2017-07-31 from FB reply by Phillip Grace
The worst is damage from leaking batteries and flood damage. The latter isn’t always apparent at first until you open up the set and find signs of sediment or corrosion. Common problems are of course failed capacitors, physical damage, and bad/failed repair attempts and tampering. Taken care of, transistor radios can be fairly reliable, the transistor devices themselves don’t fail often, but sometimes become noisy or intermittent. A can of freeze spray and a careful touch of a soldering iron applied to devices can ferret out marginal parts. Thermistors used in audio output stages often change value or fail entirely, causing distorted sound when their resistance has lowered, or poor battery life and hot running output transistors when their resistance has appreciably risen.
Speakers can be damaged by water or sunlight over time, finding replacements isn’t as easy as before, especially for odd sizes or ones that have mountings for other devices or fitments like transformers or brackets. For some unknown reason, I find a lot of radios have iron particles stuck around and to voice coils. This metal residue is tough to remove, though using a magnetized pick can slowly pull the debris out. The problem exists after this debris is removed, since the constant tug from the magnet will cause the cone to take a set and be bottomed out in the magnet structure. This can be repaired with small foam blocks between the basket and cone and some humid air and time.
Other oddball issues can be the hidden tubular ceramic caps inside the IF cans. These can fail and leave one scratching their heads knowing all the components measure good otherwise. If there is insufficient range to adjust an IF can, this is a common indication of an uncommon problem.
Volume controls and switches can be a big issue, especially in Panasonic sets. Matsushita used high quality parts in all their electronics, and these controls almost always had silver plated contacts. Depending on the environment where an old Panasonic was kept, the silver can tarnish to a point where the switches and controls will not function, even after judicial use of Deoxit. Use a multimeter to check all switch contacts and pots, and don’t be surprised if there are nearly complete open readings on otherwise closed contacts. I’ve found the Panasonic RF-2200 radios to be especially bad for this. Replacements are unobtanium, with the only solution being to remove and disassemble the switch to clean the contacts individually the last resort.
Variable capacitors are prone to damage, especially the open frame type. Look for bent or rubbing plates, dirt and misalignment. Some varicaps are mounted on insulating grommets, these often fail over time and cause noise, static and inoperative sets.
Some of the green mylar film caps degrade over time, leaving a dusty looking residue on the outside as well as hairline cracks in the plastic coating. These are easy to spot. Check them with an LCR meter if you aren’t sure.
