Murphy A72 Questions - UK Vintage Radio Repair and Restoration Discussion Forum

Neutrino · 31st Jan 2018, 3:51 pm

Two of my Murphy A72s (from about 1939) have had their paper & electrolytic capacitors replaced and are working. I have a few questions.

1) I have replaced the wiring to the magic eye and scale lamps. As the insulation on the wiring between the loudspeaker and the chassis plug has hardened I thought it might be prudent to replace it, especially considering that the circuits include both the earthed audio output and the loudspeaker field coil operating at about 320 V. My first question is how to open the Mazda Octal plug so that the new wires can be installed. I have tried to prise it open but fear that more force will break it. See the first photo.

2) The second question is what, if anything, to do about the missing and flaking metallisation on some of the valves. See the second photo.

3) A third question is what is the best way to replace some missing semi rectangular washers for the back. See the third photo. I could buy some brass strips to cut, drill and round the corners by filing but with only basic hand tools they would look home made.

4) A fourth question is how to replace the aluminium foil that is missing from the base of one of the cabinets. See photos 4 and 5. If I used kitchen foil I imagine it would only be a matter of time before it got displaced and shorted something under the chassis. I considered IKEA FIXA diffusion barrier which looks like aluminium tape but when checked with the meter it appears non conductive.

5) A fifth question is what to do about the ME41 magic eyes, one of which can only be seen in total darkness and the other has a cracked glass. One possibility is a Russian Magic Eye Tube 6E1P but this is viewed through the side and the ME41 is viewed at the end so some type of fitting would be needed to hold it in place. Another idea is an LED replacement similar to that in a previous thread but it would be require some effort to make. Is there a better and easier solution?
https://www.vintage-radio.net/forum/...ad.php?t=97804

David

G0HFCFrank · 31st Jan 2018, 9:22 pm

David,
Levering against the base of the plug will surely damage it but have you tried inserting a broad, flat-bladed screwdriver in that slot up the side of the cover and twisting? This may loosen its grip on the base. For the metallising, try "Repairing coating of metallised valves" in the search box at the top of this page. Rectangular washers; having made them out of brass, steel or even wood (old lollipop sticks?) a coat of dark paint will make them less noticeable and they are round the back. For the aluminium foil, turkey foil will be fine, stick it down on top of a coat of paint or paste and it won't move. As for the magic eye I don't know of any satisfactory substitute possibly back-light the dud tube with a green bulb or led?

Frank C.

Boater Sam · 1st Feb 2018, 6:17 am

Not a lot of point re coating the rectifier valve, later UU6 were clear glass.

Warm the octal plug a little, it will be less brittle.

ME41 last seen leaving town on a unicorn, rarer than rare.
I have heard that an octal based valve such as EM34 will fit in the socket but what to do for a 6.3V heater? And these are now expensive. Different connections, and not a single display type like the ME41.

Neutrino · 2nd Feb 2018, 8:09 pm

Thank you for the answers which will be helpful.

There was a lightbulb moment when I noticed that Russian 6E5S (6Е5С) magic eyes are reasonably priced. These are viewed from the end like the ME41 so can hopefully be adapted to fit. To increase the heater voltage from 4V to 6.3V I plan to use an autotransformer. I could wind a special but it may be easier to connect in series the secondaries of a 6VA 2x5V transformer with a resistor (select on test) to reduce the voltage from a nominal 8V to 6.3V:
https://uk.rs-online.com/web/p/chass...rmers/0504694/

David

ukcol · 2nd Feb 2018, 8:55 pm

Back in 2013 I fitted a 6E5GT in place of an AC/ME in a Murphy A40C. As well as heater voltage difference the 6E5GT is much more sensitive than the AC/ME and so I had to modify the control grid input circuit, (see circuit below).

With the exception of the base the ME41 is very similar to the AC/ME so for best results you should modify your A72 circuit to suit the 6E5 in a similar way.

I now have an ME41 so I will be undoing that mod and fitting the ME41.

Cobaltblue · 3rd Feb 2018, 6:10 pm

Hi David

I have a spare A72 Chassis (no valves sadly)

If there are any parts you want including the whole chassis Please let me know.

Its FOC post at cost or FCS

Cheers

Mike T

Neutrino · 3rd Feb 2018, 6:37 pm

Mike,
Thank you for the kind offer. However, so far I have not needed any major parts such as transformers, etc.
Best regards
David

