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Vintage tube radios - anyone know anything?
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The Robman
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PostPosted: Wed Feb 03, 2010 9:38 pm    Post subject: Reply with quote

Got a new set of tubes, and a new dial lamp bulb, in the mail today. Plugged them in and got this old radio working again.

But... one of the 4 wires from the antenna coil is loose and it's not obvious where it should go (even though the radio works without it being connected).

It's the 2nd wire from the top (of the 4 on the coil). It looked like it should go to the tuning device as there is an empty spot there, but there wasn't any solder on that spot and when I tried connecting it there, the radio didn't work, so that's not where it goes.

I'll post some pics in a bit.
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mr_d_p_gumby
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PostPosted: Wed Feb 03, 2010 10:07 pm    Post subject: Reply with quote

The Robman wrote:
Got a new set of tubes, and a new dial lamp bulb, in the mail today. Plugged them in and got this old radio working again.
Congratulations!
The Robman wrote:
But... one of the 4 wires from the antenna coil is loose and it's not obvious where it should go (even though the radio works without it being connected).

It's the 2nd wire from the top (of the 4 on the coil). It looked like it should go to the tuning device as there is an empty spot there, but there wasn't any solder on that spot and when I tried connecting it there, the radio didn't work, so that's not where it goes.
According to the schematic diagram, that wire is for connecting an external antenna. You shouldn't need to have it connected unless you are looking to pull in really distant stations.
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The Robman
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PostPosted: Wed Feb 03, 2010 10:51 pm    Post subject: Reply with quote

I was just about to post that I've followed it through on the schematic and it's the wire that ends at the triangle in the upper left corner (not that I know what the triangle is for), so I guess I can just leave it hanging then.

According to the schematic (if I'm reading it right), the tuning device is really "C2 RF" and "C4 OSC". It looks like C4 should be connected across C3. The is a wire from one side going to the 5-prong "osc. coil" but the other side has nothing connected. It doesn't look like there ever was something connected there, so I'm guessing that's OK, but I don't understand why.

Anyway, I want to thank you guys for your help, the connectivity tests confirmed that new tubes were needed, and of course the advice about getting a new light bulb.

It's very cool that you can pick up a new set of tubes so easily on ebay ($17 shipped for the set of 5). I had to special order the bulb also, that was $6.70 shipped.

You know what's a pity though, the only stations still on AM seem to be talk stations. I would have been nice if there was still an oldies station there. I've found one music channel, but it's a weak signal.
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RogerClark



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PostPosted: Wed Mar 24, 2010 11:59 pm    Post subject: Reply with quote

RobMan, I know I'm coming really late to this discussion...

But you could try attaching whats know as a "Long wire" antenna to the connector shown on the schematic as a "triangle"
As this may help you pick up the weak signal which is broadcasting music.

Any wire will do as long as its insulated.
The longer the better
Outdoors is better than indoors
As high off the ground as possible
Preferably in a straight line, i.e don't wind it around and around things, though if you want you could run the line to the end of your yard etc, then go at 90 degrees along the fence etc.

However what you may find happens, is that the radio gets overloaded and even the talk stations have interference on them, so you may just want to try 20 ft of wire to start with.


One other thought I had, is that is probably legal to broadcast a very low power signal your self with some music on it i.e. from an iPod or similar.

I'm not sure if there are any commercially available ultra low power transmitters, or possibly some kits.
I know you used to be able to buy a "spy / bug" transmitter, but that uses VHF and your radio will be medium wave or long wave etc.
Or you may be able to find a kit transmitter that you could build, but you'd need a soldering iron etc.

Ask around to see if there are any local "Radio Hams" in your area, they may be able to help you (though they're harder to find nowadays)
And yes... I'm a Radio Ham, but not an active one Sad
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The Robman
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PostPosted: Thu Mar 25, 2010 10:10 am    Post subject: Reply with quote

It's funny that you should revive this thread because I do have more work to do on this old radio. I haven't turned it on in a while because it looked like the dial light was out, and I know that can lead to other problems, so I'd been meaning to open it up to see what's going on. Well, I did that recently and had some difficulty just getting the chassis out because one of the capacitors had spewed it's guts out and the output had solidified and stuck to the casing.

