motorola: FREE radio

My wife brought this home from a yard-sale.

"Old clock radio with tubes FREE"

Motorola 62CW(1). Veneer case in good condition, slight crazing. Dust on knobs and behind face plate (which is plastic not glass).Looks good.

Usually I expect a 1952 radio will have a perished power-cord; this was in fine safe condition. So I put it in the driveway with a switch so I could turn it on from a few feet away, in case the filter cap burst. Hmmm. It humed very very low.

I fiddled the too many switches for the clock function, discovered I was turning the Tone not the Volume, and got static. I dialed down the band and got WBZ, Boston, as good as could be expected, since WBZ is hundreds of miles away! I came up the band and got the local 5KW AM station in really great quality.

The tuning cord stuck at 1500KC so I took the chassis out. When you try this, be sure the clock hands are all horizontal so they do not foul the rectangular cut-out.

The insides are in beautiful condition. Almost no dust inside. One wax-cap is oozing wax, others look fine. Several tubes are Motorola, probably original; two are RCA so vintage replacements. Dial-lamp is burned-out and an odd 117V bayonet type.

I've had it playing several hours now.  YouTube video, 29 seconds  You can hear the tone change as the camera moves near the speaker; I didn't fake this.

No sign of stress. Drifts a bit in the first 30 minutes; I remember most AM tube radios did when new.

Circuit shows an RF stage, a deluxe feature, and explains why it pulled WBZ so clearly despite distance and the racket on the power lines in my town.

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You can't tell wax cap condition from appearance. I never plug in old sets without checking leakage. There are two methods:

1) A high voltage via 2 x 1M Ohm and uA meter or a neon with 0.1uF in parallel, in series with capacitor. A DMM is only 3V to 9V, so no use to test leackage.

2) If the filaments/heaters are not series, then unplug the tubes and measure g1 voltages (should be zero) and g2 voltages (should be HT).

The anode to following grid has 50V to 300V across the capacitor depending on model, and the grid has 100K to 2M resistor to ground, thus any leakage will cause excessive bias current, short tube life and possible burnt out audio transformer, HT rectifier or mains transformer, choke or dropper.

A low voltage on the screen grids (g2) due to the 10K to 1M resistor HT and the decoupling capacitor leakage to ground won't damage anything, but makes the gain very low.

Safety:

If it's a set without an HT transformer, then there are isolation capacitors, possibly to external sockets and possibly to case. These should be replaced with 1KV rated ceramic XY class capacitors (or 2KV if the model is 220V to 240V European).

Safety (especially in Europe):

Very many USA transformerless sets with 220V adaptors (for export or travel) are totally unsafe on 220V to 240V due to inadequate rated isolation capacitors, too high a value, degraded insulation or inadequate separation as the internal 0V line can be connected easily to 220V live. So I only use USA 110V sets, even if they have a 220V adaptor (e.g. Transoceancic, Stratoworld etc) on an isolated 220V->110V transformer. The 110V mains/battery sets work on a shaver transformer (2KV isolatation and power limited to 20W). Many 220V to 110V "transformers" to convert USA appliances are unsafe as they are autotransformers, one 110V connection is direct to the 220V to 240V mains connection.

I use isolation transformer even for some European Mains/Battery sets if there is exposed metal work. Some sets with transformers and 2 core lead get a 3 core earthed lead as there is no assurance the transfomer mains part is on an isolated bobbin and hasn't been overloaded in the past.

 

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