"From the test bench I stab at thee..."

[N8YX]

"I will chase you(r drifting reference oscillator) 'round the flames of Perdition (which will make the instability problem worse, no doubt) with alignment tool and counter forever..."

Got hold of a second FT-980 a few months back and have been bringing the radio back to specs slowly but surely. There's still an issue with the CPU board and the optical encoder not tuning until a 10+ minute warmup, but I can live with that until one of the two chips I suspect completely dies and is replaced.

The more insidious problem was an inability to net the master reference oscillator. It's a 30.0000MHz crystal with a 10pF disc and 20pF ceramic trimmer in parallel; the crystal assembly is temperature compensated. Yaesu's crystals always seem to drift as they age and a previous owner attempted to rectify the problem with a pair of 22pF discs in place of the original 10pF unit.

It just so happened that I have about a dozen or so 30.0000MHz rocks in HC-18/U holders, courtesy of eBay. Swapping the OEM crystal for one and placing a counter on the test point showed the upper frequency adjustment as ~1KHz low. After subbing a few capacitors I ended up removing the fixed unit from the circuit altogether and recalibrated the oscillator circuit.

Problem fixed. Maybe.

I have a few crystals left for anyone who encounters this situation and needs a replacement. Let me qualify that comment: I plan on running that particular rig throughout my Christmas vacation and see how the reference oscillator ages first with regard to overall stability and drift.

[X-Rated]

It sounds like the drift is due to temperature changes.

The more you load the crystal, (The more capacitance that is added to the crystal load) the less the capacitive changes due to temperature and such will have on the drift. Since it had a 10pF and a 20pF cap in there before, the crystal is loaded with about 6.7pF plus maybe (guessing) 4pF of stray capacitance which will make short of 11pF load on the crystal. This should be pretty good in the first place. The tuning slope, however, gets slower at the higher loads. The frequency will come way down with the pair of 22pF caps. That would be 11pF plus those strays (maybe 4pF) making 15pF load. This should make the drift due to capacitors minimal if you used NPO or other tight tolerance caps.

The other thing is the crystal. Loading that more will do nothing else to help. Most of the crystals are AT-cut. At that rate, the best one that you can get is one with the crystal turn points near 25C so the the changes in room temperature won't change your frequency much. Hopefully, your purchase corrected the issues you had with the earlier crystal.

Voltages on the circuit and changes on the oscillator load also have effects on frequency drift. If an IC that your oscillator is on has a varying impedance over temperature, the frequency can shift from that as well.

[KC2UGV]

Yaesu? Color me stupid here, but can't you just install one of their TXCO modules? I thought they were more or less universal?

[NQ6U]

New movie coming soon to a theater near you:

Star Trek XXIII: The Wrath of Fred

[X-Rated]

Yaesu? Color me stupid here, but can't you just install one of their TXCO modules? I thought they were more or less universal?

I thought he said he already had a temperature compensated XTAL. TCXO's just aren't for me. I need more like an OCXO. Yeah baby.

[N8YX]

The temp-compensation used in this setup consists of a jacket around the crystal, to which a thermistor is attached. It is used to change the base biasing of the oscillator transistor as a function of temperature.

This isn't an oven and the circuit concept predates TCXO implementation by the majority of amateur equipment manufacturers. Better than nothing but there are more effective ways to achieve good crystal oscillator stability.

I wouldn't mind replacing the oscillator with a 30MHz TCXO if I can find one which would fit in the existing space.

May have to add capacitance - say, 4-6pF - to net the circuit at max internal temp. Will see come tomorrow after a 24h heat cycle.