So, not only do I need a new dial lamp, but I need to replace at least one capacitor. I'll probably replace all of them while I'm at it as I have to imagine the others are trashed too, they are 63 years old after all.

I don't think I have the "triangle", so would something like a wire coat hanger do?

Funnily enough, I had thought about making a little AM transmitter as a way to play CDs, etc through the radio. I know people use FM transmitters to play their iPods through their car stereos.
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RogerClark



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PostPosted: Thu Mar 25, 2010 4:39 pm    Post subject: Reply with quote

Hi Rob,

OK about the capacitor leaking.

Again, funny you should mention that... I was going to say in my posting that the capacitors often leak. The ones in your photo's did look they were likely to burst.

The schematic has a parts list, which lists the value (Capacity) of each capacitor and also its operating voltage.

Note. The capacitors are not electrolitic (aka polarized) - so when you get some replacements make sure they are not electrolitic (polarized).

It know it will be a bit of a pain, but all the capacitors really need to be replaced as there is a high likelihood that the others will leak as well Sad


Re: Antenna

The Triangle is just a symbol used to represent an external antenna. The external antenna should preferably not actually be a triangle shape.
Just get a reel of insulated wire, strip the insulation off one end, and attach the copper conductor to the screw on the back of the radio.
Then unwind the reel, and take it out through a window and into the yard for 20 ft or so. You should notice an immediate improvement in signal strength on that weak signal i.e. the one playing music.


Re: AM Transmitter

I think it would probably be worth your while posting to a dedicated forum for vintage radios, as you can't be the only person who wants to broadcast very low power AM to receive on your vintage radio.
I took a quick look on the web, and there were some lovely transmitters out there, including a valve (tube) one, but the costs were high.
Actually I would expect a valve (tube) one to cost $$, but I don't see any point in paying 100's of $ for Transistor AM transmitter for this purpose.

I'd just build one from parts in my junk box - but electronics is a bit of a hobby for me so that wouldn't be too hard Wink

BTW. All of those iPod / car transmitters are no use at all. They are FM on about 100Mhz and you need AM on about 1Mhz

Anyway. Good luck with getting the radio going again.
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The Robman
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PostPosted: Thu Mar 25, 2010 5:49 pm    Post subject: Reply with quote

RogerClark wrote:
BTW. All of those iPod / car transmitters are no use at all. They are FM on about 100Mhz and you need AM on about 1Mhz

Of course, I know that, I was just using it as an example of people using a low power transmitter for a similar purpose. If the tube radio could receive FM I'm sure I could find a station with music and I wouldn't need the transmitter.

I did do some searching a while back and did find a page that details how to build a simple AM transmitter here:
http://sci-toys.com/scitoys/scitoys/radio/am_transmitter.html
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RogerClark



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PostPosted: Thu Mar 25, 2010 8:18 pm    Post subject: Reply with quote

Hi Rob,
I've seen that circuit, but I've not personally made one. But the basic principals seem accurate.

Using a crystal oscillator would make it very stable, i.e. not change frequency as it warmed up etc. But it only works on one frequency (channel)

You'd need to get a crystal oscillator that was on a frequency that is not in use by one of those talk stations you can already pick up.

You'd probably need to tune the AM Band with modern radio with a digital readout e.g. in a car - and find a frequency that was definitely free before you splashed out on a crystal oscillator.

