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Old 24th May 2019, 3:41 pm   #21
ms660
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Default Re: Unknown radio. Identified as a Murphy A34.

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Originally Posted by LyntonP View Post
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For inter-station noise suppression, the A34 uses the cathode & g3 of the IF amplifier as a diode which is connected by way of, what are in effect (for explanation purposes) two resistances to the cathode of the detector diode, the two resistances form a potential divider to which the anode of the detector is connected, the cathode voltage of the IF amplifier is lower than that of the detector's cathode, that therefore makes the detectors anode voltage lower than the voltage on its cathode, the voltage difference being the threshold voltage for detection.

I've never owned or used one of these receivers so I've no idea how effective it is in reality.

Lawrence.
Lawrence do you have any idea where I might find more information about this system?
I'm not sure as to what information is given in the A34 manufactures manual because I don't have access to a copy, I do however have a copy of the manufactures manual for the A50 and some information about the inter-station noise suppression circuitry is given in there (basically the A50 uses the same idea as A34)

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Old 24th May 2019, 4:52 pm   #22
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Default Re: Unknown radio. Identified as a Murphy A34.

I wouldn't get too hung up over the inter-station noise suppression. I have an A50 and don't find this feature helpful so I use the radio without "silent tuning". It is also on a McMichael 364 and it was such a pain I disabled it! It looks like you have a challenge with the A34. You should have no difficulty finding an AC/TP; if you get stuck then PM me as I can supply one. You are extremely unlikely to find a good working AC/ME magic eye and the one that is fitted will almost certainly have no glow at all. It will not affect the functioning of the radio though. Cheers, Jerry
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Old 24th May 2019, 5:29 pm   #23
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Default Re: Unknown radio. Identified as a Murphy A34.

This does seem to be a bit of a "dogs Breakfast" [messy as David said Lynton] but on the other hand relatively rare. It's also an interesting example of adaptation and modification so it might be useful to keep a record of the investigation! I've always assumed that the use of a bag [p11] to enclose the Loudspeaker was to reduce the possibility of any "gunge" getting into the speaker movement, less resilient and more open then maybe? A bag/cover was perhaps not that rare. I don't think it's always necessarily replaced during restoration but others may dispute my speculations.

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Old 24th May 2019, 5:45 pm   #24
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Default Re: Unknown radio. Identified as a Murphy A34.

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It could be a case of using 2 valves instead of one,I would try for the correct one.I think you have picked a messy set to start off with as a first,due to unofficial mods

Also,the magic eye is most likely u/s.
The magic eye is seized solid in its holder! This needs careful work to release it without damaging the holder!
I am still confused with the use of two valves instead of one?
The brown Bakelite valve holder for V1 is mounted on top of the chassis and held in place with one small set screw. I loosened this to peek underneath and it is certainly not a plug in adaptor. There are short wires soldered underneath and others which emerge through it going to the extra valve on the cabinet.
I have done a couple of sets over the last six months but nothing as old as this. Some of the components underneath I have never seen before, but more information gathering from the web will help I hope. The set has obviously been well got at over the years, but the chassis is starting to clean up nicely. Not been stuck in a garage for years but kept nice and dry. The tuning dial is very unusual and the dial string had broken, and a pulley had fallen off. Fortunately this looks easy to remedy.
It would be nice however to get to the bottom of this two valve instead of one mystery.
Lynton
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Old 24th May 2019, 6:10 pm   #25
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Default Re: Unknown radio. Identified as a Murphy A34.

Hi Lynton.
In respect of the bag enclosing the loudspeaker, I think you will find that it was once commonplace to do this, particularly (but not exclusively) pre-war.
My impression was always that it was simply to exclude dust.
The models that I possess that incorporate a "bagged" loudspeaker are:
Murphy A46 (1938,) Murphy A48 (1938,) Philips 170A (1946,) and Philips 462A (1947.)
The cotton material used is frequently rather delicate after 70 - 80 years and will be full of dust; they will normally stand being carefully removed and very gently washed and dried - at least that has been my experience.
One other thing to watch out for with loudspeakers from that period is that they are frequently field energised; meaning that the speaker unit will have four wires going to it, two for the voice coil (as in a permanent magnet speaker,) and two which will have full HT on them (the field coil.)
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Old 24th May 2019, 6:25 pm   #26
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Default Re: Unknown radio. Identified as a Murphy A34.

