this instructable takes for granted that you can solder, work with small parts, read values on capacitors, and have a basic understanding of electricity.
transistor radios have been in production for over 50 years now. even early ones are common finds at yard sales and fleamarkets. they were the ipod of the 60's and 70's and many have become sought after collectibles.
in the mid 50's an american company named regency produced the world's first commercially made transistor radio. right on their heels where several other american companies and so the battle for the transistor radio market began.
transistor radio manufacturers came and went. by the late 50's, the japanese began to produce transistor radios at prices that american industry couldn't compete with. some american companies quickly moved production to japan, others just had their radios made for them. one company stuck it out till the bitter end, it's name was zenith.
zenith radios were generally better built than any of the competitors and priced accordingly. some models like the transoceanics cost as much new as a decent used car would have cost at the time. because of their quality, many still survive. this instructable will deal only with large zenith metal chassis transistor radios. much of the info provided though does carry over to other brands of metal chassis transistor radios.
Step 1: getting inside your radio
a great many of the large lunchbox sized radios came in a leather or pleather case. the leather ones were sometimes all leather with a metal face that the radio bolted into or a metal case with the leather glued on. the pleather (fake leather) radios were also made this way but it was more common to find them as a fiberboard box that was wrapped in a leather looking plastic material.
most of these covered radios used a snap clasp to keep the back of the radio shut. to change batteries you unsnapped the clasps and flipped the door up. its not uncommon to find these clasps broken off or frozen shut. if the clasps are broken off, you'll have to conjure up some way to keep that door shut while moving the radio around.
if the clasps are frozen you may be able to save them. take a can of wd40 and put the straw on it. give the clasp a shot right were both halves meet. let it sit a minute then take a pocket knife and work the blade into it trying to split both halves. if you're careful you'll pry them apart without breaking anything. the wd40 may stain a leather cased radio so keep that in mind.
on the zenith royal 1000, 2000, 3000, and 7000 series radios there is a mechanical fastener to access the battery area. these radios are much more complicated internally and beyond the scope of this instructable though some of the info presented here applies to those sets.