Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives :: Books
Support This Site: Contributors :: Advertise


It is currently Oct Fri 19, 2018 9:04 pm


All times are UTC [ DST ]





Post New Topic Post Reply  [ 26 posts ]  Go to page 1, 2  Next
Author Message
 Post subject: Field Coil or PM?
PostPosted: Feb Wed 07, 2018 8:55 pm 
Member

Joined: Dec Wed 02, 2015 9:42 pm
Posts: 151
Location: Santa Clara, CA 95051
I am looking for the radio that used to be in our living room when I was growing up. I don't know much about it. I know it was an RCA console, very similar to model 8V7, https://www.radiomuseum.org/r/rca_8v7_8_v_7_8v7.html.

It had two doors on the top of it just like that, and the AM receiver was on the right with the changer on the left. It had a wood grill in the front like the 8V7 but I recall the spokes going the opposite direction. I know the 8V7 cannot be right because it uses a PM speaker and the one thing I know about the set comes from my brother who said it had a speaker with a field coil. I know he messed with it when I was too young to understand what was going on. I think I can trust his memory about the speaker.

Can I assume that RCA quit using electromagnetic speakers and switched to PM speakers at the same time? Or did they continue to produce electromagnetic speakers while simultaneously introducing modern PM speakers? If so for how many years did they do that? I am looking for any documentation about RCA production from about that time to see if I can better nail down a time frame. I know we had the radio in 1958 and it was used then. We were a very poor family and it would not surprise me to find out it been bought used when we got it. I know it was a very basic and simple model.


Top
 Profile  
 
 Post subject: Re: Field Coil or PM?
PostPosted: Feb Thu 08, 2018 8:36 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 33346
Location: Maryland 20709, USA
Tomie wrote:
Can I assume that RCA quit using electromagnetic speakers and switched to PM speakers at the same time?

The switch from electromagnetic speakers to PM was enabled by improvements in filter capacitors.

Field coils were used because large inductors were needed to filter the B+ when using the small value capacitors available at the time. I've measured field coils over 50 Henries.

When electrolytics became available with 10 or 20 mfd values, the required inductance dropped significantly.

- Leigh

_________________
73 de W3NLB
http://www.AtwaterKent.info
Click "Grebe Stuff" for Synchrophase info


Top
 Profile  
 
 Post subject: Re: Field Coil or PM?
PostPosted: Feb Thu 08, 2018 10:14 am 
Member

Joined: Jan Mon 16, 2012 4:15 pm
Posts: 4647
Location: Near Brandon, Iowa
WW2 seems to have been the event that separated The Age of Electrodynamic Speakers from The Age of Permanent Magnet Speakers. Prior to the war, most US-built radios were made with ED speakers. After the war, most sets were made with permag speakers. It wasn't an absolutely clean break and examples of both types can be found on either side of this watershed.

Permag Rice-Kellogg style speakers were available from around 1930 but the magnet materials available then produced relatively low flux density for a given magnet mass and had low coercivity- in other words, the magnets were very heavy compared to those used in field-coil-type speakers and were also prone to eventual demagnetization, two negative properties that worked against their use in commercial radios. The Japanese invention of the Alnico alloy in 1931 offered significant improvement in both regards (flux density and coercivity) but the alloy formula wasn't optimized until sometime in WW2, at which time strong, long-lasting, yet lightweight permanent field magnets could be manufactured that weighed approximately one-fifth that of permanent field magnets available in 1940. The improved Alnico alloy formula was available to US manufacturers by the close of WW2 in 1945, in time to support the restart of consumer electronics manufacturing.


