Vintage tube radios - anyone know anything?

[The Robman]

My wife picked up a vintage 1947 RCA Radiola 61-8 radio at a thrift store, just because it looks cool. It looks like this one (only in better condition):



The radio doesn't power on, so my guess is that one of the tubes is bad, but I have no idea how you would go about figuring out which one, and then of course, good luck finding a replacement, but I bet there are places out there that have them (like Radio Daze).

So, does anybody know anything about old tube radios?

[zaphod7501]

Do the tubes light up?
Does it hum or produce noise?

Things to check for:
Power switch not making contact.
Open filament. (an open filament in one tube could keep them all dark - like Xmas tree lights)
Open windings on speaker transformer. (I have an old Crosley and the speaker transformer doubles as a power transformer)

There is usually a tube chart inside and sometimes a filament wiring chart.
If you can't locate locate pin-outs for the tubes, I can copy pages from some old books for you.

The cloth and hard rubber insulated power cords can disintegrate so be careful there. A new power cord may be at the top of your repair list.

Since "audiophiles" have rediscovered tube amps, there are still sources of some common tubes. Most are made in Russia, I believe.

[vickyg2003]

my experience with tube type radios is to hit them until they start working.

[ElizabethD]

Proposed solutions to Rob besides Vicky's wise advice:
1. Township trash dump
2. http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... 0200008641

[mr_d_p_gumby]

Here's a link to a schematic:
http://www.nostalgiaair.org/PagesByMode ... 015046.pdf

The tube filaments are in series, so you should pull each out and check them with an ohmmeter for continuity.

I repaired an old "5-tube wonder" radio for my father-in-law a couple of years back, and was able to get a complete set of tubes on ebay.

BTW, heed zaphond's warning about the power cord, and remember that the chassis is connected to one side of the AC power line.

[The Robman]

Do the tubes light up?
Does it hum or produce noise?

Nope, it does nothing. The power cord itself looks OK but the plug on the end is disintegrating so a replacement is in order.

It sure is funny looking inside at what electronics looked like before PCBs were invented. It looks like what I imagine some of our homemade JP1 interfaces look like!

What did tubes do anyway? What is the equivalent item in modern electronics, is it the transistor?

What continuity check should I do with the tubes? Should there be continuity between the first pin and the last pin?

[mr_d_p_gumby]

What did tubes do anyway? What is the equivalent item in modern electronics, is it the transistor?

Close enough. Unlike transistors, they need to light up their filaments in order to work.

What continuity check should I do with the tubes? Should there be continuity between the first pin and the last pin?

If you look on the "Volume 18" schematic in PDF linked above, in the lower right corner near the V5 35Z5-GT tube, you'll see a wiring diagram for the filaments that shows the pin numbers. V3 is pins 7 & 8; V1 is pins 7 & 8, etc. V5 is tricky as there are three pins: 2, 3 & 7. The dial lamp is wired across pins 2 & 3 of V5. (If the dial lamp is burned out, it should also be replaced.)

Checking for continuity is a quick way to tell which one of the filaments is open. Like the Xmas lights, it only takes one to burn out and none will light up.

[zaphod7501]

You should probably think of tubes as functional blocks.
Amplifier, Oscillator, Detector, Rectifier, etc.
Probably closer to integrated circuits than transistors.
Radios generally used simple tube types while later TVs used a lot of multifunction tubes to reduce space requirements.

If you remove the chassis, you can probably trace wires once you identify the filaments on one tube. That way you can check filaments without removing tubes. (sockets can fall apart)

[The Robman]

If it is the tubes, and not something else, I found the following ebay listings where I can get the complete set for $17 shipped:
http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... 0419032233
http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... 0358528223

[The Robman]

The dial lamp is burned out, so I've ordered a replacement. I have confirmed that there is voltage going to the bulb, but here's what puzzles me, the bulb (a "GE 47") is rated 6.3v but I'm measuring about 110v in the bulb socket. The schematic also shows the bulb being directly connected to the mains, so does it make sense that the bulb is only 6.3v? Could somebody have used the wrong bulb somewhere along the line?

I'm still not clear on how to test the tubes. They are easy to remove, without the sockets falling apart, so what should I test for? I didn't follow Mike's description of what connectivity I should be testing for.

