After fixing up the cabinet to this radio and enjoying staring at it for awhile, it is finally time to take a whack at the chassis. It worked fairly good as it was but it had a hum that was always in the background which gets pretty annoying after awhile. Its also not good for the tubes and what not when it does that anyway. The FM band on this radio doesnt work either. these issues will all be fixed in this instructable plus a couple of other things to make it more "usable" for newer technology with an IPOD/ aux input.
you need some tools for this plus some safety common sense
1: multi meter
2: soldering iron
4: scrap wire
5: wire snips
6: new capacitors!
7: earphone jack
8: a drill and a 1/4 inch bit
Step 1: Inventory the Caps to Be Replaced
Next, get a piece of paper to write down the values and voltages you need. I also wrote down conversions from microfarads to picofarads to help when i go to select the new ones. from what ive done research on and what the great people on this website have told me, the paper and wax capacitors are the ones that need to be changed, which is underneath the chassis. As far as the humming noise goes. that is the electrolytic capacitors in the power supply circuit, mine was located in a metal can on the top of the chassis which contains 3 capacitors inside it actually.
Make sure the radio is unplugged before you go poking at things! on most chassis, the metal case is the negative part. but while you're in there, look around and see if there is any burnt out parts or wires that need attention.
Step 2: Purchase Your New Parts!
If you replace a cap that has a lower voltage rating than what is called for, you will come to know its wrong as the radio powers up and you hear the caps explode! so be careful and write down what the voltage ratings are. you can always go above the rated voltage but NEVER lower. a capacitor will only take in the voltage its given. example: if the rating for a cap you need is 100v, you can get a 150v, 250, etc. and it will work the exact same as a 100v.
Polarity is also something to look out for, especially in polarized electrolytic caps. these are the ones cylindrical in shape and have a black stripe on one side. if these are installed backwards, it will also explode! the power supply caps i used to replace the can caps are the only polarized caps i used in this whole restore.
Keep in mind though that there are non polarized electrolytic caps out there too. they look really similar in appearance but dont have a strip on them and the box they come in says non polarized. you can install these in any position. polarity doesnt matter and they wont explode if the voltage value is where it needs to be.
Values are important in the fact that the radio wont work correctly. these values are listed in microfards on the old paper and wax caps but are abbreviated "MFD". when buying new caps, you can buy ones in picofarads too if you do the conversions for them. it is the same capacitance if you did it right. thats why i wrote it in different values for the same cap on the paper. you can look up a conversion table on google. on my radio parts list in the manual online, it says i can go +/- 20% which means i can go 20% below or 20% above the rated value and it will still work the same. older radios arent so specific i guess.
When youre at the store or online buying these things and youre like me and cant find half of the value and voltages you need, dont freak out. with capacitors you can parallel them and put them in a series to get the value you need. say if you need a 20mfd cap 250v, you can get two 10mfd 250v caps to get the 20mfd value you need. keep the voltages the same if possible or higher. just because they are in parallel the voltages is only as good as the lowest one. i didnt need to put any in a series to get a value i needed but if you need to the option is there.
one last thing, a good thing about capacitors, it more or less doesnt matter what type of cap you use as long as its value and voltage is what you need. I used polarized electrolytic in the power supply because thats what it originally had, and i mostly used ceramic and mylar caps for the rest of it simply because the ceramic caps handles alot of voltage when others dont. i used non polarized electrolytic ones where i could too.
i also purchased a decent soldering iron and solder while i was there. those battery powered cold heat ones dont seem to work for this project.
Step 3: Let Us Begin the Fun!
not me though, i started with the hardest ones i could, those being the power supply circuit caps in the metal can. three of them in that thing. not the same value either. on the side of the can it has symbols and values next to the symbols. underneath the can next to the wire post should be a symbol matching one on the side so you know what cap goes to which wire. sadly... not in my case. there is symbols on the can as you can see in the picture but no symbols underneath. in my can there is 20, 20, 40 mfd 250v caps inside. funny and true story, i spent an hour at the store looking for just these values that they didnt even have. they didnt even have the values i could parallel or series them to work. i did however find 47mfd 250v caps there. so now i had one cap that would work within 20% i could use. but inpatient me, i got to the point where i just said screw it and i got three 47mfd caps for the power supply. i read that if the capacitance is too low the radio will hum which is why it was doing that in the first place because the old ones dried out. and if the capacitance is too high, more power will flow through the circuit which is also not good.
i desoldered one wire of a post and soldered it onto the positive leg of the new cap (keeping in mind the polarity) and did that for all three now that they are the same value. then i soldered the negative legs of all three together and soldered those to a wire which was then soldered to the chassis (chassis being the negative). the metal can is a negative common for the old caps inside
you should always power up your radio after each cap you do to make sure its working properly and not going to explode etc... when i did this i didnt hear any noise or hum come from the speaker like i usually do. so i turned the volume up and all i heard was crystal clear audio! no hum whatsoever! i fixed it! my gamble with those caps paid off. no issues at all. sweet!
once these are soldered, make sure they are secure and wont contact the chassis or other components. once again, be careful when testing it after each new cap, there is DC voltage in here when powered up. i measured it at 142vdc in some spots and anywhere above and below that in certain places. DC with a little current can kill!
Step 4: Snip Snip Here We Go!
to start doing this, start with one cap and snip off the leads right at the cap to leave some lead to solder on your new one. (makes it easier to relocate if you forget and and also easier to solder). i double checked the value on the old wax cap to match up my new one. i then twisted each new cap leg around the old lead and soldered it on. these are non polarized so it dont matter which way its put in.
as you can see in the picture i paralleled a couple of ceramic caps to get the value i needed. there is also some original ceramic capacitors in here, and im told they dont need changing unless they are broken or burnt. why did they not put ceramics everywhere instead of those wax and paper ones? feel free to comment on this i would like to know!
