**this instructable takes into account that you're familiar with basic electronics, soldering, and basic hand tools.**
this instructable is written around the sylvania 4P19W transistor radio. this size radio is what was called a coat pocket radio. this radio is unique in that it only uses 4 transistors. the fact that it only uses 4 transistors gives it a slightly longer battery life than the more common 6 and 7 transistor AM radios of the time. the tradeoff is in it's performance. this radio is fine for listening to local AM stations but don't expect it to pull in signals from far away. the reflex circuit allowed a transistor to be used for two things at once, in this case it handled RF as well as audio at the same time.
this model of radio is common on ebay for $10 or less. they use older styled oval metal cased transistors so they are of interest to the vintage tech geek. the reason i have heard for these being common is they were given away as a promo when you bought a new sylvania TV set.
Step 1: Let's Dig In!
to get inside you need to remove the back cover of the radio. this is also how you'd open it up to replace the battery. many old transistor radios are setup like this were you pry open a lid to get inside. once inside you will find the circuit board (chassis) and the schematic.
note the battery holder. is it corroded from an old battery left in it? if it is, replacements can be found online. radio shack used to carry 9v clip on terminals but i'm not sure if they still do.
the circuit board is held in the plastic housing by 1/4" hex head bolts. note the two i'm pointing at in pic 4. you will be removing those two and the two on the right edge near the battery connector. no need to remove the remaining two as all they do is hold the tuning capacitor in place.
once the screws are out, you can carefully lift the board out. be careful with the speaker leads.
Step 2: Board Is Out, What Are You Looking At?
the chassis will lift out but you'll have to de-solder the two speaker wires to liberate the circuit board (chassis) from the cabinet.
you should have something similar to what's in the pic unless some joker pilfered or butchered the radio. the transistors are the oval topped, metal encased, chiclet sized devices on the board. there are 4 of them. look around the board and you will see it only has 2 electrolytic caps on it. lucky you!
electrolytics are what commonly fails on old transistor radios. this particular set had a really bad scratchy volume control. this is caused by a combination of dirty contacts on the volume control pot and failed caps causing a DC potential where there shouldn't be one. the electrolytics are the two black wrapped cylinders at the top right of pic 1. in pic 2 there is a closeup of them.
be careful replacing the caps on this radio!
modern common practice is to label the negative side of a capacitor with a black stripe. on old japanese transistor radio parts, they liked to do a band made up of "+" signs. at first glance, one can miss the little + signs and think that's a negative lead marker thus wiring in all the caps the wrong way. get a good look at those caps before you replace them and make sure you have the polarity correct.
japanese parts in an american radio?
yep. some manufacturers where early adopters of the cheap labor that was available in japan at the time. they kept their "made in USA" decals but snuck in japanese parts which saved them money.
back to the caps..
replace those two electrolytic caps with something of equal or slightly higher value. the ceramic caps that are left rarely go bad and shouldn't cause any problems. in pic 3 you see the old electrolytic caps after removal. they were sealed with epoxy. any like this you spot in an old transistor radio will likely be bad.
see the busted antenna?
the antenna on most transistor AM radios is basically a black ferrite bar with wire wound on it. this one is cracked but the wire was intact. ideally the ferrite bar should be replaced. i live in a major city with very strong local stations so i chose to mend it. i re-melted the wax around and under the antenna, securing it in place from further damage. if it comes apart again, i'll JB weld the two pieces back together. remember this radio isn't exactly collectible so no need to go overboard.
what are the metal square things and the metal round things just begging me to stick a screwdriver in them???
those are part of the RF (radio frequency) circuits. do not go messing with them unless you want a radio that will definitely not work. re-aligning a radio requires special knowledge and equipment that is beyond the scope of this instructable.
this radio is a wee bit special. on almost all transistor radios, the volume control adjusts the audio level feeding into the audio amp transistor. on this radio, they run the audio amp wide open and the volume control actually adjust the sensitivity of the radio. it's an odd way to do things but it works.. for the most part. if you are running an in house AM transmitter to play music to your old AM radios, this radio may act up. what can happen is if you are too close to the transmitter, you will find that no matter what you do, you can't turn the volume down enough for comfortable listening. it's a quirk of the design and will only be problematic if you are near an AM transmitter.
Step 3: Cleaning the Volume Pot and Tuning Cap
a dirty volume control will cause scratchiness as you adjust the volume. a really dirty one becomes downright annoying as any little tap on the radio and the volume changes. the volume pot on this radio has a small clip holding it together. carefully un-clip both sides of the retaining clip and the pot will crack open enough to get in there with contact cleaner and give it a good shot. shoot it with contact cleaner and work the control through its entire travel a few times. that should cure it.
the tuning cap may need attention as well. a dirty tuning cap will show up as crackling while you tune around to different stations. this can be caused by dust in between the small metal fins of the tuning cap and /or by poor contact to the part that turns and meshes the fins together. a can of compressed air is the best way to clean these. blow it out really good and then put a drop of light oil on the bearings as seen in pic 2. work the tuning cap back and forth through it's entire travel. if you see any excess oil dripping out, clean it up with a q-tip. don't just leave it there to cause future problems.
Step 4: All Done?
new caps in, volume pot cleaned, tuning cap cleaned, battery clip replaced if it needed one. great! solder your speaker leads back on and carefully reinsert the chassis into the cabinet. bolt it all back in.
you may have noticed it's an awfully big space for a little 9v battery in that radio? in my radio, the battery bouncing around is probably what helped crack the antenna i had to fix. i took some foam like what's used to cushion electronic parts and made a pad to secure the battery. make it thick enough to hold the battery snug when the lid is clipped on but not so thick that the back cover wants to pop off.
Step 5: All Back Together? Enjot It!
chances are that with that simple bit of work, your sylvania will come back to life and work properly. these radios are quite simple and there isn't much to go wrong.
what to do with an AM radio? most cities have a sports station and several news stations. if the yapping heads aren't your thing, you can always listen to a ballgame old-school style sitting out in the shade with your AM radio you fixed yourself.