Re: Taming a VFO - HT46
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The big offenders in the 46 VFO are C84 10pf N750, C85 82pf N80, C86 100pf NPO, C87 47pf NPO and C89 82pf NPO. These 5 capacitors are all 20 years past their useful life. I would replace all 5. You can start looking at Surplus Sales of Nebraska.
Once you get the VFO calmed down you may want to download a modernized alignment procedure at:
Good luck with project.
Walt Cates, WD0GOF
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From: [email protected] <[email protected]> on behalf of W0PWE via groups.io <j.b.hall@...>
Sent: Monday, December 2, 2024 12:17 PM To: [email protected] <[email protected]> Subject: [HallicraftersRadios] Taming a VFO - HT46 ?
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 ? |
waltcates