine.

Jon_G4MDC · 26th Mar 2016, 10:36 am

why no screws holding it down?

ms660 · 26th Mar 2016, 11:16 am

Might be fixed from the front? Screws/nut.

Lawrence

QWENIX · 27th Mar 2016, 11:21 pm

Before taking it out and throw away. Maybe you can first clean remove old dirt, grease and oxides from the axis slider contacts and ceramic insulators parts with alcohol or tunerspray (oxydex never use contactspray) clean again with alcohol and furthermore after drying lubricate the slider contact with very... very... very (point of needle: little Dow Corning Electrical Compound 4). This could help a lot. I have done this with a German Siemens E311 receiver which had bad slider contacts in the oscillator and tuning capacitor.

Will try to find an opened up receiver or else an original BFO osc trim capacitor and take a picture.

Kind regards

Qwenix

QWENIX · 28th Mar 2016, 11:26 am

I managed to take some pics. It looks original to me.

What you dont see on your picture is the axis and the two stops.

See my pics.

QWENIX · 28th Mar 2016, 11:32 am

The axis and two stops clock anti clock. The BFO osc trim is of course mounted to the chassis by a nut.

I advice you first to take out the osc trim carefully and clean and lubricate the slider contacts (a bit of a needle point) Dow Corning electrical Compound 4. If not succesfull than youll need a spare one .....

Qwenix

Jon_G4MDC · 28th Mar 2016, 11:33 am

I think I was having trouble with size/scale of it.
It's a Jackson C804 or similar type then with a shaft passing through the box wall? I don't recall them having the 2 holes in the ceramic that one has.

To me it looked like one of those slotted trimmers that need holding down with M2 screws or similar.

Tyso_Bl · 12th Apr 2016, 10:39 am

Apologies for the delay in coming back to this thread, thanks to qwenix for the pictures, and everyone else for the replies, it does look like it it an original part.

It has been removed for inspection, it does have a loose and sloppy action, we'll have a go at finding a replacement, it does look like an afterthought by the manufacturer, or they didn't expect it to get much use. Its more of a trimmer than a proper variable cap.

Other issues have put the B40 out of action for now, poss the subject of another thread.

turretslug · 12th Apr 2016, 11:44 am

I think it was an addition with the "D" revision, along with things like the more modern miniature valves, so as to make tuning into and holding data-type transmissions more tenable on the higher bands. With those few vanes and wide spacing, it was probably the equivalent of moving a finger in and out of the oscillator box! It is a bit of an after-thought, squeezed in and kind of betraying the age of what was by then a trusted and familiar radio.

That basic horse-shoe shape ceramic base had all sorts of trimmer formats built onto it, with either slots or longer shafts extending one side or the other and some with securing collars that made the two small holes superfluous.

24th Mar 2016, 4:53 pm	#1
Tyso_Bl Hexode Join Date: Jul 2014 Location: Magor, Monmouthshire, Wales, UK. Posts: 436	Murphy B40D Osc trim variable capacitor Would anyone here be able to post a pic of what the osc trim capacitor fitted to the Murphy B40D receiver is like? The reason for this slightly strange request is that the one fitted in the B40 I'm working on is responsible for some serious L.O. instability. It may be just worn out, but I'm not sure if it is even the right part. T __________________ Adapt, Improvise, Oh Bother.....

24th Mar 2016, 6:05 pm	#2
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Murphy B40D Osc trim variable capacitor I don't have a picture but I'm guessing it's physically small? It's value is given as 1 to 4.2pf in the book of words. Lawrence.

24th Mar 2016, 9:52 pm	#3
Tyso_Bl Hexode Join Date: Jul 2014 Location: Magor, Monmouthshire, Wales, UK. Posts: 436	Re: Murphy B40D Osc trim variable capacitor Ok then, I've got a pic of whats fitted, can anyone confirm if it an original fitment please? Attached Thumbnails __________________ Adapt, Improvise, Oh Bother.....

