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Old 21st Jun 2018, 7:41 pm   #61
Nuvistor
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Default Re: Murphy A855G- no sound.

Not be the first time a short on the dial lamp circuit has written a transformer off.
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Old 22nd Jun 2018, 7:52 am   #62
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Default Re: Murphy A855G- no sound.

Hi, looking at both wires from the lamp, they are clean and fresh with no signs of arcing or burning.( it's the one from the centre of the bulb holder that has become attached).
Thanks
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Old 22nd Jun 2018, 9:06 am   #63
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Default Re: Murphy A855G- no sound.

The terminal has broken from the base of the lamp, possibly with a clumsy finger!
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Old 22nd Jun 2018, 12:42 pm   #64
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Default Re: Murphy A855G- no sound.

You may as well test the primary for being open circuit, probably is but may as well confirm it. Only cure in that case is a new transformer or rewind.
Has been in the comments before, the transformer does look badly damaged and not safe.
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Old 22nd Jun 2018, 4:06 pm   #65
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Default Re: Murphy A855G- no sound.

This is a better than average player with a nice, late 1960s, transistor sound. These typically sell for c.£100.00 working. But getting a donor TX - and one with the lamp winding - may not be quite so easy. Might be worth considering a full rewind or a go get a bog standard 240V to 20 to 30V (not sure of the rail voltage) transformer and sacrifice the pilot lamp feature. Cost maybe £25.00?
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Old 22nd Jun 2018, 6:26 pm   #66
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Default Re: Murphy A855G- no sound.

Do you know where a replacement or rewind could be obtained, as I have had this for quite a while.
Do you think that the transformer failed, or something else has failed to cause this.
So can you recommend what other elements to replace to prevent a repetition ?
Thanks
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Old 22nd Jun 2018, 8:56 pm   #67
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Default Re: Murphy A855G- no sound.

Techmans post 60 has info on what could have caused the transformers demise if it was just not the transformer itself.

There are a couple of forum members who rewind transformers, post a request in the wanted section.
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Old 23rd Jun 2018, 11:20 am   #68
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Default Re: Murphy A855G- no sound.

I agree the transformer is knackered, but before replacing it find out what caused it to burn out.
The fault may turn out to be simple, or it may be expensive, you may prefer to buy a new record player.
Electrolytic capacitors and the OP transistors need to be tested and checked.
Can you supply a photo of the rest of the circuit board, or chassis so we can see the components.
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Old 23rd Jun 2018, 12:03 pm   #69
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Default Re: Murphy A855G- no sound.

I agree that more tests have to be made before attempting to fit a replacement transformer. However, stop to consider the following information supplied by Stephen, the OP.....

He told us that the record player had originally been running for around 4 hours. We need to know chapter and verse whether it was playing records continually for ALL of that time, or was it perhaps left unattended for the last part of that 4 hour period with NO records being played? If it was playing records right up until the end, was it sounding normal with NO distortion or extra 'hum' level towards the end of the playing time?

I'm not familiar with this exact model record player, but assuming that the cabinet is of standard plywood construction, and as we know that the transformer isn't mounted directly on to it, we have to consider what kind of fault condition could exist for a long enough period of time for heat of the amount to be noticed by the OP to filter through the thickness of the plywood cabinet, but leaving the unit still operating normally.

A fault in the indicator lamp circuit 'could' cause this exact fault by overloading the transformer enough to cause a burn out over a period of time, but due to having its own winding on the transformer, the overall voltage drop would probably not degrade the sound output enough for the OP to notice.

A fault in the amplifier or its power supply section could also cause this fault, but we would think that some degradation in sound would be noticed.

You're a man of few words, Stephen, but unfortunately, and as far as I know, none of us on this forum are mind readers, so we can ONLY go on what YOU tell us!

So if you want this fixing then we need a detailed description of the EXACT chain of events that led up to this transformer failure. Of course, you could have been very unlucky and the transformer just developed a fault and failed by itself and no other parts of the player are faulty - so let's have it....'chapter and verse', please!
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Old 23rd Jun 2018, 3:02 pm   #70
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Default Re: Murphy A855G- no sound.

