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 Post subject: Help: GE 218 AM/FM
PostPosted: Jun Thu 06, 2019 1:43 am 
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Location: Olathe, KS
I need some expert help. I have a strange (to me) problem on a GE 218 AM/FM radio. The radio receives well on both AM and FM. However, I cannot get the oscillator to align. An AM station at 610KC shows at 750KC on the dial. I cannot get the oscillator to go far enough to align it for 610 on the dial. I have the same problem with the signal generator and below 610KC the radio is dead. The tuning capacitor is only 3/4 meshed when reception stops. However, the Broadcast oscillator bottoms out at 958.5KC (I can see the oscillator with my scope on pin 1 of the 12Be6). A very similar problem exists on FM (a station at 88.5MC is at 93MC on the dial. I checked for shorted tuning capacitor fins (oscillator and RF) and they are OK. I replaced R10, R28 R6, C10, C12, C16 and C8 in the oscillator circuit. I made sure the dial pointer is centered for the range of the tuning capacitor. I checked the band switch and all spot weld ground connections. The broadcast and FM oscillator coils have good continuity and are not shorting. I also replaced the 12BE6 converter (twice). A clip of oscillator and converter tube circuit schematic is attached.

PS: Maybe the broadcast IF is aligned on the wrong frequency? I went through it twice at 455KC and it seemed ok. The Broadcast oscillator is at 1.143 MC when I am receiving the local AM 610KC station. It should be 1.065MC - right? What's up with that????? I double checked my signal generator with my o-scope and it is right on frequency.

Your help is appreciated,

Joe


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 Post subject: Re: Help: GE 218 AM/FM
PostPosted: Jun Thu 06, 2019 2:00 am 
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Hi Joe

The IF frequencies may not be set properly? If the IF frequency is high the oscillator operates higher than usual. When this happens it takes less capacity change on the tuner to tune a range of frequencies. This in turn causes stations to be compressed on the dial.

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 Post subject: Re: Help: GE 218 AM/FM
PostPosted: Jun Thu 06, 2019 2:11 am 
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Location: Austin, Texas
I would kill the 12BE6 oscillator (ground grid 1) and see if there is something else oscillating. Maybe something is mixing in another frequency.

Jay


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 Post subject: Re: Help: GE 218 AM/FM
PostPosted: Jun Thu 06, 2019 2:13 am 
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Thanks for your information, Norm. I'll take another look at IF alignment tomorrow. I'll start with the 2nd IF with 455KC injected into pin 1 grid of V3 (IF amp).

Joe

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 Post subject: Re: Help: GE 218 AM/FM
PostPosted: Jun Thu 06, 2019 2:14 am 
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Thanks Jay. I'll try that first!

Joe

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 Post subject: Re: Help: GE 218 AM/FM
PostPosted: Jun Thu 06, 2019 11:14 pm 
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Thanks to your help, I have the AM working. The IF alignment was off considerably.

However, the FM side still has a problem with dial/oscillator alignment. All reception is shifted higher and compressed -- like the AM side was. I went through the FM IF alignment twice more with no improvement. All of the steps in SAMS say to connect a DC probe trough a 200K resistor connected to limiter grid (pin 1). Then you connect the signal generator to the various tube grids depending on the stage you are adjusting. I am suspicious that the voltage I adjusted for maximum deflection is negative. Is this correct?

Joe

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 Post subject: Re: Still Need Help: GE 218 FM Oscillator
PostPosted: Jun Sun 09, 2019 8:48 pm 
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As posted above, the AM side of this GE218 is fixed. Now I'm really flummoxed! The FM side receives really well but the range is off and reception is blank on the low side. I am able to scope the FM oscillator using the method suggested by pixellany (Mark) on this forum. I used a 2-turn loop on the end of my scope probe and can see the oscillator.

Here's the problem: The oscillator frequency range should be 98.7 to 118.7 MHz. The range on this radio is only 72.7 to 90.7MHz. A station at 89.3 MHz is should result in an oscillator frequency 10.7MHz above or 100MHz. However the station is receiving very well at an oscillator frequency of 78.6. This is 10.7 MHz BELOW the tuned frequency. I am pretty sure the IF is peaked to 10.7MHz (I've been through it 3 or 4 times now). The gang condenser looks clean and the fins are not shorting. I've replaced the caps and resistors in the oscillator circuit.

This radio uses 300 Ohm twin lead for an oscillator coil (documented in another thread post by me about a different GE 218). I've changed the 12BE6 tube several times now and verified voltages. What the heck is going on? Why can't I get the oscillator to run at the right frequency and range?

Help please!

Joe

PS: I think the previous owner had a problem with the oscillator. There are signs of non-factory solder around the oscillator "coil" and the twin lead may have been replaced.

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 Post subject: Re: Help: GE 218 AM/FM
PostPosted: Jun Mon 17, 2019 9:27 pm 
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Still working on this radio. It is becoming an obsession! I found that there is model 218H. The only difference is that the FM oscillator runs "on the high side" of the incoming signal! So the original oscillator was supposed to run below the incoming signal.

I am going to try modifying this radio to an "H" model. The oscillator coil part numbers are the same ( a short length 300 Ohm twin lead). The capacitors/resistors and wiring are different and there is no "center Tap" on the model H oscillator coil. I managed to find a couple of feet of 300 Ohm twin lead for the new H coil to make a new one. We'll see how it goes! I will report the progress or lack thereof here.

Joe

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 Post subject: Re: Help: GE 218 AM/FM
PostPosted: Jun Thu 20, 2019 3:22 am 
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I think I found the solution. I made the mods to an "H" model. I made a new oscillator coil from 300 Ohm twin lead (without the tap). The frequency range was too low so I trimmed the length of the twin lead until the oscillator fell into the rage of 10.7 BELOW the tuned frequency. Very strange because I thought the "H" mods would make the oscillator run 10.7 above. First impression is that the FM side is performing very well. For now, I am calling it a win. :D

Joe

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