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Repair of Old Vacuum Tube Electronic Equipment

(C) Copyright 2002, by Thomas P. Gootee


I get a lot of email from people who ask me about repairing and refurbishing the electronics in old tube radios and other vacuum tube equipment. Since the answers I give are almost always the same, and since the information might be valuable to others with the same interest or need, I have posted some of my observations, experiences, information, and links, here. The text below was basically just copied and pasted from an email reply that I sent to a person who was inquiring about buying a complete set of tubes for a piece of equipment that didn't work. Even inexperienced people should be able to get an old tube radio working, quickly and cheaply, using the information below.

BUT: Be aware that tube equipment almost always has HIGH VOLTAGE, which could easily kill a person. So be very careful! (Or, as they say, "Electricity is nice... but don't get any on you...") ALSO, as pointed out to me by Peter Wieck of Wyncote, PA: Some radio designs (the "AA5" types) can have LETHAL VOLTAGE ON THE CHASSIS, ANY TIME THEY'RE PLUGGED IN!!! So ALWAYS unplug every radio, if you're going to be touching ANYTHING in it.


Since I used to (and still do, sometimes) refurbish the electronics in older tube radios and other tube equipment, I can probably save you some time and expense:

In older tube equipment, 99% of the time, the problem is just dried-out capacitors, rather than the tubes. After 30 or 40 years or so, they almost always just fail. The solid mica ones (more or less rectangular, usually, often brown, with "colored dot" codes) are almost always still OK. But, the main filter caps, and ALL of the other electrolytic caps, and any/all tubular caps, should just be replaced, right off the bat (usually before you ever even try to power it up).

You just have to write down the capacitance (uF, microfarads, usually) and voltage rating that's printed on each one, and then get new ones with similar (+ or - 20%, say) capacitances, and voltage ratings that are at least as high (higher is perfectly fine). Then, just snip the old ones' leads, near their bodies, and twist the new ones' leads onto the old leads and solder them. (Do them one at a time, so you don't get them mixed up! (A good tip, courtesy of Larry Anderson.)) It's very easy, and cheap to do. If you have a silver "can" filter cap that is mounted on top of the chassis, that contains more than one capacitance, just get separate caps, instead of trying to find a matching can. Then just leave the can in place and solder the new ones to its snipped leads, underneath the chassis. Most, if not all, of the other capacitors will be found underneath/inside the metal chassis. Don't bother trying to test them. Just replace them ALL.

NOTE! Electrolytic capacitors have polarity, i.e. positive and negative ends. You MUST replace them using the proper orientation. Also, allowable "tolerances" for capacitance values can be taken into account, when trying to find replacements. For example, for many electrolytic caps, the manufacturer's tolerance was -20%/+100%, meaning that a given cap, when new, could have a capacitance value between 20% lower and 100% higher than its listed value. So, obviously, when replacing them, you can get by with using a slightly different capacitance than the original. Paper caps, on the other hand, were often -20%/+20%, which gives you a little less leeway. And being too far off might have side-effects on the tone or sensitivity. (This paragraph's info provided courtesy of Peter Wieck, Wyncote, PA.)

Here is a good idea: After snipping the old capacitor's leads, remove most of the old crud on them, before connecting and soldering in the new capacitor, by drawing your needle nose pliers across each lead, several times, to scrape them until they're bare metal again. This will help you avoid cold solder joints.

There is a really EXCELLENT webpage about re-capping old tube radios, provided courtesy of Phil Nelson, at:

http://antiqueradio.org/recap.htm

The other main thing that you MIGHT want to check is the resistors. The old carbon-composition resistors (usually brown cylinders, with colored stripes) often went way out of tolerance, from age and heat, et al. You would need a cheap multimeter/ohmeter to check them (available at, for example, radio shack, auto parts store, hardware store, et al). But you have to keep in mind that if they're connected to other stuff (which they are), then that alone could change the resistance you measure. If, for example, they're wired in parallel with something that has a non-infinite resistance, your Ohms reading will be lower than that of the resistor. To replace them, get the same Ohms value, and a Watts rating that's at least as high. They don't usually have the watts rating on them, though. You just have to go by the size (but too large is OK). And don't replace a brown tubular striped one with a wirewound resistor (usually rectangular and ceramic or concrete filled), since the wirewound ones almost always have inductance, too (like a coil or "choke" does).

Capacitors and other parts are available from: www.jameco.com, www.mouser.com, www.radioshack.com, and www.allied.avnet.com/allied/index.html, and MANY other places. Electronic repairmen's supply stores usually have high-voltage capacitors, too, but they are usually WAY more expensive, there. (But, if you're just buying one or two, the shipping cost might even it out. Plus, you wouldn't have to wait for shipping.)

(By the way(!): If you disconnect a capacitor that was in a circuit that was (ever) powered up, it may still be charged. Since tube equipment has high voltages, be careful not to touch both leads simultaneously, before you discharge the cap. I once got 400 volts right through ("across", actually) my heart, by touching one lead with each hand. And that cap had been snipped and removed already.)

On the other hand, if you KNOW the tubes are bad, that's fine, to replace them. But, those audio tubes are usually quite expensive, since all of the "audio junkies" now want the older ones, very badly. I've seen used ones going for $30 or more apiece. But, after refurbishing a couple dozen old tube radios and other equipment, I can say that I NEVER ONCE found a bad tube. It was ALWAYS the capacitors, except for one time, when there was a melted transformer, which I replaced with one from an old tube TV from a garage sale, for $1. (That's another good source for tubes and other parts, since old stereos and TVs all had at least a couple of audio tubes and a power supply rectifier tube in them, most of which still work fine. And they usually go for less than $5, around here, and are often free, just for hauling them away. I put an ad in the newspaper, once, for old tube equipment, and had to turn down dozens of them. And this is a small rural town of only 10,000 people.) By the way: The tube manufacturer isn't really important at all; just the tube's model designation, since may companies manufactured the same tube models.

You probably won't even know if the tubes are good or bad, anyway, until you replace the capacitors and maybe check the resistors. The main filter caps, possibly in a silver "can", can make it look like nothing is working, until you replace them. By the way, in certain designs, powering up the unit while it has bad filter caps can blow the main rectifier tube in the power supply (which might be very difficult or expensive to find a replacement for!). That's why I always replaced the caps BEFORE I even tried to power up any old piece of tube equipment... (But, that's not too common of a design, luckily, at least in my limited experience...)

There is LOTS of information and help available on the rec.antiques.radio+phono newsgroup, and the sci.electronics.repair group, at www.deja.com. I always use www.deja.com (now google groups, actually) to access the Usenet newsgroups, since they now have the complete 20+ years'-worth of ALL traffic from ALL groups, back on line, in a searchable archive! It's truly a GOLDMINE, for LOTS of stuff!

You should also check out www.nostalgiaair.com, which is devoted to repair and refurbishment of old tube radios and other gear. They even have over 30,000 tube radio and equipment schematics, on line, free! (And some manuals, tube data, et al) And they have lots of good links (Antique Electronic Supply, for example).

But one of the best sites for the repair of almost anything electronic is probably this one:

http://www.repairfaq.org/

You can also look at the many links on my links page, at: http://www.fullnet.com/u/tomg/links.htm.

Sorry to have blathered-on for so long, here! Good luck!

Best regards,

Tom

Tom Gootee
tomg@fullnet.com
http://www.fullnet.com/u/tomg


If anyone has any suggestions, corrections, or helpful comments, please contact me: tomg@fullnet.com.


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