Hi,
Firstly, many thanks to all who have taken the time to provided me with their time and knowledge .
I was playing records solid for about four hours in my attic conversion which a has two dormer Windows. I left it on, but unattended for about ten minutes whist I got a cup of tea.
When I returned, there was smoke in the room and the sunlight has been coming in, directly on to the record player thorough the glass. The front corner of the player was hot externally probably from the sun, but I could feel a concentrated heat from the left hand corner as you look at it from the area where the amp is housed. This is a standard plywood cabinet and the amp is screwed directly onto the base at the from behind the grill.
I have conducted the tests as outlined in post 34 and got no reaction from the multimeter.
I checked to ensure the turntable still has power, which it has.
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Old 23rd Jun 2018, 3:22 pm   #71
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Default Re: Murphy A855G- no sound.

OK so the transformers is faulty, that’s one problem that requires attention.
You have checked the diodes and those appear ok, the large electrolytic capacitor that was connected to the diodes, check that next and then move on to check the amplifier for damaged and faulty items.
You need to be sure if the transformer was damaged or it just failed before replacing it. The pilot lamp requires repair as well.

If you know very little on how the electronics work you may find it easier to learn about simple power supplies and audio amplifiers. Any testing you do will then make more sense.
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Old 23rd Jun 2018, 6:52 pm   #72
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Default Re: Murphy A855G- no sound.

If you do want to sort the transformer out there's only three options, have it rewound if it's suitable for rewinding, find an exact replacement or fit something that will do, the latter would normally be the most common option for a job like this and for that you would need to know the secondary voltage that's needed, I haven't got the full manual so I don't know if it's given anywhere in it, if not then you can use the DC voltage output from the rectifier as an aid, I believe it's 27.5 volts DC, that voltage will be at quiescent current therefore it will be approx. the peak voltage of the AC from the transformers secondary winding going into the rectifier less the forward voltage drop across the rectifier, that voltage drop will be approx. 0.6 volts, adding that to the 27.5 volts gives 28.1 volts, that's the peak AC voltage, transformer voltages are given in RMS voltages, the RMS voltage is 0.707 of the peak voltage so in this case RMS=28.1*0.707 which is 19.8 volts so a transformer that can supply 20 volts should do at rating of 20VA (new money) or 1 amp (old money) a suitable bulb for the pilot light, of say 0.1 amp current rating can be connected across the secondary in series with a resistor eg: a 12 volt 0.1 amp bulb in series with an 82 ohm 2 watt resistor.

If that's any use.

Lawrence.
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Old 23rd Jun 2018, 8:29 pm   #73
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Default Re: Murphy A855G- no sound.

...er, that's all very well and certainly quite accurate....but is the OP going to "get" any of this? In the past I have been guilty of complex responses in just the same way.

Now, I do try to moderate my explanations hoping the OP will decide on the best option available. In many of these cases, it's often best to get 3rd party assistance. If this is just a "one off" record player fix and no more, there's no harm in that.
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Old 23rd Jun 2018, 9:40 pm   #74
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Default Re: Murphy A855G- no sound.

Err....I wouldn't call an addition and a multiplication complex.....and anyways it's not quite accurate but I missed the edit window.....

….20VA per winding if the replacement transformer has two separate secondary windings, that would be advertised as a 40VA transformer.

Also forgot to allow for some transformer regulation so maybe 18 volts would be better considering it's AD161/AD162 (I'm referring to a Bush SRP31 schematic)

New transformers that would do are available for under £20 eg: 50VA 18 volt transformer or 50VA 20 volt transformer.

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Old 23rd Jun 2018, 9:43 pm   #75
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Default Re: Murphy A855G- no sound.

I can confirm "HT" for this player is quoted as 27.5V measured across the smoothing capacitor.
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Old 23rd Jun 2018, 9:56 pm   #76
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Default Re: Murphy A855G- no sound.

