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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
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27th Jul 2018, 7:48 pm | #1 |
Triode
Join Date: Aug 2015
Location: Manchester, UK.
Posts: 32
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Does a Murphy A474 mains transformer run hot?
Hi There,
To eliminate unnecessary reading: Do you have access to a Murphy A474? If not you may go if you wish. Can you help me by measuring the input power? If not you may go if you wish. Now to the heart of the matter; I have an A474 that I am trying to restore. At first it appeared to be straightforward - of course. Replace the open circuit rectifier valve, and the short circuit main electrolytics. Replace several capacitors without even testing, and a few out-of-spec resistors. Check before switching on of course. Dead short supply line! That turned to out to be the "magic eye" anode to cathode. Now that's a new one on me. Must try to find an EM85 some day. At switch on the radio worked, but at low gain, on VHF. That's promising. And that's where it all went seriously wrong. The mains transformer rapidly became too hot to touch, and I could smell it. I later found that all secondary windings had many shorted turns and indeed were blackened in some places. I wonder which caused it; the magic eye or the main electrolytic. Anyway I stripped it all off and reassembled the transformer with only the primary left on. After about 10 minutes I would describe it as hand warm only, with a power consumption of 35 Watts. Iron losses I hoped. So I began the tedious job of winding a 1300 turn secondary in 38swg. Not easy. Then I had - by my standards - a bright idea. For test purposes at least, the one winding would do. (With a couple of 6 volt windings of course). I could use a bridge rectifier (to use both half cycles from the one winding) to feed the valve rectifier (for its slow start up) adding the other high voltage secondary if the radio finished up working properly. Now the radio is working (though I think that it still lacks gain). But after about quarter of an hour the mains transformer is uncomfortable warm, and the windings are rather hot. The trouble is it might "come with the model sir". That transformer does look a bit small to me for the job it does, so perhaps it does run hot even when good. Power consumption with all valves and dial light unplugged is now 37 Watts. Consumption when operating is 66 Watts, which is in spec. ("73 Watts max"). So here is where I need some help. Can some kind soul measure the power consumption of an A474 for me? And would you really be nice and do it off load as well? Without risking life and limb, can you hang on to the transformer after about 15 minutes of operation? And to push my luck; One of the wavechange "piano keys" is missing. Anybody got one? It would save me having to learn how to do resin casting! If the transformer won't do I am thinking of putting in a switch mode power supply, but I really hate that idea. The whole idea idea is restoration after all. A sincere thank you to anyone who wades through that lot. Tony G3PTD... |
27th Jul 2018, 7:59 pm | #2 |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
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Re: Does a Murphy A474 mais transformer run hot?
I hope most of those 35 watts were VAs. If they were real ones, the primary would get stinking hot!
Having said that, even basic grade transformer insulation can cope with 105C hot spots so a unit that is "ouch" hot may well be OK. It's also not uncommon for a cheap transformer to get nearly as hot just running magnetising current as on full load.
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27th Jul 2018, 8:29 pm | #3 |
Hexode
Join Date: Jun 2003
Location: Burntwood, Staffordshire, UK or Kabaty in Warsaw Poland.
Posts: 438
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Re: Does a Murphy A474 mains transformer run hot?
Hi I have 3 A574s which are similar to the A474, the MT runs very hot in all of them.
I also have a couple of scrap A574/A674 chassis an MT and piano key you are welcome to. Mark PS I was given a A474 a few weeks ago with a broken cabinet which as a project I am going to convert to an A574, a good cabinet of which I have. These sets sound superb when working. |
29th Jul 2018, 7:37 am | #4 |
Triode
Join Date: Aug 2015
Location: Manchester, UK.
Posts: 32
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Re: Does a Murphy A474 mains transformer run hot?
Hello Herald1360,
Thank you for the interesting reply. I had not thought of measuring the phase angle. You got me going! And that it why my reply might not be as quick as it could have been. The plug in power meter that I was using to measure power does give a power factor reading, and it measures 1.00 in both situations. However as I have never had reason to note that before I don't know what trust can be placed in it. I will try to use my 'scope, just for interest. But as I will need to make up a voltage divider to protect the 'scope and arrange a series resistor to look at current I will leave it for now rather than cause a further delay. I did measure a couple of other rather bigger transformers with the meter (off load) and got about the same readings - which proves nothing yet of course. Tony... |
29th Jul 2018, 7:41 am | #5 |
Triode
Join Date: Aug 2015
Location: Manchester, UK.
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Re: Does a Murphy A474 mains transformer run hot?
