Hi to all,

I am restoring an old radio which has a very bad speaker.
Paper cone is patched everywhere with silicone.
The speaker is not so bad at low volume, but at high volume the speaker is noisy.

I was planning to only recone , but at high volume i can see sparks between the moving coil and metal armature. That is not normal.

I decided to cut the cone and take off the moving coil, then i saw that all the varnish is gone on one side of the coil because the metal core in not perfectly centered into the moving coil.
Also the moving coil has a low resistance of 2 ohms. I believe some turns are shorting.

I was thinking to get rid of it, and use a 12" modern speaker. I have plenty of those.

So i'd like to remove the field coil of the bad speaker and install it as a filter choke on my chassis or under the chassis.

My question is: Do i still need the metal core inside the field coil ? Because without the core, the coil takes less space.
Do i need special shielding over the field coil ?

Thanks
Vianney

Sounds like, for whatever reason, there is a short between the field coil and the speaker frame, and the voice coil is grounded, or perhaps the voice coil winding of the output transformer is somehow shorting to the primary (via wiring, maybe, or fish paper deterioration). If it were my speaker, I'd re-cone it and varnish and then center the voice coil. I'd then investigate the various components of the speaker to see which one has leakage. If damp, I'd dry it out. If a wire was actually making contact to the speaker frame or two wires were touching that weren't supposed to (such as inside of the output transformer), I'd correct that. The field coil is usually insulated by some pretty thick cardboard, so maybe some windings are falling out on one of the ends, and are touching the frame or the hum bucking coil (if used).

If I could not locate the source of leakage, but the speaker otherwise functioned well, I'd make sure that the voice coil no longer used the chassis as part of its circuit, if this is done. If the voice coil is grounded to the chassis, ungrounding it will eliminate any possibility of shorts between it and the speaker frame. It might also be helpful to leave a note on the speaker indicating that its frame might be live with reference to the radio's chassis. It is always a good idea to take this precaution when handling radio speakers anyway. You should not handle a radio speaker and the chassis at the same time while the radio is powered up. A failure of insulation can lead to a shock, and I have been shocked by such leakages, even if only because of humidity.

Regarding the voice coil's resistance, 2 ohms is not uncommon for 3.5 ohm (at 400 cycles AC) voice coils, which are commonly found in antique radios. Other impedances are, of course used, but I most often run across 3.5 ohm coils, especially in the 6" size. The DC resistance will be lower than the AC impedance, and can sometimes be lower than an ohm. This does not mean that the voice coil is shorted. Varnishing the coil may also aleviate any shorts that may have developed. This can be assessed with a magnifying lens. Unless the voice coil windings have been shaved flat, it is unlikely that they are shorting.

T.

Also, if you cannot find any leakage to the speaker frame, and the speaker frame happens to be grounded to the chassis, and the voice coil is also grounded to the chassis, sparks may be developing at high volume due simply to the high currents within the voice coil from the strong audio signal, if the voice coil is scraping the speaker frame and partially shorting out. Some amplifiers develp enough speaker driving current to light a 6L6 tube to full brilliance. One such amplifier is one I own that uses 6V6 tubes in push-pull.

You can have the speaker re-coned. That would include a new voice coil too. If you were to use the old coil still as a choke, you do need the center piece to act like a transformer.

Here's a few thoughts.

Temporarily try swapping in a PM speaker in place of the old one .....but leave the field coil connections alone so the choke aspects for hum filtering stay the same. Just disconnect and re-route the output transformer secondary leads over to the temp PM speaker.

Use a good PM speaker of the approximate size you will later use just to see how it all sounds.

If this setup sounds good, then you can feel confident that you most likely don't have shorts in your windings or anything causing bad audio..

Then your choices are simple.
1. Send the old speaker out for re-coning or re-cone it yourself.

2. Scrap the old speaker but keep the field coil and use a new PM speaker.

3. Use a new PM speaker and replace the field coil with a resistor and increase the filter cap sizes to improve the filtering lost without the use of a choke.

I disconnect my audio transformer and connected it to a good PM speaker. Sound is great. My audio transformer is Ok.

I succeed to re-align the metal core of my speaker, so the moving coil is not rubbing on the speaker frame anymore. I reglue the old patched cone and retest.
At high volume, there is no sparks anymore between moving coil and speaker armature but the sound is still bad.

Nobody is reconing in my area.
I will try to recone myself before i go with a PM speaker. I have never reconed before, but found some infos about reconing on Nostalgiaair resources.
The only troublesome i might have will be the mounting of the front opening of the cone to the frame...?
I dunno how to bend the papercone.

