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


It is currently Jul Fri 25, 2014 9:58 am


All times are UTC [ DST ]



Post New Topic Post Reply  [ 19 posts ] 
Author Message
 Post subject: Fairbanks Morse 8A Questions
PostPosted: Mar Sun 15, 2009 4:23 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 234
Location: Chesapeake, VA USA
I recently picked up a FM 8A table set locally in very good and complete condition for $80 !! I'll post some pics later. I took the chassis out and in the corner was the original FB manual and tag, though a little vermin had chewed a third of it up as a pile of confetti was in the corner. The chassis is in in good shape, but I'm going to re-paint the copper painted chassis as it's got some surface rust up top. One thing nice about this deep chassis is that you can take out five screws from each side plate and remove the sides for easier access for recapping and in my case re-painting.
Here's a couple of "novice" questions to start:
1. The 16mfd cap was in a full length cardboard tube, with the little grounding strap between in the bottom. The top of the carboard tube was turned to partial confetti for the vermin bedding, so I removed the tube completely. Now, does that cardboard tube serve to totally isolate this cap from ground? Or does that sliver of grounding strap ground the cap? Remember, I'm a novice...
2. The mounting feet on this serve also as attachment holes for the mounting screws. The feet are round metal with deteriorated rubber doughnuts inbetween. Through the middle is threaded tube for the mounting screws. I looked to see if there is a way to unscrew the top or bottom round metal pieces, but they look like they maybe a tight press fit. It would help to get these off in order to re-paint.
Thanks in advance for your answers and I'll try to post the pics as soon as my wife gets back with the camera....
Britt
wbaradio


Top
 Profile  
 
 Post subject:
PostPosted: Mar Sun 15, 2009 4:58 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 4305
Location: St. Louis, MO, USA
Electrolytic capacitors have a negative and a positive connection. Usually the outside "can" or shell is the negative connection and is many times connected to chassis ground. But other times, such as in the case of the 16mfd capacitor in this radio, the negative does not connect to chassis ground, so it is necessary to isolate the "can" or shell from the chassis. This is probably what the rubber grommets or "donuts" are for. A cardbord sleeve is then put over the "can" to prevent shock should anyone touch it. It sounds like what you are calling a "grounding strap" may be the negative connection, but it should not connect to the chassis. Yes, pictures would definitely help!

But you are going to want to change this capacitor. Most guys just leave it as it is, for appearance sake, and wire in a replacement under the chassis. The new ones are very much smaller. It will be necessary to remove the wires from the old capacitor and connect them to the new one. It will be very important to pay attention to the polarity to make sure the positive and negative of the new capacitor are connected to the right places.

_________________
Terry Davidson


Top
 Profile  
 
 Post subject:
PostPosted: Mar Sun 15, 2009 6:49 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 234
Location: Chesapeake, VA USA
Terry, I've done quite a bit of re-capping on radios, but sometimes these electolytics confuse me. So, if the negative/ground side of the 16mfd is isolated from the chassis ground, then when I put in a new 16 mfd cap, where do I put the negative lead? Do I put it to that grounding strap that doesn't touch the chassis?
Thanks,
Britt


Top
 Profile  
 
 Post subject:
PostPosted: Mar Sun 15, 2009 6:54 pm 
Member

Joined: Jul Wed 26, 2006 6:25 pm
Posts: 869
Location: Coram, New York 11727
wbaradio,hi. Did the cap have two leads coming out of one of the ends? The band in the middle may be only a mounting strap. But some of those caps had it grounded. If the set has neg cap lead to B- ,not chassis, that should be a mounting strap only. If it's the neg lead of cap, then someone made error in replacing it or some sort of mounting arrangement was made to mount it without grounding to chassis which seems a complicated way to do it. PL


Top
 Profile  
 
 Post subject:
PostPosted: Mar Sun 15, 2009 8:06 pm 
Moderator

Joined: Jan Thu 01, 1970 1:00 am
Posts: 30951
Location: Livermore, CA
Hi

Here is the schematic:

http://www.nostalgiaair.org/PagesByMode ... 005757.pdf

One electrolytic has negative connected to center tap of high voltage winding on the power transformer.

