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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
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28th Jan 2023, 5:54 pm | #1 |
Pentode
Join Date: Mar 2005
Location: Portsmouth, Hampshire, UK.
Posts: 116
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Marconiphone 865 speaker field coil question.
Hello,
I've purchased one of these console sets and am very keen to restore it. I'm not very familiar with loudspeakers with energised field coils. The previous owner advised repair work had been carried out sometime in the past & the set had worked but now no reception was possible. Here's the thing. Before embarking on a repair, I thought I would test coil continuity e.g. speaker voice coil, speaker field coil, mains transformer, output transformer etc. Well, the speaker voice coil was ok but the field coil measured about 30kΩ. Oh dear. That's got to be too high. Especially as the service manual circuit diagram indicates it is 400Ω (although that seems very low). But the previous owner said it had been operating. Let's plug it in. Sure enough, valves lit, loud crackling when waveband changed & buzz from 'gramophone' input when pick-up input touched. No reception but I'll investigate that later. Here is my question which I hope some kind forum members will answer. How is there audio when the field coil is virtually open circuit? Is there just enough current going through it to produce some magnetism? Have I mis-read the circuit diagram? 400Ω stated on the cct. seems low. The coil did get slightly warm I think during operation. I really want to restore this set and would like to keep the original speaker (or replace with the same type). I don't really want to install a PM speaker unless it is necessary. I gather this would involve power supply modfication also. I understand a chap called Ed Dinning may be able to help repair the loudspeaker, so I may try to contact him if the speaker is definitely faulty. Forgot to say, CK2, the field coil, is listed in the parts section of the service manual, so hopefully was a replaceable part? All the best. S. Last edited by sabaman; 28th Jan 2023 at 6:00 pm. Reason: Typo |
28th Jan 2023, 6:27 pm | #2 |
Hexode
Join Date: Oct 2012
Location: London, UK.
Posts: 497
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Re: Marconiphone 865 speaker field coil question.
30kΩ is obviously too high. 400Ω is probably OK especially as the manual suggests this.
If it was really 30KΩ there would be very little HT and no loud crackling or buzz so all I can suggest is a measurement error. |
28th Jan 2023, 7:31 pm | #3 |
Banned
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Re: Marconiphone 865 speaker field coil question.
Oh, you lucky person! I know someone who'd give limbs to own that set - they're incredibly rare.
400 Ohms isn't particularly low in a set where there's a high current draw from the set itself; I've certainly got a few sets with 700 Ohm field coils in a six-valve table set with a large output valve. |
28th Jan 2023, 7:32 pm | #4 |
Heptode
Join Date: Mar 2017
Location: Romsey, Hampshire, UK.
Posts: 511
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Re: Marconiphone 865 speaker field coil question.
Hi Sabaman
400 ohms is a typical value for the field coil winding resistance. When passing for example 75 mA of HT current this will drop 30 volts and dissipate a couple of watts, so a little warmth will be apparent in the speaker after it has been on for a while. If the resistance was much more you would lose too much HT voltage. If you really had 30K and say 300 volts HT in, the HT current would be less than 10 mA and the HT voltage out would drop to a very low level. I rather doubt if this would give any receiver performance, but you never know. The magnetism would be much reduced but I guess you might hear something. In my experience, when probing for resistance on aged electronics you often need to use sharp meter probes and poke them quite hard and/or wriggle them so as to break through the film of oxide etc that forms on old solder joints. You might be lucky and find the coil is OK upon re-measurement. Most of the time these coils are either OK or completely open circuit. I suggest you also measure the HT voltage both before and after the field coil with it connected and the set warmed up and working. I would expect the voltage out to be roughly 90% of the voltage in, if it's appreciably less that's bad news. Your choice then would be to have the field rewound, or to fit a permanent magnet (PM) speaker instead and use a power resistor in place of the field winding. If the 100Hz hum is unacceptable you may also need a larger value smoother electrolytic. |
28th Jan 2023, 7:37 pm | #5 |
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Re: Marconiphone 865 speaker field coil question.