24th Mar 2016, 10:14 pm	#4
g4aaw pete Heptode Join Date: Jan 2008 Location: Birchington Kent, UK. Posts: 596	Re: Murphy B40D Osc trim variable capacitor On holiday at moment, but will post a picture next Monday if no one else is able to. Regards Pete

24th Mar 2016, 10:15 pm	#5
Sean Williams Dekatron Join Date: Feb 2005 Location: St.Ippolyts, Hitchin, Hertfordshire QRA IO91UW Posts: 3,518	Re: Murphy B40D Osc trim variable capacitor Looks very similar to what is fitted in mine. __________________ Engineers make things work and have spare bits when finished

26th Mar 2016, 10:36 am	#6
Jon_G4MDC Nonode Join Date: Nov 2015 Location: Welwyn Garden City, Hertfordshire, UK. Posts: 2,015	Re: Murphy B40D Osc trim variable capacitor why no screws holding it down?

26th Mar 2016, 11:16 am	#7
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Murphy B40D Osc trim variable capacitor Might be fixed from the front? Screws/nut. Lawrence

27th Mar 2016, 11:21 pm	#8
QWENIX Pentode Join Date: Jan 2006 Location: Amsterdam, The Netherlands Posts: 180	Re: Murphy B40D Osc trim variable capacitor Before taking it out and throw away. Maybe you can first clean remove old dirt, grease and oxides from the axis slider contacts and ceramic insulators parts with alcohol or tunerspray (oxydex never use contactspray) clean again with alcohol and furthermore after drying lubricate the slider contact with very... very... very (point of needle: little Dow Corning Electrical Compound 4). This could help a lot. I have done this with a German Siemens E311 receiver which had bad slider contacts in the oscillator and tuning capacitor. Will try to find an opened up receiver or else an original BFO osc trim capacitor and take a picture. Kind regards Qwenix Last edited by QWENIX; 27th Mar 2016 at 11:29 pm.

28th Mar 2016, 11:26 am	#9
QWENIX Pentode Join Date: Jan 2006 Location: Amsterdam, The Netherlands Posts: 180	Re: Murphy B40D Osc trim variable capacitor I managed to take some pics. It looks original to me. What you dont see on your picture is the axis and the two stops. See my pics. Attached Thumbnails Last edited by QWENIX; 28th Mar 2016 at 11:38 am.

28th Mar 2016, 11:32 am	#10
QWENIX Pentode Join Date: Jan 2006 Location: Amsterdam, The Netherlands Posts: 180	Re: Murphy B40D Osc trim variable capacitor The axis and two stops clock anti clock. The BFO osc trim is of course mounted to the chassis by a nut. I advice you first to take out the osc trim carefully and clean and lubricate the slider contacts (a bit of a needle point) Dow Corning electrical Compound 4. If not succesfull than youll need a spare one ..... Qwenix Attached Thumbnails Last edited by QWENIX; 28th Mar 2016 at 11:40 am.

28th Mar 2016, 11:33 am	#11
Jon_G4MDC Nonode Join Date: Nov 2015 Location: Welwyn Garden City, Hertfordshire, UK. Posts: 2,015	Re: Murphy B40D Osc trim variable capacitor I think I was having trouble with size/scale of it. It's a Jackson C804 or similar type then with a shaft passing through the box wall? I don't recall them having the 2 holes in the ceramic that one has. To me it looked like one of those slotted trimmers that need holding down with M2 screws or similar.

12th Apr 2016, 10:39 am	#12
Tyso_Bl Hexode Join Date: Jul 2014 Location: Magor, Monmouthshire, Wales, UK. Posts: 436	Re: Murphy B40D Osc trim variable capacitor Apologies for the delay in coming back to this thread, thanks to qwenix for the pictures, and everyone else for the replies, it does look like it it an original part. It has been removed for inspection, it does have a loose and sloppy action, we'll have a go at finding a replacement, it does look like an afterthought by the manufacturer, or they didn't expect it to get much use. Its more of a trimmer than a proper variable cap. Other issues have put the B40 out of action for now, poss the subject of another thread. __________________ Adapt, Improvise, Oh Bother.....

12th Apr 2016, 11:44 am	#13
turretslug Dekatron Join Date: Nov 2011 Location: Surrey, UK. Posts: 4,398	Re: Murphy B40D Osc trim variable capacitor I think it was an addition with the "D" revision, along with things like the more modern miniature valves, so as to make tuning into and holding data-type transmissions more tenable on the higher bands. With those few vanes and wide spacing, it was probably the equivalent of moving a finger in and out of the oscillator box! It is a bit of an after-thought, squeezed in and kind of betraying the age of what was by then a trusted and familiar radio. That basic horse-shoe shape ceramic base had all sorts of trimmer formats built onto it, with either slots or longer shafts extending one side or the other and some with securing collars that made the two small holes superfluous.