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The VFO in my newest HT46 has excessive drift. In standby mode it will drift about 1KHz in the first 15 minutes and then go another 600Hz in the following hour. The VFO drifts downward in frequency so it appears as though the capacitance in the tuned circuit needs more NEGATIVE temperature coefficient. It has a 10pfN750 and an 82pfN80 capacitor for compensation but they aren’t quite doing the job.

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Following some work by W7ZOI I have experimented with some different series combinations of N750 and NP0 capacitors in place of the 10pfN750. I have reduced the drift to about 10Hz/C but the series combination that produced that adds too much total capacitance to the circuit so it is not a solution. Experimental results and a few calculations would indicate I need a 10pfN1285 capacitor but of course that will be difficult to achieve.

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What bothers me about all this is that I have two other VFOs of this design - one in an SX146 receiver and the other in an old beat up and abused HT46. Both those VFOs have negligible drift. I have checked and re-checked the components in the offending VFO and I have compared them to what is in the well behaved HT46. Hopefully someone has experience with this or knows about a common cause for such behavior. 73 – Jerry – W0PWE

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The VFO in my newest HT46 has excessive drift. In standby mode it will drift about 1KHz in the first 15 minutes and then go another 600Hz in the following hour. The VFO drifts downward in frequency so it appears as though the capacitance in the tuned circuit needs more NEGATIVE temperature coefficient. It has a 10pfN750 and an 82pfN80 capacitor for compensation but they aren’t quite doing the job.

?

Following some work by W7ZOI I have experimented with some different series combinations of N750 and NP0 capacitors in place of the 10pfN750. I have reduced the drift to about 10Hz/C but the series combination that produced that adds too much total capacitance to the circuit so it is not a solution. Experimental results and a few calculations would indicate I need a 10pfN1285 capacitor but of course that will be difficult to achieve.

?

What bothers me about all this is that I have two other VFOs of this design - one in an SX146 receiver and the other in an old beat up and abused HT46. Both those VFOs have negligible drift. I have checked and re-checked the components in the offending VFO and I have compared them to what is in the well behaved HT46. Hopefully someone has experience with this or knows about a common cause for such behavior. 73 – Jerry – W0PWE

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It looks like the problem was one of those big offenders you mentioned Walt. Specifically C85 82pfN80. It measures 82pf but seems to have lost it's temperature compensation characteristics. Now the challenge is finding a replacement.?

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Surplus Sales of Nebraska seems to be the only supplier of tubular capacitors but they don't have much in that range. I considered trying a Class 1 disk ceramic but most of those are NP0 or N750. My junk box produced a 62pfN150 and I have been playing with a series and parallel NP0 to get in the ball park. That is pretty crude, not a permanent solution and I'm still not where I need to be with the drift problem. Back to googling "dogbone" "tubular ceramic" "temperature compensating capacitor" etc.?