Also, it looks like there's only 1 wire going to the speaker, which doesn't make sense to me. However, there's no sign of a wire having ever been connected to the 2nd terminal on the speaker, nor is there a loose wire floating around.

Here's a pic where you can see the 1 wire going to the speaker.



And just for the fun of it, here's a pic showing what all the wonderful circuitry inside looks like.

[3FG]

Rob,
The tubes heaters (fiaments) are all wired in series. The first digits of the tube name indicates the voltage drop of the filament. You have 3 12 volt tubes, a 35 volt tube, and a 50 volt tube. The 12 volt tubes really run on 12.6 volts by design. This all adds up to 122 volts. The 35Z5 has a tap on the filament which is intended to to provide about 6 or 7 volts to the pilot lamp. If any one of the tubes has a open in its filament, then none of the tubes will light up.

You can test each tube with an ohmmeter. A tube tester (available at any drug store--whoops, that was a while ago!) would provide filament power and external voltage and test the various amplifier tube to see if the control grid modulated the current provided by the tube, and if the gain was sufficient. I think that you are probably limited to showing that no electrode is shorted to others-- other than the filament resistance.

The speaker is driven by a transformer that is physically underneath the speaker coil. Perhaps the second connection is hidden there.

By the way, your comments lead me to suspect that you haven't understood the full import of Mike's comment that the chassis is connected to one side of the power line. This means that if the plug or the wall socket is incorrectly wired, then the chassis will be sitting at 120 volts with respect to house ground. It can kill you if you're working near a water pipe, for example.



Anyway,

[Kevin Timmerman]

The bulb is part of the series connection of tube heaters. If you measured 120V with the tubes installed, then V5 probably has an open heater.

Measure the resistance between the "H" (heater) terminals of each tube. It should be a low resistance. Do this with it unplugged!

The speaker is grounded thru it's basket (chassis).

[mr_d_p_gumby]

I'm still not clear on how to test the tubes. They are easy to remove, without the sockets falling apart, so what should I test for? I didn't follow Mike's description of what connectivity I should be testing for.

The tube filaments are just like light bulb filaments. If they are good, they will measure a fairly low resistance with an ohmmeter. Take each tube out, locate the filament pins, and measure the resistance between those pins. Practically anything other than an open circuit would tend to indicate the filament is intact. From the schematic, most of the tubes have the filament on pins 2 & 7, except for the 35Z5 (pins 2, 3 & 7) and the 12SQ7 (pins 7 & 8). For the 35Z5, measure between pins 2 & 3 first, then between pins 3 & 7.

The bulb is part of the series connection of tube heaters. If you measured 120V with the tubes installed, then V5 probably has an open heater.

I agree. That's why I said it was important to replace the lamp too. When the lamp is burned out, the tapped section of the 35Z5 filament (pins 2 & 3) has a higher voltage on it, making it the most likely to fail next.

Just make sure to do all this with the radio unplugged! If you look at the schematic diagram, the power switch is wired from one end of the AC plug to ground. Ground is connected to the chassis with a high value resistor and a very old (possibly now leaky or defective) capacitor. Even with the switch turned off, you could get a nasty surprise if you leave it plugged in. And radios of this vintage do not have polarized AC plugs, so you have no idea which end is "hot" and which end is ground.

[The Robman]

By the way, your comments lead me to suspect that you haven't understood the full import of Mike's comment that the chassis is connected to one side of the power line. This means that if the plug or the wall socket is incorrectly wired, then the chassis will be sitting at 120 volts with respect to house ground. It can kill you if you're working near a water pipe, for example.

I don't know much about electronics, as you can tell, but I did understand Mike's comments. I tested the connectivity of the power cable to see which wire went to the volume pod and which went to the tubes, so I could label the hot and cold wires which in turn let me plug it in the right way around. Plus, when it was plugged in, I didn't touch the chassis.

[The Robman]

It appears that the 35Z5GT and the 50L6GT tubes are bad and the 12SA7, 12SQ7 AND 12SK7 tubes are good.

I found a very good site that described the pinouts for all the tubes here:
http://www.nostalgiaair.org/Tubes/

And this site has info about capacitors:
http://www.justradios.com/captips.html