[Edisla]

I just finished my 40-180 restoration and now that it is all together I am hearing a slight hum.

I can disconnect the speaker and get a little hum I guess just from the transformer itself resonating through the cabinet but once I connect speaker it gets a tad louder.

I don't recall hearing this when it was on the bench.

Do these units usually exhibit a slight hum or do I have something else going on?

Most might not hear it but I can of course

[KCMike]

Some of my radios have a low level hum in them. I have also found that it can be a light or appliance in the house wiring circuit causing the hum.
Be careful with unplugging the speaker on some radios you could do damage to the circuit.

[Edisla]

Some of my radios have a low level hum in them. I have also found that it can be a light or appliance in the house wiring circuit causing the hum.
 Be careful with unplugging the speaker on some radios you could do damage to the circuit.

Its not enough to deal w/ so I have it plugged in and will enjoy it until something blows up

[DeckApe]

My 40-190 has a similar slight hum to it, not a huge one (really just a mumble). I'm told that the wizardry 'neath the chassis has a "hum-bucker" in it, which can either be utilized or defeated by reversing the speaker wires. Upshot: wire in backwards, hum in speaker. (Mine, like yours, is small enough to leave it alone.) Anyone able to elaborate?

[morzh]

Hum-bucker is in the speaker, not in the chassis.

It is a turn (few turns) of a wire around the field coil, connected in series with voice coil, but so the winding goes opposite to it; this way the influence of the field coil to the voice coil is countered.

However your slight hum might be due to:

1. Not enough filtering.
2. House wiring (I have that). Good way of checking - have a transistor battery radio on the same frequency and see if it also hums. Short of taking care of your house's wiring there is not much that can be done.
3. Hum-bucker wired in backwards.
4. Transformer buzzing.
5. Power line close by. Also not much can be done.

[mikethedruid]

Chasing down a tiny hum like that can be maddening. One possible cause also might be bad grounding of some of the decoupling caps through rivets. I would check the grounds on all these connections also, see what they read to chassis. If one reads a few ohms instead of a dead short, you may have found your culprit. Best of luck.

[morzh]

In my case of Philco 20 (2 weeks ago) it was bad Ground connection in the 27 tube's socket.
Yes, rivets are often culprits.

Also bad grounding of the coil shield cans. Simple twisting motion restores it by breaking the oxide layer that has formed in all those years.