Hello Mark,
Thank you for the helpful reply reply about the transformers in the A574s. So mine might be OK. I apologise for the delay in my reply. As you might have gathered Herald1360's reply got me started on more measurements. If I may, I would very much like to take you up on your offer. But I will "PM" you about that. Not only would it save me the time need to learn how to to cast a push key - about which I have no experience, but also what I suspect would be a lot of waste material trying to get the colour right. And the spare transformer too would help to ease my mind about the one in my radio. But even more important - I really was not looking forward to winding the other secondary! Although, if the temperatures are comparable I might do that anyway (as the urgency will have been taken out of it). It might just be needed someday. Tony... |
29th Jul 2018, 7:08 pm | #6 |
Dekatron
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Re: Does a Murphy A474 mains transformer run hot?
A half decent transformer on no load should run a lowish PF with primary current effectively limited by the primary reactance. Sounds like your power monitor has a problem.
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29th Jul 2018, 7:44 pm | #7 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
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Re: Does a Murphy A474 mains transformer run hot?
A decent transformer with no load should run cool and stay cool if run for a long period. Getting hot is a real indication of true power dissipation. Most power indicators are scaled to suit larger loads and can read very misleadingly on low power things. Some of them even just measure the current and assume that the voltage is nomilal and the power factor is perfectly dissipative.
If the transformer gets too hot, then something needs doing. If that radio model has a reputation for hot transformers, then a design problem is the chief suspect. The question then becomes one of finding whether it's a copper limitation or an iron limitation. It may need a bigger transformer to be efficient and reliable. DAvid
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30th Jul 2018, 12:48 pm | #8 |
Triode
Join Date: Aug 2015
Location: Manchester, UK.
Posts: 32
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Re: Does a Murphy A474 mains transformer run hot?
Hi again,
Thank you for the comments about transformers and power. I tried a phase angle measurement using the 'scope last night. Unfortunately the 'scope is earthed and mains neutral has a couple on volts on it wrt mains earth. I didn't think it would matter but the ridiculously sensitive house breaker went off in the middle of Emerdale. Did I hear about it from downstairs! Worst "improvement" I ever did, being talked into having the good old fashioned fuse panel to be replaced. I will try again with a floating 'scope and not in the evening. Tony... |
30th Jul 2018, 2:43 pm | #9 |
Moderator
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Re: Does a Murphy A474 mains transformer run hot?
Now I think about it, I've noticed that the mains transformer runs a bit warm in a number of Murphy radios from this period - not alarmingly hot, just warmer than average. They must have designed them that way.
If a transformer is overheating, the temperature will continue to rise until bad smells and smoke become obvious. If this doesn't happen within an hour or so then there's nothing to worry about. |
8th Aug 2018, 7:58 am | #10 |
Triode
Join Date: Aug 2015
Location: Manchester, UK.
Posts: 32
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Re: Does a Murphy A474 mains transformer run hot?
Hello again.
I thought that I should appear to let you all know that I haven't gone away and ignored you. Thank you to Radio Wrangler and Paul Sherwin for the additional comments about transformer temperature. I am working on it. Pilot Mariner very kindly sent me an A547 transformer (and a couple of the "piano" keys as well) so I am well set up. I measured its power consumption, phase and temperature on the bench, and that of my transformer operating in the radio. I intended to then swap transformers, do the tests again, and then post the results in case they might help others. But there has been a complication with the new transformer that I won't bore you with (yet!) that has delayed that. So it will be a few more days, but I will be back with the conclusions. Tony... |
8th Aug 2018, 9:42 pm | #11 |
Hexode
Join Date: Jun 2003
Location: Burntwood, Staffordshire, UK or Kabaty in Warsaw Poland.
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Re: Does a Murphy A474 mains transformer run hot?
Not wishing to "spill the beans" on Tonys transformer complication (if this is it), the A474 transformer has a separate heater winding for the ez80 one side of which is also connected to the ez80 cathode.
In the A574, all heaters (including the ez80) are paralleled off the same winding, oneside of which is connected to chassis. There is no separate ez80 heater winding. What is the advantage of doing it the A474 way? Is there any reason why the A474 heaters can't be re-wired as in the A574? (Note that there is an additional vhf valve in the A474) |
9th Aug 2018, 10:09 am | #12 |
Rest in Peace
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Location: Matlock, Derbyshire, UK.
Posts: 1,378
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Re: Does a Murphy A474 mains transformer run hot?
A separate winding for the rectifier reduces the stress on the valve heater/cathode insulation at a cost.
I have not looked up the permitted voltage on the EZ80. EDIT: If I have interpreted it correctly, the limit is 500V. Last edited by TrevorG3VLF; 9th Aug 2018 at 10:18 am. |
16th Aug 2018, 11:40 pm | #13 |
Pentode
Join Date: Mar 2008
Location: London, UK.
Posts: 121
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Re: Does a Murphy A474 mains transformer run hot?
I had a A474 and the transformer did run very hot, you could not keep your finger on it for more than one second and the radio was using about 55 watts if I remember correctly.
