>
>On Sun, 3 Aug 1997 20:12:10 +0000 w8jitom@postoffice.worldnet.att.net
>writes:
>
>>
>>It seems to me the capacitor could easily be well within its rating.
>>The limiting factor would be tuning drift, and it would appear LONG
>>before the rating was exceeded.
>>
>>>it appears that the amplifier's designer made an engineering
>>>error.
>>
>>It's tough to tell since your long drawn-out condemnations
>>are based on incorrect, misinterpreted, or missing data.
>
>IMO the above statement should be applied to its author as well.
Amen, Carl. Before the grate vhf parasitics debate got going, I was
warned that one of the prime Rauchian debating weapons was the Text
Avalanche, which Rauch uses to bury those who dare question his "expert"
status. (thanks, John F.) [Will's archive is replete with many
impressive examples.]
>At least
>on this list we have an administrator that permits the discussions to
>play out without heavy handed censorship.
>
Hear, hear, Carl. The quickest way to poison a lively discussion is with
a control-freak censor who coddles whiny lap-dogs.
>>
>>Rich, it seems really silly to spend so much time publishing
>>incorrect information just to find fault with one single little
>>$5 doorknob in a $3000 amplifier.
>>
The fault was not with the doorknob capacitor, Mr. Rauch. A faulty
designer chose a capacitor that was not capable of reliably carrying the
current in the circuit.
RE: 'incorrect information': Using Mr. Rauch's numbers: the power
level is 4000w,. The Z is 50 ohms. Using Ohm's law: E=(P*R)^0.5,
E=447Vrms. At 1.9MHz, the current in a 1000pF cap. would be 447v/83.7
ohms = 5.3a. In a 500pF cap., the current would be 2.65a. It appears
that amplifier's designer either did not read the capacitor
specifications, or did not calculate the RF current burden on the
capacitor, or all of the above.
>>Have there been a rash of field failures in this component?
>
>Rash? No but a rather steady history of failures over the years. The two
>100pf HT50 type doorknobs that are switched in on 80M are more failure
>prone when compared to the additional 500pf that is used on 160M. This
>may be due to the simple fact that a stock 77 (of any flavor) can output
>considerably more power on 80.
>Since I am not in the position to analyze exactly why the failure
>occurred I will not add to the considerable verbiage. My contribution is
>strictly as a supplier of replacement capacitors over the past 11 years.
>Many customers have opted for the larger 857 (HT57) series ...
A good engineering choice, albeit delayed, based on AC Circuit Analysis
calculations.
>...while others
>have stuck with the original type.
>
... ...an invitation to another failure down the road.
>There also appears to be a misreading of capacitor readings for the
>500pf. In the HEC catalog they list the HT-58 and HH-58. The 500pf HT-58
>with a N-4700 TC is rated at 3.2A at 1 MHz. The 500pf HH-58 with a X5T
>TC is rated at only 1.4A at 1 MHz. ...
Not only are the current ratings grossly exceeded, Neg. 750 ppm/degree C
is pretty much tops for use in a tuned circuit. Neg. 4700 ppm/degree C
or X5T types are for coupling and bypass applications--not tuned
circuits.
Rich---
R. L. Measures, 805-386-3734, AG6K
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