Top
 Profile  
 
 Post subject: Re: Field Coil or PM?
PostPosted: Feb Thu 08, 2018 3:48 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 1739
Location: Lafayette, CO
Even with alnico speakers becoming common, RCA still had consoles for the 1949 model year with electromagnetic speakers. That's about the last use l've seen. Craig


Top
 Profile  
 
 Post subject: Re: Field Coil or PM?
PostPosted: Feb Thu 08, 2018 4:19 pm 
Member

Joined: Jan Mon 16, 2012 4:15 pm
Posts: 4647
Location: Near Brandon, Iowa
analog.tv wrote:
Even with alnico speakers becoming common, RCA still had consoles for the 1949 model year with electromagnetic speakers. That's about the last use l've seen. Craig

Midwest produced some models (216, for example) that still were using field-coil speakers as late as the 1952 model year. Some postwar Scotts likewise continued to use field-coil type speakers; although the sets designed under Meck ownership in the early 1950s used permag speakers with large steel magnet covers to (possibly) give a casual viewer the impression that a massive magnet lurked under it.

Beside improving technology, another thing that might have contributed in a small way to the decline of field-coil speakers was the rise of hi fi audiophile culture in the 1950s, which eschewed factory-integrated systems in favor of the "one good piece at a time" component approach. While it was certainly possible to create a decent standalone-powered electrodynamic speaker (it was done as early as the 1920s), by the time the "hi-fi" craze happened there was no reason to do so due to the availability of efficient permanent magnet units (which also totally eliminated the possibility of residual AC hum coming from the (now nonexistent) field magnet).


Top
 Profile  
 
 Post subject: Re: Field Coil or PM?
PostPosted: Feb Thu 08, 2018 8:41 pm 
Member

Joined: Dec Wed 02, 2015 9:42 pm
Posts: 151
Location: Santa Clara, CA 95051
Thanks everybody,

My question about the speaker configuration was to try to establish if I should be looking for models that are older than the RCA 8V7 I found which is similar or if the age has that much to do with it? I was reminded that I was born in 1949 and as a young child I used to hide behind that radio. So we had the set in 1952-53-54? The 8V7 was built in 1949 and it would not surprise me to discover that our old radio was built about then or prior to that.
So I guess a better question might be does anyone know of a catalog or other source that might list both model numbers and pictures of those early day RCA console sets? I need a model that looks very similar to the 8V7 but with an Electrodynamic Speaker. Our radio was built around the same style as the 8V7.
I found a better picture posted here but I'm also sending it to my brother. I will ask him again about the speaker to try to confirm it was an Electrodynamic and not PM.


Attachments:
RCA 8V7 vintage-rca-victor-victrola-console-radio-phonograph---.jpg
RCA 8V7 vintage-rca-victor-victrola-console-radio-phonograph---.jpg [ 73.02 KiB | Viewed 1283 times ]
Top
 Profile  
 
 Post subject: Re: Field Coil or PM?
PostPosted: Feb Fri 09, 2018 6:30 pm 
Member

Joined: Dec Wed 02, 2015 9:42 pm
Posts: 151
Location: Santa Clara, CA 95051
Mystery solved. We now have a good picture and after a family wide inquiry I got this response from one of my brothers, "I always thought it had a permanent magnet speaker because I remember getting a screwdriver attracted to the back of the speaker once." Everyone else thinks it looks just like our old radio.
So it is the model 8V7, https://www.radiomuseum.org/r/rca_8v7_8_v_7_8v7.html that I am looking for.

Thanks everyone for all the help.

Tomie


Top
 Profile  
 
 Post subject: Re: Field Coil or PM?
PostPosted: Feb Tue 13, 2018 7:37 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 5280
Location: 253 Blanche St. Plymouth, MI USA
Leigh wrote:
Tomie wrote:
Can I assume that RCA quit using electromagnetic speakers and switched to PM speakers at the same time?

The switch from electromagnetic speakers to PM was enabled by improvements in filter capacitors.

Field coils were used because large inductors were needed to filter the B+ when using the small value capacitors available at the time. I've measured field coils over 50 Henries.

When electrolytics became available with 10 or 20 mfd values, the required inductance dropped significantly.

