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Author Topic: GE BT-20-A Oscillator  (Read 5921 times)
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Rick K5IAR
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« on: September 24, 2007, 03:57:48 PM »

Steve and all other BC Transmitter Gurus!  I have the BC-20-A ready to convert, but the 1885 kc rock will not oscillate in the circuit the way it is set up.  I checked the crystal in my signal generator and it works perfectly, right on frequency (naturally, I got from Bry!) but it won't do a thing in the oscillator circuit.  The oscillator and transmitter works great on it's native frequency of 1450 kc.  The oscillator coil (L2) is set to the highest tap.  Do I need to add/subtract some capacitance?  When I apply power and try to dip the oscillator I get zero change in oscillator current.  Any ideas?

Thanks,
Rick/K5IAR
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Rick K5IAR
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« Reply #1 on: September 24, 2007, 05:47:29 PM »

Hi Mack,

Thanks for the input.  The coil has three sections and two are shorted out leaving very few coil windings as you said.  I should have been more descriptive.  It doesn't appear to be oscillating at all on 1885, but it works perfectly on 1450, the original frequency.  I even tried feeding it with a signal generator and still got no oscillation even on 1450.  I guess the characteristics of the crystal are a lot more important than I suspected.

Thanks,
Rick/K5IAR
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Rick K5IAR
Guest
« Reply #2 on: September 24, 2007, 09:41:20 PM »

Well, I didn't look at the circuit good enough.  It's actually a Colpitts oscillator and the coil I have been referring to is the plate coil, not the oscillator coil and is used in conjunction  with a variable capacitor to peak the output of the oscillator, not assist with oscillation.  So, if anything needs adjusting it would be the split tank capacitors in the control grid circuitry.  I have since discovered that a different crystal, one that I use with the BC-610's, oscillates just fine and it is at 1985 kc, so the 1885 kc should have no problem.  The 1885 crystal works great, just not in the GE!  Go figure...

Rick/K5IAR
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N3DRB The Derb
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« Reply #3 on: September 25, 2007, 01:02:31 AM »

WB3HUZ owned one, we swapped it back and forth a couple of times. he might be able to tell you something on the osc. He put a lot more work into it than I did. he might have a manual too.
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Steve - WB3HUZ
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« Reply #4 on: September 25, 2007, 09:23:55 AM »

The crystal in the BT-20 was avacuum sealed with an octal socket, about the size of the 6V6. I don't know the type of crystal used (type of cut) and the capacitance of the holder, but those might be causing you problems. I never used the crystal on mine. Just ran a VFO straight to the grid of the oscillator tube.
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Rick K5IAR
Guest
« Reply #5 on: September 25, 2007, 10:31:11 AM »

Thanks guys...

It's the weirdest thing.  I have one of the original crystals like Steve described and the transmitter works great with it on 1450 kc.  I also have a couple of rocks I took out of the RCA box. I took them out of the ovens and plugged into the GE and they worked great too!  They were on 1400 kc.  Then I got the bright idea to jury rig a FT-171-B crystal from the BC-610 and it worked (not quite as well) on 1975 kc.  I know it sounds like the 1885 kc crystal is no good, but when I put it in my old RCA signal general it works great.  This is a real puzzler to me.  I tried feeding it directly from the signal generator, but the cable was too long and I got nothing.  I can switch the original crystal to either of the two sockets in the GE and it works fine.  I think the original crystal in it's vacuum can and likely special cut does make a big difference like Steve said.  I have a VFO in the shed I'll try next.

Thanks again for all the input and help.  If you guys think of anything else, please, let me know.

Rick/K5IAR
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Steve - WB3HUZ
Guest
« Reply #6 on: September 25, 2007, 12:28:49 PM »

If you are set on using a crystal, consider opening one of the GE crystal units and grinding the crystal up to 1885.
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Rick K5IAR
Guest
« Reply #7 on: September 25, 2007, 12:43:14 PM »

Thanks Mack.. this one is sure kicking my butt right now!

Steve, thanks for the idea, but my grinding skills are non existent.  I've tried a few, but never been very successful.  I'm cobbling together a small power supply to use with my Viking VFO, I think I'll be less likely to mess things up!  Grin

Rick/K5IAR
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