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Old 8th Jul 2023, 4:51 pm   #121
agardiner
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Default Re: What did I get myself into? Murphy A40C.

Hi All,

Yes, the gram socket has left me a bit baffled. Looks overly complex and I am really not sure what is going on. It is switched, but there are 5 connections. I attach the part of the schematic which shows this. I am sure there must be a simple explanation. Can anyone shed any light on it please?
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Old 8th Jul 2023, 5:21 pm   #122
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Default Re: What did I get myself into? Murphy A40C.

Quote:
Originally Posted by agardiner View Post
Hi All,

Yes, the gram socket has left me a bit baffled. Looks overly complex and I am really not sure what is going on. It is switched, but there are 5 connections. I attach the part of the schematic which shows this. I am sure there must be a simple explanation. Can anyone shed any light on it please?
When the pickup is plugged in it disconnects the output from the detector from the control grid of the audio amplifier and references the suppressor grid to the cathode via a fixed resistance instead of being connected to the ATC circuit.

Lawrence.
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Old 9th Jul 2023, 11:04 am   #123
agardiner
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Default Re: What did I get myself into? Murphy A40C.

Thanks Lawrence. So how would one connect a modern socket? I guess it would have to be a switched type like a jack socket, but what about the suppressor grid? Ignore it and just break the detector/control grid circuit?
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Old 9th Jul 2023, 12:09 pm   #124
Leon Crampin
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Default Re: What did I get myself into? Murphy A40C.

Why not leave the socket as it is and use a 1/4" post office type jack plug to make your external audio input connection. The A40 would then be correctly configured to accept the input.

Leon.
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Old 9th Jul 2023, 12:28 pm   #125
ms660
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Default Re: What did I get myself into? Murphy A40C.

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Originally Posted by agardiner View Post
Thanks Lawrence. So how would one connect a modern socket? I guess it would have to be a switched type like a jack socket, but what about the suppressor grid? Ignore it and just break the detector/control grid circuit?
According to the schematic, the N.O. contacts on the gram input socket are in parallel with the contacts of S1m when S1m is switched to gram. If that's the case you could use a N.C. switched jack socket if you wanted too.

Lawrence.
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Old 12th Aug 2023, 12:50 pm   #126
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Default Re: What did I get myself into? Murphy A40C.

Quote:
Originally Posted by Leon Crampin View Post
Why not leave the socket as it is and use a 1/4" post office type jack plug to make your external audio input connection. The A40 would then be correctly configured to accept the input.

Leon.
Not sure why I never thought of this but of course like with everything else on this radio it isn't exactly that easy.

I assume a 1/4" post office type jack plug is exactly the same size as modern 6.35mm jack and that seems to be a few mm short. Still, it gave a few obvious pointers how this socket is connected and how it all works.
Any ideas what sort of a contraption suppose to fit in there?
I think a GPO plug is similarly sized so will be too short too.
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Old 13th Aug 2023, 11:43 am   #127
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Default Re: What did I get myself into? Murphy A40C.

I use a 3.5mm stereo jack to 1/4" mono converter plug. Just plug my bluetooth in and it works perfectly. Sounds great too, a lot more bass than the radio gives.

Bill
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Old 13th Aug 2023, 12:09 pm   #128
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Default Re: What did I get myself into? Murphy A40C.

Thank Bill, could you post a pic of it and let me know what's the length of the metal bit from tip to where it rests on the socket? As you can see in my pic that's a standard stereo converter and that appears to be slightly too short, just wanna make sure that if I order a mono converter it will reach the bottom pin in the radio.
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Old 13th Aug 2023, 6:03 pm   #129
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Default Re: What did I get myself into? Murphy A40C.

Will do.
Probably be tomorrow though as still not back home from work

Bill
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Old 14th Aug 2023, 8:38 pm   #130
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Default Re: What did I get myself into? Murphy A40C.

Ok, here are measurements:

length of plug shaft is 30mm tip to shoulder
Thickness of socket protruding from chassis is 6mm (see photo)

Bill
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Old 15th Aug 2023, 10:38 am   #131
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Default Re: What did I get myself into? Murphy A40C.

Ok, I will have to revisit this and look at the socket and all blades that pin suppose to touch, from a quick look before, mine at 31mm was too short to contact the right blade so either this suppose to be like this or I might need to adjust something.

Still need to modify power supply for it(to basically safety improve it by adding some diodes, fuses and extra caps), might finally get to it this week so will post more pics and details soon.
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Old 15th Aug 2023, 1:18 pm   #132
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Default Re: What did I get myself into? Murphy A40C.

