01-06-2012, 03:58 PM
I'm fairly new at this so please bear with me. I do have a basic understanding of tube radio circuits (I've been helped greatly by a studying a copy of Elements of Radio Servicing by William Marcus -- highly recommended.)
I replaced all of the electrolytic and paper caps and checked all the resistors. Injecting a modulated 455 hz signal at the IF stage gives a good output on the speaker. However, the same signal at the input of the mixer gives a softer tone from the speaker. Tuning the radio to about 600 hz and moving the tuning dial on the signal generator doesn't give any tone when rotated through 600 hz. I'm suspecting that the oscillator is not running. The schematic calls for 110 volts (phono) 45 volts (radio) at the grid of the 7C5 oscillator. I get 118 and 47 respectively with my multi-meter. Since the original readings were with a 1000 ohm per volt meter I suspect my slightly higher readings are ok. However, the voltages at the plate should be 220 volts (phono) and 83 volts (radio) but I get 233 (phono) and 168 (radio). That last one seems too high. I did find a 15,000 ohm resistor (buried under all the wiring for the pushbuttons) that is in the plate circuit that I had missed before. It measures 21,000 ohms in circuit so I replaced it with a 15,000 ohm resistor. No help.
My problem is that I'm not able to understand the oscillator circuit since all the pushbutton switches and wiring is confusing me. The schematic shows the all pushbuttons in the "out" position but when I'm trying to test the radio I have the "broadcast" button pushed in. I don't understand what the schematic should look like with a button pushed in.
Any help understanding the effects on the circuit with the button in or any advice on what to check next would be greatly appreciated.
I replaced all of the electrolytic and paper caps and checked all the resistors. Injecting a modulated 455 hz signal at the IF stage gives a good output on the speaker. However, the same signal at the input of the mixer gives a softer tone from the speaker. Tuning the radio to about 600 hz and moving the tuning dial on the signal generator doesn't give any tone when rotated through 600 hz. I'm suspecting that the oscillator is not running. The schematic calls for 110 volts (phono) 45 volts (radio) at the grid of the 7C5 oscillator. I get 118 and 47 respectively with my multi-meter. Since the original readings were with a 1000 ohm per volt meter I suspect my slightly higher readings are ok. However, the voltages at the plate should be 220 volts (phono) and 83 volts (radio) but I get 233 (phono) and 168 (radio). That last one seems too high. I did find a 15,000 ohm resistor (buried under all the wiring for the pushbuttons) that is in the plate circuit that I had missed before. It measures 21,000 ohms in circuit so I replaced it with a 15,000 ohm resistor. No help.
My problem is that I'm not able to understand the oscillator circuit since all the pushbutton switches and wiring is confusing me. The schematic shows the all pushbuttons in the "out" position but when I'm trying to test the radio I have the "broadcast" button pushed in. I don't understand what the schematic should look like with a button pushed in.
Any help understanding the effects on the circuit with the button in or any advice on what to check next would be greatly appreciated.