_________________
Norm


Top
 Profile  
 
 Post subject:
PostPosted: Mar Sun 15, 2009 9:03 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 234
Location: Chesapeake, VA USA
Guys, the 16 mfd has a single + post. It had a cardboard full length sleeve over it. The cardboard/cap was secured with a screw/nut band clamp attached to the chassis. It appears to me that this arrangement isolated the cap metal can from being grounded to the chassis. There is a small metal strap that snugs up against the can, but still appears to be isolated from the chassis ground. A wire is connected to the strap which goes to speaker pin and looks to have connecting wire to the transformer (center tap?). So, does this can need to have a new cardboard tube re-inserted to keep it from being grounded to chassis? I would post a pic, but the Gallery button gives an error message.
Britt
wbaradio


Top
 Profile  
 
 Post subject:
PostPosted: Mar Sun 15, 2009 10:18 pm 
Silent Key

Joined: Jan Tue 16, 2007 11:48 pm
Posts: 9664
Location: Hueytown, AL
The strap attaching the cap to the chassis is insulated from the cap. The other strap that is attached to the can is the negative terminal of the cap and correctly is connected to the speaker field and the xfmr center tap. It must be insulated from the chassis as it is normally 60 to 100 volts negative with respect to the chassis so use your cardboard sleeve. The voltage is just an estimate. Varies according to radio make and model.


Top
 Profile  
 
 Post subject:
PostPosted: Mar Mon 16, 2009 2:11 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 4305
Location: St. Louis, MO, USA
There's ususally more than one way to do these things, but what I would do, if I understand the description right, is:

[1] Remove the wire that goes from the non-grounded strap to the speaker field and the transformer center tap.

[2] Then connect the negative lead of your replacement capacitor to the speaker field and transformer center tap. There should also be a resistor, #59 on the schematic, connected there.

[3] Then remove the wire(s) from the positive post of the old capacitor and connect it to the positive wire of the new cap.

Now you have the old cap completely disconnected and you don't have to worry about whether it's grounded or not.

_________________
Terry Davidson


Top
 Profile  
 
 Post subject:
PostPosted: Mar Mon 16, 2009 6:50 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 13246
Location: Tennessee,USA
Hi Britt,

I think you should try a little bit to follow your wiring by looking at the diagram, then look at your chassis to repair. That way you can see exactly how the filter cap is connected and to where.


I have an 8A but in a console. I recapped it about 5 yrs agi and I can't get to it right now to see how I did the filter cap. But the guys hers hould have some good advice.

As said before, some cap's negative goes to the chassis. Some goes to a "floating ground". That is a B- or negative side of the high voltage source. It has it's "own" ground, which is isolated from the chassis.


When I did my 8A I restuffed all the paper caps, as they were the originals and had the Fairbanks Morse markings on them. The inderside doesn't look touched at all.

If you don't restuff the caps, at least save the originals in a plastic bag.
I have the original hang tag also in good shape. It might be the same as yours. If you have another tag, I might be interested in a scan if possible. I will see what I have.

Take care,

_________________
Gary Rabbitt


Top
 Profile  
 
 Post subject:
PostPosted: Mar Tue 17, 2009 3:06 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 234
Location: Chesapeake, VA USA
Guys, thanks for the insight on the 16mfd cap. I'll probably just connect the neg of the new 16mfd to that strap after I find a suitable replacement cardboard tube to isolate the strap from chassis ground. If I don't do that, I'll just connect the neg to the trans center tap wire that is connected to the strap per Old Bear!

Gary, thanks for clearing up the difference between chassis ground and "floating ground." My "novice" electronic mind always thought that electrolytics were always grounded to chassis ground. Looking at the Rider schematic, I don't see where the "neg' side of the cap is located. Also, I have already re-stuffed the paper caps. All were original caps as best I can tell. Most were marked Fairbanks Morse, but others were Sprague. Also, the paper tag is still on the original power cord and there is another hanging FB tag along with a half eaten manual. Gary, I might get with you later on the "dial string" set-up on this set. Did you use a rubber O ring for the dial, or some type of dial string?
Thanks again to ALL the posters
Britt


Top
 Profile  
 
 Post subject:
PostPosted: Mar Tue 17, 2009 5:34 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 13246
Location: Tennessee,USA
Hi Britt,

Here is the section of the diagram showing the filters.
Image

The 16mf cap negative does not touch the chassis, as others have said it's insulated from chassis ground.
This is not an example of a true 'floating ground' as only one of the filters neg is connected here.