- Leigh

More like during WW2 MAGNETS were improved... became cheaper too, so that caused the move to PM speakers after the war. Oddly, GE radios had PM speakers before WW2 in their consoles 1940-42, not RCA.
Mark Oppat


Top
 Profile  
 
 Post subject: Re: Field Coil or PM?
PostPosted: Feb Tue 13, 2018 5:07 pm 
Member
User avatar

Joined: Jan Mon 18, 2010 2:13 am
Posts: 17196
Location: Dayton Ohio
Mark has it right.

It was all about magnet strength and longevity, not about being a filter reactor.

Being a filter reactor was only a cost saving ploy which worked so well practically everyone did it that way.

GE was the first to use AlNiCo magnets starting about 1939. Everyone else had steel magnets which loose their strength over time. (mostly in Farm radios)
After WWII, was a huge shift to AlNiCo.

-Steve

_________________
Radio Interests
-Zenith
-Sparton
-Pre-War FM
Consoles and floor models, the bigger, the better!


Top
 Profile  
 
 Post subject: Re: Field Coil or PM?
PostPosted: Feb Thu 15, 2018 1:41 am 
New Member

Joined: Jan Sat 13, 2018 7:28 pm
Posts: 13
Location: Syracuse, NY 13078
This might be the correct place to post my question. I'm trying to resurrect an RCA 9K3. I've recapped it and now, bringing up the voltage with my variac, I'm finding good B+ but the electro magnet coil at the speaker is smoking a little and smells bad. The schematic says the coil should measure 2K ohms but I'm reading about 750 ohms. That, I think, would explain the smoke. At the same time, I've injected an audio signal on the power out tube and can hear it though not real loud. I guess I'm looking for an answer to can I get a replacement speaker? Can I, should I replace it with a PM speaker? Is there an upgrade/method to do so. I'm thinking it would be easier to replace the speaker with same type.

Ken B.


Top
 Profile  
 
 Post subject: Re: Field Coil or PM?
PostPosted: Feb Thu 15, 2018 2:27 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 5280
Location: 253 Blanche St. Plymouth, MI USA
Ken, your field coil could well be partially shorted now due to damage done by the previous owner when the caps were bad. First I would unplug the speaker and measure from the load side of that to chassis, with power off of course. Basically this is across the second filter cap in the power supply. Use an analog meter if possible with all these vintage radio jobs, you dont want to be distracted by dithering, just want to know if there is a short or low value to chassis from the B+ line to chassis. Should not read below about 50K or so. If it does, you still have some issue. No matter, if the speaker is reading 750 ohms you need to get that fixed or get another speaker. I may have one in my stock but would have to look. The 9K3 was a good seller in 1936 so they are not super rare but good speakers are not easy to find anymore as so many repairable ones have been carelessly discarded.
Mark Oppat


Top
 Profile  
 
 Post subject: Re: Field Coil or PM?
PostPosted: Feb Sat 17, 2018 2:05 am 
New Member

Joined: Jan Sat 13, 2018 7:28 pm
Posts: 13
Location: Syracuse, NY 13078
Matt,
All I have is a couple DVMs. I long ago got rid of my analog meters. I'll see what it reads with my DVM just for grins. Yes, would you see if you have one of these speakers? It's a shame, the cone on my speaker is flawless. I can't see how to disassemble the coil. There are a couple screws that appear the hold the Field coil bracket to the the speaker frame but removing those doesn't allow the bracket to be removed. There must be more to it than I can see. Let me know if you have one and what you would need for it if you do. Thanks Matt.
Ken


Top
 Profile  
 
 Post subject: Re: Field Coil or PM?
PostPosted: Feb Sat 17, 2018 7:10 pm 
Member

Joined: Dec Wed 02, 2015 9:42 pm
Posts: 151
Location: Santa Clara, CA 95051
Hi Ken,

Do you have a picture of the speaker? I'm just curious about how the field coil is attached to the speaker frame. My friend and I have worked on some of these in the past and I may or may not be able to share how to repair it, if that's even possible. I am unfamiliar with the model you have.

