Radio on the fritz ...

[tonyb]

I'd much rather post this query here than in any old-radio-exclusively online community, seeing how it's far less likely anyone here will look down his or her nose at my ill-informed and naive self.

Here's the deal: My old Sparton tabletop radio, 1940s-vintage, worked okay for the couple-three I've had it. It would light up and warm up and pick up AM stations. This morning I turned it on. It lit up. No noise comes out. Thought perhaps the station's transmitter was down. Nope. The other radio picks it up. Smell of burning something. Switch off radio and unplug it.

First steps?

 

[LizzieMaine]

Is it an AC or AC-DC set -- in other words, is there a power transformer? Look in the back -- if you see a big metal transformer mounted next to one of the tubes, that's it. If there is -- without plugging it back in -- sniff the transformer. If it smells like burnt pitch or tar, you've got a short circuit somewhere that pulled extra current and overheated the secondary winding of the transformer. Most likely it's one of the filter capacitors, which will fail with age and cause such problems. It could also be an AC coupling capacitor, which bypasses radio noise off the AC line and which, if it shorts, could burn up the primary winding.

If it's an AC-DC set, it's most likely a shorted or open filter capacitor, which will cut off B voltage to the tubes and result in no sound. Unplug the set, take it out of the case, and look at the bottom for a large wax-covered paper tube. THere'll be several of these, but you're looking for the biggest one. Examine it to see if there's evidence of wax dripping out of it, which is a sign of an overheated part caused by a short.

Another possibility might be a resistor failing, but these don't usually fail unless a capacitor has shorted first. The smell of a burning resistor is much more acrid than a shorted capacitor, and will usually produce visible smoke and charring on the surface of the failed part.

These parts are easily replaced if you're handy with a soldering iron or know someone who is.

 

[tonyb]

There's this box-like thing in there, about three inches square and a couple of inches tall, with a smaller box-like thing atop it. That's a transformer, then? Which means I have an AC (rather than an AC-DC) radio set?

It still smells of burnt electric something, by the way. And its unfortunate event occurred some eight hours ago now.

 

[LizzieMaine]

Is there a model number visible? It'd be easier to diagnose if I could look up the schematic.

That might be an audio or even an RF transformer you're describing, but some sets had their power transformers potted in a metal casing, so it's best to check to be sure.

 

[tonyb]

As luck would have it, I found without too much time and effort a source online that says my radio is a Sparton 6521. The photo on the website shows a radio identical to mine.

http://www.radioatticarchives.com/archive.htm?page=s5#Sparto

(Second row from the bottom, on the far right.)

 

[LizzieMaine]

That's definitely an AC model, probably made in 1939-40. The filter condensers are the likely problem -- they're the round metal cans on top of the chassis, with solder-lug connections on the bottom. Look around the mounting holes under the chassis for evidence of electrolyte leakage -- white crystals or gooey exudate. In any case these should be replaced as a matter of course.

 

[tonyb]

What else might be zapped? Would the filter condensers be what I'm smelling?

 

[LizzieMaine]

It's likely, if the condensers went, that they also took out some resistors in that circuit, or possibly even the field coil in the speaker -- let's hope it wasn't the latter, because that means finding an exact replacement speaker or doing some serious juryrigging to get another one to work. In radios of this vintage, there was no permanent magnet in the speaker -- instead there was an electrical coil wound around a metal slug at the rear of the speaker frame which also served as a filter choke in the power supply. This coil provided the magnetism required for the speaker to work, but it also carried the entire B+ supply for the tubes. If the field coil burns out or otherwise opens, there will be no sound at all from the radio, not even a quiet hum.

In the absolute worst case scenario your power transformer cooked, but you would have *really* smelled that, there would be sticky tar under the transformer, and there likely would have been visible smoke. Replacing a power transformer is the radio equivalent of a heart transplant -- it can be done, but it's difficult, and you have to find a replacement unit that doesn't just fit physically but also electrically. I've done it, but it's no fun to do.

Most radios do have a distinctive burnt smell under the chassis which comes from the layers of dust and condenser wax that tend to gently bake as the radio operates. This is distinct from the sharp, nasty electrical smell of a smoked component.

 

[tonyb]

I'm inclined to think it's a "smoked component." I detected no smoke, but the smell is something which wasn't there prior to this morning's unfortunate event. It was detectable from several feet and even now, 15 hours on, I can detect it if I put my nose within a foot of the radio and give a good sniff.

 

[LizzieMaine]

I'll see if I can dig up a schematic -- my Riders' Manuals are out in the garage, so it might be a couple of days. But seeing the exact circuit might help to narrow things down a bit.

In the meantime, do you have, or could you borrow, an ohmmeter? The first thing I'd do in servicing a set like this is unplug the speaker and check the resistance of the field coil, just to confirm that it isn't burned open.

 

[tonyb]

An ohmmeter, eh? I'll look into it. Can one be had cheap?

 

[LizzieMaine]

You can usually get a cheap voltmeter/ammeter/ohmmeter for twenty bucks or so at Radio Shack or an auto-parts store. The modern ones are digital for easy reading, but otherwise there's no particular advantage over the old analog type. If you get one, I'll tell you which pins on the speaker plug to check.