I suspect that the mods around V1 may well have been done during the war as "make do and mend". Valves were in very short supply, and the owner wouldn't have minded if it got the set running again.

If the mod is safe and works well, I'd probably keep it as-is, it's a part of the set's history. Others may disagree and return it to ex-factory. Each to his own, vive le difference
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Old 24th May 2019, 6:26 pm   #27
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Default Re: Unknown radio. Identified as a Murphy A34.

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It would be nice however to get to the bottom of this two valve instead of one mystery.
Lynton
The valve in the valve holder that was screwed to the cabinet is a triode (AC/HL) and could be serving as the oscillator, the valve in the V1 position could be a pentode (VP4 etc) and could be serving as the mixer.

Langrex were showing 20 AC/TP's on their stock list yesterday if you get absolutely stuck for the original type.

Lawrence.
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Old 24th May 2019, 8:15 pm   #28
60 oldjohn
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Default Re: Unknown radio. Identified as a Murphy A34.

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The valves are listed as Mazda AC/TP, AC/VP2, AC/PenDD, UU4, and AC/ME. Are such items still available??

Lynton

The valves are still available. If you are lucky it will not need any valves. The output valves and rectifier valves wear out the fastest due to being run at near their limits, (and get very hot) these are usually more expensive than the rest of the valves. The other valves will most lightly be ok. The capacitors are the weak link, not the valves in vintage sets.


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Old 24th May 2019, 8:41 pm   #29
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Default Re: Unknown radio. Identified as a Murphy A34.

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Hi Lynton.
One other thing to watch out for with loudspeakers from that period is that they are frequently field energised; meaning that the speaker unit will have four wires going to it, two for the voice coil (as in a permanent magnet speaker,) and two which will have full HT on them (the field coil.)
I noticed this when I took the chassis out. Two wires for the speaker are plugged in on the rear panel, and a further two connected to the transformer marked "Field". I marked them so they go back in the same place.
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Old 24th May 2019, 9:42 pm   #30
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Default Re: Unknown radio. Identified as a Murphy A34.

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Were there valves in all of the 4 valve holders on the chassis (not including the magic eye mounted by the dial) as well as a valve in the additional valve socket fitted sideways?
Mike Yes all four valves mounted on the chassis are present but in place of the AC/TP (V1) is a V4/HP and the additional valve that is mounted sideways is a AC/HL.
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Last edited by LyntonP; 24th May 2019 at 9:44 pm. Reason: Typo
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Old 24th May 2019, 9:57 pm   #31
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Default Re: Unknown radio. Identified as a Murphy A34.

The original AC/TP is the mixer/oscillator valve. It is actually 2 valves in one envelope; the triode part acts as the oscillator while the pentode part acts as the mixer. This is very common in valve radios. However, there is no reason why the 2 functions cannot be performed satisfactorily using 2 separate valves. Indeed this is done by design in some radios. As others have said, in your radio it looks like a bodge job has been done to work around the unavailability (at the time) of an AC/TP. Unless you have to remove the magic eye for some reason I would leave it alone until such time as you have powered the set. There is always the off chance it might work and in removing it you might damage it. Cheers, Jerry
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Old 24th May 2019, 10:14 pm   #32
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Default Re: Unknown radio. Identified as a Murphy A34.

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Originally Posted by LyntonP View Post
The valves are listed as Mazda AC/TP, AC/VP2, AC/PenDD, UU4, and AC/ME. Are such items still available??

Lynton
The valves are still available. If you are lucky it will not need any valves. The output valves and rectifier valves wear out the fastest due to being run at near their limits, (and get very hot) these are usually more expensive than the rest of the valves. The other valves will most lightly be ok. The capacitors are the weak link, not the valves in vintage sets.

John.
I totally agree, I've managed to get about 6 pre-war Murphy sets working really well and I've never yet had to replace a valve......capacitors yes, loads of them.
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Old 24th May 2019, 10:29 pm   #33
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Default Re: Unknown radio. Identified as a Murphy A34.

