**this instructable takes into account that you have some basic electronics knowledge, you can solder, and you can use basic hand tools.**
i have quite a few radios in my collection and no other basic AM portable has the heft or the classy looks of the GE P-780B! this radio has an all metal, chrome plated front. it's design is reminiscent of the interior of a 1950's classic car, big, shiny, and heavy.
not only do these radios look good, they are excellent performers for those into DX listening. DX listening is a hobby in which you try to pick up far away stations not normally heard in your area. this model has a big, rather directional antenna and full sound. no tinny pocket transistor squawky sound here!
Step 1: Restore or Just Repair?
i had been hunting one of these for a while and snagged one on ebay for what i considered to be a very fair price. it had some cosmetic issues and was deemed as broken by the seller. that didn't deter me. a little work brings most old transistor radios back to life.
this radio came with the following issues..
1) the chrome is pitted in some places
2) the knobs are incorrect, these look like they came off a 70's era ford automobile
3) the battery compartment had extensive battery acid damage (a common problem with old radios)
4) the battery door had a broken latch (another common flaw on this model)
restore or repair?
considering the price paid and the flaws, i decided to repair this radio back to operating condition. i wanted one i could take outdoors with me and not be too worried about it getting scuffed or bumped around in the car. proper restoration was not my goal. i wanted to get it working and useable again.
it's important to asses whether you want to restore or just repair. some repairs can actually take away from the collector value.
Step 2: Let's Dig In!
this is a real easy radio to get into. flip it over and you will see two philips head screws that hold the back cover on (pic 1), remove them (pic 2). the back cover will come off but be careful as it will be connected to the radio chassis via two wires. the green is negative and the red is postive and labeled as such on the battery compartment terminals if you look closely to where they are soldered (pic 3 shows them already de-soldered).
you will need to de-solder the battery compartment wires to liberate the back cover from the chassis. in pic 4 you see the chassis free and clear to work on.
be careful once you are at this stage. it's best to lay the radio on a towel folded over as a cushion until you learn to recognize what's delicate in these old radios. once you gain experience, you'll know which way to support the chassis so nothing is damaged.
if you have a variable DC supply, this is a good chance to see if your radio works at all. as we saw earlier, red is + and green is - on the battery compartment leads. how many cells did this radio originally use? it used (6) 1.5v batteries. that gives us 9vdc. almost all antique AM transistor radios will work on voltages a few volts lower than they were designed for.
i set my power supply for 6vdc and powered up. it took about 30 seconds but then the radio came to life. the old caps probably needed to come to life before the juice once again flowed. turns out my radio got laid up not because it had a circuit issue, it was because of the battery leakage. the battery box was all corroded so there was no contact being made when somebody tried to install fresh batteries in it. to a geek, this radio is repairable. to a non geek, this radio is busted and thus sold for cheap on ebay.
since your radio is powered up go ahead and check the dial lamps. with the radio on, push the white "lite" button next to the volume control. their are two light bulbs located behind the "eight transistor" sign that light the dial from the bottom up. if they need replacing, you will be able to get to them now that the radio is opened up.
Step 3: Ok, Chassis Is Out. Now What?
let's cover what you're looking at.
in the pic you see the back of the chassis, the radio is upside down. in the center is the speaker. just below it there are two cardboard covered cylinders, those are capacitors. these early transistor lunchbox sized radios often used similar parts to their vacuum tube predecessors.
in the pic you will see several aluminum rectangular objects with pointed tops and tape over them. those are RF (radio frequency) transformers. don't mess with those! you will cause yourself unnecessary headaches as the radio will then not only require whatever repairs needed to be made but also an alignment. alignment is beyond the scope of this instructable.
to the right of the speaker you see the tuning capacitor. it has the large pulley on it. to the left of the speaker you see two transformers. one is a matching transformer to match the impedance of the transistors to the speaker, the second is a coupling transformer between sections of the radio. early transistor radios required these. as transistor technology advanced, these were eventually designed out. you wont find these in more modern transistor radios.
this radio has a circuit board. some very early transistor radios and certain model zeniths didn't use a circuit board.
Step 4: Careless Owner, Bad Batteries...
battery leakage is the nemesis of old transistor radios. this one fell victim to owner neglect. look at all the pics in this page and you will see damage caused by battery acid.
plenty of radio manufacturers knew the consumer would forget to take the batteries out during long term storage and batteries of the time were known for leaking. this is why this radio, like many others of the time, were built with the batteries on the bottom and the electronics up top. this usually prevents damage unless the radio was laid on its back during storage like this one was. that's why there is corrosion in multiple places on the chassis. fortunately this will clean up.
use a brass bristle brush and small screwdriver to scrape away as much of this corrosion as you can without further damaging anything. battery acid residue remains corrosive even after 30 or more years. don't touch your face or anything else for that matter when cleaning up old corrosion. wash your hands when you're done.
