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Old 25th Jan 2023, 11:46 pm   #41
Katar83
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Default Re: What did I get myself into? Murphy A40C.

Well, that sucks, cobbled up some capacitors in series to replace the dead 8uf cap, plugged in new UU5 valve, powered it all up and everything started glowing nicely but not a sound from speaker.
Switched to LW and I could just about hear static from somewhere inside the tunning capacitor, eventually even tuned in to something but couldn't tell what as was too quiet. Checked everything, still not a sound from speaker. Was about to give up for the evening but as last thing checked the speaker itself and of course its dead, no idea why I haven't checked it first
Plugged in temporarily my spare wharfedale 8 ohm speaker and of course it all works, BBC on LW is the only station I could tune to, AM just lots of static, SW too although I havent figured out how this actually works. I've used a piece of wire connected to actual antenna.
Both cords snapped quickly so repaired them temporarily with some dental floss

So two questions for now, what dial cord size should be in there, original looks closer to 0.6mm to me but its so old and degraded that it may as well been 1mm before the war.
What speaker should I look for, I take it, it should be a 15 ohm speaker. Looks like there's some 10-12" Wharfedale 15 ohm speakers on eBay right now from similar era. Would that be a good choice for a replacement?
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Old 26th Jan 2023, 12:24 am   #42
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Default Re: What did I get myself into? Murphy A40C.

The service manual says 3ohm speaker impedance which is typical for that era.

Was the output transformer replaced when the AXIOM was fitted? It sits on the main chassis. Also, what has the original field coil been replaced with?
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Old 26th Jan 2023, 1:42 am   #43
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Default Re: What did I get myself into? Murphy A40C.

I assume this was converted somehow when original speaker was replaced. I couldn't tell you if the output transformer was replaced either and if the one in the pic and my chassis is original or not as the Axiom was fitted well over 40 years ago and according to previous owner radio worked fine when he bought it 43 years ago.

If the values in the service manual are for resistance than this actually shows 124ohms / 171ohms.

Not sure what the original field coil was replaced with, not sure what to look for either.

PS

The temp Wharfedale 100W 6ohm(not 8 as I thought earlier) bookshelf speaker actually sounded very good, just a bit quiet.
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Old 26th Jan 2023, 4:49 am   #44
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Default Re: What did I get myself into? Murphy A40C.

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I assume this was converted somehow when original speaker was replaced. I couldn't tell you if the output transformer was replaced either and if the one in the pic and my chassis is original or not as the Axiom was fitted well over 40 years ago and according to previous owner radio worked fine when he bought it 43 years ago.
That output transformer looks original and considerably older than 43 years.
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Old 26th Jan 2023, 7:39 am   #45
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Default Re: What did I get myself into? Murphy A40C.

'100W' bookshelf speakers are usually rather inefficient, so that might explain some of the quietness.

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Old 26th Jan 2023, 8:41 am   #46
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Default Re: What did I get myself into? Murphy A40C.

It's more to do with a lot of half exploded waxies still in the set and probably a number of other out of spec components than just a speaker itself that obviously isn't correct for this set. The clarity of the sound is still there though and in the process I figured out the tone selector switch, which got me baffled at first as it is a selector switch, not a pot.
Anyway, if the transformer looks original, that would mean that someone just slapped an axiom speaker there without any modifications and connected it as is and I should look at something closer to original 3 ohm impedance instaed of axiom 15 ohm?
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Old 26th Jan 2023, 9:10 am   #47
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Default Re: What did I get myself into? Murphy A40C.

Ah i see, so when they fitted axiom they needed to either replace the field coil with a big whopping resistor or a choke and a smaller resistor. I take it the big and fried wound up resistor on top L31 isn't original, it's 260 ohms resistor stuck between field coil pins.
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Old 27th Jan 2023, 12:10 am   #48
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Default Re: What did I get myself into? Murphy A40C.

Sort the missing field coils out first. The service data is a bit confusing and it would be good if someone who owns one of these can confirm that the console has a single speaker and the field coil is 400 ohms. The total voltage drop on the field coil is given as 80V in the service data which gives us 200mA and 16W dissipation.

Ideally you should find a choke with a similar series resistance and current capability but a 200mA choke will be a much lower resistance so you would still need a series resistance. I would therefore go for a suitably rated 400ohm resistor but you will need to find a suitable place to mount it as it is going to get HOT.

A request for a 12" 4ohm speaker here might bring up something.

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Old 27th Jan 2023, 12:39 am   #49
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Default Re: What did I get myself into? Murphy A40C.

Yes, it would be great if someone could confirm what's the original field coil resistance so I could match resistor to it, I'll see if I can find suitable choke in my parts bin too. I take it I should probably also use bigger electrolytics too.

The current resistor is neatly sitting on top of L31 coil on the transformer board. Its a 260ohms resistor currently with 50V across it so around 10W going into it. Its clearly been fried a few times in its life.

I will have a look at 4 ohms speakers then, thanks for confirming!

Also to answer my own question, dial cord is 1mm, pulled it all out and in a few small undamaged sections its clearly 1mm thick
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Old 27th Jan 2023, 6:29 am   #50
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Default Re: What did I get myself into? Murphy A40C.

There is only one loudspeaker fitted to the A40C.
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Old 27th Jan 2023, 10:30 am   #51
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Default Re: What did I get myself into? Murphy A40C.

An extra 30V on the HT will increase the current consumption and risk of damage so I suggest a new resistor is required. Does the set have a 240/250 high AC mains tap?
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Old 27th Jan 2023, 11:18 am   #52
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Default Re: What did I get myself into? Murphy A40C.

