4 tube 1948 portable AM Radio Repair - Page 1
Forum section: electronics / repair

author: lordvader88
author_rank: Frequent Contributor
timestamp: January 07, 2017, 10:11:29 am
content: Hi I got very lucky and found an old 1948 4 tube AM radio, a Rogers Majestic R109. Hardly anyone has old stuff around here it seems. It can be plugged in or run off 3 batteries. It weighs about 4.8lb/2.2kg, without batteries I guess. Its 4 x 7.5 x 5 inch / 102 x 191 x 127 mmhttp://www.radiomuseum.org/r/rogerstube_r109.htmlI even found the schematic pdfhttp://elektrotanya.com/rogers_majestic_r109_sm.pdf/download.htmlI'll add my images at the bottom NO I CAN'T THEY ARE ALL OVER 2MB, so how do I post pictures ?Its pretty clean inside, no signs of prior entry. The tubes don't even seem to glow at all, I'll have to turn off lights. But after 5-10 minutes they weren't even hot, so wow. 3 of them use 1V heaters, and the audio amp tube uses a 3V heater, so thats why.The cord was cut through in places so I finally spliced on another 1 and fired it up. And it only partially works......WHICH IS GREAT because now I get to poke around and get it working better and better. I've barely spent 5 minutes probing around, just looking for places not to touch and get zapped really.It has what seems to be 60Hz hum thats always at what I guess is the maximum volume, regardless of what the volume knob is set to.The antenna is built into the flip open lid, that hides the volume and tuner knobs, and releases a spring loaded on/off switch. The switch is only pushed down or let free to be on, there's no latching it seems, thats annoying.At 1st I wasn't getting any radio at all, but if I touch the spring/hinge of the flip cover (which contains the loop antenna) , I do get CBC 640AM here in St.John's, Newfoundland, Canada. Just by fluke I had the knob set right there somehow.It comes through pretty distorted, and mixed with that persistent full volume 60Hz hum, but I can make out the words.When the radio is out of the full metal case, the top lid w/antenna just attaches with 2 plugs for either end of the antenna. Moving that lid around makes the thing gocompletelysilent, and jiggle it a bit more and the sound comes back. But its not turning it off, because when its turned off/on, the volumes starts a bit low then ramps up over 1-2 seconds as the tubes get going. When it comes back from going silent, it just comes back in an instant. Not sure what thats about yet, shorting something, never checked voltages during that yet.So there's about +8 paper/waxed capacitors that I'll replace 1 at a time in order by the schematic, to see what changes. And 2 big multi-capacitor cans I believe. No doubt they should go too. They are in cardboard tubes, I'll try and save them for the retro look.There's one of those sand covered resistors I know nothing about yet, its leads are pretty corroded and I've knocked sand of it cause I didn't even know what it was a few months ago.1 Tube seems to have some residue build up inside, I'm new to this and can't remember what all those repair videos say that is. But they work, how well who knows. Same for the variable inductor cans, and whatever else is built into them.There's 1 component with 120VAC (I think, writing this before going to work) across it, no idea what it is, its picture 2-4. Maybe cooling fins over something. I've barely looked at this radio yet, but I've had it for months.So I have a fun project to work on and I even tracked down the SCHEMATIC. So I'll learn a bunch along the way. Just wish I had more stuff like this to play with.

author: chris_leyson
author_rank: Super Contributor
timestamp: January 07, 2017, 12:31:53 pm
content: Nice radiogood idea to replace the waxed paper caps after 65 years some if not all of them have gone leaky, resistors will tend to go high in value so some will need to be replaced. Check the grid voltages on all of the valves, a leaky cap will bias the grid to a higher than normal voltage and the valve will draw excessive current which is probably the reason for the distortion. Thinking about it, it's probably best to replace the caps around the audio amp and work backwards from there. Good luck with the repairs.