Vianney

There are many different sources for pre-made speaker cones. Here is an example of an inexpensive source:

http://cgi.ebay.com/Speaker-Cones-12-2-per-pack-/160377189460?pt=LH_DefaultDomain_0&hash=item255739b454

www.electronix.com sells speaker cones in the miscellaneous section of their speaker section, but, unfortunately, they do not sell 12" cones, or at least they do not list them.

Some other suppliers sell high end cones of various styles, but the inexpensive will probably suit you well. >>Be sure that the voice coil opening is the same as your cone, and that the depth of the cone is the same.<< If the opening is smaller than you want, you can enlarge it. If it is larger than you want, if the center of your original cone is in good condition, you can glue the new cone to the center of the old cone.

If you wish to make your own cone, this isn't the easiest task, but, regarding the flexible edge, you can use lightly rubberized cloth as a roll to make your flexible edge.

Whatever you do, the depth of your new cone should be the same as your old cone so that the voice coil is centered in the pole pieces, and doesn't lie above or below the pole pieces. If necessary, you may be able to add an extension to the voice coil if the cone is too shallow, or add shims to the edge of the speaker frame if the cone is too deep. ...within reason.

If you use a foam edge cone, the foam edge may interfere with the baffle, and will push the cone in, uncentering the voice coil. If so, take a piece of cardboard of the appropriate thickness and cut out an opening that will allow the foam edge to fit through. Place this between the speaker and the baffle when mounting your speaker.

Regarding sparking, you centered your voice coil and now sparking is gone, but you should still see if there is a leak between the field coil and the speaker frame, and, if possible, correct the electrical leak. Use your ohmmeter to test for leakage between the speaker frame and the voice coil.

Also, regarding poor sound quality, be sure that your spider is in good condition, or else you will have to repair/rebuild/replace it. Be sure that the voice coil is properly glued to the spider, and that the spider is firmly attached (though if screwed in place, only snug the screws firmly...don't crank on them or you might break or distort the spider). You might consider dipping the voice coil in thin varnish to be sure that the coils are firmly attached to the coil form, and that the coil form is strengthened. After many years of aging, these things start to come apart, and though not necessarily readily noticeable to the eye, can cause all sorts of rattles and obnoxious noises in the speaker.

Good luck! I hope you go the speaker-restoration route, but I know it can be somewhat difficult. You don't have much to lose, though. The speaker sounds terrible now, so you may as well try it and hoan your skills at speaker rebuilding.

T.

Hi,

I checked the DC resistance between the field coil and the voice coil. There is no leakage at all.

The field coil has a DC resistance of 578 ohms.
I thought it should be around 2000 ohms.

If i end with a PM speaker, you say some people are replacing the field coil by a resistance and increase the value of the filter capacitors. Even the first capacitor of the filter ? I thought i had to respect the tube rectifier specs.
Is there any chance i end with HUM if i don't stay with the field coil ?

I don't know if it's original, but there is a lug connected to the voice coil and screwed to the speaker frame. So one lead of the moving coil is grounded to the speaker frame which is isolated from the radio frame.

By the way, there is a third winding somewhere on the top of the field coil. I can't see it, but i see 2 small black wires entering by the top of the field coil. I see it on the schematic too.
What is the purpose of that one ?

Thanks
Vianney

That 3ed winding is a hum bucking coil. It is a reverse wound coil to cancel AC. If this radio is in good shape other than the speaker. It's almost a shame to rid the field coil ( filter choke ) and hum bucking coil. You do have the option of adding a frame type choke, and adding a resistor in series if the choke has little resistance.
If you use just a resistor and up the values of both capacitors there will some slight hum.

Vianney,

Another possibility at:

http://www.antiquewireless.org/otb/resto0504.htm

Clifton

:Hi,
:
:I checked the DC resistance between the field coil and the voice coil. There is no leakage at all.
:
:The field coil has a DC resistance of 578 ohms.
:I thought it should be around 2000 ohms.
:
:If i end with a PM speaker, you say some people are replacing the field coil by a resistance and increase the value of the filter capacitors. Even the first capacitor of the filter ? I thought i had to respect the tube rectifier specs.
:Is there any chance i end with HUM if i don't stay with the field coil ?
:
:I don't know if it's original, but there is a lug connected to the voice coil and screwed to the speaker frame. So one lead of the moving coil is grounded to the speaker frame which is isolated from the radio frame.
:
:By the way, there is a third winding somewhere on the top of the field coil. I can't see it, but i see 2 small black wires entering by the top of the field coil. I see it on the schematic too.
:What is the purpose of that one ?
:
:Thanks
:Vianney
:
:

Thanks Clifton,

This solid-State Filter Choke as a Field Coil replacement is interesting.
I keep this in my files.

