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Old 17th Feb 2023, 11:24 am   #101
B.Taylor
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Default Re: What did I get myself into? Murphy A40C.

Be careful when soldering the components back onto the tags or you risk melting the soldering you have done inside the box.
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Old 17th Feb 2023, 11:39 am   #102
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Default Re: What did I get myself into? Murphy A40C.

Yep, get those tags nice and shiny and pre-tinned, again, quickly.
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Old 17th Feb 2023, 12:59 pm   #103
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Default Re: What did I get myself into? Murphy A40C.

Well, I've messed something up or fixed and found another issue in the process.
Soldered everything back together and when I switch it on now, I get static ok for a second or two with what appears to be correct volume and then throbbing/pulsing sound from speaker
Sounds like capacitor charging/discharging but bit lost now what this might be.
I've checked all wires going into condenser block and all are connected correctly.
I've quadruple checked condenser block and all seems fine and connected correctly.
If I turn the tone control between the settings(I assume its off then), radio seem to work fine, just quiet.
Oh and I think I found reason why R52 was open circuit, with neither of AC4/PEN valves in the set, it gets very hot, rather quickly, although I think that's unrelated.
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Old 17th Feb 2023, 1:13 pm   #104
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Default Re: What did I get myself into? Murphy A40C.

Well, if it worked prior to you refitting the cap block, then in theory it has to be related to that? As has been suggested, has a lead come unsoldered inside the block as a result of soldering on to the tags?
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Old 17th Feb 2023, 3:06 pm   #105
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Default Re: What did I get myself into? Murphy A40C.

Block is fine but I've solved it, got confused by the 'underside' schematics with the block pictured, its basically missing a 'dot' for the ground for C8 but its there in the main schematics, so added that and radio works, need to tidy it all up again so will get that sorted and report back. I knew it was something silly!
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Old 17th Feb 2023, 7:24 pm   #106
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Default Re: What did I get myself into? Murphy A40C.

Started looking into SW. From day one SW reception was either non existent or barely there, sometimes when I've hit the tuning capacitor I was able to find quite few(mainly foreign stations).
Well, found the problem now. Fixed it too.
This radio is an absolute monster of a puzzle at times

I went over the whole set again and found 4 more of these too(basically wires came off at slightest push), along couple more of faulty silver mica caps, one looked like it delaminated, was showing correct capacity as far as I squeezed it.

It's finally at least functional. Cord and more voltages checking next.
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Old 17th Feb 2023, 7:28 pm   #107
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Default Re: What did I get myself into? Murphy A40C.

Short wave stations are mainly foreign stations.
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Old 18th Feb 2023, 3:42 pm   #108
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Default Re: What did I get myself into? Murphy A40C.

Yes, it appears so. Got next two questions on this set.

The R52, through which the anode of S.W. H.F. Amplifier, V12, AC/VP2 is fed was o/c when I got this, I've replaced it with a 2W 15k resistor, but its clear that this is running too hot and wont last so I'm thinking I'll put a much bigger ceramic one in there instead. I take it, this is down to missing field coils. The R54 also gets warm but nowhere near as bad as R52 so should be ok I think?

Also, how does the noise suppression works on this set? Is it just removing the inter station noise? Is it suppose to work on all bands? The switch on this does absolutely nothing it seems but there are some voltages present on the switch contacts and its probably next thing I'd like to investigate.
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Old 18th Feb 2023, 10:46 pm   #109
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Default Re: What did I get myself into? Murphy A40C.

Something is amiss. R52 should be dropping 95 volts so at 15k it ought to be dissipating 0.6 Watts. If a 2 Watt resistor is getting excessively hot then something is wrong. The most likely cause is if C71 is leaky. It could be a duff AC/VP2 but less likely. Check the voltages either side of R52 and work out how much current is passing through it. Are you sure that's a 15k resistor? Neither of them look like it.... Jerry
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Old 18th Feb 2023, 11:12 pm   #110
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Default Re: What did I get myself into? Murphy A40C.

According to Post #7 of this thread https://www.vintage-radio.net/forum/...d.php?t=147650 muting is achieved by making the suppressor grid (Grid 3) of V5 the AC/SP1 pentode 20 volts negative with respect to the cathode. This action cuts off the anode current (but not the screen current) thus muting the audio. I can't tell from the circuit whether this function is intended to work on all bands. I can imagine it would be fairly useless on SW. Jerry
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Old 19th Feb 2023, 1:29 am   #111
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Default Re: What did I get myself into? Murphy A40C.

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Originally Posted by cathoderay57 View Post
Something is amiss. R52 should be dropping 95 volts so at 15k it ought to be dissipating 0.6 Watts. If a 2 Watt resistor is getting excessively hot then something is wrong. The most likely cause is if C71 is leaky. It could be a duff AC/VP2 but less likely. Check the voltages either side of R52 and work out how much current is passing through it. Are you sure that's a 15k resistor? Neither of them look like it.... Jerry
Well, looks like I have dead valve, valve anode was dropping eventually to ~22V so at that point if my math is right resistor was dissipating more than 2W. C71 was ok but I replaced it anyway, then swapped the AC/VP2 and that immediately settled to around 110V which is what it suppose to be. Thanks for pointing me in the right direction!
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Old 19th Feb 2023, 4:43 pm   #112
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Default Re: What did I get myself into? Murphy A40C.

Fixed another issue, an open L26 coil in the a.t.c. circuit. Still non wiser how the N.S suppose to function here or how I could test it.
Also whoever thought it was a good idea to put resistors and capacitors inside the valve cap hopefully was fired
Think I'll get the cords replaced now.
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Old 19th Feb 2023, 5:04 pm   #113
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Default Re: What did I get myself into? Murphy A40C.

