[Raleigh]

I have been restoring a Crosley 106CP for a friend.  This is a 10 tube chassis with AM/SW and phono input from a console.  I think it is either immediately pre-war or immediately post-war, as it was full of the crumbling rubber-coated wire.

It works perfectly in my basement workshop, but installed in the cabinet at the owner's house, there is a fairly strong modulation hum.  It hums whenever I tune into a station.  In between stations, no hum.

I've tried all the common solutions for modulation hum listed in Marcus and Levy.  1) Line filter cap, 2) RF Tube, 3) Converter Tube.  I've made sure the antenna is connected properly.  Nothing made a bit of difference.

I figured I missed something, so I brought it back home to check out again.  I dummied up a loop antenna of the same dimensions and orientation as in the cabinet, and the radio performs strongly with no hum.  But it hums in the cabinet at the owner's house all the same.

If anyone has a possible solution or something else to check, I would appreciate it.

[Paul Philco322]

Maybe something at his house, did you try another set in the area of the cabinet, maybe it is a problem in his home?

You never know......maybe something not related to set

Paul

[Raleigh]

Thanks, I actually didn't think of testing the location with a similar radio.  Next time I go over there I will take a one to test in the same outlet.

[Radioroslyn]

Hi John,
You may find at the customers location has some strong interference from modern electronics. Chargers can be pretty awful and cfl's are top on my list for rfi.

Happy hunting!

[Raleigh]

I was pretty sure I'd missed something in my restoration until I brought the chassis and speaker back home. I had tested it originally with a random wire test antenna, so I figured maybe there was enough hash and static that I didn't notice the modulation hum. But when I dummied up a loop similar to the Crosley original and it was quiet with no hum, I figured it had to be something at the site.

I'll have him turn off lights and so forth if he has fluorescent lights. Thanks for the tip about chargers. I was wondering if there could be anything wrong with the way the outlet was wired that would make this happen, like if it had a bad ground connection or something. Any thoughts?

But here is one more question. If it were interference at the site, I can't figure out why it would show up as modulation hum instead of humming across the band and not just on as station.

[Paul Philco322]

There is so much stray RF and electric appliance intereference who can say, in a world of no wall warts, old incandescent bulbs and no computers, surge protectors, reception was easier!

Good luck.

Paul

[Arran]

I should point out that one of the real winners for RF noise are cheap switch mode power supplies, and even some that are not so cheap like computer power supplies. I had an old computer that ruined AM reception when it was one, not nearly as bad as one of the old DVD players I had though, that would completely destroy AM reception even when it was off. In terms of wall warts the new cell phone charger ones are usually switch mode, and those can cause hash, the old style "wall warts" with a linear power supply, with a proper power transformer, didn't cause that many problems. Another suspect can be some power supplies with a capacitive dropper, but those are more localized for interference such as if the radio is plugged into the same outlet as one, not over the whole house. You were wondering about whether the outlet may have been wired wrong, well it could have, but not so much with the ground prong as with having the hot and neutral wires reversed, 
Regards
Arran

[Raleigh]

This radio uses only one .01 uf line filter cap from the power switch to ground. I put a polarized plug on the chassis, with the hot side going through the switch, so only the hot side is connected to the line filter cap. There isn't a filter on the neutral side. Because the plug is polarized, I can't try reversing the plug to see if that makes a difference.

By the way, the filter is now a new safety cap. It is definitely not open.

Similar Philcos put a .01 filter on each side of the line to ground. If I put a second .01 safety cap from the neutral to ground, could that make a difference, especially if the hot and neutral in the socket are reversed? I've tried this on my workbench using test leads, and it makes no difference whatsoever, but I don't have any modulation hum here. There is an easy spot to mount a cap from neutral to a good ground nearby.

The reason I keep coming back to this is that Marcus and Levy stress the line filter(s) as a cause of modulation hum.

Next time I go to the customer's house, I'll take a 41-255 for comparison and a socket tester.

[Radioroslyn]

Don't be fooled by the .01 across the ac line thinking that it will bypass all of the noise on the ac line. It helps some but a lot can get by it. I used to think that a set w/a power transformer would block rfi and be impervious to pass line noise because of the inductance in the primary winding. I was chatting w/Mike about this issues and he brought up the fact that do to the close proximity of the pri to the sec winding there is some capacitance coupling between the winding and that the rfi can pass though.
As a practical matter I find that sets that have series strung heaters are more susceptible to rfi  than a set w/a pt.

[Mondial]

John, have you tried grounding the chassis with a separate wire to the electrical outlet ground? With the line bypass cap connected to the hot side of the line and no external ground connection, the chassis is floating at 120 V AC. This can cause all sorts of hum effects.

You can try making an adapter so you can reverse the plug polarity and see if it makes a difference. In general, if you are using only one line cap and no external ground, the cap should be connected between chassis and neutral, so the chassis is at AC and RF ground potential.

[Arran]

I had a look at the schematic for this set, it's a 1946-47 model by the way, and it shows a .01 uf capacitor connected between one leg of the transformer primary and ground, which is the same side that is connected to the power switch. Many non Philco radios, such as RCAs and G.Es, used what is called an electrostatic shield to reduce noise coming in from the power line, it's a strip of copper foil wrapped around the outside of the transformer windings and connected to ground.
Regards
Arran

http://www.nostalgiaair.org/Resources/191/M0003191.htm

[Raleigh]

Arran, is the electrostatic shield inside the transformer? Do you need to take the transformer apart to see if it is there?

I used a polarized plug. The power switch, fuse, and the .01 filter cap are on the hot side of the line. I decided to add a second .01 safety cap from the neutral side to ground.

Thanks for the help, everyone. You've given me a few things to check next time I go to the customer's house.

[Radioroslyn]

Something else that came to mind is to try some snap on ferrite cores over the line cord. This may help reduce incoming rfi from the house wiring.

[Arran]

Yes;
  If there is an electrostatic shield it is inside of the transformer, sometimes they wrap some of the copper foil on the outside of the iron, but most of the time you would have to remove one of the end bells (covers) to see it. However if the set had one it would be indicated on the schematic as a line parallel to those denoting the iron laminations, but with a ground symbol attached.
Regards
Arran

[Raleigh]

Thanks for all your help and suggestions!