If you do a search on e-bay for "capacitor doorknob" and you'll be surprised
at the values you find. I'm looking at one right now with 5 of the same
capacitors I used for $24.50 "Buy it now" price. Here is the link if you
want to look:
http://cgi.ebay.com/470-pF-16-KV-Doorknob-Capacitors-Brand-New-Lot-of-5_W0QQitemZ200422100559QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item2eaa168e4f
Ron N6IE
www.N6IE.com
----- Original Message -----
From: "John McAlpine IV" <kz4b@hotmail.com>
To: <tentec@contesting.com>
Sent: Tuesday, December 29, 2009 8:31 PM
Subject: Re: [TenTec] 238 caps
>
> I have two Model 238 and two older Model 229 (electrically identical to
> 238 but earlier cosmetic design) TT Tuners to go with my two legal limit
> TITAN 1 (425)'s and two Ameritron AL1500 Linears that I operate on both AM
> & SSB.
> Due to frequent (nominal 400 picofarad) disc ceramic switched "shunt"
> capacitor failures--I have changed each of them out to small 5kV 400 pfd
> "doorknob type capacitors mounted four in a row on the bottom of each of
> the tuners in a line just under the central shaft connecting the capacitor
> selector knob on the front panel to the shunt switch wafer at the back of
> the tuner. These capacitors appear to fail due to high RF Current (they
> get hot) and not excessive voltage. With this modification I have had zero
> tuner failures of any kind even with challenging antennas and max legal
> power with this mod in place.
> The only problem with the above mod is the increasingly high cost of
> ceramic doorknob type capacitors. I was very lucky in finding sixteen
> blue-colored 5/8 inch diameter surplus doorknob capacitors at the Shelby
> Hamfest approximately five years ago for only $5.00 each! I have NO extra
> capacitors available for sale, don't remember the name of the Vendor, and
> haven't confirmed any current sources. Try RF Parts and Surplus sales of
> Nebraska for starters. Good luck in your hunt.
> John R. McAlpine IV, P.E. / KZ4B 110 Lakefront DriveMooresville, N.C.
> 28117704-663-1364
>
>> Date: Tue, 29 Dec 2009 19:23:45 -0600
>> From: ranchorobbo@gmail.com
>> To: tentec@contesting.com
>> Subject: Re: [TenTec] 238 caps
>>
>> I get it now, thanks Ron and Jim. I have not observed any trouble
>> with mine yet but I use it to tune my inverted L on 160 when I run a
>> 250 w. carrier there. some of my transmissions get uh, relatively
>> long. the pep is around 1.2 kw. I may pull mine out and look inside
>> to see if there are any signs of stress. I smoked a supposedly legal
>> limit low pass filter a few months ago, the type that cuts off at 30
>> MHz, but in that case the failure was torroid inductors.. the caps,
>> similar to the ones in the 238 were all fine.
>>
>> 73
>>
>> Rob
>> K5UJ
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>
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