04-16-2009, 11:07 PM
I was told when I got this radio that a shop could not solve the problem. I did a complete chassis overhaul. All new caps, resistors, wiring etc. I've rung it out three times now and all connections are correct. I could list all voltages but they are all within spec. I've checked resistance on all IF's and they all read within tolerances. So let me just tell you what I have now.
Power up radio and there's nothing except dial lights and a soft hum from speaker. With my DMM neg. lead attached to chassis ground I can touch the positive lead to the plate of the 1st detector XXL tube and music comes out of the speaker. I am tuned to 1260 on the dial. Strongest local station. Moving the tuning capacitor changes nothing. I still hear the same station. Only difference is that the speaker seems to sound like a sweep oscillator but the station still comes in. I did an oscillator check with a known good radio tuned to 1255Khz and the 41-285 tuned to 800Khz and the oscillator is working. The volume control works when listening to this station as well as the tone pot. I disconnected one end of the .01ufd cap (#38 on schematic). I connected a test lead to the plate of one of the 41 output tubes and the other end of that lead to a .001ufd cap. A lead to the other side of the .001ufd cap I connected to the high side of the volume pot where the #38 was disconnected. I powered up the set and slowly cranked up the volume. This produced a noise from the speaker that also sounded like a sweep oscillator so I'm confident my audio circuit is good.
So this is where I'm at. I do not know how to induce a signal into the individual stages to check the IF circuits. Would this be the next step? I have a frequency generator that does give me a solid 455Khz signal if necessary. I sure could use some help with this one folks.
Thanks much,
Johnny
Power up radio and there's nothing except dial lights and a soft hum from speaker. With my DMM neg. lead attached to chassis ground I can touch the positive lead to the plate of the 1st detector XXL tube and music comes out of the speaker. I am tuned to 1260 on the dial. Strongest local station. Moving the tuning capacitor changes nothing. I still hear the same station. Only difference is that the speaker seems to sound like a sweep oscillator but the station still comes in. I did an oscillator check with a known good radio tuned to 1255Khz and the 41-285 tuned to 800Khz and the oscillator is working. The volume control works when listening to this station as well as the tone pot. I disconnected one end of the .01ufd cap (#38 on schematic). I connected a test lead to the plate of one of the 41 output tubes and the other end of that lead to a .001ufd cap. A lead to the other side of the .001ufd cap I connected to the high side of the volume pot where the #38 was disconnected. I powered up the set and slowly cranked up the volume. This produced a noise from the speaker that also sounded like a sweep oscillator so I'm confident my audio circuit is good.
So this is where I'm at. I do not know how to induce a signal into the individual stages to check the IF circuits. Would this be the next step? I have a frequency generator that does give me a solid 455Khz signal if necessary. I sure could use some help with this one folks.
Thanks much,
Johnny