Still working on this fine old TRF, and its working and very sensitive. However, there is distortion in the audio. At first, turned way up, the audio would go dead, then recover after about a minute. One 45 output tube developed a faint blue glow when volume up, so replaced the tube. Now, it doesn't cut out, but there is still distortion. The on-line Riders says 45 bias should be 46v. I measure essentially nothing. I assume that 46v is to ground. Measured the grid-to-grid resistor (part #3423/100k) and got 104k. Measured the Secondary of the input transformer and got 3.48k and 3.03k each side to ground. Shouldn't they be the same? I get 1.37K across the transformer's primary. There is 300v on each plate. Finally, I notice little or no difference when using either tube alone. Anyone have an idea what's going on? Thanks for the help. Jim.

Hi Jim

Measure voltage from grid to filament on each #45 tube. Both tubes must have the grid around 40 volts negative in relation to filament. The radio can operate with only one side right but with distortion.

Center tapped transformers will not be exactly the same. One half of the winding will be inside. Although both have the same number of turns the winding inside has less wire, will have lower resistance.

Norm

:Still working on this fine old TRF, and its working and very sensitive. However, there is distortion in the audio. At first, turned way up, the audio would go dead, then recover after about a minute. One 45 output tube developed a faint blue glow when volume up, so replaced the tube. Now, it doesn't cut out, but there is still distortion. The on-line Riders says 45 bias should be 46v. I measure essentially nothing. I assume that 46v is to ground. Measured the grid-to-grid resistor (part #3423/100k) and got 104k. Measured the Secondary of the input transformer and got 3.48k and 3.03k each side to ground. Shouldn't they be the same? I get 1.37K across the transformer's primary. There is 300v on each plate. Finally, I notice little or no difference when using either tube alone. Anyone have an idea what's going on? Thanks for the help. Jim.

Hi Jim

Measure voltage from grid to filament on each #45 tube. Both tubes must have the grid around 40 volts negative in relation to filament. The radio can operate with only one side right but with distortion.

Center tapped transformers will not be exactly the same. One half of the winding will be inside. Although both have the same number of turns the winding inside has less wire, will have lower resistance.

Norm

:Still working on this fine old TRF, and its working and very sensitive. However, there is distortion in the audio. At first, turned way up, the audio would go dead, then recover after about a minute. One 45 output tube developed a faint blue glow when volume up, so replaced the tube. Now, it doesn't cut out, but there is still distortion. The on-line Riders says 45 bias should be 46v. I measure essentially nothing. I assume that 46v is to ground. Measured the grid-to-grid resistor (part #3423/100k) and got 104k. Measured the Secondary of the input transformer and got 3.48k and 3.03k each side to ground. Shouldn't they be the same? I get 1.37K across the transformer's primary. There is 300v on each plate. Finally, I notice little or no difference when using either tube alone. Anyone have an idea what's going on? Thanks for the help. Jim.

Jim

One other thing. Be sure coupling cap to previous stage has been replaced and has no measurable leakage. Positive on the 1st audio amp grid will cause distortion.

http://ww3.nostalgiaair.org/schematics/NCC/_NCC_1-4.gif

Norm

:Hi Jim
:
: Measure voltage from grid to filament on each #45 tube. Both tubes must have the grid around 40 volts negative in relation to filament. The radio can operate with only one side right but with distortion.
:
: Center tapped transformers will not be exactly the same. One half of the winding will be inside. Although both have the same number of turns the winding inside has less wire, will have lower resistance.
:
:Norm
:
::Still working on this fine old TRF, and its working and very sensitive. However, there is distortion in the audio. At first, turned way up, the audio would go dead, then recover after about a minute. One 45 output tube developed a faint blue glow when volume up, so replaced the tube. Now, it doesn't cut out, but there is still distortion. The on-line Riders says 45 bias should be 46v. I measure essentially nothing. I assume that 46v is to ground. Measured the grid-to-grid resistor (part #3423/100k) and got 104k. Measured the Secondary of the input transformer and got 3.48k and 3.03k each side to ground. Shouldn't they be the same? I get 1.37K across the transformer's primary. There is 300v on each plate. Finally, I notice little or no difference when using either tube alone. Anyone have an idea what's going on? Thanks for the help. Jim.

Hello, Norm. Thanks. I will try and check it out again tonight and let you know what I find. I think I checked the previous stage coupling, but will check that again also. I notice the voltage chart for bias on the previous stage calls for a positive voltage. That would be to cathode, and can't be right. More later.
Jim

Hi JIm

Bias on a tube with cathode is between grid & cathode. Noticed Rider shows positive grid voltages. They left out the minus signs. All those numbers should be negative.

Some radios do have a slightly positive grid on the detector. Not this one.

Norm

:Hello, Norm. Thanks. I will try and check it out again tonight and let you know what I find. I think I checked the previous stage coupling, but will check that again also. I notice the voltage chart for bias on the previous stage calls for a positive voltage. That would be to cathode, and can't be right. More later.
:Jim

Norm, Where do I begin? Well, the radio is playing good now. With all the voltage problems, I decided to back up and lowered the input electrolytic to the filter to 2mfd. That dropped the voltage on the 45's to 270v, and the bias to -38v. The screens on the three RF amps are now 90v...plates at 175v. 1st audio and detector plates now 160v (I am assuming the schematic voltage chart has another misprint there regarding the detector). After checking and changing a couple resistors the 1st audio bias now -10.4v; detector -6.1v. 1st and 2nd RF bias at -3.2v. Here's the really weird part: 3rd RF bias was out of the ballpark. In looking it over I noticed cathode not tied into the gain/pot configuation with the previous stages. It went directly to a resistor and ground. Tied it into the other two, and now bias at -5.3v. Most of the distortion gone now, plus no more cut-out due to front end overload. Hard to tell if it was a long ago repairman's error (caps were replaced in the area), or maybe just one stage of circuit evolution that eventually got changed in later models. Anyway, 90% better. Some crud still in the audio, but I think I'll quit while still ahead....unless you think any of this looks like there may be something left to do. What do you think? Jim.