Hi Katar83

I have had a look at the Murphy data sheet. As far as I can work out, the HT current is about 68 mA for each output valve and 53 mA for the rest of the set. This gives a total of 68 + 68 + 53 = 189 mA.

The sheet quotes 80 volts total across the field coils. For the radiogram this means L30 and L40. Taking 80 volts and 189 mA implies a total field resistance of 423 ohms and a dissipation of 15 watts. That's 212 ohms and 7.5 watts per speaker. The data sheet quotes 200 ohms for both L30 and L40 so that tallies.

The console model has L30 listed as 400 ohms, so they must have used a different design of loudspeaker. A resistor field alternative would be 390 ohms with a rating of at least 10 watts. I would suggest a 'metalclad' conduction cooled type, bolted to the chassis with a smear of heatsink paste underneath. The alternative would be a ceramic bodied type spaced well away from other components (its surface is designed to be hot) with a free airflow around it. Your present 260 ohms is too low, and will be increasing the HT by about 26 volts.

The HT ripple reduction will be less than for a choke, but given the other chokes still in circuit you may not notice. If 100 Hz hum is too high, try increasing the value of C62. Don't increase the reservoir C64 as this would stress the rectifiers.

Regarding speaker voice coil impedance, the radiogram used the two speakers in series and my guess is that they were both 3 or 4 ohms. Since the console speaker had a different field coil, it might well have had a different voice coil too e.g. 6 or 8 ohms, but who knows. The same output transformer is shown for radiogram and console chassis, so the output stage will 'expect' to see 6 to 8 ohms. Having said this, don't get too hung up on exact speaker impedance as its not that critical.

Last edited by Mr 1936; 27th Jan 2023 at 11:24 am.
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Old 27th Jan 2023, 1:03 pm   #53
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Default Re: What did I get myself into? Murphy A40C.

Amazing, thanks guys, much appreciated and very reassuring!

I'll get that all sorted and come back here.

Re speakers, I had a look at speakers on eBay and its going to be tricky to find something that isn't rubbish, too old and/or damaged, stupidly expensive or wrong spec/size.
If impedance isn't critical, my option would probably be something like wharfedale super range, that's around 10 ohms speaker, full range and still very good.
Old lower impedance speakers would be blown by modern types but then again, the latter would be half the size so might look a bit odd in such a big cabinet and yet again I'll have to recreate the speaker board as currently have original 12 inch one and new 10 inch speaker board.
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Old 27th Jan 2023, 2:32 pm   #54
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Default Re: What did I get myself into? Murphy A40C.

Here's the BBC engineers report on this set

https://downloads.bbc.co.uk/rd/pubs/reports/1945-11.pdf
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Old 27th Jan 2023, 3:46 pm   #55
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Default Re: What did I get myself into? Murphy A40C.

Got this one, interesting reading. I think the only paperwork I'm missing for it is an actual instructions manual if one was ever made for it.

One thing I don't understand is how the SW works in this set. LW and AM is same as in any other radio but SW has an extra tuning knob, not entirely sure how I'm suppose to use it and what to select.

I had a look at the old Axiom speaker now too and although after removing the cone I could clearly see a break in the coil under the glue but no luck after repair either, it disintegrated after I pulled it out of the speaker.
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Old 27th Jan 2023, 3:56 pm   #56
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Default Re: What did I get myself into? Murphy A40C.

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One thing I don't understand is how the SW works in this set. LW and AM is same as in any other radio but SW has an extra tuning knob, not entirely sure how I'm suppose to use it and what to select.
Set the small SW knob to the band you want - it should click into position, for the sake of repeatability - then the main tuning scale functions as a bandspread scale for the band you've selected.

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Old 27th Jan 2023, 3:59 pm   #57
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Default Re: What did I get myself into? Murphy A40C.

The original speaker in the console version was one 3 Ohm Impedance speaker.

The DC resistance of the voice coil would probably measure 2.5 Ohms.

Car speakers are usually 4 Ohms. Vintage radio speakers were usually 3 Ohms.

The BBC report says it can put out 21W of audio power so be careful with the volume control if using a vintage radio speaker.
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Old 27th Jan 2023, 4:00 pm   #58
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Default Re: What did I get myself into? Murphy A40C.

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Here's the BBC engineers report on this set

https://downloads.bbc.co.uk/rd/pubs/reports/1945-11.pdf
The BBC were rather scathing in their report. That said, in my teens I repaired an A40C for a friend of my father's (burnt out mains transformer) and had a chance to compare it side by side with a fairly average Philips 653A radiogram. Admittedly the 653A was a design from 20 years later and had a rotatable ferrite aerial. I was very impressed by the complexity of the A40C and its huge energised speaker but both in terms of radio performance and of sound quality the 653A won hands down.

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Old 27th Jan 2023, 4:05 pm   #59
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Default Re: What did I get myself into? Murphy A40C.

Unless the BBC did a full service on the A40C, it would still have the 7 year old wax capacitors in it when they did the test.
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Old 27th Jan 2023, 5:15 pm   #60
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Default Re: What did I get myself into? Murphy A40C.

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Quote:
Originally Posted by skodajag View Post
Here's the BBC engineers report on this set

https://downloads.bbc.co.uk/rd/pubs/reports/1945-11.pdf
The BBC were rather scathing in their report. That said, in my teens I repaired an A40C for a friend of my father's (burnt out mains transformer) and had a chance to compare it side by side with a fairly average Philips 653A radiogram. Admittedly the 653A was a design from 20 years later and had a rotatable ferrite aerial. I was very impressed by the complexity of the A40C and its huge energised speaker but both in terms of radio performance and of sound quality the 653A won hands down.

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