The true folating ground will run the whole circuitm and anything that is "grounded" will be connected to a common "ground bus" that is present in the wiring. You might not see a straight wire that things are connect to . It's just a set of points that are a common connection. This is also known as the B- , the negative side of the high voltage source.



The 18mf cap negative does touch the chassis as far as I can see.

On a diagram with a floating ground, the standard ground symbol will be shown. On those diagrams, the Chassis ground symbol is shown as a rake looking symbol.

Image

With the 8A the standard ground symbol is showing chassis ground.

The cardboard tube you removed, might not be the insulator, generally when a chassis mounted can is mounted, and the neg is isolated, there will be a fiber washer between the cap and the chassis. A washer/terminal lug is provided for the negative lead (can outside) to go where it should go. In this case to one side of the field coil, and also the center tap of the transformer.

If the cap can negative is ok to touch the chassis, then the metal part of the can will touch the chassis, no fiber insulator. The Pos. side will be just one wire coming from the can. I can't get the chassis pulled to have a good look at it right now.

I have the brown and orange tag with my set, I will check and see if I have a different tag. If I do, I can scan and email it to you to make a repro of it. I think there may have been a green one.

Saw your photos in the gallery, that's a really nice looking set.

Well hope this helps, and good to hear that you restuffed the caps. One thing you might want to do, is make a small notation inside the chassis, saying those caps were replaced and the date. That way if there is anew owner, they will not cut the new caps out thinking they are still old :shock:

Take care,

_________________
Gary Rabbitt


Top
 Profile  
 
 Post subject:
PostPosted: Mar Wed 18, 2009 12:48 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 234
Location: Chesapeake, VA USA
Gary, thanks for the schematic analysis. Being the novice, the - neg / +pos you penned in is what I'm looking for in schematics when replacing electrolytic filter caps. The Rider 8A schematic is well numbered, but there's no key in the Rider for number part descriptions. I wonder if there's one in Sam Photofact?
I would have posted the pics in this posting, but got an error message when trying. Yes, it is a nice set, I've been looking for this one for quite some time as I like the tuning eye table sets and consoles. The local guy I got this set from has what he says is the same set in a console sitting in his storage unit in WV. Hopefully, it's the 9 or 12 tube set ...
I have two FB tags, the orange and brown 8" x 3.5" there abouts with FB 1938 Radio Model 8AT-8 listing all the features on the back with PRICE at the bottom. The other FB tag is an inspection tag wired to the orginal power cord which lists serial number of the radio. I'll scan them when I get the chance.
Britt


Top
 Profile  
 
 Post subject:
PostPosted: Mar Fri 20, 2009 5:57 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 13246
Location: Tennessee,USA
Hi Britt,
The console version is the same as the table model. I looked at my console today and it had 7 tubes plus the eye tube.


The main difference is the console has a large speaker with special surround on it. I had some pics in the gallery but I think they were deleted. Although the output is not 'push-pull' the sound is very good.
ALso, I was able to pick up lots of shortwave stations with a short antenna, maybe 5 ft. was all.


I did not find a Sam's folder listing for it, and you're right, there is not much in the way of a parts list for the 8A. I relied on the original parts for the specs, I can't remember if the filters were 450v on one and 475v on the other filter?

I will get back in a few minutes after I scan the tags I have. Yours may be in better shape, I"d like to have a scan of those whenever you get the time. Any other literature will be nice too.
Take care,
Gary.

EDITED>>>
Here are the scans of the two tags I have. I tried to clean up the first one a bit, removing creases, but I did not finish it. These are in the F-M gallery in case someone wants to make a repro.

As you can see the 8A C2 has different features.

