Stabilizing Frequency - what do you do?

[Lkaskel]

Hey Everyone,
I guess the 1st thing to do is explain the case study. I have a 198? President Jackson export radio with the PB 042 board in it (see attached pic). I have completed a full recap. removed the glue and aligned. The only mod is an unlocked clarifier and the rest of the radio is set to spec. The truth is, it's a great receiving radio and I get nothing but great reports on the audio (mainly SSB) using a Turner +2 mic. The radio never gets turned off only down (0 volume). It's been running for about 3 weeks now. As the temprature has dropped here in Maryland the overall temp in the house may drop only a few degrees. The interesting thing is, every few days the radio is raising in frequency. When the radio was aligned I think the house was about 72 degrees and it is currently 67. It's currently 250Hz high on all bands. I am assuming that this is due to the temprature.

What do you do to stabilize the frequency in radios like this? Would you have any other thoughts as to why the frequency would rise like this other than temprature?

 

[sp5it]

Use DDS synthesiser.
Not worth the effort IMO.
Mike

 

[Enterprise312ok]

I have been looking in to the qh40a crystal heaters, they use them on drifty ham radios.

 

[9C1Driver]

Some radios are more prone to drift than others. Generally processor controlled radios are more stable then say a Galaxy sailboat on SSB. You might do some research on a TCXO circuit and see if it could be added to your radio. Sometimes clarifier mods have a negative effect on frequency stabilization as well. Good luck, nice old school radio.

 

[brandon7861]

Turn up the thermostat and check again tomorrow.

What frequency changed? If all modes moved the same, it suggests the oscillator that is drifting is common to all modes. Being you have already done an alignment, I assume you can figure out which oscillator is the culprit.

Random thoughts follow...

Do new electrolytic capacitors need time to reform? If they sat for a few years at the factory, their characteristics may have changed in the first week of use. If a cap is used to stabilize a tuning voltage, maybe that slight change in leakage resistance cause it to shift.

Maybe there is a bad trim cap or pot that changed value from being slightly jarred around. Had that happen in my washington. Alignment went great, radio worked fine, a few days later, I was 400Hz off. Moving the radio on the desk was enough to make it abruptly change frequency. Lightly tapping on the trim cap sent my frequency all over the place, problem found! A wore spot in the clarifier pot can do this too.

What about wax? Every radio I ever had with wax around the oscillators was rock solid. Maybe that is enough?

I can't remember where I heard this, but I remember something about physical shocks causing the drift characteristics in a crystal to "reset". I wish I had dug further into that when I came across it because I know nothing more about the subject. I have no first hand observations of this happening, but if it is true, perhaps giving the crystals a little tap before alignment might reduce the odds of it being off frequency later down the road if the radio does get bumped. I'm not advocating hitting your radio with a hammer, just set it down hard a couple times before alignment. It's going to happen eventually.

 

[Amperex]

Only solution is a crystal heater kit. Wire it to operate when radio is turned off and keep power supply on.