:Vianney,
:
:Another possibility at:
:
:http://www.antiquewireless.org/otb/resto0504.htm
:
:Clifton
:
::

What is the model number of this radio? Not all radios have a 2K ohm field coil, though if the schematic calls for one, then it should have one. My Crosley 1117 has a 575 ohm field coil. This is a rather large radio, and a lot of current is drawn through the field coil, so enough magnetism is generated with only a 575 ohm field coil.

My DeWald BT-100 television has a 62 ohm field coil. So much current is drawn through this field coil that it doesn't need to be very big at all.

If your field coil is supposed to be 2000 ohms, then there's something wrong with it, but if it's supposed to be between 500 and 600 ohms, then it's just fine.

Regarding the voice coil sparking to the frame (back when it was mis-aligned), if one side of the voice coil circuit is connected to the frame, sparks could have occurred at high volume when there was high current in the voice coil, because, as I said before, larger radio amplifiers generate enough current at the voice coil to light a 6L6. ...So in fact, as you have observed, you may, and do probably not have any leakage between the field coil and the speaker frame. The sparking was only generated by the voice coil voltages. With your voice coil centered this should no longer be a problem.

I believe that you can make this speaker work well again, if the field coil is of the correct resistance.

T.

Hi Thomas.

:What is the model number of this radio?

It's a Marconi model 161.

The part list doesn't tell the resistance of my field coil. Only a part number. I suppose the field coil is good. There is no sparks anymore. Only rattles from the cone at hight volume.
I will try to recone this week.

Thanks for all the infos.
Vianney

Before you recone, be sure to dip the voice coil into some very thin varnish and allow to dry for a few days. The varnish should just leave a thin film so that it doesn't interfere with the movement of the voice coil. This will assure good insulation of the voice coil, and also assure that the voice coil is firmly attached to the voice coil form so that rattles cannot develop here.

Also, during reconing, be sure that the spider is well attached to the voice coil, and to whatever hardware that attaches it to the frame, or if the spider is glued directly to the frame, that it is glued well.

T.

:Hi Thomas.
:
: :What is the model number of this radio?
:
:It's a Marconi model 161.
:
:The part list doesn't tell the resistance of my field coil. Only a part number. I suppose the field coil is good. There is no sparks anymore. Only rattles from the cone at hight volume.
:I will try to recone this week.
:
:Thanks for all the infos.
:Vianney
:
:Hi Vianney, I am currently working on a similar problem with a Zenith 12 tube (12S370) where the field coil was open, and I was unable to repair same. here are some of the issues I ran into. Replaced the field coil with a filter choke with 155 Ohms resistance. Field coil was 670 Ohms, so added a 560 Ohm 15 watt resistor in series. The set worked great, but run into further problems. Had recapped the radio including electrolytic caps. the Two HT caps were 12 MF from the rectifier and 20 MF other side of the field coil. The 12 MF 450 working volt electrolytic blew up. The radio was designed to work on about 115 volt supply. The Rectifier in this set was a 5Y4. DC voltage from the rectifer should have been 320, but with 121 volt supply which is quite common in most homes today, the DC voltage was actually 360. Put a analogue volt meter across the hot side of the 12 MF capacitor and voltage spiked to 475 on power up,far above the working voltage, did a test run by putting two 30 MF, 450 caps in series, replacing the 12 MF cap and radio seems okay now. But replacing electro-magnetic speaker field coils can be more demanding than origionally thought.
:
:
:
:

Have used these guys a couple of times. A 12 inch re-cone job is $45.00 + S&H ..

http://www.johnsvintageradio.com/recone.html

Then there is good old Hank Brazeal hankspkr@charter.net

Hi to all,

Reconing my speaker has not been a success.
The central metal core was too loose and tend to shift off-center. The moving coil is still rubbing the speaker frame at high volume.

I decided to dismantle the field coil and put it over the magnet of a PM speaker. The side of the field coil has been choosen so when it is energized, it is attracted by the permanent magnet. And vice-versa.
I put some clear silicone under the field coil.

I did some HUM tests with and without the field coil frame, with and without the field coil metal core, and with and without the humback coil.

I didn't see any difference with and without the field coil frame. So i didn't installed it. It's less heavy this way. But may be inductance is not the same ...
B+ voltage stay the same.

I didn't see any difference with and without the hum bucking coil. It would be easy to reinstall it over the field coil, but i don't know what would be the right polarity of the coil...Is there any test i could do to figure out ?

Then i noticed an important increase of the HUM when i pull out the core of the field coil. So i kept it inside the coil.

The sound of the set is good now.. However the DC resistance of my PM speaker (voice coil) is now 7.5 ohms instead of the original 2.5 ohms.

I might have less power volume i suppose ?

Thanks
Vianney