I just kept moving one capacitor at a time working my way through each section and then finished on the other side of the radio on the am/fm circuit caps. there is also some RF chokes and transformers in here, it would be a good idea to ohm these out with a multi meter to make sure they dont have an open connection somewhere. check wires too, as you can see in the picture they are the fabric covered kind that could short against the chassis if a hole is rubbed in it. mine checked out great and the material is still really strong.
Step 5: Replace Tubes If Necessary
there is an antique store in vegas that i recalled going to that had a huge basket of old radio tubes in it. i went there and spent probably 2 hours going through all the tubes looking for some replacements for my radio and i actually found every kind except the 12AT7.some tubes looked brand new. i did some research before i left to see if i could substitute different ratings for the original and discovered a couple they had there would work in place of the 12AT7. so i got one 12AX7 and a 12AU7 which the AX has a slightly higher gain than the AT, and the AU has a lower gain than the AT. so i got one higher and one lower. the gain just makes it sound a little different i read online.
I took my "new to me" tubes and went to buy them. there was 6 tubes i wanted and i talked to the lady who works there and got the price down to $10 for the 6 tubes. which in my research wasnt bad at all for all them compared to buying them online. i priced them on ebay later and a couple of them were going for $10 each while others were around $2 on average. buying those 6 tubes off ebay including shipping would have cost me almost $60. so it really does save money to do your research. plus i didnt have to wait to get them!
i came home and plugged them in, and all of them work great, as far as all the heaters lighting up brightly, but the radio didnt act any different or sound any better by me doing this. on the bright side i got some spare tubes for the radio.
it still has no FM band, for the fact that i did look up other radios on the internet of the same exact brand and model and their FM worked, so i know i missed something, but its no big deal. i came up with a better solution to the problem. IPOD input!
Step 6: Can It Have an AUX Input?
some people used AM transmitters or FM transmitters and even bypassing everything directly to the speaker to do this, but thats just not the same as the sound actually being produced by the radio. plus it wouldnt have that old radio sound. i spent 3 days reading up on this and which tube i hopefully can hook it up to. i came across this website: http://tubedata.tubes.se/sheets/093/6/6T8.pdf which is for the 6T8 tube, which is the preamp tube. this tube has 3 inputs two are for the AM/FM detector and the third is beginning of the audio part.
i used the schematic to find the FM input post on the bottom of the tube and put a wire on that and another wire on the audio input post. then to be sure i turned it on and put it on a multi meter to make sure i wasnt hooked into power, and it showed 0 volts. next step, i ran the wires to my cd player and plugged it in, turned it on, and like it was meant to be, perfect clear sound coming from the radio speaker. yay! i turned it to AM with the cd player still going, and it was just AM so my new input wasnt on the AM circuit. turned it back to FM and it was there and the radio controlled the volume like i wanted so everything works like i wanted it.
Step 7: Installing the IPOD / AUX Input
i did a "dry" run without solder to confirm it will work before i solder it on. as i did this i noticed that once the jack is on the chassis the metal on the jack is the same as the ground post on the back so it worried me a little bit because i didnt know if that being grounded to the chassis would be an issue. i put one wire on the terminal and then touched the jack to the chassis while everything was on and it worked better than just with the two original wires! awesome, so now i only need one wire to make it work!
I tried the one wire on the number 2 post that goes to the FM but it wasnt as loud as the number 1 post. i soldered the wire onto a lead coming from the ratio detector to the number 1 post on the 6T8 tube, and tested it again for sound and it worked great, only able to hear it on the FM band and not AM. (AM still working the same.) i guess the ratio detector is also FM? last thing i needed to do was run the wire to the jack and solder it on and tighten it up. confident i was now done i double checked for crossed wire and leads and then put the radio back together in its cabinet with a big smile on my face.
Step 8: Hook It and Enjoy!
After its all put back together the only way anyone can tell anything has been done is by an almost invisible jack below the antenna. other than that one wire soldered onto the tube terminal this radio now had an AUX input that almost zero changes have been made to the radio. i didnt cut any wires or alter the circuit in any way whatsoever. i could have just soldered a headphone plug on it and been done but it would always be there in view.. this way i can unplug the cord like its never been there and no one would really know. and if one day the FM decides to start working again on its on by some miracle it will be able to.
i tried my CD player, IPOD, and cell phone on this radio with the input and it all sounds great. it has clear sound with no hum or any annoying tones at all. it still has the distinct tube radio sound like it should, the volume can be controlled by its own knob. the AM works perfectly like it should, no static from the volume control either. out of curiosity i plugged a mircrophone into the input and i could hear my voice coming over the speaker too. i dont see a use for this but it can be done.
this couldnt have had a better outcome than what has happend. everything works as it should and almost as if it was designed with this and originally had it. all tubes work and are operating at max quality with new capacitors. ive had this radio on for almost 3 straight days before i wrote this and it has worked perfectly, still no hum, or funny stuff. im in the clear.
here are a few resources that i used to accomplish this. all credit goes to them.
- http://www.nostalgiaair.org/PagesByModel/238/M0001238.pdf service manual, schematics, parts list for my radio
- http://tubedata.tubes.se/sheets/093/6/6T8.pdf tube data for my 6T8 tube
- http://antiqueradio.org/recap.htm how to replace capacitors in a radio for beginners
- http://antiqueradios.com/ antique radio resource website
- http://instructables.com everyone's help and their projects helped me alot