Crystal Controlled Novice Transmitter Chirp

[KB5JO]

Am recently interested in Boatanchors generally and old Novice transmitters in particular.  A recently acquired  Eico 723 chirps on 40M.  I owned one of these as a kid, and chirp wasn't ever a problem.  The crystal is just an HC49 crystal mounted inside an FT243 case and several OMs have told me that the tiny crystals will chirp in a hollowstate oscillator, and eventually fail.   Are these common issues with using small crystals in vacuum-tube oscillators?

I've ordered an FT-243 ( for 80M ) from a ham who grinds down lower frequency crystals, we'll see how that works out.  Also a new HC-6 with larger pins from ICM.  I'm going to try 80M with the genuine FT-243 to see how that works.

Are there any other alternates, like maybe changing somehow changing the voltage the crystal is seeing, besides using a VFO? 

73, Curt KB5JO

[W0GSQ]

I also have a 723.  I am using a mix of old WWII crystals and some of the newer ones in the old holders from AF4K, I have chirpers in both bunches. One thing that did help was to regulate the oscillator tube screen grid voltage with a OA2 voltage regulator tube.
One pin to ground and the other to pin 3 on the 6CL6 tube socket, simple,cheap,and made a big improvement.

[N8CMQ]

Chirp has many causes. Voltage fluctuations, over coupling, who can tell, until you start trouble-shooting.

[W8JI]

Chirp comes from:

A sluggish starting oscillator, just on the edge of starting

A modest or slow speed voltage change in the oscillator

A "slow" change in loading on the oscillator by a later stage

Rapid RF heating of the frequency determining components

Unstable feedback in the oscillator ( see    http://www.w8ji.com/johnson_vfo_chirp_jump.htm and  http://www.w8ji.com/globe_scout_modifications.htm )

When you use a small crystal, current can heat the element. The crystal current has NOTHING to do with grid current. It entirely is a function of feedback levels to the crystal. Too much feedback can shatter a crystal without any grid current at all.

My Globe Scout had so much feedback it shattered a crystal on 160 meters. I had to change capacitor values to reduce feedback, and for safety I added a small 50 mA 12 volt lamp in series with the crystal. That lamp actually glows a dull red, so it likely has about 10 mA of crystal current. Prior to changing feedback capacitor values, it lit almost normal brightness on some bands.

My guess would be there is truth to what you were told. It is logical that a smaller crystal element would get hotter, chirp more, and could eventually shatter. The safest thing to do would be to modernize the oscillator with solid state or use an additional tube stage at low power, and use the new stage to drive the original oscillator tube. They tried to minimize stages and components too much, and that makes stage operation needlessly critical.

73 Tom

[W0GSQ]

Chirp comes from:

A modest or slow speed voltage change in the oscillator

A "slow" change in loading on the oscillator by a later stage

Rapid RF heating of the frequency determining components

Unstable feedback in the oscillator ( see    http://www.w8ji.com/johnson_vfo_chirp_jump.htm and  http://www.w8ji.com/globe_scout_modifications.htm )

When you use a small crystal, current can heat the element. The crystal current has NOTHING to do with grid current. It entirely is a function of feedback levels to the crystal. Too much feedback can shatter a crystal without any grid current at all.

My Globe Scout had so much feedback it shattered a crystal on 160 meters. I had to change capacitor values to reduce feedback, and for safety I added a small 50 mA 12 volt lamp in series with the crystal. That lamp actually glows a dull red, so it likely has about 10 mA of crystal current. Prior to changing feedback capacitor values, it lit almost normal brightness on some bands.

My guess would be there is truth to what you were told. It is logical that a smaller crystal element would get hotter, chirp more, and could eventually shatter. The safest thing to do would be to modernize the oscillator with solid state or use an additional tube stage at low power, and use the new stage to drive the original oscillator tube. They tried to minimize stages and components too much, and that makes stage operation needlessly critical.

73 Tom

I guess that explains why a crystal that chirps in one BA transmitter doesn't in another one. Thanks Tom, I learned something new today.