31st Jan 2018, 3:51 pm	#1
Neutrino Pentode Join Date: Oct 2016 Location: Maldon, Essex, UK. Posts: 184	Murphy A72 Questions Two of my Murphy A72s (from about 1939) have had their paper & electrolytic capacitors replaced and are working. I have a few questions. 1) I have replaced the wiring to the magic eye and scale lamps. As the insulation on the wiring between the loudspeaker and the chassis plug has hardened I thought it might be prudent to replace it, especially considering that the circuits include both the earthed audio output and the loudspeaker field coil operating at about 320 V. My first question is how to open the Mazda Octal plug so that the new wires can be installed. I have tried to prise it open but fear that more force will break it. See the first photo. 2) The second question is what, if anything, to do about the missing and flaking metallisation on some of the valves. See the second photo. 3) A third question is what is the best way to replace some missing semi rectangular washers for the back. See the third photo. I could buy some brass strips to cut, drill and round the corners by filing but with only basic hand tools they would look home made. 4) A fourth question is how to replace the aluminium foil that is missing from the base of one of the cabinets. See photos 4 and 5. If I used kitchen foil I imagine it would only be a matter of time before it got displaced and shorted something under the chassis. I considered IKEA FIXA diffusion barrier which looks like aluminium tape but when checked with the meter it appears non conductive. 5) A fifth question is what to do about the ME41 magic eyes, one of which can only be seen in total darkness and the other has a cracked glass. One possibility is a Russian Magic Eye Tube 6E1P but this is viewed through the side and the ME41 is viewed at the end so some type of fitting would be needed to hold it in place. Another idea is an LED replacement similar to that in a previous thread but it would be require some effort to make. Is there a better and easier solution? https://www.vintage-radio.net/forum/...ad.php?t=97804 David Attached Thumbnails

31st Jan 2018, 9:22 pm	#2
G0HFCFrank Pentode Join Date: Oct 2011 Location: Oldham, Greater Manchester, UK. Posts: 167	Re: Murphy A72 Questions David, Levering against the base of the plug will surely damage it but have you tried inserting a broad, flat-bladed screwdriver in that slot up the side of the cover and twisting? This may loosen its grip on the base. For the metallising, try "Repairing coating of metallised valves" in the search box at the top of this page. Rectangular washers; having made them out of brass, steel or even wood (old lollipop sticks?) a coat of dark paint will make them less noticeable and they are round the back. For the aluminium foil, turkey foil will be fine, stick it down on top of a coat of paint or paste and it won't move. As for the magic eye I don't know of any satisfactory substitute possibly back-light the dud tube with a green bulb or led? Frank C.

1st Feb 2018, 6:17 am	#3
Boater Sam Banned Join Date: Dec 2013 Location: Middlewich, Cheshire, UK. & Winter in the Philippines. Posts: 3,897	Re: Murphy A72 Questions Not a lot of point re coating the rectifier valve, later UU6 were clear glass. Warm the octal plug a little, it will be less brittle. ME41 last seen leaving town on a unicorn, rarer than rare. I have heard that an octal based valve such as EM34 will fit in the socket but what to do for a 6.3V heater? And these are now expensive. Different connections, and not a single display type like the ME41.

2nd Feb 2018, 8:09 pm	#4
Neutrino Pentode Join Date: Oct 2016 Location: Maldon, Essex, UK. Posts: 184	Re: Murphy A72 Questions Thank you for the answers which will be helpful. There was a lightbulb moment when I noticed that Russian 6E5S (6Е5С) magic eyes are reasonably priced. These are viewed from the end like the ME41 so can hopefully be adapted to fit. To increase the heater voltage from 4V to 6.3V I plan to use an autotransformer. I could wind a special but it may be easier to connect in series the secondaries of a 6VA 2x5V transformer with a resistor (select on test) to reduce the voltage from a nominal 8V to 6.3V: https://uk.rs-online.com/web/p/chass...rmers/0504694/ David

2nd Feb 2018, 8:55 pm	#5
ukcol Rest in Peace Join Date: May 2003 Location: Harlaxton, Lincolnshire, UK. Posts: 3,944	Re: Murphy A72 Questions Back in 2013 I fitted a 6E5GT in place of an AC/ME in a Murphy A40C. As well as heater voltage difference the 6E5GT is much more sensitive than the AC/ME and so I had to modify the control grid input circuit, (see circuit below). With the exception of the base the ME41 is very similar to the AC/ME so for best results you should modify your A72 circuit to suit the 6E5 in a similar way. I now have an ME41 so I will be undoing that mod and fitting the ME41. Attached Thumbnails

3rd Feb 2018, 6:10 pm	#6
Cobaltblue Moderator Join Date: Dec 2010 Location: Exeter, Devon and Poole, Dorset UK. Posts: 6,876	Re: Murphy A72 Questions Hi David I have a spare A72 Chassis (no valves sadly) If there are any parts you want including the whole chassis Please let me know. Its FOC post at cost or FCS Cheers Mike T __________________ Invisible airwaves crackle with life or at least they used to Mike T BVWS member. www.cossor.co.uk

3rd Feb 2018, 6:37 pm	#7
Neutrino Pentode Join Date: Oct 2016 Location: Maldon, Essex, UK. Posts: 184	Re: Murphy A72 Questions Mike, Thank you for the kind offer. However, so far I have not needed any major parts such as transformers, etc. Best regards David