My new console radio

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Re: My new console radio

Post by Ragwire on Mon Oct 27, 2014 10:20 pm

Still haven't got the FM side working, but I believe I have narrowed it to the tuning section.
--The oscillator is running...-3.4V on the oscillator grid of the 6BA7 converter tube until I kill it with a finger on the oscillator section of the tuning gang...and I can also pick up the signal from it on another FM radio.

--I have correct voltage on all the tube's pins.

--I can inject a low level 10.7 MHz IF from another FM radio into the Coronado's converter plate circuit and get stations through the IF and audio sections and loud and clean out the speaker.

--I can do the same to the converter input or antenna terminals and it works.

--I can jump a FM broadcast frequency signal directly from the Coronado's FM antenna terminals (hooked to my homebrew FM dipole) into the oscillator grid of the converter tube, and receive stations...a bit weak beacause of background noise, but plenty loud from the speaker. When this is done, I can tune in various stations, but they are not in the right places on the dial.

--If I jumper this same off-the-air signal into the antenna grid of the converter tube--wher it is supposed to go in--I have nothing but soft static. Tuning around yields nothing.

It seems like the individual components are working, but the tuning gang for the FM and/or the oscillator is/are way off frequency.
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Re: My new console radio

Post by 75X11 on Mon Oct 27, 2014 10:39 pm

Wonder if any of your mica caps in that area may be changing in value?
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Re: My new console radio

Post by Ragwire on Tue Oct 28, 2014 8:41 am

Could be(?) Last night I unlugged the radio and put the signal generator across the FM antenna terminals and the signal tracer to the high side of the FM part of the tuning gang to try to see where it is resonating. A 108 MHz input signal seems to be peaking the tone near the middle-bottom of the dial scale. Seems like it needs more capacitance...I want to get the RF tuning about right so I can then work with the oscillator frequency. I'll post back when I try bridging new trimmers.
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Re: My new console radio

Post by Bill Cahill on Tue Oct 28, 2014 11:53 am

Sounds like bad mica caps, or, off value resistors to me. There were also some small round very sensitive white round mica caps usually rated at one percent tolerance used by manufacturers that occasionaaly short, or, open.

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Re: My new console radio

Post by 75X11 on Tue Oct 28, 2014 12:31 pm

Sometime the micas can be influenced to do their stuff by a light application of heat. That can work for those that are not a total fail, but are on their way south.
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Re: My new console radio

Post by Ragwire on Tue Oct 28, 2014 7:04 pm

Thanks guys. I replaced the cap on the antenna coil tank with a trimmer and adjusted the coil a little and got it closer. Now it picks up 106 MHz on 108, but the tracking needs more work. I am getting frustrated and decided to shelve this project for a while and work on something else. It looks like a long road ahead getting the Coranado fully functional, and it stopped being fun. I'll get back to it at some point.
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