Title: GE BT-20-A Oscillator Post by: Rick K5IAR 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 Title: Re: GE BT-20-A Oscillator Post by: Rick K5IAR 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 Title: Re: GE BT-20-A Oscillator Post by: Rick K5IAR 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 Title: Re: GE BT-20-A Oscillator Post by: N3DRB The Derb 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.
Title: Re: GE BT-20-A Oscillator Post by: Steve - WB3HUZ 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.
Title: Re: GE BT-20-A Oscillator Post by: Rick K5IAR 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 Title: Re: GE BT-20-A Oscillator Post by: Steve - WB3HUZ 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.
Title: Re: GE BT-20-A Oscillator Post by: Rick K5IAR on September 25, 2007, 12:43:14 PM Thanks Mack.. this one is sure kicking my butt right now! AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
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! ;D Rick/K5IAR |