I'm just curious.

Tomie


Top
 Profile  
 
 Post subject: Re: Field Coil or PM?
PostPosted: Feb Sat 17, 2018 8:24 pm 
New Member

Joined: Jan Sat 13, 2018 7:28 pm
Posts: 13
Location: Syracuse, NY 13078
Yep, here are four pics.


Attachments:
IMG_3037.jpg
IMG_3037.jpg [ 80.35 KiB | Viewed 852 times ]
IMG_3038.jpg
IMG_3038.jpg [ 87.19 KiB | Viewed 852 times ]
IMG_3039.jpg
IMG_3039.jpg [ 103.34 KiB | Viewed 852 times ]
IMG_3040.jpg
IMG_3040.jpg [ 93.67 KiB | Viewed 852 times ]
Top
 Profile  
 
 Post subject: Re: Field Coil or PM?
PostPosted: Feb Sun 18, 2018 7:32 pm 
Member

Joined: Dec Wed 02, 2015 9:42 pm
Posts: 151
Location: Santa Clara, CA 95051
Hi Ken,
I did rebuild a speaker field coil very similar to that one. I undid that center nut just like you have done, but the “U” shaped retainer lifted right off of mine. It was not the same speaker as yours, but looking at the pictures it seems to be constructed in a very similar manner. I'll try to find a picture to share if I have one.
If that retainer is still attached to the main speaker frame I'm betting it is mechanical because I can't see them putting spot weld heat that close to the windings.
Whatever is holding that clamp on there is obviously not on the top. If you run your finger between the frame and the cone directly below where the edge of that clamp is sitting, do you feel anything under there? Spot weld even? There are 4 places, 2 on each side of that clamp where it looks like the base of the clamp has extended down through the frame and that's most likely what's holding it on there.
Once removed you should find that the field coil is just a big roll of wire and it should lift right off of there. Take pictures and keep track of the parts. There are folks around who will rewind that coil for you or you can do it yourself in your shop. It's tedious but not terribly difficult.
So IMHO it should be possible to repair that speaker field coil.
One note of caution, don't attempt to cut those retainers, whatever they are, with something like a drummel. Tiny bits of metal will get between the voice coil and the magnet and ruin the speaker. Voice of experience, lots of masking and tape did not keep it out. There may be just enough residual magnetism to attract the shavings.

Tomie


Top
 Profile  
 
 Post subject: Re: Field Coil or PM?
PostPosted: Feb Sun 18, 2018 7:53 pm 
New Member

Joined: Jan Sat 13, 2018 7:28 pm
Posts: 13
Location: Syracuse, NY 13078
My problem is that the nut is stuck on the threads right where you see it. I can't get it to tighten or loosen any further. The screw rotates with the nut. My only thought is to cut a slot across the nut deep enough to also slot the screw... of course with a Dremmel tool as you suggest not to. I could then use a screw driver to hold the screw and turn the nut with a wrench. I think I may be screwed on this one but better to try and fail than to walk away from it. That's a sure fail. I'll let you know what happens, thanks for the input.
Ken


Top
 Profile  
 
 Post subject: Re: Field Coil or PM?
PostPosted: Feb Sun 18, 2018 9:52 pm 
Member

Joined: Dec Wed 02, 2015 9:42 pm
Posts: 151
Location: Santa Clara, CA 95051
That always makes things fun.
:)
How badly is that nut stuck? If it's just a bit and a small amount of gentle pressure will hold the bold, I have slipped something as mundane as a putty knife in the slot and pressed hard against the bolt while gently turning the nut. That may or may not work because on closer look it seems as though they have peened that nut and bolt to help hold it tight.
If that fails I think I would use the drummel. Given what you have I agree why not try? I would use a big piece of something like tag board, something that is thin but has some strength. Cut a hole as close to the diameter as the bolt/threaded shaft as possible then slit the piece on one side enough that you can slide it between the nut and clamp. Then get a roll of masking tape and start sealing things up. Use the drummel to cut the nut off trying hard to maintain the threads on the bolt. Use a magnet to occasionally gather up the shavings.

