Powering up that old Radio or TV?

Do's & Don'ts


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WAIT!  Don't plug that old piece of electronic equipment in!

Unless you know the radio has been restored properly or has been working well recently.....DO NOT APPLY POWER! I'm referring here to radios, televisions, amplifiers etc. with a power transformer, not the small 4 or 5 tube AC/DC radios.

There are precautions that should be made when powering up an early power transformer operated radio or transformer powered piece of equipment. Electroinic devises that have not been used in years has been doing more than just sitting gathering dust. Chances are the filter condensers (capacitors) have deteriorated to the point where they are shorted or very close to being a "short circuit". Applying power to them for the first time in many years can be catastrophic. Few of these old radios (and this applies to old TV's as well) have no fuse protection. When something shorts out (such as a filter capacitor), something's going to smoke or burn out. This can happen very suddenly or it can take a few moments.

Here's what can happen when you apply power to an old electronic instrument that's been setting around for years:

Click on the thumbnail images below to see what happened to a Radiola 60 (from 1929) when it was recently plugged in for the first time in 60 years:

click on above to enlarge

When the owner powered up his recent find it first made as loud noise popping sound, then the smoke rolled out, then silence. The incident left his shop with a foul burnt smell that lingered for several hours. He later learned that the condenser pack had shorted out. The sudden in-rush of AC current  caused the canned package of components to overheat, which spewed out all of the can and splattering the surrounding tubes and surfaces with the black potting substance.

Here's what you can do to protect that old keepsake
 or restoration project from going up in smoke:

First make a simple test fixture like described below:

Many think simply using a Variac will protect from damage but beware! Although some protection is offered, 100% protection is not assured.  For example; if the instrument has a short circuit, bringing up the power slowly will only help if you monitor the current draw while doing so. This requires an AC ammeter and the knowledge of what's normal for the instrument's current draw, so you can terminate the (slow) power up when excessive current is reached.

Below is a diagram of a simple device you can make sure when you apply power, the power transformer will be protected. You need to make this simple test jig using a light bulb, lamp socket and AC receptacle. If you are not capable of doing this, I suggest that you find someone who is and have them check out your radio so you do not cause irreparable damage.

If you can determine the power consumption of the radio (this is often on the model or ID plate/sticker). Choose a lamp of a wattage size that is equal or slightly higher than the radio's power consumption. Example, the radio draws 85 - 90 watts, use a 100 watt lamp.

If you can't determine the power rating, choose a lamp of the proper size from the  chart below:

    No of tubes the radios has               Lamp Size (watts)

               5 -6 ......................... 50 - 75
               7- 8 ......................... 75 - 100
               9 -10 ...................... 100 - 150
               11 - 15 ....................150 - 200

The lamp will be in series with the radio under test, if the radio has a short circuit or draws more current than it was designed to do, the lamp will work as overload protection and draw the excess power. On the other hand, the radio cannot function properly with the lamp in place as it will be operating below it's normal voltage requirements.

Now for the test; after choosing the lamp size, remove the rectifier tube (usually a #80, 5Y3, 5Y4, 5V4 or some other tube # beginning with "5"). Now plug the set into the test jig's socket and turn it on. The lamp should not glow or glow or glow very dim. If the lamp glows bright at this point, either you are using a lamp of too low wattage or the radio has a serious short. Do not proceed ant further until you can or have someone resolve the short circuit problem.

If the bulb does not glow or glows very dim, proceed to the next step by plugging the rectifier tube back in it's socket. Observe this tube and the lamp, should the lamp glow bright or the tube glow purple or arc inside the glass, this is not normal, stop the test and seek technical assistance. This could be a faulty tube or internal short in the radio. If the tube glows normal, the lamp should increase a bit in brightness and you may start to hear a slight hum from the radio within a few minutes (give it time to warm up).

If after a few minutes you start to hear a hum that's getting louder and the bulb starts to glow much brighter, this is an indication that all is well with the power transformer but the filter capacitors (condensers) are faulty. It's rare that they would not be faulty.

If you hear no or very little hum and the lamp is dim, chances are that it will now be safe to now apply full power to the radio by removing the plug from the test fixture and plugging the radio directly into the wall receptacle. Do so carefully while observing the rectifier tube. Should a loud hum develop or the rectifier tube arc, flash or glow purple inside the glass, immediately remove the power, cease any further test and find a knowledgeable technician.

The above info is meant for the novice radio enthusiast or non-technical person to help them determine the condition of an old radio's power supply.

I do not suggest or recommend using an old radio for music or entertainment unless it has been properly electronically restored and fused for safe operation. Once a proper restoration has been done and the power circuits have been fused for short circuit protection, these old radios can be a wonderful source for great sound of the good old music being played in most areas on stations that has the "Music of Your Life" format.

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Sonny, the RadiolaGuy 

© C.E. Clutter


Member of:
Northwest Vintage Radio Society

Member of:
Antique Wireless Association