I also had low gain on FM and substitution of C52 cured the problem. It was a .003 cap sitting between pin 7 and 8 of V4 according to my notes. |
17th Aug 2018, 7:26 am | #14 |
Nonode
Join Date: Aug 2007
Location: Walsall Wood, Aldridge, Walsall, UK.
Posts: 2,853
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Re: Does a Murphy A474 mains transformer run hot?
Hi!
Murphy were obviously aware of this problem as nearly all of their a.c. designs, both live and isolated chassis, had temperature fuses shown on the mains transformers of their circuits! Quarrington specifically refers to this point in Vol. 2 of "Radio and Television", chapter 1 " Power Supply and Decoupling", in which he states, "all too often the necessary wattage output of mains transformers has been obtained by using too much iron and too little copper!", a common penny–penching technique of setmakers! It's quite acceptable to operate the heaters of 6.3V rectifiers at chassis potential when only one L.T. winding is available – all of them are made to withstand the rectified max r.m.s. value of anode voltage between cathode and heater. My experience with receiver mains transformers is as Member Mark said, they nearly are always too hot to touch comfortably – most were designed for a temperature rise of 40–50°C on full rated load! Intsresting point about a defective m.e. causing a h.t. short, I've never come across that one, degradation of phosphor is the only fault I've ever seen with eye indicators, altho' another member made reference to "modulation problems" in one set cured by fitting a new indicator – can anyone elaborate further on this? Both C.A. Quarrington in "Radio & Television" and E.J.D. Lewis in "Radio Servicing And Maintainance" make reference to magic–eyes going "soft", but again, I've never come across a case of this! Chris Williams
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21st Aug 2018, 9:08 pm | #15 |
Triode
Join Date: Aug 2015
Location: Manchester, UK.
Posts: 32
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Re: Does a Murphy A474 mains transformer run hot?
Hi there,
I notice that there are several new posts, and I will reply to them - but not now. What i have here is quite long enough! I have some test results here that largely answer the question that I posed at the start, and might hopefully help someone else with transformer problems in this Murphy A474 series. Firstly I did a temperature run on the transformer that Pilot Mariner nicely sent to me. Note that the transformer was open on the bench. The temperatures have been normalised to the ambient temperature at the time and therefore show temperature rise. Power consumption 25 Watts. Phase angle about 80 degrees (but see later comment). Measurements were taken at 5 minute intervals. 5 10 15 20 25 30 35 40 Minutes 1 4 5 6 10 11 11 11 Degrees C After this there was no significant change. There was very little difference between the temperature of the core and that of the windings. Because the original transformer was at this time in the radio the off load measurements on that one were actually done later. But they are here for comparison: Power consumption 40 Watts Phase angle 70 degrees (comment later) 5 10 15 20 25 30 35 40 Minutes 4 6 10 13 15 16 20 20 Degrees C With the same comments as for the new transformer. My transformer, when installed in the radio, but with the back open yielded the following results: Power consumption 68 Watts Phase angle about 36 degrees current lag 5 10 15 20 25 30 Minutes 5 11 11 12 14 15 Degrees (Winding) 25 32 40 45 51 48 Degrees (Laminations) With little change after this Where the new transformer measured: Power consumption: 63 Watts Phase angle about 36 degrees current lag 5 10 15 20 25 30 35 40 Minutes 4 7 10 12 13 14 15 16 Degrees (Winding) 17 25 32 35 37 45 47 48 Deg (Laminations) Again little change after this. The difficulty in measuring the phase angle arose because my (tapped down) mains voltage was significantly flat topped, and the current waveform (votage across 10 Ohms) had a very high second harmonic content, so the zero crossing was wide and flat! That may enable anyone with a suspect transformer to decide whether it actually is or not. BUT, there is a big trap that I have not mentioned! You might remember that my original transformer was violently overheating because of shorted turns. This meant that I had no chance to measure anything before I took it apart. So the no load consumption of about 35 watts was after re-assembly. This looks a lot more than the 25 Watts of the new transformer. However I also had good reason to dismantle and re- assemble the new transformer as well. (Well it seemed good at the time!) After I re-assembled it the consumption was - guess what. 38 Watts! And that is why the off load figures for temperature rise are rather different, but the "installed" figures are very much the same. Sorry about that. But I can't undo the dismantling! Summing up: If a transformer in these radios consumes about 25 watts off load it is probably OK. Installed it should be around 55 Watts (according to the service manual). And the answer to my original question is - Yes, they get quite hot. My next move will be to take the old transformer apart again. I might add the second high voltage secondary (if I can make it fit). But either way I will clean up the laminations, apply the thinest varnish spray that I can, and measure the consumption after (if) it goes back together. I hope that that was of at least a little interest. Tony... |
21st Aug 2018, 9:41 pm | #16 |
Dekatron
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Location: Leominster, Herefordshire, UK.