Well I have trawled the internet tonight and found that an HC/HL is a triode (which partially fits), but I can find no reference at all to a V4HP P= Pentode??. I need to check the pin outs of the original valve and see where they go on the circuit diagram. But I'm crossed eyed now so it will have to wait until tomorrow!
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Old 25th May 2019, 1:14 pm   #34
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Default Re: Unknown radio. Identified as a Murphy A34.

All points noted gents. Many thanks for the advice.
Regarding the re wiring is there any convention regarding the colours of the wires to use?
I have seen that there are kits containing several colours.
Any thoughts?
Lynton
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Old 25th May 2019, 1:34 pm   #35
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Default Re: Unknown radio. Identified as a Murphy A34.

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I can find no reference at all to a V4HP
Me neither so far, any chance of a photo showing the markings?

Lawrence.
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Old 25th May 2019, 2:22 pm   #36
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Default Re: Unknown radio. Identified as a Murphy A34.

In regard to rewiring, I don't think there is much of a convention regarding colours. The existing wiring will (as you have no doubt discovered) be either DCC (double cotton coloured) which, though eighty years old, will likely be perfectly serviceable and not need replacing, or rubber which will have perished, become rock hard and will crumble away if moved. The cotton covered wiring that Murphy tended to use had a brown base colour with a coloured stripe. The rubber wiring used could be many colours but as it was natural rubber the colours were rather muted. A suitable modern replacement would (in my view) be silicone covered of the nearest diameter, personally I use 20 AWG. Silicone has several advantages, the insulation is much more heat resistant than PVC, it is highly flexible and the colours are much more muted than PVC.
One thing that I would suggest watching out for in particular (if you propose replacement) is the wire gauge of the heater chain. Although only carrying 4 volts A.C, it carries significant current (several amps) and is normally quite heavy gauge. One possibility is to retain the original wiring, crumble off the rubber and replace with silicone sleeving of an appropriate diameter as there's nothing wrong with the conductor(s,) just the insulation.
You may find this strategy particularly useful when it comes to the loudspeaker field coil wiring (if rubber and decayed) as it frequently disappears straight into the coil without any tags and may be rather fragile.
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Last edited by Nanozeugma; 25th May 2019 at 2:31 pm.
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Old 25th May 2019, 2:45 pm   #37
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Default Re: Unknown radio. Identified as a Murphy A34.

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I can find no reference at all to a V4HP
Me neither so far, any chance of a photo showing the markings?
Attached are photos of both valves. The V4HP has no other markings at all.

I had another look at the “adaptor base” the wiring is in such poor condition it needs replacement but some of said wires disappear under the adaptor. I can get it loose from the chassis but can only lift it about 5mm upwards. I think the original valve base is still fastened to the chassis and ‘connection pins’ with wires go to terminals on the adaptor. Apart from desoldering all the wires I have no idea how to remove it.
I have replaced the dial cord and the mechanism works fine now.
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Old 25th May 2019, 3:13 pm   #38
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Default Re: Unknown radio. Identified as a Murphy A34.

What is a V4HP, I cant see any reference to it anywhere?

Mike
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Old 25th May 2019, 3:14 pm   #39
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Default Re: Unknown radio. Identified as a Murphy A34.

I have a number of reference sources for valves, but none of them list a V4HP. I would suggest it comes from a minor manufacturer or reseller, quite possibly the same "Gloria" brand as the other valve given the similarity of type face. Maybe it just means V = Valve, 4 = 4 Volts Heater, H = High Frequency, P = Pentode ? However, this wouldn't fit with being an oscillator/mixer.
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Old 25th May 2019, 3:39 pm   #40
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Default Re: Unknown radio. Identified as a Murphy A34.

Providing the wiring is replaced, the 2-valve bodge might just work, especially since this set is MW/LW only. However, IMHO it is a messy set-up and would be better reverted to the correct valve. The AC/TP is on a 9-pin valve holder (see pics) and the one shown is my only spare as I use it with an adaptor on my valve tester so I can't let it go unfortunately. However, Mike Lewis can probably supply one if you choose to replace it. Mike can be contacted on 01582 580367. Probably has the valve to go with it! Cheers, Jerry
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