Garey Barrell <[EMAIL PROTECTED]> made an utterance to the drakelist gang ---------------------------------------------------------------------- Dino -
Those caps were chosen for both their voltage rating AND their current handling capacity. There are two to divide the RF currents further as well as to reduce the series inductance. Another pair are used as the plate coupling cap at the "other" end of the RFC for the same reasons.
Since that is a rather unusual value for a disc ceramic capacitor, I suspect Drake chose it for a reason. The L-4B used a pair of 0.001 uF disc ceramics in each of those two places, and there was almost certainly a reason WHY they changed. The most likely reason is to compensate (eliminate) possible resonances in the RFC circuitry. While the L-4B was designed with the "legacy" ham bands in mind, with a little excursion for MARS, etc., the L-7 was marketed as a "Wideband" linear amplifier and included the WARC bands plus general frequency coverage. Another clue is C41, the 47 pF capacitor which is connected across a portion of the RFC. Contrary to popular belief, Drake engineers didn't just pick seemingly "odd" value components out of thin air. These caps don't appear to be used in any other product, so they weren't just using what was in the stock room either. So they were ordered specifically for this application, certainly at greater expense than using a standard 0.001 uF cap like they used in the L-4B.
So. If John Kriner <[EMAIL PROTECTED]> doesn't have an original part, _three_ of the 0.0005 caps from RF Parts might be a good substitute. A 1500 pF "doorknob" capacitor could also be a good substitute, but I would try to stay pretty close to that value.
As for what caused the problem, it could be any number of things. Most like would be a parasitic resonance of some sort resulting from some combination of tuning, loading, antenna insulator flashover, a bird flying into the antenna ... :-) It could also be a random failure of that one part due to a manufacturing defect the was brought to the surface by one or more or none of the above. I'm not aware of any plague of failure of this particular cap, so more than likely it's a random occurrence that might never happen again.
73, Garey - K4OAH Glen Allen, VA Drake 2-B, 4-B & C-Line Service Supplement CDs <www.k4oah.com>
On 10/23/07, Dino Papas <[EMAIL PROTECTED] <mailto:[EMAIL PROTECTED]>> wrote: > > Dino Papas <[EMAIL PROTECTED] <mailto:[EMAIL PROTECTED]>> made an utterance to the drakelist gang > ---------------------------------------------------------------------- > A friend's L7 gave up the ghost mid-QSO with the self destruction of > one of the two disc ceramic 760pF caps that bypass the HV line at the > cold end of RFC5. They're rated at 4KV....anyone know of a good > source for replacements? Also obviously gotta discover what made one > of the two fail. RF Parts has some 0.0005 uf 10KV disks that may > work in this application. BTW, his L7PS is fine as tested with his > L-4B after the L7 failure. > > Thanks much in advance! > > 73 -- Dino KL0S/4 >
---------------------------------------------------------------------- Submissions: [EMAIL PROTECTED] Unsubscribe: [EMAIL PROTECTED] - unsubscribe drakelist in body Hopelessly Lost: [EMAIL PROTECTED] - help in body of message Zerobeat Web Page: www.zerobeat.net - sponsored by www.tlchost.net ----------------------------------------------------------------------