Ok, went to the workshop and had another look and I did get it wrong, the tip not suppose to touch the right blade and mono 6.35mm to RCA in my case will work well, so its a 'double switched' socket basically, that's why there are 5 connection with two being NC and NO.
I really need to triple check all things on this radio
Many thanks to Bill and Leon for pointing me in the right direction!
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Old 29th Aug 2023, 11:27 pm   #133
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Default Re: What did I get myself into? Murphy A40C.

Right, I have finished modifications to the power supply which should much better protect this set going forward.
As mentioned on this forum the original Murphy PSU design is probably the worst thing about this radio as when one of the HT rectifiers fail, remaining valve will supply all the current from just one half of the HT secondary, which can effectively kill the transformer. The attached modifications basically fixes that issue. More detail about it can be found in the BVWS bulleting, Spring 2019.

I have also liked the idea of adding an additional fuse, which was suggested by Kevin aka mister valve on this forum, the original transformer has two separate HT windings which are linked together externally at a 'star type' earth tag attached to the frame of the transformer(in the pic) and the fuse went between HT centre-tap and chassis itself. I've used an existing hole at the back of the PSU for the fuse holder so it will be invisible, even with the back cover off and the remaining free earth tag at the bottom of the PSU.

I have tested everything today and can confirm all works well. Initially used the AM transmitter supplied by Derek aka coopzone, then switched to my BT module plugged in directly through the 6.35mm jack to RCA and that worked great too, some excellent LBC podcasts this evening

I can confirm that the BT module plugged in directly somehow makes up for far more bass than any station that radio can pick up on its own. Not entirely sure why. Replacement 6 ohms Oxford speaker is extremely loud and the sound is nice and clear even at max volume.

Now just need to fit replacement power cord and start putting it all back together. More pics probably later in the week.
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Last edited by Katar83; 29th Aug 2023 at 11:32 pm.
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Old 30th Aug 2023, 11:40 pm   #134
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Default Re: What did I get myself into? Murphy A40C.

Finished!

Almost died trying to get the chassis into the cabinet, needed some proper gymnastics to get it in, I really wish Murphy made the cabinet a centimetre taller, would be so much easier.

Replaced PSU rubber grommets, the old ones were hard as rock, routed new power cable, the PSU was missing one original bolt and of course they are some weird imperial size so decided to tap the hole to standard M6, it was the same with the main chassis, only two original bolts, tapped the two front ones to M6 size and fitted some spare suspension bolts from my Jaguar that were the right length

I have also added a small board for the spare tubes, these are some various Mazda tubes and two spare output 'Ever Ready' A70E tubes(think rebagged Mullards) to eventually replace unobtanium AC4/PENs. Think it's matching rest of the radio nicely and this way I definitely wont loose the spare tubes.

I have then almost died for the second time moving the whole thing from the workshop to the house but its in now and running off TV socket, surprisingly it picks lots and lots of stations and reception is generally excellent. I'm just using the aerial socket with no ground as the latter mutes everything except for a few stations with best reception. One way or another I must be picking up at least 20 stations so cant really complain!
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Old 30th Aug 2023, 11:43 pm   #135
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Default Re: What did I get myself into? Murphy A40C.

More pics.
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Old 30th Aug 2023, 11:45 pm   #136
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Default Re: What did I get myself into? Murphy A40C.

Cabinet itself. Very happy with it, hope it survives another 90 years
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Old 31st Aug 2023, 10:34 am   #137
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Default Re: What did I get myself into? Murphy A40C.

What a first class job I do like the valve storage shelf ,well done with restoration , it brings back happy memories when I restored mine I wish I still had it .Mick.
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Old 31st Aug 2023, 11:41 am   #138
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Default Re: What did I get myself into? Murphy A40C.

Looks good; the little spare-valve shelf is a nice touch.

Must admit though, I wonder about the overall ergodynamics of the radio; to see the tuning scale you would have to stand over it and look down, then readh down to the tuning knob.

Half an hour of that and my back/shoulders would be aching terribly!
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Old 31st Aug 2023, 8:16 pm   #139
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Default Re: What did I get myself into? Murphy A40C.

Thanks! To be honest its probably easier on your back than a standard radio, you get to it, left hand on top of the radio, for support, the dial is slightly angled and right hand on the tuning knob. of course its made for people under 170cm tall so I have to crouch but still not too bad. Its weight is definitely much worse for any back

I forgot to mention that the 'Ever Ready' A70B valves seem to produce exactly the same sound as the AC/4PEN, I believe these are rebadged Mullards PEN4VA , its been mentioned here that they sound worse but with replacement speaker I couldn't tell a difference, maybe it just affects original style speaker. Radio was on all day today and worked perfectly, I'll keep it on for the next few days, just to make sure its all working as expected. It is stupidly loud on anything but lowest volume too, entire neighbourhood could hear it if it was on in the garden at full volume
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