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28th Jan 2023, 8:48 pm | #6 |
Nonode
Join Date: Jun 2007
Location: Coulsdon, London, UK.
Posts: 2,152
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Re: Marconiphone 865 speaker field coil question.
I think I have read somewhere that multimeters can get confused when they try to measure the resistance of a coil with large inductance.
I would measure it with an AVO. |
28th Jan 2023, 9:26 pm | #7 |
Pentode
Join Date: Jan 2008
Location: Derby, Derbyshire, UK.
Posts: 107
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Re: Marconiphone 865 speaker field coil question.
I’m the proud owner of one of these sets. I’ll measure the resistance of my field coil tomorrow and see how it compares with yours.
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28th Jan 2023, 10:13 pm | #8 |
Dekatron
Join Date: Nov 2011
Location: Surrey, UK.
Posts: 4,385
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Re: Marconiphone 865 speaker field coil question.
That's one lovely radio!
Definitely- whilst I habitually use a modern DMM for most work, a simple analogue ohm-meter is a welcome sanity-check for things that are or may be significantly inductive, some DMMs in my experience can either scroll around restlessly or read something way wrong. |
28th Jan 2023, 10:35 pm | #9 | |
Dekatron
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Re: Marconiphone 865 speaker field coil question.
Quote:
If the speaker is working, there must be continuity on the field coil. Mark |
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29th Jan 2023, 10:33 am | #10 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,364
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Re: Marconiphone 865 speaker field coil question.
I think the most likely explanation is that you do indeed have a damaged field coil. The high voltage at this coil means that if there are (and from the sound if it there will be) corroded windings (green spot) then there will be arcing and inter-winding shorts such that the coil's continuity might self-heal at least for a time. Unfortunately it will eventually go open circuit permanently. A rewind is expensive but for a set such as this it must be worth it. Dismantling the speaker is fiddly so make sure that you take photographs and notes as to how the coils, including any hum-bucking winding, are connected. Reassembly after rewind must be done with care to avoid damaging the speech coil or its former. Radiospares used to supply replacement field coils back in the day but, obviously, no longer. You might be able to get a suitable replacement coil from Mike Lewis so if you don't have his contact details then drop me a PM. Jerry
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29th Jan 2023, 11:08 am | #11 |
Nonode
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Re: Marconiphone 865 speaker field coil question.
Are the points from where your taking your readings clean and corrosion free? I might be worth re-soldering the joints and measuring again.
The circuit shows the H.T. voltage, how does yours compare? David
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29th Jan 2023, 11:19 am | #12 |
Pentode
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Re: Marconiphone 865 speaker field coil question.
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29th Jan 2023, 4:56 pm | #13 |
Pentode
Join Date: Mar 2005
Location: Portsmouth, Hampshire, UK.
Posts: 116
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Re: Marconiphone 865 speaker field coil question.
Hello,
Thank you all so much for your replies. I wanted to include your remarks as quotes in my reply & respond individually but had difficulty doing this, so please forgive me for the summary. Mr 1936 & Radio_Dave suggested measuring the H.T. before and after the coil. Why didn't I do this to begin with 322v before & 260v after. So the coil is ok! winston_1 was right. It was a measurement error. Thank you to Silicon & turretslug for their advice regarding DVMs. I've worked in electronics for a long time and was unaware (or had forgotten?) about problems measuring resistance combined with inductance. Must remember that in future. I sold my AVO some time ago for some reason & now just rely on the two meters shown in the photo. They both gave the same erroneous result. I expected better of the Celtek. Thank you Cathovisor, for your reply & kind remarks. It is a lovely radio, hence my determination to do it justice & restore it. I feel very fortunate. steve102, what a beautiful & impressive example of the radio. If I can get mine looking anywhere near as good as that, I'll be very happy. Thanks also for taking the time & trouble to measure the field coil resistance - I'm most grateful. I'm guessing you used an old AVO! When doing the initial measurements, I did wonder about tarnishing/corrosion on the measurement points, so made sure I was used sharp points on the meter probes and then even tried resoldering the points I was measuring as suggested by Radio_Dave & Mr 1936. Thank you,cathoderay57, for the detailed explanation of possible field coil failure and suggestions of help. Most kind of you. I'm hoping now though, that I can bypass this step. Thanks mark pirate for your help also. As you say, 'If the speaker is working, there must be continuity on the field coil.' I'm most grateful for all of your help. Now I can get on with tracing the RF fault. And get an AVO S. |
29th Jan 2023, 6:51 pm | #14 | |
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Re: Marconiphone 865 speaker field coil question.