Good luck.


Top
 Profile  
 
 Post subject: Re: Field Coil or PM?
PostPosted: Feb Mon 19, 2018 1:02 am 
New Member

Joined: Jan Sat 13, 2018 7:28 pm
Posts: 13
Location: Syracuse, NY 13078
I covered everything up with news paper and made just a hole for the nut to stick through as I worked on it. I cut a slot across it with my dremel deep enough to cut a groove in the screw and was able to then hold the screw with a screw driver while turning the nut with a crescent wrench. Worked like a charm. I was unable to disassemble the coil any further than before. Somehow the entire cone and all would have to come out the front and I just don't see how I can do that. The big U-bracket surrounding the coil is welded on and inside the speaker frame, I just don't see how to get things out. Enclosed are some pics. When I was convinced I can go no further, I put a new nut back on, it's just a standard 1/4-20.


Attachments:
IMG_3044.jpg
IMG_3044.jpg [ 58.6 KiB | Viewed 795 times ]
IMG_3045.jpg
IMG_3045.jpg [ 64.15 KiB | Viewed 795 times ]
IMG_3046.jpg
IMG_3046.jpg [ 66.01 KiB | Viewed 795 times ]
IMG_3047.jpg
IMG_3047.jpg [ 87.42 KiB | Viewed 795 times ]
IMG_3048.jpg
IMG_3048.jpg [ 77.28 KiB | Viewed 795 times ]
Top
 Profile  
 
 Post subject: Re: Field Coil or PM?
PostPosted: Feb Mon 19, 2018 1:06 am 
New Member

Joined: Jan Sat 13, 2018 7:28 pm
Posts: 13
Location: Syracuse, NY 13078
And on another note, I was measuring the voltage across the field coil and it was about 71 Volts. No burning or smelling. Then after a minute or so, the voltage will jump up to over 100 volts and the burning and smelling starts. I'm wondering if the output 6L6 is breaking down. I've replaced all the electrolytic and paper waxed caps in the radio.
Ken


Top
 Profile  
 
 Post subject: Re: Field Coil or PM?
PostPosted: Feb Mon 19, 2018 2:10 am 
Member

Joined: Dec Wed 02, 2015 9:42 pm
Posts: 151
Location: Santa Clara, CA 95051
They sure made a lier out of me didn't they?
I think you need a new speaker.
There is no question that if you get way less than 2K ohms DC resistance across that field coil it is shorted someplace. Windings melt through and into adjacent turns. They can move around during load and the conditions of the short might vary, more or less. But there is no question with those kinds of test results you need a new speaker.
If you could cut through the spot welds maybe there is still a possibility of getting that one off. But I have my doubts. It also tells us the nut you were trying to remove holds the field coil into that bracket which was then spotted in place. The problem with cutting, if that's possible, is the frame has to go back when your done and the question comes how will you hook it back down? It has to be centered to work properly.
You could swap a tube but without proper B+ it wouldn't tell you much and proper B+ can't happen without a good field coil.
I don't think the output tube is your issue.
You have a shorted field coil and I back off saying it can be repaired.
I couldn't do it.

Tomie

As an added thought, powering up that radio with a shorted field coil can put a heavy load on the mains transformer. I would be very careful doing that as you might soon need that transformer too.


Top
 Profile  
 
Post New Topic Post Reply  [ 26 posts ]  Moderator: sofaslug Go to page 1, 2  Next

All times are UTC [ DST ]


Who is online

Users browsing this forum: Tarpawns and 22 guests



Search for:
Jump to:  
























Privacy Policy :: Powered by phpBB