[mikethedruid]

First, looking at the schematic, the resistor that goes between the center tap and chassis ground is 395 ohms, so that is what I would expect to see between center tap and chassis. that 395 ohm resistor appears to set the bias voltage for the grid of the #42 Audio output amplifier. The capacitor across it is to remove ripple from this bias voltage. Bigger capacity would work even better. I notice two capacitors marked number 10, both are .09mfd decoupling caps. If either of those are bad or miss wired, or cold soldered, you could have hum from them. I would replace them with .1mfd 600V poly caps, which you may already have done. Check and check your wiring again.

[loosecannon]

thanks mike.

yes, the two #10 caps were originally in one case together, and i did indeed replace them with .1uF caps.

in the info i have, part #36 is a 325 ohm wirewound resistor. i have replaced it with a 330 ohm resistor.

i will be checking for bad solder joints and wiring errors tonight.

in searching the forum i came across a thread where someone had gotten two of the speaker leads mixed up and got a hum from that.

i thought i labeled things pretty well, but just in case this happened; would i still hear the radio stations if two of the wires got crossed?
when i saw the term, "hum bucking" in reference to the transformer on the speaker, it reminded me of a guitar pickup, and if those are wired backwards you are sure to get hum.

just a thought.
LC

[jerryhawthorne]

Yes LC reversing of the wires on the hum bucking coil would result in well HUM. It would not stop the radio from receiving stations.
Jerry

[loosecannon]

thanks for the quick reply Jerry!

i was hoping someone would respond before i went home tonight.

this is going to be the first thing i try tonight!

LC

[morzh]

Bah!
Hum buck!

[Radioroslyn]

If it is hum you are trying to get rid of try grounding any ground connection that is connected thru a rivet. Use a wire and solder it to a gnd lug and hook it to the part that is to be grounded. I know there is one that is on the 42 tube that can be a problem.
GL
Terry

[loosecannon]

thanks for the replies.

i wish i would have messed up the wiring to the bucking coil because that would have been an easy fix, but alas, i had it right the first time.

i started poking around the chassis, moving parts a bit to see what i could change, and i temporarily put a .01uF cap from the non-switched wire of the AC cord to chassis ground and the hum instantly cut almost in half.
still there but much better.

essentially, i added cap #32 back in to the circuit.

feeling hopeful, i added another .01uF cap from the other AC cord wire (at the switch) to the chassis, and it cut the hum down even more.

i really have no frame of reference as to what an acceptable amount of hum is in a 1937 radio, as im only 39 years old and have never heard one. LOL

all i can say is that the hum is there, but once you turn the volume up your ears can start to dismiss it when you are paying attention to what the radio announcer is saying.

i only have about a three foot piece of wire connected as an antenna right now, and it is obvious by touching the end of the wire that a better antenna would only help matters.

i will try attaching a wire to the chassis and then touching it to various parts of the radio that should be grounded and see if that helps.

still open to more ideas, but at this point i think the radio is listenable and thats the real point.
LC

[jerryhawthorne]

Your making real progress LC. There really shouldn't a "noticable" hum but if your fine with the volume to a listening level, that's great! I would still try the 10mfd cap across the resistor from the center tap to ground. As indicated, that resistor sets up a negative voltage to bias the amp. It needs to be a clean DC and the earlier schematic that used it seems to make sense. Any ripple in this will add some hum.
Again, I'm dumb on this stuff.
Jerry

[morzh]

Do you have any other radio that is known to work in this AM band?
Can you make sure it does not hum?

[loosecannon]

you are right Jerry, i will try the 10uF cap.

Morzh, that is a good idea. i have a radio shack DX-160 shortwave radio set up right on my repair bench, and i will check to make sure it doesnt have a hum.
i see where you are going with this, but im already pretty sure that it doesnt.
LC

[morzh]

Well,

All my radio have the same exact hum that is a function of my house wiring and which subides quite a bit when the chandelier in my lobby turns on.
Inluding a small pocket transistor one I have.

This is not a prblem with the radios but with wiring.

Same is the case with my dad's Westinghouse radio I gave him.
I checked, it is present in my car's radio when parked near his garage.

Only on BC band.

I want to make sure this is the radio and not you Mains.

[loosecannon]

well the radio is finished!

thanks for all the help and suggestions, and for bearing with my questions.

i believe that the suggestion about checking rivets and bolts attached to the chassis was the key to eliminating the hum issue.

the reason i say " i believe" is because i basically tore the whole thing down again and built it up better than i had previously.

in the first incarnation, i had left some of the original wiring, figuring that it was fine.
and i still believe it was, however i can be a bit of a perfectionist and just decided to do it right.

all the parts and wiring came out, all the chassis rivets and bolts were tightened if possible (bolts) and given the pro gold treatment.
i think this really helped all the old oxidized metal.

everything looks so much better than the way i had it before, and the hum is all but non existent.

i did make one pretty big boo boo though.

its kind of embarrassing to admit, but when i put the parts back in, i somehow put the 10uF cap in backwards!

i had the chassis on its side, with the speaker hooked up, and was doing the "turn it on and listen to the beautiful music" test. no noise at all from the speaker, but turning the sensitivity trimmer cap produced a few clicks from it.
in an unbelievably lucky twist, my wife came in to the radio room to tell me i'd been in there too long, so i stood up and turned my back on the still "on" radio to tell her that i was almost done.

then...

BOOOOM!!!

big plume of smoke and a shriek from my wife. i turned back around to find capacitor remnants everywhere.

just moments before the cap blew, i had my face pretty close to that chassis.
thank goodness for her checking up on me.

after turning the radio off and unplugging it faster than the flash himself, i peered in to ponder my predicament.

and there was my mistake staring me right in the face.
the arrows on the side of the cap pointing right to the pin on the rectifier tube.
DOH!

again luckily, i had the forethought to order two of them when i made my parts order for this radio.

one new cap soldered in, cardboard fluff blown out, a THOROUGH check for any other mistakes, and the radio came to life like it hadnt in 50+ years.

no hum, and after a few tweaks to the trimmer caps, she was picking up nearly every AM station in town with a 6 foot piece of wire.

i gotta tell ya, the sound had a timbre to it that id not heard before. it was really addictive and made me want to sit and listen to sports radio and pretend it was 1944.
(whatever that was like LOL)

i delivered it to the person who had me restore it, and it worked just as well at his house. thank goodness again.

im really proud of this one, and if i ever come across one again, i will surely grab it up to do one for myself.

i sure learned a lot doing this project, and it was nice to not worry about burning up sensitive PC board traces for once!

i do have some before and after pics on my phone, including the blown cap.
so after i figure out how to get them from my phone in to my email, i will post them here for posterity.

thanks again to a wonderful group of very helpful and knowledgeable people.
you guys rock.
LC

[morzh]

Well, all is well that ends well.
The beginning is always trouble, as the Russians say (and they should know )
They also say, "The first crepe aways comes out scrambled".
I always add - but still tasty.

It works, no one got harmed, ad you had fun (I hope). And this is what counts.
Mazltov!