Quote:
Originally Posted by Station X View Post
I can confirm "HT" for this player is quoted as 27.5V measured across the smoothing capacitor.
Cheers for that.

Lawrence.
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Old 24th Jun 2018, 1:22 pm   #77
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Default Re: Murphy A855G- no sound.

Now we get a clear picture of what happened.

You don't say whether it was still playing a record when you left it unattended, but reading between the lines I'm guessing that it had stopped playing, but the sound had been normal up until that time. Guessing that you had been operating it with the lid closed, as you would do for best sound, you probably left it like this when you went to make a cup of tea. I think it may well be that with it being in the sun all day and having been operating over a longish period, it just got too hot, and it was in the last ten minutes when left that it just couldn't cope any more.

So you've got your transformer replacement options sorted. It's now just a case of whether it was just the transformer that overheated and failed, or whether the amplifier went into thermal runaway, drawing too much current and finished off the already hot transformer.

The next step is to test the amplifier. This is where a proper bench power supply with current limiting and voltage and current metering would come into play, but unfortunately you're unlikely to have such a thing. After checking for any obvious short circuits on the supply line, I would advise getting three 9 volt batteries in series and with your meter on a current range in series with the + line, connect it briefly and measure the current drain. Three PP9 batteries would be ideal, but I think you'd get away with three decent PP3 types. For this test I would grab three alkaline types from the 'Pound Shop' at a quid each (you can always use them in something else after the test), and wire them in series + to - etc. etc. Your meter will probably have a 200 mA range as its highest normal range, which under no fault condition would be ok, However, due to the fact that you're going to connect your test supply ACROSS that main smoothing capacitor which will take a high surge current on initial charge and the distinct possibility that there may be a fault condition on the amplifier, then you should use the 10 amp range which is likely to require you to connect your positive test lead into a dedicated socket on the your meter. Connect the amplifier to the batteries with ammeter in series as described, observing correct polarity. Do this very quickly and take a current reading and report this to members on this thread - don't leave it connected for more than a few moments in case there's a fault. If you're unsure about any of the above, then ask for clarification from members before you go ahead.

If the reading you get turns out to be normal, then you could try playing a few records with the amplifier connected in this way, carefully monitoring the current all the while for any significant increase, reporting back readings taken.

I think Lawrence meant the SRP51 circuit as the SRP31 is valves.

With the option of using a generic transformer with just a single tapped winding, wiring the indicator lamp across the supply (you'll need to buy a new bulb holder) with a dropper resistor is the way to go and will probably have the added bonus of the lamp dipping in brightness slightly to the beat of the music - so win-win!

Last edited by Techman; 24th Jun 2018 at 1:27 pm.
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Old 24th Jun 2018, 1:39 pm   #78
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Default Re: Murphy A855G- no sound.

Yes, SRP51 not SRP31, here's the snippet I was using:

https://www.vintage-radio.net/forum/...1&d=1237544959

Lawrence.

Last edited by ms660; 24th Jun 2018 at 2:02 pm.
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Old 24th Jun 2018, 8:46 pm   #79
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Default Re: Murphy A855G- no sound.

You could use a replacement transformer with just a secondary winding of 18-20V and no centre tapping, if you fitted a bridge rectifier instead of the existing two diodes.

It should be possible to glue an LED into the back part of the lampholder, which will both take care of the broken contact and be easier to sort out power for than a filament bulb. A 4.7kΩ series resistor will give about 5mA from a 27.5V supply, which should be plenty for any modern LED. Decrease the value if it is too dim, or increase it if it is too bright.
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Old 29th Jun 2018, 10:59 pm   #80
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Default Re: Murphy A855G- no sound.

Hi!

Just one small point worth making – I believe you can now get 24V m.e.s./l.e.s. LED replacement "white" bulbs – one of these can be screwed into the original lamp holder, and a 100 ohm 2W resistor interposed in series between the bulb centre–contact and the +27.5V line, with the screw–cap terminal going to the negative side of the 27.5V supply – this saves you mounting problems and any accidental s/c across the lamp holder will do no further damage!

Chris Williams
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