[pbunn]

I am reworking a National NC 60 that had been revenged in the power supply but otherwise in nice shape. I have redone the power supply and recapped the paper caps . It works some what but has a loud background buzz. It sounds like bad interference, but my ham transceiver with the same antenna does not hear it. It is worse on the AM band.

I have not cleaned the band switch or the tube sockets. Tubes are all NOS 


Any ideas?

Thanks in advance

[Radioroslyn]

Hi Pat and welcome to the Phorum.

It could be the noise is coming thru the ac line and not the air if it is interference (not hum). There are a couple of other possibilities. A miswired cap from the ac voltage over to anything on the dc side of things. A short in the heater to cathode or cathode to grid in any of the tubes will give you a 60cy hum. An open grid circuit in the audio or somewhere else can produce a hum situation.

Try plugging the ac plug around the other way and see if that makes a change. Is there any sort of "ground" connection to the set, I know this thing is an ac/dc job.

Have fun chasing electrons!

de N3GTE

[David]

Short write up on your set.   https://antiqueradio.org/NationalNC-60.htm

[pbunn]

Thanks for the assistance. I don't think it is interference coming in the power supply. I have worked hard to quieten the shack and I am not hearing it in other radios. The radio has had some modifying done in a pretty crummy way. I have rewored the entire power supply.
I replaced the multi filter cap with one from Hayseed Hamfest . The noise seems to be coming from ahead of the volume control as it is relatively quire with the volume center tap disconected. Only slight 60Hz hum as you would expect in a single ended amplifier.

The antenna input has been messed with. The BNC antenna connector has nothing connected to the center pin and a strap conncted from the chassis to the other pin. The screw terminal 2 connector strip antenna connector does not have a chassis connection. It ties to the B minus bus and the other side to the RF front end. I got a nice shock from the antenns "hot" terminal on the antenna screw terminal and chassis ground. Something is miswired in the front end.

I have two of these radios in good cosmetic condition. I may repair this one as designed. The other one I may modify with a power transformer and a full wave solid state rectifier removing the 35W4 and replacing the 50C5 with a different power tube. I have a hammond power transformer with a 250 volt center tap and 6.3 volts at 2 amps. I also have a small 50 mA 7 H choke that will fit.

Should be a fun project and not all that difficult as the radio has a B minus bus rather than a hot chassis. Withe a 6AQ5 or 6AS5 I have the needed heater capacity.

[Radioroslyn]

So if you really want to dig into this thing here's a few ideas.

Lose the 35W4 and replace it w/a complementary output tube. Resize the hole where the 12av6 tube was to a 9pin socket and replace w/a 12ax7. This will be your 1st audio/phase inverter for the p/p output. Add a 1N34 for the detector diode. Resize hole for 6BA7 mixer 9pin socket and add same. Much quieter and more gain than the 12be6.

It won't turn your star roamer into a HRO 60 but it will give it a bit of a step up.

[pbunn]

Running out of room on the modification. Installed a Hammond power transformer, a fuseholder on the back panel, A new 3 wire line cord, a 10H choke, and a new output transformer. Just enough room to install a terminal strip with diodes and filter caps. Hope to get the audio and power supply working this weekend. Not enough room for a push pull output stage.