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Title: New Vacuum Cap Post by: W7SOE on November 22, 2010, 05:35:28 PM Just bought this for the 813 rig. Jennings 250pF
Seems ok. There is no "stop" to keep the shaft from unscrewing all the way out as you turn CCW towards maximum capacitance. Is this normal? Thanks Rich Title: Re: New Vacuum Cap Post by: KC2FXE on November 22, 2010, 06:01:24 PM Hello Rich :
That normall for the shaft to come out if turned cc all theway. but 250 pf may be a lite i "m useing a 350pf vacuumm cap in my 813 transmitter. John kc2fxe Title: Re: New Vacuum Cap Post by: W7SOE on November 22, 2010, 06:06:22 PM Hello Rich : That normall for the shaft to come out if turned cc all theway. but 250 pf may be a lite i "m useing a 350pf vacuumm cap in my 813 transmitter. John kc2fxe Yea, I hope it is big enough, it will be matched to a B&W 850A tank coil assembly. Another question. This one has the mount that is insulated from the chassis on the shaft side. Is it correct to think that this should be reversed so the HV/RF side is away from the shaft side which would then become ground? Rich Title: Re: New Vacuum Cap Post by: K1JJ on November 22, 2010, 06:35:41 PM Rich -
That cap is real old timer - an early design as far as vac caps go .... ;D I have a few like it in plate tank service too. Yes, the shaft/tuning should be grounded while the opposite end should be insulated from ground. Having HV on the shaft would ruin your day if you didn't use an insulated shaft, but don't risk it. Normally the shaft can screw out without a problem. However, one time I had one unscrew and the threaded core slammed against the vac variable and it imploded. It was under tension until the shaft fell out. Not sure if that was the design or a flaw, but it ruined an otherwise good cap. T Title: Re: New Vacuum Cap Post by: WA1GFZ on November 22, 2010, 08:38:33 PM Rich,
That should work fine with the 850A. Don't over tighten it in the CW direction which can cause failure. I had one of those once. Title: Re: New Vacuum Cap Post by: W7SOE on November 23, 2010, 03:13:06 PM Rich - That cap is real old timer - an early design as far as vac caps go .... ;D I have a few like it in plate tank service too. Yes, the shaft/tuning should be grounded while the opposite end should be insulated from ground. Having HV on the shaft would ruin your day if you didn't use an insulated shaft, but don't risk it. Normally the shaft can screw out without a problem. However, one time I had one unscrew and the threaded core slammed against the vac variable and it imploded. It was under tension until the shaft fell out. Not sure if that was the design or a flaw, but it ruined an otherwise good cap. T Well that makes sense. I will probably use an insulating joint and maybe fiberglass shaft to boot! Thanks Rich Title: Re: New Vacuum Cap Post by: WA1GFZ on November 23, 2010, 03:25:24 PM Why float the lead screw. you need to attach to the back of the 850A anyway.
Just swap the two clamps. Then the lead screw will be at ground, kiss Title: Re: New Vacuum Cap Post by: W7SOE on November 23, 2010, 03:30:09 PM Why float the lead screw. you need to attach to the back of the 850A anyway. Just swap the two clamps. Then the lead screw will be at ground, kiss Yes, yes, you are right. Too much paranoia. Rich Title: Re: New Vacuum Cap Post by: flintstone mop on November 23, 2010, 03:46:11 PM Rich - That cap is real old timer - an early design as far as vac caps go .... ;D I have a few like it in plate tank service too. Yes, the shaft/tuning should be grounded while the opposite end should be insulated from ground. Having HV on the shaft would ruin your day if you didn't use an insulated shaft, but don't risk it. Normally the shaft can screw out without a problem. However, one time I had one unscrew and the threaded core slammed against the vac variable and it imploded. It was under tension until the shaft fell out. Not sure if that was the design or a flaw, but it ruined an otherwise good cap. T YUP happened to me and that was the end of a very nice 15KV vac cap. FRED Title: Re: New Vacuum Cap Post by: K1JJ on November 23, 2010, 04:29:04 PM Well that makes sense. I will probably use an insulating joint and maybe fiberglass shaft to boot! Thanks Rich Yes, correct. Even though the shaft may be grounded at the chassis, I have gotten bitten when touching the shaft of a vac variable under operation. Not sure why there wud be induced RF voltage there, but I always use insulated couplers anyway. The test wud be to fire the rig up and carefully take a grounded clip lead (connected to a fiberglass rod) and touch the shaft. If you see any tiny sparks, then you know it's a hot shaft. (Like Shaft) T Title: Re: New Vacuum Cap Post by: KM1H on November 23, 2010, 05:26:12 PM I use Millen shaft bushings and have never been RF excited. ;D
Title: Re: New Vacuum Cap Post by: KB2WIG on November 23, 2010, 05:53:33 PM " If you see any tiny sparks, then you know it's a hot shaft. "
Damn right. klc Title: Re: New Vacuum Cap Post by: W7SOE on November 23, 2010, 06:09:17 PM The test wud be to fire the rig up and carefully take a grounded clip lead (connected to a fiberglass rod) and touch the shaft. If you see any tiny sparks, then you know it's a hot shaft. (Like Shaft) T OK, The fiberglass shaft and insulating coupling is back in! Title: Re: New Vacuum Cap Post by: K1JJ on November 23, 2010, 06:18:46 PM I use Millen shaft bushings and have never been RF excited. ;D Me too. I mentioned this problem to Chuck/K1KW one time. He told me I should ALWAYS use insulated shafts due to RF on the vac metal rod. I notice all his BIG rigs have used them. (I've never known the Tron or Chuck to be wrong when it comes to RF... ;D |