I'm not sure what the quality of the AM would be using the crystal oscillator unit, as crystal oscillators are generally designed to give a fixed output power and you'd normally "Modulate" the AM signal from the fixed output of the oscillator using an additional "stage" i.e. a transistor and a few ancillary components.
But I guess it would give some some sound. Wink
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The Robman
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PostPosted: Sat Apr 03, 2010 1:27 pm    Post subject: Reply with quote

I found this great site that helped explain what sort of capacitors I would need (apparently, the old ones were called "paper" capacitors and the new ones are called "film" capacitors).

http://www.justradios.com/captips.html

I ordered the replacement capacitors and dial lamp, and they arrived today, so I'm going to try the repair today also.

Plus, as luck would have it, our coffee maker died, so I now have a replacement power cord. Could someone confirm for me the correct way to connect the power cord? Instinct would say that the hot wire should go to the volume/on/off pod, but looking at the schematic, it appears that pod is directly wired to the ground, so that makes me thing that the cold wire should go to the volume pod, is that correct?
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Last edited by The Robman on Wed Apr 07, 2010 10:26 am; edited 2 times in total
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3FG
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PostPosted: Sat Apr 03, 2010 2:20 pm    Post subject: Reply with quote

Rob,
There are two kinds of ground symbols in the schematic, and you can see them defined at the left edge of the schematic. The volume pod is not connected directly to chassis ground, but instead to the isolated ground. Look just to the right of the leftmost tube (in the schematic) to see that chassis ground is connected to the isolated ground by a 220K ohm resistor, and a .1uF cap.

So it is designed to work with either polarity. When the on-off switch is open, once the temperature of the filament and thus the cathode drops, current won't be conducted by the rectifier. So with the switch open, and the rectifier effectively open, current won't go anywhere.

However, I believe that the radio is designed to have chassis ground close in voltage to the isolated ground, and the hot AC lead should go to the plate of the rectifier tube, and neutral should go the on-off switch (volume pod). I also think that is the safest way to wire it, if someone were to touch the volume knob with really wet hands.
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The Robman
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PostPosted: Sat Apr 03, 2010 7:38 pm    Post subject: Reply with quote

Thanks Dave. The power cable has been replaced, as have most of the capacitors and the dial lamp and the radio is up and running again. I say "most" of the capacitors because digi-key sent the wrong part for one of them, so I had to leave one of the old paper capacitors in there (but I will replace it as soon as I can get a new part).
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The Robman
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PostPosted: Fri Apr 09, 2010 5:41 pm    Post subject: Reply with quote

Here's some before and after shots of the circuitry with the old capacitors and then with the new ones (and the new power cord).



and after...


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zaphod7501



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PostPosted: Fri Apr 09, 2010 6:06 pm    Post subject: Reply with quote

Sometimes people will hollow out the old caps and put the new ones inside of the shells so it still looks original. Very Happy
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The Robman
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PostPosted: Fri Apr 09, 2010 11:57 pm    Post subject: Reply with quote

Now why didn't you suggest that before I started, and threw out all the old caps! Smile

I thought about the fact that replacing the caps would mean that the radio is no longer "all original parts" but then again, isn't changing the caps kinda like changing the oil on a car, or maybe the tires? Caps are not designed to last forever.
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KISS



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PostPosted: Sat Sep 11, 2010 10:36 pm    Post subject: Reply with quote

I'm late to this discussion as well. I used to repair these things when I was a kid. I still have a couple of tube testers. One is military. I put a tube Blaupunkt radio in my second car and designed a solid state vibrator replacement for it. The Blaupunkt was 3 band and the dial showed wavelength rather than frequency, so everything was backwards. Still have the radio somewhere.

The thing you forgot to do, was put sleaveing on the long wires of the new caps to prevent shorts. I hope you paid attention to the voltage ratings.

You forgot the big black one. That's the one the when it leaks, creates hum. It probably needs to be replaced with two.

Here's a funny story. I worked on a McIntosh tube amt that someone wire the output of one amp into the input of the other to get more power. He definitely let out the magic smoke.

I have a few radios that could use some restoration. How about a portable that used a 45, 22.5 battery and I think a 1.5V battery? One company makes a battery eliminator for these batteries.

Thanks for saving it.
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