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Re: Does a Murphy A474 mains transformer run hot?
When you say "watts" and quote a phase angle are those real watts- ie the VA is significantly greater or are they actually VA when the actual watts (power disspated in the transformer) are somewhat lower?
If the V and I are reasonably sinusoidal, actual watts are VAcosφ where φ is the phase angle between V and I.
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23rd Aug 2018, 8:05 pm | #17 |
Triode
Join Date: Aug 2015
Location: Manchester, UK.
Posts: 32
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Re: Does a Murphy A474 mains transformer run hot?
Hi Murphy 512,
According to the service manual 55W is about right. Thank you specifically for the added comment about C52. It is interesting how one can get stuck in a mental groove. Because the wax, and black, and "torpedo" capacitors are almost always leaky one can become blind to other possibilities. Because I was becoming short of suitable capacitors, by the time I got to C52 I resoldered it back in when it proved not to be leaky. I did mean to replace it later of course, and there are a few Hunts capacitors in less important positions to get the treatment to! But I was becoming increasing puzzled about low gain on VHF and a Magic eye that barely moved. (though I thought that that might be the EM81 that I had had to fit.) After reading your comment I found that 3000pF capacitor to be 15pF. Oh dear. However the gain did not come up. That was because someone must have miss-aligned T4 to compensate. After proper alignment the gain came up tremendously - and the EM81 now works well on VHF too. So thank you. Tony... |
23rd Aug 2018, 8:06 pm | #18 |
Triode
Join Date: Aug 2015
Location: Manchester, UK.
Posts: 32
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Re: Does a Murphy A474 mains transformer run hot?
Hi again,
Pilot Mariner is quite right - the complication was the lack of a separate heater winding for the rectifier that the A474 has. I made a couple of attempts to add a second heater winding on the transformer but I simply couldn't fit it in. I think that the manufacturer of the A574 (and therefore probably the A674 also) have used much more insulation on the high voltage windings, thus using up the space. So I was going to have to modify the radio. I was bothered about the heater cathode insulation of the rectifier, but I found some interesting things on the internet. Like everything else on the internet of course these could be "alternative facts" but they do make sense: The 6 volt heaters were introduced specifically to allow cheaper transformers to be used, and Phillips deliberately produced a 500V htr/cathode rating on all such rectifiers to allow for a common (earthed) heater supply. (I can only hope Mazda did the same.) They also gave these valve heaters a slower warm up time to allow other valves to be conducting by the time the rectifier did. So I began to feel happier about the modification. But what really did it is when I bought the A474 service manual I read (extracting from Page 4) "Modifications... A new mains transformer is now supplied which has no separate winding for the rectifier heater. .... must be connected in parallel with the other valve heaters (pin 5 to chassis)". So there - it's official I really wish I had not taken the transformer apart now! Incidentally, the rectifier does seem to warm up later that the others and the cathode in my set never gets above 285 Volts. One thing I did, and would suggest is a good idea on all these radios where there isn't a fuse in sight, is to take the secondary centre tap to chassis via a fuse. Just in case of a shorted htr/Cathode - or even a magic eye short! I used a 250mA A T fuse and it hasn't failed on start up yet. If you do that you will need to remove the link to heater ground behind the transformer panel. Tony... |
23rd Aug 2018, 8:09 pm | #19 |
Triode
Join Date: Aug 2015
Location: Manchester, UK.
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Re: Does a Murphy A474 mains transformer run hot?
Hi Herald 1360,
That's a fair question. I didn't actually do any calculation to get the power figures. I just used the in-line domestic power meter that I referred to earlier. It is a Globaltronics DEM1379 and was probably bought from Aldi. I think that it is probably a chinese product and appears with a hundred other badges! However, as a check on it I today applied mains 240V to a 40 watt incandescent lamp, a 10 ohm resistor and a 2.7 microfarad capacitor in series. I measured the voltages and calculated the current from the voltage across the resistor. As this is related to the topic, but drifting a bit off it I won't show calculations; just the measurements. Vlamp 167V Vc 159V Vr 1.35V Anyone who cares will easilly find that the power dissipation (essentially that of the lamp) should give a reading of 22 Watts. The power meter actually showed 33 Watts. So it is showing power on the assumption that the current is in phase. I calculate it as 46 degrees, and measured 50 degrees. The power meter insists that the power factor is still 1.00! So my readings of the power taken by the radio will be slightly high, but I still think that my results will enable someone to check for a functioning transformer - especially if they don't get too scientific about it, and subtract the 10 Watts or so I inadvertedly added by dismantling the transformers, which was in one case unavoidable anyway. Tony... |