Unknown R-C bridge [Mullard GM4140] - UK Vintage Radio Repair and Restoration Discussion Forum

G3PIJpeter · 9th Dec 2018, 6:31 pm

I bought a nice crackle-finish test device at today's BVWS swapmeet at RWB today (Dec 9th). It was billed as a Phillips R-C bridge of pre-war vintage. Although the valves are side-contact, the components appear more 1950s to me. The magic eye is bright and the sensitivity control works - but I cannot obtain a null with any combination of R or C attached to the top terminals (centre one is earth).

Does anyone recognise this device? Does it have a name? Can I obtain working instructions or a circuit diagram?

All suggestions gratefully received.

Peter G3PIJ

Barnmead · 9th Dec 2018, 6:42 pm

I have a Mullard GM140 which is very similar it seems many variations were made have a look here: http://www.richardsradios.co.uk/cr_bridge.html

G3PIJpeter · 9th Dec 2018, 8:29 pm

Many thanks Richard. I knew that someone out there would have the answer to all my questions. I shall now go back to my recent acquisition and try using it according to the book.

Best wishes

- Peter

MotorBikeLes · 9th Dec 2018, 10:57 pm

Whilst I know the saying "Replace all caps" is depracated, I think all those black/brown ones WILL be faulty. Check and replace one at a time, then expect it to work correctly. Check the values first though, they MAY be close tolerance, and labelled 1%, 2% or similar, depending on circuit location. The circuit is very likely almost identical to a host of similar ones. For example, Hunts (of known infamy here) made one.
Les.

David G4EBT · 10th Dec 2018, 1:59 pm

Looks like someone has robbed it of its magic eye.

Nothing new there then. Shame really, but that's the way of the world.

Test gear generally gets little use, so as often as not, the 'eye' is in excellent condition, and of course worth far much more than the bridge.

ms660 · 10th Dec 2018, 2:07 pm

Post #1.

Lawrence.

David G4EBT · 10th Dec 2018, 3:15 pm

Ooops - egg on face - couldn't see it in the hole on the front panel, didn't read the post as carefully as I should have.

WME_bill · 10th Dec 2018, 9:51 pm

Unknown RC bridge. Mullard GM4140 or Admiralty type 5000.
I have one of these, the first instrument I acquired.

I still remember the face of the Mullard Executive in 1971 when I went into the Head Office in Torrington Place, Tottenham Court Road and asked for a manual. He kindly received this schoolboy, produced the manual for me, and sent me on my way with best wishes for a successful future.

My version is the GM4140, with the EM4 side contact magic eye, and EF36 and EB34.
I still use it. The electrolytics and the two 0.05 wax capacitors have been replaced, but the standards seem alright still.

This machine is interesting as the scale covers two decades, from 0.1 to 1 to 10, which reduces the numbers of ranges and standards required.
I have fitted a switch to short out the two 85 ohm expansion resistors to give a single decade, which I find easier to read. Using external standards.
And for use with a transformer ratio adapter, based upon a Scroggie design.
Also a neon lamp flasher to test wax capacitors for leakage, which I found very useful when every radio seemed to have TCC paper capacitors for interstage coupling.
I attach the manual and a 1954 specification leaflet.
wme_bill.

G3PIJpeter · 10th Dec 2018, 11:54 pm

Many thanks to all for the extra details and documents posted here. My GM4140 works after a fashion and gives a rough indication of the values of resistors and capacitors. I say rough because the EM4 indicator 'petals' do not narrow at one setting of the dial but rather across a range.

The next job will be to replace the internal capacitors and resistors as suggested. This bridge does seem to have had a variety of incarnations, with the Phillips version that I have perhaps earlier than the Mullard version - see http://frank.pocnet.net/instruments/...4140_flyer.pdf

- Peter G3PIJ

Refugee · 11th Dec 2018, 12:16 am

I have got a GM4249 badged Phillips.
It is mainly an AC resistance unit but with an external standard it can do capacitance and chokes. Like most of these instruments the magic eye is still nice and bright.
It has recently begun to fall out of use because I have acquired a Marconi TF1313A.

WME_bill · 12th Dec 2018, 1:36 pm

Unknown R-C Bridge GM4140
My posting #8 of 10thDec was getting a bit modern. My visit to the surprised Executive in Mullard Head Office was in 1951, not 1971. So the unit must be well before 1950, as I bought it second hand.
I have just switched it on again. The waxy capacitors have all gone, so replaced. They were 68 years old. And one dry joint. The resistors are old fashioned cracked carbon, and seem very stable. All the wiring is rubber covered with cotton waxed and still seems good. The standards seem fine.
I am always doubtful about this philosophy of removing every component for test and replacement. I suspect it can cause more failures later. A check on working voltages seems much the best way to identify problems.
Your problem sounds like a bad contact on the range switch. Check that you got the 2v AC across the two outer test terminals, and then across the ratio potentiometer and does the slider voltage vary.
wme_bill.

