Nuts and Volts - July 2014

and don't play with any caps in the "front end" of the receiver as these are rarely paper except for a few that may be in the power supply decoupling circuits. These, you replace.

A leaky coupling cap on the grid of an audio preamp in a higher power output amp can cause that tube to go into full conduction and drive one of the output tubes full-on to the point that it will burn out the primary of the output transformer — a common problem in many old console radios.

Your best friends will be found on the forum at www.antiqueradios .com. These guys — if they don't own your particular model — will know about it, and will be able to help you tremendously in its repair and restoration. Don't visit the forum as a lurker. Sign up. It's free (but they'd like a donation, if possible). Then, you can be set to be notified when there's activity on your thread.

Also, don't throw away all those tubes you replaced. There's probably nothing wrong with them.

Dean Huster via email

#2 Since you are producing "static," you know the power supply, final amplifiers, and speakers are okay. Short of having a schematic and using a signal generator and oscilloscope to check for proper operation, I would replace all of the capacitors. Old capacitors have a tendency to go bad more so than the inductors. An old technique involved "jumpering" a known good capacitor around a suspect one, but in the case of an old unit, replacement may be the best approach.

Also check for cracked resistors and loose wiring (I am assuming this unit pre-dates circuit boards), and clean the switches, contactors, and tuning capacitors. IF this fails, you need a signal generator (frequencies covering RF and IF for the unit) and an oscilloscope to check the circuit for the faulty stage(s). Tim Brown PhD EE, PE Honea Path, SC

#3 We need more information about the receiver. What bands (frequency range) does it cover and what antenna are you using? Generally, you need a long wire type antenna that is 100' or longer mounted as high as you can get it. Propagation will affect what stations you can hear. You may be listening to a "dead" band that will come to life at another time of day.

Also, many shortwave stations have been relocated to different frequencies in the last 10 years to free up spectrum for other use. It's possible your radio is listening in the spectrum where stations have vacated. Finally, the restoration of a "boat anchor" is often more complex than just a re-tube and clean-up can fix. A common failure is with old electrolytic and paper capacitors.

Perry Ogletree Murfreesboro, TN

[#5144 - May 2014]

What Wall Wart?

While on vacation, I managed to lose the power supply wall wart to a cheap vintage portable no-name brand shortwave radio. On the back of the radio, the power jack says 9 VDC, and has a C shaped circle and a dot in the center with a plus sign on the dot. I’m pretty sure this means nine volts; positive tip. However, what it doesn't state is the milliamp rating. If I use a power supply that has too high or low an amperage rating, am I in danger of damaging the radio?

#1 The symbol you see on the radio indicates a "coaxial" power adapter plug with "tip positive" polarity. Go to your local RadioShack and get a "Universal" wall wart that will deliver 9 VDC at 1,500 (or more) mA DC.

(IMPORTANT: Take your radio to the store so you can get the proper "adaptaplug" to use with the wall wart!)

I suggest a 1,500 mA (i.e., 1.5A) or larger current rating to ensure you'll have enough power. DO NOT FEAR: A wart with a large current rating WILL NOT harm the radio. However, a smaller-than-needed current rating will quickly burn out the wall wart (or cause overheating and/or fire hazard).

Hope this helps, and happy listening to your radio.

Ken Simmons Auburn, WA

#2 When choosing a wall wart, you have to check three things and look out for a problem with the wall warts.

1. Does the voltage match?

2. Does the polarity match?

3. Does the wall wart put out enough current?

The catch is with the voltage. Wall warts come in four different flavors:

1. Transformer — unregulated.

2. Transformer — regulated.

3. Switching — unregulated (very rare).

4. Switching — regulated.

The belief that the general public has about power supplies is that the supply will drive whatever current it is rated at.

This is WRONG in capital letters. The device will draw whatever current it is rated at for the rated voltage. The power supply must be able to supply at least as much current as the device needs. If it can't, the voltage will fall off. If it is rated at more current than necessary, this won't hurt anything as long as the maximum voltage is not exceeded. This is the catch.

This is why it’s important to check unregulated wall warts at their rated current. A nine volt unregulated power supply may read as high as 18 volts with no load on it. So, if an unregulated supply is rated at one amp at nine volts and the device only needs 250 mA or 1/4, amp the voltage output of