05-19-2009, 09:20 PM
Voltage and resistance checks:
Field coil: Spec = 680 ohms, actual = 814 ohms
Voltage divider: Spec = 140 and 27 ohms, actual = 137 and 29 ohms.
Once again I'm not sure if the differences are significant or not on the power supply end of the circuit. I suspect this part of the circuit is fine.
Output transformer: Black, white and center tap black-white reads 252, 265 and 250 volts. I applied a 10V, 400 Hz sine wave (with a 0.1 uf cap between the circuit and the function generator) on the grid of the number 42 output tubes. Audible with nice tone. Looks like the speaker works.
I checked all of the resistors from the first audio to the speaker and decided to replace them since some were reading high and share a common junction with two or more resistors. I ended up changing all of them. I'm still getting low volt on the second audio number 37 tubes: 168 and 163 volts. Voltage at the cathode (?) is also low: spec 95 reading 69. Tubes? When I applied <10V, 400 Hz sine wave on the grid, big time volume. I'm not sure if that tells me much.
Voltage off the phase inverter looks normal. Spec 140, actual of 186. There is (what I believe) a capacitor that makes a low pass filter on the grid of this tube. It reads 360 pf and the spec is 250 pf. I guess all that will do is draw off some highs. Not sure if it's important. This number 37 tube glows brighter than the audios.
Interesting note on the first audio tube. There are two inputs to the tube that are bridged by a 2.2 Mohm resistor that I replaced because it simply looked old. I've got a a decent multimeter and when I try to measure the voltage at each node. One of them clicks and the other actually makes reception better - good enough to manually tune in a station. The radio also plays better when the ground is disconnected. This is odd to me.
One the whole the signal is absent. Depending on the tone setting I get either load white noise or lower white noise (and I don't believe that's motorboating). I can only hear the radio signal through the noise faintly. I've got a 75 foot long wire connected as an antenna. It's connected to the one of four posts that gives the best reception, which isn't very good at all.
I'm at a loss now where to begin although it sure looks like teh second audio stage is the problem. Perhaps new tubes.
Field coil: Spec = 680 ohms, actual = 814 ohms
Voltage divider: Spec = 140 and 27 ohms, actual = 137 and 29 ohms.
Once again I'm not sure if the differences are significant or not on the power supply end of the circuit. I suspect this part of the circuit is fine.
Output transformer: Black, white and center tap black-white reads 252, 265 and 250 volts. I applied a 10V, 400 Hz sine wave (with a 0.1 uf cap between the circuit and the function generator) on the grid of the number 42 output tubes. Audible with nice tone. Looks like the speaker works.
I checked all of the resistors from the first audio to the speaker and decided to replace them since some were reading high and share a common junction with two or more resistors. I ended up changing all of them. I'm still getting low volt on the second audio number 37 tubes: 168 and 163 volts. Voltage at the cathode (?) is also low: spec 95 reading 69. Tubes? When I applied <10V, 400 Hz sine wave on the grid, big time volume. I'm not sure if that tells me much.
Voltage off the phase inverter looks normal. Spec 140, actual of 186. There is (what I believe) a capacitor that makes a low pass filter on the grid of this tube. It reads 360 pf and the spec is 250 pf. I guess all that will do is draw off some highs. Not sure if it's important. This number 37 tube glows brighter than the audios.
Interesting note on the first audio tube. There are two inputs to the tube that are bridged by a 2.2 Mohm resistor that I replaced because it simply looked old. I've got a a decent multimeter and when I try to measure the voltage at each node. One of them clicks and the other actually makes reception better - good enough to manually tune in a station. The radio also plays better when the ground is disconnected. This is odd to me.
One the whole the signal is absent. Depending on the tone setting I get either load white noise or lower white noise (and I don't believe that's motorboating). I can only hear the radio signal through the noise faintly. I've got a 75 foot long wire connected as an antenna. It's connected to the one of four posts that gives the best reception, which isn't very good at all.
I'm at a loss now where to begin although it sure looks like teh second audio stage is the problem. Perhaps new tubes.
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