author: lordvader88
author_rank: Frequent Contributor
timestamp: January 07, 2017, 12:58:39 pm
content: Yeah I'm going to replace them all, but 1 at a time in order, so that I'll learn more. I've barely fixed anything, I've only getting serious about EE this past year.Inside is compact but its sort of a rats nest since its a 3-dimensional layout.I can't find transistor radio's around this city to play with and try to fix. I'm sure there's some but 99% just tossed them years ago I'd say.

author: Seekonk
author_rank: Super Contributor
timestamp: January 07, 2017, 01:35:16 pm
content: Why that's just like the heavy portable radio Hitachi Mitsubishi carried around to his ear in a train station. He heard a group of nuns talking. One said "Hurry or you'll miss the train sisters". At that moment he thought to himself, "Transistors, that just might work."

author: lordvader88
author_rank: Frequent Contributor
timestamp: January 07, 2017, 02:52:42 pm
content: LOLDam this thing will be hard to unsolder with my cheap solder pump and plus only 2 hands. Solder wick is in the mail with 2-4weeks to go yet.Lots of parts are buried, and most component (or some) component leads are wrapped around or through and around pins/holes, so its tough, but hard to take apart.I'm not out to preserve this thing (I would if I had the means), but I'm not just going cutting stuff out and then bodging it back together if I don't have to.There's stuff riveted together too, its quite the piece of work, looks great TBH. Lots of little wire lead covers cut to perfect lengths too.

author: Seekonk
author_rank: Super Contributor
timestamp: January 07, 2017, 03:03:25 pm
content: Just use a pair of side cutters to remove parts. Terminal strips and sockets can be fragile at this age. I like the symbol for electrolytic capacitors.

author: TimFox
author_rank: Super Contributor
timestamp: January 07, 2017, 03:38:38 pm
content: You may need to replace the HV rectifier, which is probably selenium (stacked wafers). A normal 1N4007 should suffice. Note that one side of the power supply connects directly to the AC line.

author: Seekonk
author_rank: Super Contributor
timestamp: January 07, 2017, 04:01:41 pm
content: Those selenium have quite a voltage drop. You will likely have to double the resistance of R4 if replaced with silicon. I imagine the design point for this radio is about 90V if it will run with a 67V battery.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: January 10, 2017, 02:57:10 pm
content: Quote from: lordvader88 on January 07, 2017, 02:52:42 pmOthers may have better approaches, but what works best for me on this style of construction is cutting the lead near the body of the component. This leaves something to grab while unwinding the rest of the lead from the solder lug while unsoldering. This works when the part you are removing is the top attachment to the lug. You are stuck with more challenging work when it is not on top, particularly if you must or want to salvage the other components.While the older lugs can get brittle with age, replacements are still available so all is not lost if you break one. Still worth avoiding because of the amount of work and possible collateral damage that can happen as you transfer all of the components from all of the lugs. The good news is that all of the components are far less heat sensitive than modern components. You can fry them, but not nearly as easily.

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: January 10, 2017, 05:14:07 pm
content: Quote from: Seekonk on January 07, 2017, 04:01:41 pmI find the easiest way to spec the correct resistor is to either get the voltage after the rectifier from a schematic, or by measuring, and then put in a hefty resistor to obtain the desired voltage. Testing is easy if calculation is not coming out precisely.

author: PaulAm
author_rank: Frequent Contributor
timestamp: January 10, 2017, 06:07:02 pm
content: The power supply caps are shot for sure (as are the wax paper caps). For gear of that vintage, it's also possible for the resistors to change in value; most of those were 20% tolerance anyway, but they might be worth a check.If you want to get some real expertise check outwww.antiqueradios.com/Folks over there work on that vintage equipment all of the time.You should use an isolation transformer if you're poking around with that thing plugged in to the mains

author: Ozo
author_rank: Newbie
timestamp: January 13, 2017, 03:57:12 pm
content: +1 to PaulAm's comment. AntiqueRadios forum is my favorite. It is an international forum too.