The only Murphy I have restored is an A46. I found that the wires were easier to remove from tabs as they weren't wrapped round as in many other sets.

As a matter of interest was that the case with your set or was it just something that happened in the one I have?
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Old 19th Feb 2023, 5:57 pm   #114
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Default Re: What did I get myself into? Murphy A40C.

Quote:
Originally Posted by Katar83 View Post
how does the noise suppression works on this set? Is it just removing the inter station noise?
A description is given on mag. page 197 in the link below if that helps:

https://worldradiohistory.com/UK/Wir...ld-1938-03.pdf

Lawrence.
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Old 19th Feb 2023, 8:17 pm   #115
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Default Re: What did I get myself into? Murphy A40C.

Thanks, will go over it, see if I can understand anything out of it!

Redid dial cords now, the main one was super easy, the S.W. one was a bit fiddly, having a lot of spare cord, definitely helps. Indexing works well now with the new cord.
Cleaned up both dials, can just about see the red lettering on the main dial for LW.
Replaced 3 bulbs for the main dial too, I think I'm at the end of what I can do with it, just need to fit new power cable and need to have a think about aerial for it and see how it works with something better than a few meters of copper cable.
Currently at home I get BBC on LW, maybe couple stations on MW(barely) and a few on SW, this is improved when set it is in the garage but still feels like the whole thing is missing aerial.
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Old 19th Feb 2023, 8:24 pm   #116
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Default Re: What did I get myself into? Murphy A40C.

Quote:
Originally Posted by PaulR View Post
The only Murphy I have restored is an A46. I found that the wires were easier to remove from tabs as they weren't wrapped round as in many other sets.

As a matter of interest was that the case with your set or was it just something that happened in the one I have?
I'd say half were wrapped around, then another quarter with wires through the tabs so easy to pull out and remaining quarter just soldered against the tab. Looks like you were lucky.
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Old 19th Feb 2023, 8:53 pm   #117
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Default Re: What did I get myself into? Murphy A40C.

Quote:
Originally Posted by Katar83 View Post
Thanks, will go over it, see if I can understand anything out of it!
No problem, basically when the interstation mute is operating, V5's suppressor grid (g3) is made -ve with respect to V5's cathode, the -ve voltage is developed across the two resistors in the cathode circuit (R24 & R25) the voltage developed across them will be approx. 20 Volts, the bottom end of the cathode resistor R25 is connected to ground as is V5's suppressor grid via R26, the switches, R43 and R42, the cathode of V5 is approx. 20 Volts +ve with respect to ground which means that V5's suppressor grid is 20 Volts -ve with respect to the cathode, -ve 20 Volts on the suppressor grid with respect to the cathode is enough to cut off the current flow to the anode, hence the inter-station noise is muted, the valve data for the AC/SP1 (V5) can be seen via the link shown below, it includes information about using g3 (the suppressor grid) for noise suppression and details of g3's cut off voltage etc:

http://www.r-type.org/pdfs/ac-sp1.pdf

Lawrence.
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Old 21st Feb 2023, 11:08 pm   #118
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Default Re: What did I get myself into? Murphy A40C.

Thanks Lawrence, that's very helpful! I'll go through all this info during this week but a quick update for now! Managed to source an excellent condition AC/ME magic eye which arrived today and its working like brand new valve!

Original one was barely glowing and could only be seen in dark really. This was at 1pm, beautiful green
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Old 3rd Jul 2023, 12:08 pm   #119
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Default Re: What did I get myself into? Murphy A40C.

Update on this and more questions.

Managed to arrange a radio alignment with Adrian Gardiner, this didn't exactly go as planned as at some point radio developed a fault in the discriminator circuit and this was effectively first step in alignment instructions so the radio went back to my work bench and sure enough a cold solder joint was found in one of the cans in discriminator circuit and as soon as I blew some air at the inside of the can the wire came right off. I've fixed the other can few months back with the coil wire that was broken, just didn't notice this one at the time.

Radio is going back to Adrian shortly, hopefully it was just that as everything else seems fine in there.
One thing I've noticed, MW reception was quite good when Adrian tested it and was almost non existent on LW. Its completely opposite at my place.
We both use a fairly short length of wire and are 10 minutes apart

Another thing we both struggled with is the gram socket pinout. I'd like to ideally rewire it separately so I can connect a Bluetooth adapter to it later. Could anyone offer some help with it? Socket itself has 5 wires connected to it and we weren't sure what's what.
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Old 6th Jul 2023, 11:35 am   #120
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Default Re: What did I get myself into? Murphy A40C.

More questions while radio is with Adrian.

I've been on a mission to sort out speaker, the one kindly supplied by one of the forum members here finally gave up, trying to find a 10" cone with a 1" voice coil hole seems almost impossible(found one in US), currently trying eastern europe.
Only other suitable speakers I've found so far are 6 and 8 ohms Oxfords though, not tested yet but I assume anything up to 8 ohms should technically be still fine and not cause too much strain on either transformer or one of my fairly weak AC4/Pen output valves?
Except for drop in max volume I do not expect any other issues, would that be correct?

That also brings me to the AC4/PEN valves. I've read a number of times not to go with the replacement Mullards but since the Mazda one are effectively unobtanium, I've managed to source a couple of ever ready rebadged Mullards, to future proof it when the original one inevitably dies. For anyone that actually tried these is the sound really that much worse with Mullards?
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