Image
Image
Image
Image

_________________
Gary Rabbitt


Top
 Profile  
 
 Post subject:
PostPosted: Mar Fri 20, 2009 7:17 am 
Member

Joined: Jul Wed 26, 2006 6:25 pm
Posts: 869
Location: Coram, New York 11727
Britt, if that is a can cap on top of chassis, some do not mount to the chassis at all. The clamp or strap mounts to the chassis and is clamped tight to the can which has been insulated by the cardboard tube over it. Many had a ground terminal which was nothing more than a slim piece of metal that was slipped between the can and cardboard sleeve and became the neg lead. It was not soldered to the can (can is aluminum) just the pressure between cardboard tube and can made contact. The mounting band, of course, helped with the pressure. I have made new cardboard tubes from toilet paper tubes. Slit down one side and remove enough of side of tube to get it to fit snugly around can, soak tube in water to soften, place around can and hold with rubber bands till dry. You can also roll over one end of tube so when slipped on it will stop when seated agains bottom of can. After formed in snug fit, laquered same, dry, place on can. I placed electrical tape on inside of mounting clamp and trimmed that so when tightened onto can (over cardboard tube) you can't tell its there. PL


Top
 Profile  
 
 Post subject:
PostPosted: Mar Sat 21, 2009 2:12 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 234
Location: Chesapeake, VA USA
PL, Thanks for the info, I to had thought about the toilet roll , and your system appears to be well thought out to look original...! I've got another parts spare FB 8A chassis coming my way. If the cap is there with the original carboard tube I'll probably use it, otherwise, it's the toilet roll!!
Britt


Top
 Profile  
 
 Post subject:
PostPosted: Mar Sat 21, 2009 2:19 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 234
Location: Chesapeake, VA USA
Gary, those are the same exact tags that I have. The only difference on the long tag is mine is listed as Model 8A5-8. Your tags are in a little bit better condition than mine. I'll see if I can scan them. Also, I'll probably try to remove the green sticker tag from the chassis with acetone before painting the chassis. I'd like to scan that tag and reproduce it, even though some of the edges re missing. Like I said, the original FB manual was in with the tag, but the mouse ate out about 1/5 of the manual.
Britt

[/img]


Top
 Profile  
 
 Post subject:
PostPosted: Mar Sat 21, 2009 2:42 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 13246
Location: Tennessee,USA
Hi Britt,

Too bad the manual was eaten.
As far as the painting the chassis and the label, if I had to repait, I would make every effort to not remove the label. It is pretty much glued on. I know acetone can remove some labels such as on a bakelite cabinet, but it takes a few hours and you have to keep the cloth over the label wet with acetone.

I doubt that water will dissolve the glue inderneath, and if and when you get the label of, it will be highly fragile.

As the color you choose will probably match the original color you can mask off the label, the only thing you may see it 1/8" of original color around the label.

I mask off a label by putting the masking tape along the edge of the masking paper. Use a good quality 3M masking tape. If you don't , the paint will undercut and bleed onto your label. Quality tape will prevent this.
I leave about 1/8" of stickiness showing. Then I make sure the tape touches only the chassis itself and not the label.

So basically, you have 1/8" of tape touching the chassis, and masking paper touchng your label.


You should be able to find a good metallic copper color to match. Try the Duplicolor line from an auto parts store.

Clean up the chassis as best you can, removing dust and dirt. You may want to clean off your IF cans, tuning cap, or whatever before painting. That way, you will not contaninate your new paint.

BEFORE you do any masking or anything to the label. take some high resolution sharp photos of the label just in case you do make a mistake. You will have a backup. I do that with any label I find.

The better and sharper of photo you take, the better a repro will be, just in case.

I guess as a backup to a backup, I have a good photo of my label. :)
Keep us informed.

_________________
Gary Rabbitt


Top
 Profile  
 
 Post subject:
PostPosted: Mar Sun 29, 2009 5:13 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 234
Location: Chesapeake, VA USA
Finally figured out was wrong with adding pics to my posts. Here's the back label... certainly like to get it reproduced. Also a couple of photos that I couldn't post earlier...maybe
Britt

Image

Image

Image[/img]


Top
 Profile  
 
 Post subject:
PostPosted: Jan Wed 06, 2010 10:01 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 4893
Location: Wilmington, NC 28412 USA
Very excited.

Just found me an 8A console in very nice shape. Got it for $135
Going to pick it up this weekend.

Found a stash of radios , wetting my pants waiting to get there.

Update got my Radio See Thread :

http://www.antiqueradios.com/forums/vie ... 61#1027161



Bob T.


Top
 Profile  
 
Post New Topic Post Reply  [ 19 posts ]  Moderators: Marcc, Norm Leal

All times are UTC [ DST ]


Who is online

Users browsing this forum: No registered users and 6 guests



Search for:
Jump to:  



















Privacy Policy :: Powered by phpBB