Step 5: Getting Into the Battery Compartment.
on the bottom of the back cover you will see a series of philips head screws that hold the battery box in place. remove them and the battery box will tip out and up from the back cover. note the box is held with screws on one edge and plastic fingers on the other. be careful with random bits of rust and battery acid residue.
the pics show the screws to remove, the battery box in the back cover, and the inside of the battery box once it's been removed.
note the extensive corrosion to the battery box terminals and the hardware that holds the terminals in place. this box is saveable if you want to put the work into it. i decided not to bother on my radio with all the cleanup and simply removed all the corroded hardware and washed the box out with soap and water. on my radio i opted to replace the original pack with a AA battery pack. more on that later.
Step 6: Old Electrolytic Capacitors, They Need to Go
the cardboard covered cylinders you see in the center of pic 1 directly above the speaker are electrolytic capacitors. these go bad with age and should be replaced on any antique radio you work on. since this is a repair for functionality and not a proper restoration, we'll just be replacing them with modern parts.
why replace the caps?
they go bad with age. they normally have two modes of failure, to short out or go open. on battery powered transistor radios, shorted caps will result in a dead or barely working radio. caps that are drying out and going open will result in raspy or weak tinny sound.
on early transistor lunchbox sized radios like this, it wasn't uncommon for the manufacturer to use whatever they had on hand from their tube radio parts stock. in pic 2 you will see that this radio has an electrolytic cap in it rated at 450vdc! the radio runs off (6) 1.5v batteries and no provision for external AC power. there will never be 450v in this radio. there's no need to replace caps like this with such high voltage parts in old transistor sets. electrolytic caps with a rating of at least twice it's max battery voltage are fine. if you don't feel comfortable making these decisions you can just go with a part of the same value as the original but keep in mind finding high voltage caps isn't as easy as it used to be. it's really overkill for battery powered transistor radios.
in pic 3 you see two vertically mounted orange cylinders. those are electrolytics. they need to be replaced. in pic 4 you see one of the old 8mfd caps that was vertically mounted next to a 10mfd 250v modern cap. i had tons of these 250v caps and that's why i used them but the voltage rating is definitely overkill. granted the large amount of free space in this radio, it doesn't hurt it. you can generally go a little higher in capacitance (mfd rating) without detriment.
when replacing caps always be sure to note their polarity BEFORE you remove them. old caps can be marked by which end is positive, negative, or both ends could be marked. if you have the unfortunate luck of having one where the markings are damaged and you can read them. the lead connected to the outer aluminum shell is the negative lead. try to put the replacement part in the same position as the old part. the overall alignment of the radio can be affected by sloppy workmanship when replacing parts.
i'll show you how to get to the caps in the next step.
Step 7: The Two Hidden Caps and Circuit Board Access
in pic 1 you will see two caps that are hidden behind the transformer connected to the speaker. they are buried in a back corner of the circuit board. one of these will present a bit of a dilemma for you as you will soon find out.
in order to gain access to the bottom of the circuit board, you will need to remove the antenna (pic 2) and remove the aluminum RF shield (pic 3). be VERY careful with the antenna once its loose. look at the wires that connect to it. they are hair thin and absolutely no fun to repair if they break. if you do accidentally break one, you will have to use fine sandpaper to remove the enamel paint insulation on the wire and solder the broken ends back together. in pic 4 you see the bottom of the circuit board.
so... here's your dilemma.
if you follow the leads on the two vertical orange caps in the back corner you will see they are dangerously close to the tuning dial string and a bunch of wires. the wires can be moved out of the way by unbolting the volume control and setting it aside. you'll have to remove the volume knob to get the control lose. the tone control lever can stay on as you can fish the control out with the lever on it.
with the volume control out of the way, one of the two capacitors is much more accessible. this leaves only one that you can't quite get to. the leads on this cap are right next to the tuning dial string. tuning dial strings are the bane of any antique radio repair tech. they are frustrating to replace and many a restoration has been abandoned due to dial string headaches. the heat from your soldering iron WILL snap the string.
you can't replace it without risk of burning the dial string or without taking the tuning dial mechanism partially apart. see the dilemma here? since my radio did play before i replaced the caps, i opted to leave that one capacitor in place. technically all the electrolytics should be replaced but since i had a working radio to begin with, the risk of burning the dial string is not worth it.
