looking over the chassis I notice someone added a capacitor the existing wet cap, next to the rectifier tube. it is connected to the stem of the original cap, the transformer, the rectifier and one lead of the speaker.
I'd disconnect both. Chances are the wet cap is no good and the dry one isn't too far behind it. Lose both so you won't be looking for a power transformer or burn your house down.
As Mike and Terry said, that cap that was tacked in under the chassis was probably an old repair to replace the original electrolytic cap that had died. The old cap is probably an aluminum can that sits above the chassis with the lug below. You can take off the replacement cap and stuff new electrolytic cap into the can. There are a number of threads showing how to do that but here's one to start that gives you and idea of what to do. You will need to replace all electrolytic and paper caps. mica caps are likely still good. Also check and replace resistors that have drifted high (>20%). Some folks replace all resistors any way. 1940 sets may have rubber coated wires that need replacing as well. See this thread.
Yes, that can was added as a old repair, the original wet electrolytic probably dried up and in effect went open circuit. There were a lot of service shops, and even more amateurs that used to do that, even though every technician's course, and even some factory service manuals, told them not to.
This very afternoon I was removing a very similarly mounted can, though I am not sure why it was added as the originals were, and still are, functional. They connected a two section twist lock can under the chassis with each of it's 10 uf 450 v caps in parallel with the two main filter caps in the original can, wired in directly below. My guess is that they wanted a hum free set so they decided to add the extra caps under the chassis. However even though two of the sections of the original twist lock can were good, the third 10 uf 450 v cap was starting to fail, but it was connected to the plate of the RF amplifier tube. I'm replacing them all, they may work now but the newer can was put in there in 1962, the original in 1940, so time for something made within the past decade.
Regards
Arran
(07-18-2015, 04:35 PM)klondike98 Wrote: Welcome to the Phorum!!!!
As Mike and Terry said, that cap that was tacked in under the chassis was probably an old repair to replace the original electrolytic cap that had died. The old cap is probably an aluminum can that sits above the chassis with the lug below. You can take off the replacement cap and stuff new electrolytic cap into the can. There are a number of threads showing how to do that but here's one to start that gives you and idea of what to do. You will need to replace all electrolytic and paper caps. mica caps are likely still good. Also check and replace resistors that have drifted high (>20%). Some folks replace all resistors any way. 1940 sets may have rubber coated wires that need replacing as well. See this thread.
Thanks - yes I'm a newbie/am, but can use a multimeter and can solder. The original cal looks like it's insulated from the chassis. so is there a ground somewhere?
Also I believe the repair was made because the clamp screw is facing the radio chassis and you'd have to remove the entire frame assembly to access the screw.
My 2nd question is what is the best way to check the resistors? there seems to be close to 800 of them.
And so when I replace all the caps and drifted resistors when can I plug it in to see if it works? I don't want to let any smoke out. Does anyone have an amateurs checklist of what can be done with a multimeter and soldering iron and capable of reading a schematic , but doesn't have any of the fancy tech gear?
I get the feeling that without oscilloscopes, tube testing dodads and a doctorates in schematic reading is it easier to send it to a shop and have a professional do this?
PS - We had a house fire 5 years ago. I DO NOT want to go through that again. 3 daughters, a wife, two dogs in a double wide on the front yard for a year. There are only a few people I would wish that on!!
800 resistors.... yes, you did not pick the easiest radio for your first restoration. There are folks out there who restore vintage radios if you decide to go that route. If you DIY you will need a good readable schematic. You can order docs here for just a few bucks. A free dowload can be found here but its difficult to read.
Electrolytic caps do not always have the negative lead connected to the chassis. In your case cap 78 neg lead is connected to the center tap of the power transformer and thus the can is insulated from the chassis. Cap 77 however has its neg lead to chassis ground. Also note the two 0.01uF caps in bakelite block #85 should be Y2 safety caps. You can read about safety caps for use on power line input here.
You check resistors with an ohm meter one at a time. In many cases you can just put the ohm meter leads across the resistor and check it but if there are other paths for DC current then you must unsolder one side of the resistor and then check it. As I mentioned, some folks just go ahead and replace them all with modern resistors of the correct value.
As someone who has revived a grand total of one radio, I will say that I just replaced all the resistors in mine. I figured "I'm under the hood anyway" and some of the tighter corners were best dealt with my removing components A, B, C and XX to get to capacitor ZZ9 Plural Z-Alpha; why not just go ahead and renew A, B, C and XX while I'm there.
As someone who has revived a grand total of one radio, I will say that I just replaced all the resistors in mine. I figured "I'm under the hood anyway" and some of the tighter corners were best dealt with my removing components A, B, C and XX to get to capacitor ZZ9 Plural Z-Alpha; why not just go ahead and renew A, B, C and XX while I'm there.
Makes sense - I subscribe to the "you made it to the gym honey - do the D**n ab cruncher and stop whining"....Guess I can take the same medecine!!!
Ok so next question is all the resistors, caps and wire tidbits are twisted together and soldered as one hunk of love. Do I unsolder the entire mess or snip tight and solder a new mess onto the old mess?
Also - if I measure resistance directly across the resistor, does it matter where it's located in a circuit? Should I get the resistance between the two point where I measure from??
Yes it does matter where the resistor is in the circuit. As I mentioned earlier.. In many cases you can just put the ohm meter leads across the resistor and check it but if there are other paths for DC current then you must unsolder one side of the resistor and then check it.
Re: the twisted wire mess shown in your photo....if you restuff the original capacitor can, the leads from the replacement cap will come out. That should help clean things up.
(07-21-2015, 10:43 AM)klondike98 Wrote: Yes it does matter where the resistor is in the circuit. As I mentioned earlier.. In many cases you can just put the ohm meter leads across the resistor and check it but if there are other paths for DC current then you must unsolder one side of the resistor and then check it.
Re: the twisted wire mess shown in your photo....if you restuff the original capacitor can, the leads from the replacement cap will come out. That should help clean things up.
got it - looking at the tube sockets for instance it looks like the original work was done by twisting the resistor wires and caps around the tabs and soldering them at once. To replace these resistors and caps would i de-solder and remove all that original solder and wire? I think I may have answered my own question...
Ah, now I understand your question...just a little slow this morning. Best to get a solder sucker and take as much solder off as possible, then work the leads off the tube socket lugs. Once the leads are off you can suck off the last off the solder and have a relatively clean lug to attach the new leads to. Sometimes the area you are working in can be crowded so there have been times I've clipped the old lead but left enough of it to use as a lug to solder the new lead to, but I usually take as much of the old out as reasonable.
I'm with Klondike on this one. I did my best to clean up as much of the chassis as possible--including complete removal of old components wherever possible (unless, as noted, I needed the lead extension) and the whacking great wads of solder from the previous connection. Less to heat up in case my new component fails or (more likely) I hooked it up to the wrong lug. (Ahem.)
yes, you did not pick the easiest radio for your first restoration. There are folks out there who restore vintage radios if you decide to go that route. If you DIY you will need a good readable schematic. You can 
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