For years I have been heartlessly slaughtering innocent Heathkit Q multipliers, just to get these variable capacitors. Dean KK4DAS has joined me in this mayhem. And recently Wes W4JYK has also started hunting down QF-1s.
I am sorry to report that my love for this variable capacitor has taken a hit. The capacitance is perfect. And it has a reduction drive built into the shaft. But I have found that that reduction Drive has a form of backlash. It is really just a dead spot that you find when tuning in a signal. Turning in one direction, all is good. But when you then try to turn in the other direction, you have to turn the shaft a bit before the cap blades begins to move (see video above). This makes tuning SSB signals a bit difficult.
What should I do? Should I pull out a Dremel and cut off the reduction drive, then use a better (external) reduction drive to move the vanes? Any other ideas?
These are the kinds of problems that homebrewers face...
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SI5351.....problem solved.
ReplyDeleteBill, old dry thick grease within the reduction assembly may be the problem. When I was a field TV repair tech in the late 70's, I would get 100's of service calls where the UHF tuners (which were tuned by an air variable cap with a ball bearing planetary reduction mechanism) would do this exact thing. To fix this we would dissemble UHF tuners and wash the small ball bearings and shaft in a solvent then apply new grease. With your cap, disassembly may not be possible, so I would try to soak the entire cap in a solvent overnight turning the shaft occasionally or perhaps boiling water for a time, then blow out the mechanism with compressed air and apply a light oil to the areas that need it.
Delete
DeleteWow, that is the advice I need. This may explain why I seem to have had better luck with these parts in earlier rigs. This allows me to avoid physically cutting off the reduction drive, or -- worse! -- giving up by going to an Si5351! Thanks. I will try this.
73 Bill
Following that logic why not just by an IC-7300? BOOM! DONE! PROBLEM SOLVED!
ReplyDeleteI don't think I'd dremel off the reduction drive. I would probably try to compensate either electrically by increasing the bandspread, or mechanically by putting a large diameter knob on it. I know neither of those actually fixes the problem, but they could make it more manageable by making tuning less critical.
ReplyDeleteThanks to you I bought a $2 QF-1 last year at Dayton for this very capacitor. It may have a bit of backlash, but for 2 dollars, I can live with it.
Well, I tried adding a padder and a trimmer. This spread the frequencies out nicely. But the problem with the dead space remains. I find that the dead spot makes tuning difficult. I have fixed this problem by using other capacitors, but to make this capacitor useful in a transceiver, I'm afraid that this reduction drive has to go. A bigger knob is not going to help. Have you tried using this capacitor in a rig? 73 Bill
DeleteI haven't yet. I haven't found the right project for it yet, plus I've gone down the rabbit-hole of varactor tuning lately. I did pick up a couple of vernier drives at Dayton as well, so I I guess if the backlash is too bad, I'll follow your lead and lop it off to replace with an external reduction drive. Do keep us posted with what you decide to do.
DeleteSometimes desperate times call for desperate measures. This might be one of them. Assuming that the dead spot is caused by some kind of contaminate and is not a flat spot on a bearing, maybe immersion of only that section in Isopropyl alcohol could help, with rotating the drive several times. This could take some time.
ReplyDeleteIf this doesn't remove or at least move the dead spot, my next suspicion that its a mechanically flat spot.
Forgot to mention: Should this work, the drive probably could use a drop of light oil, like Singer's Sewing Machine oil.
ReplyDeleteI use oil (sparingly) that on my Yamada (Lafayette Radio) YN-series dials.
"Should I pull out a Dremel and cut off the reduction drive, then use a better (external) reduction drive to move the vanes?"
ReplyDeleteYeah, I think you answered your own question,Bill.
I have used that shaft reducer on some VXO rigs; they are okay when they work. But if you are going through the effort of homebrew, the external reduction type makes sense. Unless your "Plunder Trove" has one already, a $15~$35 reduction drive may be worthwhile.
I can hear the dremel side-cutter already!
Come on Bill! You can’t have everything all at the same time. The touchy tuning was part of the excitement. Could you tune in that elusive signal or not successfully?
ReplyDeleteOr do what I do to sticky tuning shafts on my TenTec gear: drip WD-40 down the shaft and turn for a few minutes, then drain it out and drip in some silicon trombone oil. It might take a few days to let the WD-40 get down in and out, same with the oil. Repeat as many times as needed. It works for a few more years without tearing it apart. The old green grease dries out. Repairing the old tuning device (PTO) is a major project.