 

[LizzieMaine]

Okey, I've looked up the schematic, and there are no AC line bypass condensers in the primary circuit of the transformer. So we can rule that out. The filter condenser unit is a dual-element metal can type with 15 and 30 mfd elements rated at 450 volts, and the can itself connected to chassis ground. There's a pretty good chance that one or both of the elements have shorted to ground, and that's what caused your problem. We can rule out any problem with the filament winding of the power transformer, since the dial lights are part of that circuit and they lit up when you turned on the set. Since the failure was immediate, the condenser likely blew at the first surge of high voltage when you turned the set on.

The question now is whether or not the high-voltage winding of the power transformer got cooked. If you shut the set off as soon as you realized something was wrong, there's a good chance it's still OK -- the smell you got was from an overheating transformer, but unless you see the tar oozing out of the casing, there's a chance it didn't completely burn out.

You can check that winding with an ohmmeter to see if it's open or not by taking out the 5Y3 rectifier tube -- with the set unplugged from the wall -- and inserting the ohmmeter probes into holes number 4 and 6 in the tube socket. Locate these holes by looking under the chassis at the bottom of the socket and finding the alignment notch. Pin 1 is at the immediate left of that notch and pin 8 is at the immediate right. Just count the holes from either end and find 4 and 6. The diagram doesn't list an exact resistance for this winding, but it should be somewhere between about 50 ohms and about 300 ohms -- the exact resistance at this stage isn't important, we just want to be sure the winding isn't open. If the winding *is* burned out, the ohmmeter needle won't move at all -- you'll have infinite resistance, the sign of an open circuit, and you need a new power transformer.

If your transformer is still good, the next step is to replace the filter condenser unit. You can either get another can-type unit -- these are made today for the electric-guitar-amp crowd and install that, connecting each element as shown by the markings on the spec sheet or printed on the side of the can, or you can get tubular electrolytic condensers and mount them under the chassis, soldering the negative end to some convenient ground point on the chassis and the positive ends to the connections removed from the appropriate points on the existing can type unit. If you use tubulars be careful to secure them so they don't move around and keep the leads as short as is practical.

All of this is assuming the chassis is still as original. There might have been work done by some other service person over the course of the life of the set -- if you get a chance, post a photo of the underside of the chassis and I should be able to tell if this has been done and change the instructions accordingly.

 

[tonyb]

Well, ask and you shall receive.

Thanks. Now I gotta go gets me one of them ohmmeter things. (And I still haven't found my digital camera.)

 

[Stanley Doble]

There may be some old time radio enthusiasts in your area or a ham radio club. Somebody who could help or have a look at your radio. I say this because it sounds like you are not too familiar with electronics.

By the way the instrument you need is called a "multimeter".

http://www.ehow.com/list_5966249_uses-digital-multimeters.html

They are a common item at hardware stores, harbor freight, auto parts stores etc. I have a dozen of them around the place ranging in age from 5 to 70 years (my father's). The last one I bought cost $13 but I have bought them for as little as $5 bucks. For your purpose, the cheapest one will do as well as the most expensive.

 

[Stanley Doble]

Lizzie I did not mean to slight you or your excellent advice, I thought tony could use some training wheels so to speak.

 

[tonyb]

... it sounds like you are not too familiar with electronics. ...

What ever gave you that idea?

 

[LizzieMaine]

The thing with basic radio repair is that you don't really need to know a lot of umpety-ump electronic theory to do it. A basic radio receiver has only three main sections -- a section that receives, detects, and tunes the signal, a section that converts the signal to audio, and a power supply. Figuring out the problem is simply a matter of tracking back from the speaker to the point where the problem appears.

Components are pretty simple too -- tubes amplify or otherwise convert the signal and turn AC into DC for the power supply. Resistors reduce the voltage at various points in the circuit, condensers allow the signal current to pass while blocking the DC current, and coils are used in combination with condensers to tune the signal. Transformers are simply specialized coils that change voltages for the power supply and pass radio-frequency and audio-frequency signals between the sections of the set. And a speaker is a speaker.

There's nothing mysterious about a radio once you get to where you can recognize each type of component by sight. And anybody who can handle a soldering iron can fix one. I have no background or formal training in electronics at all, other than years of working in radio stations surrounded by antiquated equipment. I picked up everything I know by dealing with emergencies as they happened.

 

[Stanley Doble]

Yes, I know anyone who can handle a soldering iron, a multimeter, a screwdriver, and understand simple instructions can fix a radio. But it can be a little off putting at first.

 

[tonyb]

What little I know about the workings of machinery and electronics and what have you I learned mostly hands-on. I'm no car painter, but I've painted cars. I'm no mechanic, but in my early driving days a young man of modest means learned how to fix his car, lest he prefer walking or taking the bus.

There's no urgency and really no necessity in fixing this old radio. But I don't like to be the person who failed in his stewardship of interesting, stylish, and often still quite useful old things. This radio is something like 75 years old now. It's not at all valuable, leastwise not in any monetary sense. But it's an attractive design, and cosmetically it's in a good unrestored condition. It would bring me some satisfaction to get the thing working again, one way or another.