one option here for those that have their heart set on replacing this cap is to follow the traces on the bottom of the board and see if there is a safer way to solder that cap in place. you might end up soldering it to two different locations that are not exactly where it was intended. check and double check you are on the correct traces and your polarity is correct.
as far as getting this poorly located cap out. one option is a bit destructive but i have used it with success. use needle nose pliers and twist the cap a few turns till it breaks its leads. if you're lucky, you may even end up with some wire nubs you can wrap the leads of the new cap in place with. if you solder quickly, you may luck out and get the new cap soldered to the old nubs and not melt the solder on the bottom of the board.
sounds like a hassle right? this is why i left that one old cap in place. verifying if your radio has any life in the first place makes such decisions a little easier. can this old cap fail? yep. is it out of spec? yep. is my radio working? yep. eventually this old cap will completely die so i'm on borrowed time.
while your in there...
this radio has two tiny light bulbs that illuminate the dial. if you need to replace them, now is the time. they are located under the dial scale behind the "eight transistor" nameplate so they illuminate the dial from the bottom up.
Step 8: It's Like a Transistor Museum in a Box
here's a unique bit of trivia about this radio, it's got transistors from three different era's in it!
GE was obviously clearing out their parts shelves when they made these. not only does it have tube era capacitors, it's got oval top hat transistors, round top hat transistors, and standard round can transistors in it. in the pic you see an oval and a round top hat transistor.
is this of concern? not really, AM radios are very simple devices. there's plenty of leniency in parts choices. in a matter of just 6 years or so, transistor production went from having to hand test and select transistors for radio use to mass production and automated testing. by the time this radio was made, transistors were still new but manufacturing had advanced quite a bit.
Step 9: A New Battery Holder
the original terminals in this radio's battery holder were trashed. since this isn't a restoration, i opted to modify the original battery pack setup.
these old transistor radios can run for a very long time on a set of batteries. we're talking a year or more with daily use. they are extremely efficient. modern high capacity AA batteries have higher Ah (amp hour) ratings than the best D cells of 1959 did. for this reason i went with an AA battery holder and abandoned the original design of using D batteries.
a 4 cell AA battery holder fits neatly inside the original battery compartment. the only mod needed was to bring the battery pack leads out into the old battery compartment and make them long enough so the AA pack can be fished out when they need replacing. you will need to fish the wires through the holes in the old pack, put the back cover on the radio, and finally solder the AA holder to the leads you just ran. make sure and leave enough length so that the cover can be taken off if needed.
bet your saying... um.. the original pack was for 6 batteries and your using 4?
yep. in order to prevent what happened to this radio from ever happening again, i load all my antique transistor radios with lithium AA batteries. Lithium's don't leak, they have extremely long shelf lives, they have much higher capacity than alkaline AA's and they produce a slightly higher voltage.
lithium AA's will measure 1.7v when new. my 4 pack reads 6.8vdc. my radio plays loud with just 6v as i verified on the power supply so it's quite happy at 6.8v. the higher cell voltage *might* be a problem if i actually loaded the radio with 6 cells. at 4 cells, i'm fine.
you may be saying "lithium AA's are expensive!!".
yes they are when you buy them at your local store. if you get them online in lots of 30-40 they can be found for as low as $1 per cell shipped. they are well worth purchasing considering their many benefits. as an added bonus they are very light. imagine this radio loaded with 6 duracell D batteries?
Step 10: Clean and Reassemble
gojo to the rescue!
gojo creme (no pumice) is amazing at cleaning up grungy plastic and metal cabinets. if you want to clean up a grungy cabinet, take small dollops of gojo and rub them all over the plastic cabinet. let it sit for a bit and do same to chrome edges of radio. use a terry cloth towel to wipe off the gojo. you will be amazed at the results.
before putting the cover back on dont forget to give the volume and tone controls a shot of contact cleaner. they have a convenient hole to shoot into right were the wires are soldered on. this will take care of any scratchiness.
put the cover back on, reinstall knobs, and load the radio with batteries.
Step 11: Enjoy Your Radio!
an antique radio's worst enemy is lack of use. use your radio!
every major city has sports and news talk stations on AM. if the talking heads aren't your thing, there's a weekly broadcast of old american standards (sinatra and such) that many AM broadcasters carry. you can find out what stations carry it here..
one other cool activity plenty of AM listeners enjoy doing is DX-ing. this is the pastime of trying to tune in far away stations. this old GE might be old but it's no slouch. this radio has a very directional antenna. as you tune around the dial, if you hear something interesting try physically turning the radio for strongest signal. this might result in the radio not facing you but you may be surprised at it's ability to pinpoint one signal and ignore another on the same frequency. nigh time outdoors is the best time to try DX-ing.
hope you enjoyed the instructable and you got your radio working right!