9th Dec 2018, 6:42 pm	#2
Barnmead Pentode Join Date: Dec 2002 Location: Norfolk, UK. Posts: 168	Re: Unknown R-C bridge I have a Mullard GM140 which is very similar it seems many variations were made have a look here: http://www.richardsradios.co.uk/cr_bridge.html __________________ Richard

9th Dec 2018, 8:29 pm	#3
G3PIJpeter Hexode Join Date: Nov 2011 Location: Chippenham, Wiltshire, UK. Posts: 322	Re: Unknown R-C bridge Many thanks Richard. I knew that someone out there would have the answer to all my questions. I shall now go back to my recent acquisition and try using it according to the book. Best wishes - Peter

9th Dec 2018, 10:57 pm	#4
MotorBikeLes Nonode Join Date: Dec 2012 Location: Kirk Michael, Isle of Man Posts: 2,350	Re: Unknown R-C bridge Whilst I know the saying "Replace all caps" is depracated, I think all those black/brown ones WILL be faulty. Check and replace one at a time, then expect it to work correctly. Check the values first though, they MAY be close tolerance, and labelled 1%, 2% or similar, depending on circuit location. The circuit is very likely almost identical to a host of similar ones. For example, Hunts (of known infamy here) made one. Les.

10th Dec 2018, 1:59 pm	#5
David G4EBT Dekatron Join Date: Feb 2003 Location: Cottingham, East Yorkshire, UK. Posts: 5,761	Re: Unknown R-C bridge Looks like someone has robbed it of its magic eye. Nothing new there then. Shame really, but that's the way of the world. Test gear generally gets little use, so as often as not, the 'eye' is in excellent condition, and of course worth far much more than the bridge. __________________ David. BVWS Member. G-QRP Club member 1339.

10th Dec 2018, 2:07 pm	#6
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Unknown R-C bridge Post #1. Lawrence.

9th Dec 2018, 6:31 pm	#1
G3PIJpeter Hexode Join Date: Nov 2011 Location: Chippenham, Wiltshire, UK. Posts: 322	Unknown R-C bridge [Mullard GM4140] I bought a nice crackle-finish test device at today's BVWS swapmeet at RWB today (Dec 9th). It was billed as a Phillips R-C bridge of pre-war vintage. Although the valves are side-contact, the components appear more 1950s to me. The magic eye is bright and the sensitivity control works - but I cannot obtain a null with any combination of R or C attached to the top terminals (centre one is earth). Does anyone recognise this device? Does it have a name? Can I obtain working instructions or a circuit diagram? All suggestions gratefully received. Peter G3PIJ Attached Thumbnails

10th Dec 2018, 3:15 pm	#7
David G4EBT Dekatron Join Date: Feb 2003 Location: Cottingham, East Yorkshire, UK. Posts: 5,761	Re: Unknown R-C bridge Ooops - egg on face - couldn't see it in the hole on the front panel, didn't read the post as carefully as I should have. __________________ David. BVWS Member. G-QRP Club member 1339.

10th Dec 2018, 11:54 pm	#9
G3PIJpeter Hexode Join Date: Nov 2011 Location: Chippenham, Wiltshire, UK. Posts: 322	Re: Unknown R-C bridge Many thanks to all for the extra details and documents posted here. My GM4140 works after a fashion and gives a rough indication of the values of resistors and capacitors. I say rough because the EM4 indicator 'petals' do not narrow at one setting of the dial but rather across a range. The next job will be to replace the internal capacitors and resistors as suggested. This bridge does seem to have had a variety of incarnations, with the Phillips version that I have perhaps earlier than the Mullard version - see http://frank.pocnet.net/instruments/...4140_flyer.pdf - Peter G3PIJ

11th Dec 2018, 12:16 am	#10
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,553	Re: Unknown R-C bridge I have got a GM4249 badged Phillips. It is mainly an AC resistance unit but with an external standard it can do capacitance and chokes. Like most of these instruments the magic eye is still nice and bright. It has recently begun to fall out of use because I have acquired a Marconi TF1313A.

12th Dec 2018, 1:36 pm	#11
WME_bill Octode Join Date: Oct 2007 Location: Exeter, Devon, UK. Posts: 1,554	Re: Unknown R-C bridge [Mullard GM4140] Unknown R-C Bridge GM4140 My posting #8 of 10thDec was getting a bit modern. My visit to the surprised Executive in Mullard Head Office was in 1951, not 1971. So the unit must be well before 1950, as I bought it second hand. I have just switched it on again. The waxy capacitors have all gone, so replaced. They were 68 years old. And one dry joint. The resistors are old fashioned cracked carbon, and seem very stable. All the wiring is rubber covered with cotton waxed and still seems good. The standards seem fine. I am always doubtful about this philosophy of removing every component for test and replacement. I suspect it can cause more failures later. A check on working voltages seems much the best way to identify problems. Your problem sounds like a bad contact on the range switch. Check that you got the 2v AC across the two outer test terminals, and then across the ratio potentiometer and does the slider voltage vary. wme_bill.