Quote:
I tend to use a Fluke 179 as my day-to-day meter in my professional capacity, but I would like to have one good AVO 8 and one good AVO 7 for 'vintage' work - the last so I don't have to make allowances for meter load on old service sheets! Am I right in thinking this is the Marconi version of the HMV 659? To me it is so much more attractive and - dare I say it - has a very 'American' look to it. |
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29th Jan 2023, 7:07 pm | #15 | |
Pentode
Join Date: Jan 2008
Location: Derby, Derbyshire, UK.
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Re: Marconiphone 865 speaker field coil question.
Quote:
It would be worth checking if your set has the revised circuit shown in the manufacturer's service manual as it affects some of the resistance measurement points shown in the continuity check table, i.e. L19 is now connected to R42 instead of chassis. This caused me some head scratching until I realised mine had the revised circuit! |
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29th Jan 2023, 9:05 pm | #16 | ||
Pentode
Join Date: Feb 2015
Location: Chepstow, Monmouthshire, UK.
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Re: Marconiphone 865 speaker field coil question.
Quote:
After restoring it, and soak testing it on the bench, the sound would be as expected, but, occasionally the sound would drop to about half or quarter volume after a while, and sometimes it would be half/quarter volume on turning on. This set (being a TRF with reaction) is quite fiddly to tune in, and it can drift after a while. So I initially assumed the sound drops were drift, and/or being unable to get the tuning and reaction "just right". But, after a while it became clear it must be due to a fault somewhere. When in it's "low" sound mode it would also become unstable. Tests showed the HT in "low" sound mode was about 30 volts too high, which was causing the output valve to become unstable (235 volts on anode with voltage drop across output transformer primary, normally it would be about 203 volts). I traced the fault to a bad connection on the field coil. With the field coil completely disconnected the radio would continue to work, albeit in "low" sound mode. The key to the "mystery" of how this was so is because the field coil in this set (GEC mains universal 3 of 1934) is not involved in smoothing the HT, and it is connected directly across the rectifier output. So the disconnected field coil didn't cause lack of HT, in fact the reduced load meant it rose by about 30 volts. But it still meant the loudspeaker was working without the field coil being energised, but at a reduced volume. I assume this was due to residual magnetism. |
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29th Jan 2023, 11:57 pm | #17 | |
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Re: Marconiphone 865 speaker field coil question.
Quote:
And yes, residual magnetism would have explained the sound. Bush used PM speakers in their pre-war AC/DC sets until the 70-series came along, which were boringly conventional: gone were the PM speakers and the barretter tubes. |
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30th Jan 2023, 10:54 pm | #18 |
Pentode
Join Date: Mar 2005
Location: Portsmouth, Hampshire, UK.
Posts: 116
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Re: Marconiphone 865 speaker field coil question.
Hello,
Cathovisor, it is, I believe, the Marconi version of the HMV 659 (according to the service manual). I had a quick look on the web but couldn't find a picture of the HMV version so can't really comment, other to say that I am quite taken by the Marconiphone's stately deco appearance. Nice Fluke meter by the way. steve102, thank you for your kind words and advice given on the revised A.V.C. circuit. I have not had a chance to see what 'version' I have yet. Hope to do so this weekend. Question is, do I revise the circuit in the set if it is presumably some sort of modification improvement? I guess get it going first. Catkins, what an interesting fault! Thank you for sharing the details. That would have certainly kept me guessing. All the best. S. |